use {crate::rect::Rect, crate::tree::NodeId};

const MIN_SHRINK_DENOMINATOR: i32 = 8;

#[derive(Clone, Debug)]
pub struct MultiphaseRequest {
    pub bounds: Rect,
    pub windows: Vec<MultiphaseWindow>,
    pub clearance: i32,
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct MultiphaseWindow {
    pub node_id: NodeId,
    pub from: Rect,
    pub to: Rect,
    pub hierarchy: MultiphaseWindowHierarchy,
}

impl MultiphaseWindow {
    pub fn new(node_id: NodeId, from: Rect, to: Rect) -> Self {
        Self {
            node_id,
            from,
            to,
            hierarchy: Default::default(),
        }
    }

    pub fn with_hierarchy(
        node_id: NodeId,
        from: Rect,
        to: Rect,
        hierarchy: MultiphaseWindowHierarchy,
    ) -> Self {
        Self {
            node_id,
            from,
            to,
            hierarchy,
        }
    }
}

#[derive(Copy, Clone, Debug, Default, Eq, PartialEq)]
pub struct MultiphaseWindowHierarchy {
    pub source: MultiphaseHierarchyPosition,
    pub target: MultiphaseHierarchyPosition,
    pub transition: MultiphaseHierarchyTransition,
}

impl MultiphaseWindowHierarchy {
    pub fn new(source: MultiphaseHierarchyPosition, target: MultiphaseHierarchyPosition) -> Self {
        let transition = if !source.parent_is_mono && target.parent_is_mono {
            MultiphaseHierarchyTransition::EnteringMono
        } else if source.parent_is_mono && !target.parent_is_mono {
            MultiphaseHierarchyTransition::ExitingMono
        } else if source.parent.is_none() || target.parent.is_none() {
            MultiphaseHierarchyTransition::Unknown
        } else if target.depth < source.depth {
            MultiphaseHierarchyTransition::Ascending
        } else if target.depth > source.depth {
            MultiphaseHierarchyTransition::Descending
        } else {
            MultiphaseHierarchyTransition::SameLevel
        };
        Self {
            source,
            target,
            transition,
        }
    }

    fn reversed(self) -> Self {
        Self {
            source: self.target,
            target: self.source,
            transition: self.transition.reversed(),
        }
    }
}

#[derive(Copy, Clone, Debug, Default, Eq, PartialEq)]
pub struct MultiphaseHierarchyPosition {
    pub parent: Option<NodeId>,
    pub depth: u16,
    pub sibling_index: Option<u16>,
    pub split_axis: Option<PhaseAxis>,
    pub nearest_horizontal_split_depth: Option<u16>,
    pub nearest_vertical_split_depth: Option<u16>,
    pub parent_is_mono: bool,
    pub mono_active: bool,
}

#[derive(Copy, Clone, Debug, Default, Eq, PartialEq)]
pub enum MultiphaseHierarchyTransition {
    #[default]
    Unknown,
    SameLevel,
    Ascending,
    Descending,
    EnteringMono,
    ExitingMono,
}

impl MultiphaseHierarchyTransition {
    fn reversed(self) -> Self {
        match self {
            Self::Unknown => Self::Unknown,
            Self::SameLevel => Self::SameLevel,
            Self::Ascending => Self::Descending,
            Self::Descending => Self::Ascending,
            Self::EnteringMono => Self::ExitingMono,
            Self::ExitingMono => Self::EnteringMono,
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct MultiphasePlan {
    pub phases: Vec<MultiphasePhase>,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct MultiphasePlanned {
    pub plan: MultiphasePlan,
    pub explanation: MultiphasePlanExplanation,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct MultiphasePlanExplanation {
    pub strategy: PlanStrategy,
    pub phases: Vec<PhaseExplanation>,
    pub validation: ValidationExplanation,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct PhaseExplanation {
    pub action: MultiphasePhaseAction,
    pub reason: PhaseReason,
    pub nodes: Vec<NodeId>,
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct ValidationExplanation {
    pub continuous_overlap_passed: bool,
    pub final_rects_matched: bool,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub enum PlanStrategy {
    NoOp,
    SingleAction,
    MixedSinglePhase,
    HierarchyOrderedScales,
    OrientationChange { from_axis: PhaseAxis },
    SwapLanes { axis: PhaseAxis },
    SpaceThenOrthogonalGrowth { axis: PhaseAxis },
    ReversedForwardPlan { original: Box<PlanStrategy> },
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub enum PlanDirection {
    Forward,
    Reverse,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct RejectedStrategy {
    pub direction: PlanDirection,
    pub strategy: PlanStrategy,
    pub reason: MultiphasePlanFailure,
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub enum PhaseReason {
    SingleAction,
    SameAxisRedistribution,
    MixedAxisActions,
    ShrinkIntoLanes {
        lane_axis: PhaseAxis,
    },
    MoveThroughFreedSpace,
    GrowOutOfLanes,
    CreateSpaceForAscendingChild,
    MoveAscendingChildAfterSpaceExists,
    OrthogonalGrowthAfterMove,
    ParentAxisBeforeChildAxis {
        parent_axis: PhaseAxis,
        parent_depth: u16,
        child_axis: PhaseAxis,
        child_depth: u16,
    },
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct MultiphasePhase {
    pub action: MultiphasePhaseAction,
    pub steps: Vec<MultiphaseStep>,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub enum MultiphasePhaseAction {
    Uniform(PhaseAction),
    Mixed(Vec<PhaseAction>),
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct MultiphaseStep {
    pub node_id: NodeId,
    pub from: Rect,
    pub to: Rect,
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct PhaseAction {
    pub kind: PhaseKind,
    pub axis: PhaseAxis,
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub enum PhaseKind {
    Move,
    Scale,
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub enum PhaseAxis {
    Horizontal,
    Vertical,
}

impl MultiphasePhaseAction {
    fn from_step_actions(actions: Vec<PhaseAction>) -> Self {
        debug_assert!(!actions.is_empty());
        let first = actions[0];
        if actions.iter().all(|action| *action == first) {
            Self::Uniform(first)
        } else {
            Self::Mixed(actions)
        }
    }

    fn action_for_step(&self, idx: usize) -> Option<PhaseAction> {
        match self {
            Self::Uniform(action) => Some(*action),
            Self::Mixed(actions) => actions.get(idx).copied(),
        }
    }

    fn as_uniform(&self) -> Option<PhaseAction> {
        match self {
            Self::Uniform(action) => Some(*action),
            Self::Mixed(_) => None,
        }
    }
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub enum MultiphaseError {
    EmptyBounds,
    EmptyWindow,
    DuplicateWindow,
    InitialOverlap,
    FinalOverlap,
    NoPlan,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct MultiphasePlanDiagnostic {
    pub forward: MultiphasePlanFailure,
    pub reverse: Option<MultiphasePlanFailure>,
    pub attempted: Vec<RejectedStrategy>,
}

impl MultiphasePlanDiagnostic {
    fn legacy_error(self) -> MultiphaseError {
        match self.forward {
            MultiphasePlanFailure::Request(error) => error,
            _ => MultiphaseError::NoPlan,
        }
    }
}

impl ValidationExplanation {
    fn passed() -> Self {
        Self {
            continuous_overlap_passed: true,
            final_rects_matched: true,
        }
    }
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub enum MultiphasePlanFailure {
    Request(MultiphaseError),
    NoPattern,
    ShrinkBound {
        axis: PhaseAxis,
        available: i32,
        required: i32,
    },
    InvalidPhaseStep {
        action: PhaseAction,
        node_id: NodeId,
    },
    Validation(MultiphaseValidationError),
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub enum MultiphaseValidationError {
    DuplicatePhaseStep {
        phase: usize,
        node_id: NodeId,
    },
    PhaseActionCount {
        phase: usize,
        actions: usize,
        steps: usize,
    },
    UnknownPhaseStep {
        phase: usize,
        node_id: NodeId,
    },
    StaleStepStart {
        phase: usize,
        node_id: NodeId,
    },
    PhaseOverlap {
        phase: usize,
        a: NodeId,
        b: NodeId,
    },
    FinalMismatch {
        node_id: NodeId,
    },
}

#[derive(Clone, Debug, Eq, PartialEq)]
struct PlanForwardFailure {
    reason: MultiphasePlanFailure,
    attempted: Vec<RejectedStrategy>,
}

pub fn plan_no_overlap(request: &MultiphaseRequest) -> Result<MultiphasePlan, MultiphaseError> {
    plan_no_overlap_with_diagnostics(request).map_err(|diagnostic| diagnostic.legacy_error())
}

pub fn plan_no_overlap_with_diagnostics(
    request: &MultiphaseRequest,
) -> Result<MultiphasePlan, MultiphasePlanDiagnostic> {
    plan_no_overlap_explained(request).map(|planned| planned.plan)
}

pub fn plan_no_overlap_explained(
    request: &MultiphaseRequest,
) -> Result<MultiphasePlanned, MultiphasePlanDiagnostic> {
    if let Err(error) = validate_request(request) {
        return Err(MultiphasePlanDiagnostic {
            forward: MultiphasePlanFailure::Request(error),
            reverse: None,
            attempted: vec![],
        });
    }
    if request
        .windows
        .iter()
        .all(|window| window.from == window.to)
    {
        return Ok(MultiphasePlanned {
            plan: MultiphasePlan { phases: vec![] },
            explanation: MultiphasePlanExplanation {
                strategy: PlanStrategy::NoOp,
                phases: vec![],
                validation: ValidationExplanation::passed(),
            },
        });
    }
    if let Some(failure) = target_shrink_bound_failure(request) {
        return Err(MultiphasePlanDiagnostic {
            forward: failure,
            reverse: None,
            attempted: vec![],
        });
    }
    let forward = match plan_forward(request, PlanDirection::Forward) {
        Ok(plan) => return Ok(plan),
        Err(error) => error,
    };
    let reversed = reverse_request(request);
    match plan_forward(&reversed, PlanDirection::Reverse) {
        Ok(plan) => Ok(reverse_planned(plan)),
        Err(reverse) => {
            let mut attempted = forward.attempted;
            attempted.extend(reverse.attempted);
            Err(MultiphasePlanDiagnostic {
                forward: forward.reason,
                reverse: Some(reverse.reason),
                attempted,
            })
        }
    }
}

pub(crate) fn validate_phase_paths(
    request: &MultiphaseRequest,
    paths: &[Vec<(Rect, Rect)>],
) -> Result<MultiphasePlan, MultiphasePlanFailure> {
    if paths.len() != request.windows.len() {
        return Err(MultiphasePlanFailure::NoPattern);
    }
    let phase_count = paths.iter().map(Vec::len).max().unwrap_or(0);
    if phase_count == 0 {
        return Err(MultiphasePlanFailure::NoPattern);
    }
    let mut phases = vec![];
    for phase_idx in 0..phase_count {
        let mut steps = vec![];
        let mut actions = vec![];
        for (window_idx, path) in paths.iter().enumerate() {
            let Some((from, to)) = path.get(phase_idx).copied() else {
                continue;
            };
            if from == to {
                continue;
            }
            let step = MultiphaseStep {
                node_id: request.windows[window_idx].node_id,
                from,
                to,
            };
            let Some(action) = classify_step(step) else {
                return Err(MultiphasePlanFailure::NoPattern);
            };
            steps.push(step);
            actions.push(action);
        }
        if !steps.is_empty() {
            phases.push(MultiphasePhase {
                action: MultiphasePhaseAction::from_step_actions(actions),
                steps,
            });
        }
    }
    let plan = MultiphasePlan { phases };
    validate_plan_continuous_diagnostic(request, &plan)
        .map(|_| plan)
        .map_err(MultiphasePlanFailure::Validation)
}

pub(crate) fn partition_motion_groups(
    windows: &[MultiphaseWindow],
    clearance: i32,
) -> Vec<Vec<usize>> {
    let clearance = clearance.max(0);
    let mut groups = vec![];
    let mut seen = vec![false; windows.len()];
    for start in 0..windows.len() {
        if seen[start] {
            continue;
        }
        seen[start] = true;
        let mut group = vec![];
        let mut pending = vec![start];
        while let Some(idx) = pending.pop() {
            group.push(idx);
            let bounds = motion_bounds_with_clearance(windows[idx], clearance);
            for other in 0..windows.len() {
                if seen[other]
                    || !bounds.intersects(&motion_bounds_with_clearance(windows[other], clearance))
                {
                    continue;
                }
                seen[other] = true;
                pending.push(other);
            }
        }
        group.sort_unstable();
        groups.push(group);
    }
    groups
}

fn validate_request(request: &MultiphaseRequest) -> Result<(), MultiphaseError> {
    if request.bounds.is_empty() {
        return Err(MultiphaseError::EmptyBounds);
    }
    for (idx, window) in request.windows.iter().enumerate() {
        if window.from.is_empty() || window.to.is_empty() {
            return Err(MultiphaseError::EmptyWindow);
        }
        for other in &request.windows[..idx] {
            if other.node_id == window.node_id {
                return Err(MultiphaseError::DuplicateWindow);
            }
        }
    }
    if overlaps(request.windows.iter().map(|window| window.from)) {
        return Err(MultiphaseError::InitialOverlap);
    }
    if overlaps(request.windows.iter().map(|window| window.to)) {
        return Err(MultiphaseError::FinalOverlap);
    }
    Ok(())
}

fn target_shrink_bound_failure(request: &MultiphaseRequest) -> Option<MultiphasePlanFailure> {
    let min_width = sane_min_size(request.bounds.width());
    let min_height = sane_min_size(request.bounds.height());
    for window in &request.windows {
        if window.to.width() < min_width {
            return Some(MultiphasePlanFailure::ShrinkBound {
                axis: PhaseAxis::Horizontal,
                available: window.to.width(),
                required: min_width,
            });
        }
        if window.to.height() < min_height {
            return Some(MultiphasePlanFailure::ShrinkBound {
                axis: PhaseAxis::Vertical,
                available: window.to.height(),
                required: min_height,
            });
        }
    }
    None
}

fn plan_forward(
    request: &MultiphaseRequest,
    direction: PlanDirection,
) -> Result<MultiphasePlanned, PlanForwardFailure> {
    let mut rejection = None;
    let mut attempted = vec![];
    match plan_single_action_phase(request) {
        Ok(plan) => return Ok(plan),
        Err(error) => {
            record_rejection(&mut attempted, direction, PlanStrategy::SingleAction, error);
            if error != MultiphasePlanFailure::NoPattern {
                rejection.get_or_insert(error);
            }
        }
    }
    for axis in [PhaseAxis::Horizontal, PhaseAxis::Vertical] {
        match plan_space_then_orthogonal_growth(request, axis) {
            Ok(plan) => return Ok(plan),
            Err(error) => {
                record_rejection(
                    &mut attempted,
                    direction,
                    PlanStrategy::SpaceThenOrthogonalGrowth { axis },
                    error,
                );
                if error != MultiphasePlanFailure::NoPattern {
                    rejection.get_or_insert(error);
                }
            }
        }
    }
    match plan_hierarchy_ordered_axis_scales(request) {
        Ok(plan) => return Ok(plan),
        Err(error) => {
            record_rejection(
                &mut attempted,
                direction,
                PlanStrategy::HierarchyOrderedScales,
                error,
            );
            if error != MultiphasePlanFailure::NoPattern {
                rejection.get_or_insert(error);
            }
        }
    }
    for axis in [PhaseAxis::Horizontal, PhaseAxis::Vertical] {
        match plan_orientation_change(request, axis) {
            Ok(plan) => return Ok(plan),
            Err(error) => {
                record_rejection(
                    &mut attempted,
                    direction,
                    PlanStrategy::OrientationChange { from_axis: axis },
                    error,
                );
                if error != MultiphasePlanFailure::NoPattern {
                    rejection.get_or_insert(error);
                }
            }
        }
    }
    for axis in [PhaseAxis::Horizontal, PhaseAxis::Vertical] {
        match plan_axis_crossing_lanes(request, axis) {
            Ok(plan) => return Ok(plan),
            Err(error) => {
                record_rejection(
                    &mut attempted,
                    direction,
                    PlanStrategy::SwapLanes { axis },
                    error,
                );
                if error != MultiphasePlanFailure::NoPattern {
                    rejection.get_or_insert(error);
                }
            }
        }
    }
    Err(PlanForwardFailure {
        reason: rejection.unwrap_or(MultiphasePlanFailure::NoPattern),
        attempted,
    })
}

fn record_rejection(
    attempted: &mut Vec<RejectedStrategy>,
    direction: PlanDirection,
    strategy: PlanStrategy,
    reason: MultiphasePlanFailure,
) {
    attempted.push(RejectedStrategy {
        direction,
        strategy,
        reason,
    });
}

fn plan_single_action_phase(
    request: &MultiphaseRequest,
) -> Result<MultiphasePlanned, MultiphasePlanFailure> {
    let mut uniform_action = None;
    let mut is_uniform = true;
    let mut steps = vec![];
    let mut step_actions = vec![];
    for window in &request.windows {
        if window.from == window.to {
            continue;
        }
        let step = MultiphaseStep {
            node_id: window.node_id,
            from: window.from,
            to: window.to,
        };
        let Some(step_action) = classify_step(step) else {
            return Err(MultiphasePlanFailure::NoPattern);
        };
        if step_action.kind == PhaseKind::Scale {
            let (available, required) = match step_action.axis {
                PhaseAxis::Horizontal => (window.to.width(), sane_min_size(request.bounds.width())),
                PhaseAxis::Vertical => (window.to.height(), sane_min_size(request.bounds.height())),
            };
            if available < required {
                return Err(MultiphasePlanFailure::ShrinkBound {
                    axis: step_action.axis,
                    available,
                    required,
                });
            }
        }
        if uniform_action.is_some_and(|action| action != step_action) {
            is_uniform = false;
        }
        uniform_action.get_or_insert(step_action);
        steps.push(step);
        step_actions.push(step_action);
    }
    if steps.is_empty() {
        return Err(MultiphasePlanFailure::NoPattern);
    }
    if !is_uniform {
        return build_validated_plan(
            request,
            PlanStrategy::MixedSinglePhase,
            [phase_draft_mixed(
                steps,
                step_actions,
                PhaseReason::MixedAxisActions,
            )],
        );
    }
    let action = uniform_action.unwrap();
    build_validated_plan(
        request,
        PlanStrategy::SingleAction,
        [phase_draft_uniform(
            action,
            steps,
            single_action_reason(action),
        )],
    )
}

fn plan_hierarchy_ordered_axis_scales(
    request: &MultiphaseRequest,
) -> Result<MultiphasePlanned, MultiphasePlanFailure> {
    let mut changed_axes = vec![];
    for axis in [PhaseAxis::Horizontal, PhaseAxis::Vertical] {
        if request
            .windows
            .iter()
            .any(|window| interval_changed(window.from, window.to, axis))
        {
            changed_axes.push(axis);
        }
    }
    let [first_axis, second_axis] = changed_axes
        .try_into()
        .map_err(|_| MultiphasePlanFailure::NoPattern)?;
    let order = hierarchy_scale_axis_order(request, first_axis, second_axis)
        .ok_or(MultiphasePlanFailure::NoPattern)?;
    let mut current: Vec<_> = request
        .windows
        .iter()
        .map(|window| (window.node_id, window.from))
        .collect();
    let mut phases = vec![];
    let reason = PhaseReason::ParentAxisBeforeChildAxis {
        parent_axis: order.axes[0],
        parent_depth: order.depths[0],
        child_axis: order.axes[1],
        child_depth: order.depths[1],
    };
    for axis in order.axes {
        let mut steps = vec![];
        for window in &request.windows {
            let (_, rect) = current
                .iter_mut()
                .find(|(node_id, _)| *node_id == window.node_id)
                .unwrap();
            let next = with_main_interval(
                *rect,
                axis,
                main_start(window.to, axis),
                main_end(window.to, axis),
            );
            if next == *rect {
                continue;
            }
            if main_size(*rect, axis) == main_size(next, axis) {
                return Err(MultiphasePlanFailure::NoPattern);
            }
            steps.push(MultiphaseStep {
                node_id: window.node_id,
                from: *rect,
                to: next,
            });
            *rect = next;
        }
        if steps.is_empty() {
            return Err(MultiphasePlanFailure::NoPattern);
        }
        phases.push(phase_draft(PhaseKind::Scale, axis, steps, reason));
    }
    let [first, second] = phases
        .try_into()
        .map_err(|_| MultiphasePlanFailure::NoPattern)?;
    build_validated_plan(
        request,
        PlanStrategy::HierarchyOrderedScales,
        [first, second],
    )
}

fn hierarchy_scale_axis_order(
    request: &MultiphaseRequest,
    first_axis: PhaseAxis,
    second_axis: PhaseAxis,
) -> Option<HierarchyScaleAxisOrder> {
    let first_priority = hierarchy_axis_priority(request, first_axis)?;
    let second_priority = hierarchy_axis_priority(request, second_axis)?;
    match first_priority.cmp(&second_priority) {
        std::cmp::Ordering::Less => Some(HierarchyScaleAxisOrder {
            axes: [first_axis, second_axis],
            depths: [first_priority, second_priority],
        }),
        std::cmp::Ordering::Greater => Some(HierarchyScaleAxisOrder {
            axes: [second_axis, first_axis],
            depths: [second_priority, first_priority],
        }),
        std::cmp::Ordering::Equal => None,
    }
}

#[derive(Copy, Clone)]
struct HierarchyScaleAxisOrder {
    axes: [PhaseAxis; 2],
    depths: [u16; 2],
}

fn hierarchy_axis_priority(request: &MultiphaseRequest, axis: PhaseAxis) -> Option<u16> {
    request
        .windows
        .iter()
        .filter(|window| interval_changed(window.from, window.to, axis))
        .flat_map(|window| {
            [
                split_depth_for_axis(window.hierarchy.source, axis),
                split_depth_for_axis(window.hierarchy.target, axis),
            ]
        })
        .flatten()
        .min()
}

fn plan_axis_crossing_lanes(
    request: &MultiphaseRequest,
    axis: PhaseAxis,
) -> Result<MultiphasePlanned, MultiphasePlanFailure> {
    let moving_windows: Vec<_> = request
        .windows
        .iter()
        .copied()
        .filter(|window| window.from != window.to)
        .collect();
    if moving_windows.len() < 2 {
        return Err(MultiphasePlanFailure::NoPattern);
    }
    let orth_min = request
        .windows
        .iter()
        .map(|window| orth_start(window.from, axis))
        .min()
        .ok_or(MultiphasePlanFailure::NoPattern)?;
    let orth_max = request
        .windows
        .iter()
        .map(|window| orth_end(window.from, axis))
        .max()
        .ok_or(MultiphasePlanFailure::NoPattern)?;
    if moving_windows.iter().any(|window| {
        orth_start(window.from, axis) != orth_min
            || orth_end(window.from, axis) != orth_max
            || orth_start(window.to, axis) != orth_min
            || orth_end(window.to, axis) != orth_max
            || main_start(window.from, axis) == main_start(window.to, axis)
    }) {
        return Err(MultiphasePlanFailure::NoPattern);
    }
    let clearance = request.clearance.max(0);
    let lane_count = moving_windows.len() as i32;
    let available = (orth_max - orth_min) - clearance.saturating_mul(lane_count - 1);
    if available <= 0 {
        return Err(MultiphasePlanFailure::ShrinkBound {
            axis: axis.other(),
            available: 0,
            required: sane_min_size(orth_max - orth_min),
        });
    }
    let lane_size = available / lane_count;
    let mut lane_remainder = available % lane_count;
    let required = sane_min_size(orth_max - orth_min);
    if lane_size < required {
        return Err(MultiphasePlanFailure::ShrinkBound {
            axis: axis.other(),
            available: lane_size,
            required,
        });
    }

    let mut windows = moving_windows;
    windows.sort_by_key(|window| lane_sort_key(*window, axis));
    let mut phase1 = vec![];
    let mut phase2 = vec![];
    let mut phase3 = vec![];
    let mut phase4 = vec![];
    let mut lane_start = orth_min;
    for (idx, window) in windows.iter().enumerate() {
        let extra = if lane_remainder > 0 {
            lane_remainder -= 1;
            1
        } else {
            0
        };
        let lane_end = lane_start + lane_size + extra;
        let lane_from = with_orth_interval(window.from, axis, lane_start, lane_end);
        let lane_to = with_main_interval(
            lane_from,
            axis,
            main_start(window.to, axis),
            main_end(window.to, axis),
        );
        let lane_move = crossing_lane_move_rect(lane_from, window.to, axis);
        push_step(&mut phase1, window.node_id, window.from, lane_from);
        push_step(&mut phase2, window.node_id, lane_from, lane_move);
        push_step(&mut phase3, window.node_id, lane_move, lane_to);
        push_step(&mut phase4, window.node_id, lane_to, window.to);
        if idx + 1 < windows.len() {
            lane_start = lane_end + clearance;
        }
    }
    build_validated_plan(
        request,
        PlanStrategy::SwapLanes { axis },
        [
            phase_draft(
                PhaseKind::Scale,
                axis.other(),
                phase1,
                PhaseReason::ShrinkIntoLanes {
                    lane_axis: axis.other(),
                },
            ),
            phase_draft(
                PhaseKind::Move,
                axis,
                phase2,
                PhaseReason::MoveThroughFreedSpace,
            ),
            phase_draft(
                PhaseKind::Scale,
                axis,
                phase3,
                PhaseReason::SameAxisRedistribution,
            ),
            phase_draft(
                PhaseKind::Scale,
                axis.other(),
                phase4,
                PhaseReason::GrowOutOfLanes,
            ),
        ],
    )
}

fn crossing_lane_move_rect(from: Rect, target: Rect, axis: PhaseAxis) -> Rect {
    let size = main_size(from, axis);
    if main_start(target, axis) > main_start(from, axis) {
        let end = main_end(target, axis);
        with_main_interval(from, axis, end - size, end)
    } else {
        let start = main_start(target, axis);
        with_main_interval(from, axis, start, start + size)
    }
}

fn lane_sort_key(window: MultiphaseWindow, axis: PhaseAxis) -> (usize, i32, i32, u32) {
    let delta = main_start(window.to, axis) - main_start(window.from, axis);
    let direction = match delta.cmp(&0) {
        std::cmp::Ordering::Greater => 0,
        std::cmp::Ordering::Less => 1,
        std::cmp::Ordering::Equal => 2,
    };
    (
        direction,
        main_start(window.from, axis),
        main_start(window.to, axis),
        window.node_id.0,
    )
}

fn plan_space_then_orthogonal_growth(
    request: &MultiphaseRequest,
    axis: PhaseAxis,
) -> Result<MultiphasePlanned, MultiphasePlanFailure> {
    if request.windows.len() < 2 {
        return Err(MultiphasePlanFailure::NoPattern);
    }
    let orth_axis = axis.other();
    let min_width = sane_min_size(request.bounds.width());
    let min_height = sane_min_size(request.bounds.height());
    let mut phase1 = vec![];
    let mut phase2 = vec![];
    let mut phase3 = vec![];
    for window in &request.windows {
        if window.to.width() < min_width {
            return Err(MultiphasePlanFailure::ShrinkBound {
                axis: PhaseAxis::Horizontal,
                available: window.to.width(),
                required: min_width,
            });
        }
        if window.to.height() < min_height {
            return Err(MultiphasePlanFailure::ShrinkBound {
                axis: PhaseAxis::Vertical,
                available: window.to.height(),
                required: min_height,
            });
        }
        let main_changes = main_start(window.from, axis) != main_start(window.to, axis)
            || main_end(window.from, axis) != main_end(window.to, axis);
        let orth_changes = orth_start(window.from, axis) != orth_start(window.to, axis)
            || orth_end(window.from, axis) != orth_end(window.to, axis);
        let mut orth_from = window.from;
        if main_changes && main_size(window.from, axis) == main_size(window.to, axis) {
            let after_move = with_main_interval(
                window.from,
                axis,
                main_start(window.to, axis),
                main_end(window.to, axis),
            );
            push_step(&mut phase2, window.node_id, window.from, after_move);
            orth_from = after_move;
        } else if main_changes {
            let target_size = main_size(window.to, axis);
            let after_main_scale = if main_start(window.from, axis) == main_start(window.to, axis)
                || main_end(window.from, axis) == main_end(window.to, axis)
            {
                with_main_interval(
                    window.from,
                    axis,
                    main_start(window.to, axis),
                    main_end(window.to, axis),
                )
            } else if main_start(window.to, axis) < main_start(window.from, axis) {
                with_main_interval(
                    window.from,
                    axis,
                    main_end(window.from, axis) - target_size,
                    main_end(window.from, axis),
                )
            } else {
                with_main_interval(
                    window.from,
                    axis,
                    main_start(window.from, axis),
                    main_start(window.from, axis) + target_size,
                )
            };
            push_step(&mut phase1, window.node_id, window.from, after_main_scale);
            orth_from = after_main_scale;
            if main_start(after_main_scale, axis) != main_start(window.to, axis)
                || main_end(after_main_scale, axis) != main_end(window.to, axis)
            {
                let after_move = with_main_interval(
                    after_main_scale,
                    axis,
                    main_start(window.to, axis),
                    main_end(window.to, axis),
                );
                push_step(&mut phase2, window.node_id, after_main_scale, after_move);
                orth_from = after_move;
            }
        }
        if orth_changes {
            push_step(&mut phase3, window.node_id, orth_from, window.to);
        }
    }
    if phase1.is_empty() || phase2.is_empty() || phase3.is_empty() {
        return Err(MultiphasePlanFailure::NoPattern);
    }
    build_validated_plan(
        request,
        PlanStrategy::SpaceThenOrthogonalGrowth { axis },
        [
            phase_draft(
                PhaseKind::Scale,
                axis,
                phase1,
                PhaseReason::CreateSpaceForAscendingChild,
            ),
            phase_draft(
                PhaseKind::Move,
                axis,
                phase2,
                PhaseReason::MoveAscendingChildAfterSpaceExists,
            ),
            phase_draft(
                PhaseKind::Scale,
                orth_axis,
                phase3,
                PhaseReason::OrthogonalGrowthAfterMove,
            ),
        ],
    )
}

fn plan_orientation_change(
    request: &MultiphaseRequest,
    from_axis: PhaseAxis,
) -> Result<MultiphasePlanned, MultiphasePlanFailure> {
    if request.windows.len() < 2 {
        return Err(MultiphasePlanFailure::NoPattern);
    }
    let to_axis = from_axis.other();
    let min_lane_size = sane_min_size(main_size(request.bounds, to_axis));
    let target_start = request
        .windows
        .first()
        .map(|window| main_start(window.to, from_axis))
        .ok_or(MultiphasePlanFailure::NoPattern)?;
    let target_end = request
        .windows
        .first()
        .map(|window| main_end(window.to, from_axis))
        .ok_or(MultiphasePlanFailure::NoPattern)?;
    let source_start = request
        .windows
        .first()
        .map(|window| main_start(window.from, to_axis))
        .ok_or(MultiphasePlanFailure::NoPattern)?;
    let source_end = request
        .windows
        .first()
        .map(|window| main_end(window.from, to_axis))
        .ok_or(MultiphasePlanFailure::NoPattern)?;
    if request.windows.iter().any(|window| {
        main_start(window.from, to_axis) != source_start
            || main_end(window.from, to_axis) != source_end
            || main_start(window.to, from_axis) != target_start
            || main_end(window.to, from_axis) != target_end
            || main_size(window.to, to_axis) < min_lane_size
    }) {
        return Err(MultiphasePlanFailure::NoPattern);
    }

    let mut phase1 = vec![];
    let mut phase2 = vec![];
    let mut phase3 = vec![];
    for window in &request.windows {
        let lane = with_main_interval(
            window.from,
            to_axis,
            main_start(window.to, to_axis),
            main_end(window.to, to_axis),
        );
        let moved = with_main_interval(
            lane,
            from_axis,
            main_start(window.to, from_axis),
            main_start(window.to, from_axis) + main_size(lane, from_axis),
        );
        push_step(&mut phase1, window.node_id, window.from, lane);
        push_step(&mut phase2, window.node_id, lane, moved);
        push_step(&mut phase3, window.node_id, moved, window.to);
    }
    if phase1.is_empty() || phase3.is_empty() {
        return Err(MultiphasePlanFailure::NoPattern);
    }
    build_validated_plan(
        request,
        PlanStrategy::OrientationChange { from_axis },
        [
            phase_draft(
                PhaseKind::Scale,
                to_axis,
                phase1,
                PhaseReason::ShrinkIntoLanes { lane_axis: to_axis },
            ),
            phase_draft(
                PhaseKind::Move,
                from_axis,
                phase2,
                PhaseReason::MoveThroughFreedSpace,
            ),
            phase_draft(
                PhaseKind::Scale,
                from_axis,
                phase3,
                PhaseReason::GrowOutOfLanes,
            ),
        ],
    )
}

struct MultiphasePhaseDraft {
    action: MultiphasePhaseActionDraft,
    steps: Vec<MultiphaseStep>,
    reason: PhaseReason,
}

enum MultiphasePhaseActionDraft {
    Uniform(PhaseAction),
    Mixed(Vec<PhaseAction>),
}

fn phase_draft_uniform(
    action: PhaseAction,
    steps: Vec<MultiphaseStep>,
    reason: PhaseReason,
) -> MultiphasePhaseDraft {
    MultiphasePhaseDraft {
        action: MultiphasePhaseActionDraft::Uniform(action),
        steps,
        reason,
    }
}

fn phase_draft(
    kind: PhaseKind,
    axis: PhaseAxis,
    steps: Vec<MultiphaseStep>,
    reason: PhaseReason,
) -> MultiphasePhaseDraft {
    phase_draft_uniform(PhaseAction { kind, axis }, steps, reason)
}

fn phase_draft_mixed(
    steps: Vec<MultiphaseStep>,
    actions: Vec<PhaseAction>,
    reason: PhaseReason,
) -> MultiphasePhaseDraft {
    MultiphasePhaseDraft {
        action: MultiphasePhaseActionDraft::Mixed(actions),
        steps,
        reason,
    }
}

fn build_validated_plan<const N: usize>(
    request: &MultiphaseRequest,
    strategy: PlanStrategy,
    phases: [MultiphasePhaseDraft; N],
) -> Result<MultiphasePlanned, MultiphasePlanFailure> {
    let mut explanations = vec![];
    let phases: Vec<_> = phases
        .into_iter()
        .filter_map(|draft| {
            if draft.steps.is_empty() {
                return None;
            }
            let mut nodes: Vec<_> = draft.steps.iter().map(|step| step.node_id).collect();
            nodes.sort_by_key(|node_id| node_id.0);
            let action = match draft.action {
                MultiphasePhaseActionDraft::Uniform(action) => {
                    MultiphasePhaseAction::Uniform(action)
                }
                MultiphasePhaseActionDraft::Mixed(actions) => {
                    debug_assert_eq!(actions.len(), draft.steps.len());
                    MultiphasePhaseAction::from_step_actions(actions)
                }
            };
            explanations.push(PhaseExplanation {
                action: action.clone(),
                reason: draft.reason,
                nodes,
            });
            Some(MultiphasePhase {
                action,
                steps: draft.steps,
            })
        })
        .collect();
    for phase in &phases {
        for (idx, step) in phase.steps.iter().enumerate() {
            let action = phase.action.action_for_step(idx).unwrap();
            if classify_step(*step) != Some(action) {
                return Err(MultiphasePlanFailure::InvalidPhaseStep {
                    action,
                    node_id: step.node_id,
                });
            }
        }
    }
    let plan = MultiphasePlan { phases };
    validate_plan_continuous_diagnostic(request, &plan)
        .map(|_| MultiphasePlanned {
            plan,
            explanation: MultiphasePlanExplanation {
                strategy,
                phases: explanations,
                validation: ValidationExplanation::passed(),
            },
        })
        .map_err(MultiphasePlanFailure::Validation)
}

fn validate_plan_continuous(request: &MultiphaseRequest, plan: &MultiphasePlan) -> bool {
    validate_plan_continuous_diagnostic(request, plan).is_ok()
}

fn validate_plan_continuous_diagnostic(
    request: &MultiphaseRequest,
    plan: &MultiphasePlan,
) -> Result<(), MultiphaseValidationError> {
    let mut current: Vec<_> = request
        .windows
        .iter()
        .map(|window| (window.node_id, window.from))
        .collect();
    for (phase_idx, phase) in plan.phases.iter().enumerate() {
        if let MultiphasePhaseAction::Mixed(actions) = &phase.action
            && actions.len() != phase.steps.len()
        {
            return Err(MultiphaseValidationError::PhaseActionCount {
                phase: phase_idx,
                actions: actions.len(),
                steps: phase.steps.len(),
            });
        }
        for (idx, step) in phase.steps.iter().enumerate() {
            if phase.steps[..idx]
                .iter()
                .any(|prev| prev.node_id == step.node_id)
            {
                return Err(MultiphaseValidationError::DuplicatePhaseStep {
                    phase: phase_idx,
                    node_id: step.node_id,
                });
            }
            let Some((_, rect)) = current.iter().find(|(node_id, _)| *node_id == step.node_id)
            else {
                return Err(MultiphaseValidationError::UnknownPhaseStep {
                    phase: phase_idx,
                    node_id: step.node_id,
                });
            };
            if *rect != step.from {
                return Err(MultiphaseValidationError::StaleStepStart {
                    phase: phase_idx,
                    node_id: step.node_id,
                });
            }
        }
        let motions: Vec<_> = current
            .iter()
            .map(|(node_id, rect)| {
                let to = phase
                    .steps
                    .iter()
                    .find(|step| step.node_id == *node_id)
                    .map(|step| step.to)
                    .unwrap_or(*rect);
                RectMotion { from: *rect, to }
            })
            .collect();
        for (idx, motion) in motions.iter().enumerate() {
            if let Some((other_idx, _)) = motions[idx + 1..]
                .iter()
                .enumerate()
                .find(|(_, other)| motions_overlap_during_phase(*motion, **other))
            {
                return Err(MultiphaseValidationError::PhaseOverlap {
                    phase: phase_idx,
                    a: current[idx].0,
                    b: current[idx + 1 + other_idx].0,
                });
            }
        }
        for step in &phase.steps {
            let (_, rect) = current
                .iter_mut()
                .find(|(node_id, _)| *node_id == step.node_id)
                .unwrap();
            *rect = step.to;
        }
    }
    for window in &request.windows {
        if !current
            .iter()
            .find(|(node_id, _)| *node_id == window.node_id)
            .is_some_and(|(_, rect)| *rect == window.to)
        {
            return Err(MultiphaseValidationError::FinalMismatch {
                node_id: window.node_id,
            });
        }
    }
    Ok(())
}

#[derive(Copy, Clone)]
struct RectMotion {
    from: Rect,
    to: Rect,
}

fn motions_overlap_during_phase(a: RectMotion, b: RectMotion) -> bool {
    let mut interval = TimeInterval::unit();
    interval.intersect_less_than(edge_delta(a.from.x1(), a.to.x1(), b.from.x2(), b.to.x2()))
        && interval.intersect_less_than(edge_delta(b.from.x1(), b.to.x1(), a.from.x2(), a.to.x2()))
        && interval.intersect_less_than(edge_delta(a.from.y1(), a.to.y1(), b.from.y2(), b.to.y2()))
        && interval.intersect_less_than(edge_delta(b.from.y1(), b.to.y1(), a.from.y2(), a.to.y2()))
        && interval.is_non_empty()
}

fn edge_delta(a0: i32, a1: i32, b0: i32, b1: i32) -> LinearDelta {
    let from = a0 as i64 - b0 as i64;
    let to = a1 as i64 - b1 as i64;
    LinearDelta {
        start: from,
        velocity: to - from,
    }
}

#[derive(Copy, Clone)]
struct LinearDelta {
    start: i64,
    velocity: i64,
}

#[derive(Copy, Clone)]
struct TimeInterval {
    lower: Rational,
    lower_open: bool,
    upper: Rational,
    upper_open: bool,
}

impl TimeInterval {
    fn unit() -> Self {
        Self {
            lower: Rational::new(0, 1),
            lower_open: false,
            upper: Rational::new(1, 1),
            upper_open: false,
        }
    }

    fn intersect_less_than(&mut self, delta: LinearDelta) -> bool {
        if delta.velocity == 0 {
            return delta.start < 0;
        }
        let boundary = Rational::new(-delta.start, delta.velocity);
        if delta.velocity > 0 {
            self.tighten_upper(boundary, true);
        } else {
            self.tighten_lower(boundary, true);
        }
        self.is_non_empty()
    }

    fn tighten_lower(&mut self, value: Rational, open: bool) {
        match value.cmp(&self.lower) {
            std::cmp::Ordering::Greater => {
                self.lower = value;
                self.lower_open = open;
            }
            std::cmp::Ordering::Equal => {
                self.lower_open |= open;
            }
            std::cmp::Ordering::Less => {}
        }
    }

    fn tighten_upper(&mut self, value: Rational, open: bool) {
        match value.cmp(&self.upper) {
            std::cmp::Ordering::Less => {
                self.upper = value;
                self.upper_open = open;
            }
            std::cmp::Ordering::Equal => {
                self.upper_open |= open;
            }
            std::cmp::Ordering::Greater => {}
        }
    }

    fn is_non_empty(&self) -> bool {
        match self.lower.cmp(&self.upper) {
            std::cmp::Ordering::Less => true,
            std::cmp::Ordering::Equal => !self.lower_open && !self.upper_open,
            std::cmp::Ordering::Greater => false,
        }
    }
}

#[derive(Copy, Clone, Eq, PartialEq)]
struct Rational {
    num: i64,
    den: i64,
}

impl Rational {
    fn new(mut num: i64, mut den: i64) -> Self {
        if den < 0 {
            num = -num;
            den = -den;
        }
        Self { num, den }
    }

    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        (self.num as i128 * other.den as i128).cmp(&(other.num as i128 * self.den as i128))
    }
}

fn classify_step(step: MultiphaseStep) -> Option<PhaseAction> {
    let same_x = step.from.x1() == step.to.x1() && step.from.x2() == step.to.x2();
    let same_y = step.from.y1() == step.to.y1() && step.from.y2() == step.to.y2();
    let same_size = step.from.size() == step.to.size();
    match (same_x, same_y, same_size) {
        (false, true, true) => Some(PhaseAction {
            kind: PhaseKind::Move,
            axis: PhaseAxis::Horizontal,
        }),
        (true, false, true) => Some(PhaseAction {
            kind: PhaseKind::Move,
            axis: PhaseAxis::Vertical,
        }),
        (false, true, false) => Some(PhaseAction {
            kind: PhaseKind::Scale,
            axis: PhaseAxis::Horizontal,
        }),
        (true, false, false) => Some(PhaseAction {
            kind: PhaseKind::Scale,
            axis: PhaseAxis::Vertical,
        }),
        _ => None,
    }
}

fn single_action_reason(action: PhaseAction) -> PhaseReason {
    match action.kind {
        PhaseKind::Move => PhaseReason::SingleAction,
        PhaseKind::Scale => PhaseReason::SameAxisRedistribution,
    }
}

fn reverse_request(request: &MultiphaseRequest) -> MultiphaseRequest {
    MultiphaseRequest {
        bounds: request.bounds,
        clearance: request.clearance,
        windows: request
            .windows
            .iter()
            .map(|window| MultiphaseWindow {
                node_id: window.node_id,
                from: window.to,
                to: window.from,
                hierarchy: window.hierarchy.reversed(),
            })
            .collect(),
    }
}

fn reverse_plan(plan: MultiphasePlan) -> MultiphasePlan {
    MultiphasePlan {
        phases: plan
            .phases
            .into_iter()
            .rev()
            .map(|phase| MultiphasePhase {
                action: phase.action,
                steps: phase
                    .steps
                    .into_iter()
                    .map(|step| MultiphaseStep {
                        node_id: step.node_id,
                        from: step.to,
                        to: step.from,
                    })
                    .collect(),
            })
            .collect(),
    }
}

fn reverse_planned(planned: MultiphasePlanned) -> MultiphasePlanned {
    let mut phases = planned.explanation.phases;
    phases.reverse();
    MultiphasePlanned {
        plan: reverse_plan(planned.plan),
        explanation: MultiphasePlanExplanation {
            strategy: PlanStrategy::ReversedForwardPlan {
                original: Box::new(planned.explanation.strategy),
            },
            phases,
            validation: planned.explanation.validation,
        },
    }
}

fn overlaps(rects: impl IntoIterator<Item = Rect>) -> bool {
    let rects: Vec<_> = rects.into_iter().collect();
    for (idx, rect) in rects.iter().enumerate() {
        if rects[idx + 1..].iter().any(|other| rect.intersects(other)) {
            return true;
        }
    }
    false
}

fn motion_bounds(window: MultiphaseWindow) -> Rect {
    window.from.union(window.to)
}

fn motion_bounds_with_clearance(window: MultiphaseWindow, clearance: i32) -> Rect {
    let bounds = motion_bounds(window);
    Rect::new_saturating(
        bounds.x1().saturating_sub(clearance),
        bounds.y1().saturating_sub(clearance),
        bounds.x2().saturating_add(clearance),
        bounds.y2().saturating_add(clearance),
    )
}

fn interval_changed(from: Rect, to: Rect, axis: PhaseAxis) -> bool {
    main_start(from, axis) != main_start(to, axis) || main_end(from, axis) != main_end(to, axis)
}

fn split_depth_for_axis(position: MultiphaseHierarchyPosition, axis: PhaseAxis) -> Option<u16> {
    match axis {
        PhaseAxis::Horizontal => position.nearest_horizontal_split_depth,
        PhaseAxis::Vertical => position.nearest_vertical_split_depth,
    }
}

fn push_step(steps: &mut Vec<MultiphaseStep>, node_id: NodeId, from: Rect, to: Rect) {
    if from != to {
        steps.push(MultiphaseStep { node_id, from, to });
    }
}

fn sane_min_size(size: i32) -> i32 {
    (size / MIN_SHRINK_DENOMINATOR).max(1)
}

fn main_start(rect: Rect, axis: PhaseAxis) -> i32 {
    match axis {
        PhaseAxis::Horizontal => rect.x1(),
        PhaseAxis::Vertical => rect.y1(),
    }
}

fn main_end(rect: Rect, axis: PhaseAxis) -> i32 {
    match axis {
        PhaseAxis::Horizontal => rect.x2(),
        PhaseAxis::Vertical => rect.y2(),
    }
}

fn main_size(rect: Rect, axis: PhaseAxis) -> i32 {
    main_end(rect, axis) - main_start(rect, axis)
}

fn orth_start(rect: Rect, axis: PhaseAxis) -> i32 {
    main_start(rect, axis.other())
}

fn orth_end(rect: Rect, axis: PhaseAxis) -> i32 {
    main_end(rect, axis.other())
}

fn with_main_interval(rect: Rect, axis: PhaseAxis, start: i32, end: i32) -> Rect {
    match axis {
        PhaseAxis::Horizontal => Rect::new_saturating(start, rect.y1(), end, rect.y2()),
        PhaseAxis::Vertical => Rect::new_saturating(rect.x1(), start, rect.x2(), end),
    }
}

fn with_orth_interval(rect: Rect, axis: PhaseAxis, start: i32, end: i32) -> Rect {
    with_main_interval(rect, axis.other(), start, end)
}

impl PhaseAxis {
    fn other(self) -> Self {
        match self {
            Self::Horizontal => Self::Vertical,
            Self::Vertical => Self::Horizontal,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn id(raw: u32) -> NodeId {
        NodeId(raw)
    }

    fn rect(x1: i32, y1: i32, x2: i32, y2: i32) -> Rect {
        Rect::new_saturating(x1, y1, x2, y2)
    }

    fn window(raw: u32, from: Rect, to: Rect) -> MultiphaseWindow {
        MultiphaseWindow::new(id(raw), from, to)
    }

    #[derive(Clone)]
    enum TestTree {
        Leaf(u32),
        Split {
            id: u32,
            axis: PhaseAxis,
            weights: Vec<i32>,
            children: Vec<TestTree>,
        },
    }

    struct TestLeaf {
        node_id: NodeId,
        rect: Rect,
        hierarchy: MultiphaseHierarchyPosition,
    }

    fn leaf(raw: u32) -> TestTree {
        TestTree::Leaf(raw)
    }

    fn split(id: u32, axis: PhaseAxis, weights: &[i32], children: Vec<TestTree>) -> TestTree {
        TestTree::Split {
            id,
            axis,
            weights: weights.to_vec(),
            children,
        }
    }

    fn layout_tree(tree: &TestTree, bounds: Rect) -> Vec<TestLeaf> {
        let mut leaves = vec![];
        layout_tree_inner(
            tree,
            bounds,
            TestHierarchy {
                parent: None,
                depth: 0,
                sibling_index: None,
                split_axis: None,
                nearest_horizontal_split_depth: None,
                nearest_vertical_split_depth: None,
            },
            &mut leaves,
        );
        leaves.sort_by_key(|leaf| leaf.node_id.0);
        leaves
    }

    #[derive(Copy, Clone)]
    struct TestHierarchy {
        parent: Option<NodeId>,
        depth: u16,
        sibling_index: Option<u16>,
        split_axis: Option<PhaseAxis>,
        nearest_horizontal_split_depth: Option<u16>,
        nearest_vertical_split_depth: Option<u16>,
    }

    fn layout_tree_inner(
        tree: &TestTree,
        bounds: Rect,
        hierarchy: TestHierarchy,
        leaves: &mut Vec<TestLeaf>,
    ) {
        match tree {
            TestTree::Leaf(raw) => leaves.push(TestLeaf {
                node_id: id(*raw),
                rect: bounds,
                hierarchy: MultiphaseHierarchyPosition {
                    parent: hierarchy.parent,
                    depth: hierarchy.depth,
                    sibling_index: hierarchy.sibling_index,
                    split_axis: hierarchy.split_axis,
                    nearest_horizontal_split_depth: hierarchy.nearest_horizontal_split_depth,
                    nearest_vertical_split_depth: hierarchy.nearest_vertical_split_depth,
                    ..Default::default()
                },
            }),
            TestTree::Split {
                id: split_id,
                axis,
                weights,
                children,
            } => {
                assert_eq!(weights.len(), children.len());
                let rects = split_rect_by_weights(bounds, *axis, weights);
                for (idx, (child, rect)) in children.iter().zip(rects).enumerate() {
                    let depth = hierarchy.depth.saturating_add(1);
                    let mut child_hierarchy = TestHierarchy {
                        parent: Some(id(*split_id)),
                        depth,
                        sibling_index: Some(idx.min(u16::MAX as usize) as u16),
                        split_axis: Some(*axis),
                        nearest_horizontal_split_depth: hierarchy.nearest_horizontal_split_depth,
                        nearest_vertical_split_depth: hierarchy.nearest_vertical_split_depth,
                    };
                    match axis {
                        PhaseAxis::Horizontal => {
                            child_hierarchy.nearest_horizontal_split_depth = Some(depth);
                        }
                        PhaseAxis::Vertical => {
                            child_hierarchy.nearest_vertical_split_depth = Some(depth);
                        }
                    }
                    layout_tree_inner(child, rect, child_hierarchy, leaves);
                }
            }
        }
    }

    fn split_rect_by_weights(bounds: Rect, axis: PhaseAxis, weights: &[i32]) -> Vec<Rect> {
        let total_weight: i32 = weights.iter().sum();
        assert!(total_weight > 0);
        let total_size = match axis {
            PhaseAxis::Horizontal => bounds.width(),
            PhaseAxis::Vertical => bounds.height(),
        };
        let mut pos = match axis {
            PhaseAxis::Horizontal => bounds.x1(),
            PhaseAxis::Vertical => bounds.y1(),
        };
        let mut remaining_size = total_size;
        let mut remaining_weight = total_weight;
        let mut rects = vec![];
        for (idx, weight) in weights.iter().enumerate() {
            let size = if idx + 1 == weights.len() {
                remaining_size
            } else {
                total_size * *weight / total_weight
            };
            let rect = match axis {
                PhaseAxis::Horizontal => {
                    Rect::new_sized_saturating(pos, bounds.y1(), size, bounds.height())
                }
                PhaseAxis::Vertical => {
                    Rect::new_sized_saturating(bounds.x1(), pos, bounds.width(), size)
                }
            };
            rects.push(rect);
            pos += size;
            remaining_size -= size;
            remaining_weight -= *weight;
            if remaining_weight == 0 {
                assert_eq!(remaining_size, 0);
            }
        }
        rects
    }

    fn generated_request(old: &TestTree, new: &TestTree, bounds: Rect) -> MultiphaseRequest {
        let old_leaves = layout_tree(old, bounds);
        let new_leaves = layout_tree(new, bounds);
        assert_eq!(old_leaves.len(), new_leaves.len());
        let mut windows = vec![];
        for old_leaf in &old_leaves {
            let new_leaf = new_leaves
                .iter()
                .find(|leaf| leaf.node_id == old_leaf.node_id)
                .unwrap();
            windows.push(MultiphaseWindow::with_hierarchy(
                old_leaf.node_id,
                old_leaf.rect,
                new_leaf.rect,
                MultiphaseWindowHierarchy::new(old_leaf.hierarchy, new_leaf.hierarchy),
            ));
        }
        MultiphaseRequest {
            bounds,
            windows,
            clearance: 0,
        }
    }

    fn assert_generated_case_plans(old: &TestTree, new: &TestTree, bounds: Rect) {
        assert_generated_case_plans_deterministically(old, new, bounds);
    }

    fn assert_generated_case_plans_deterministically(
        old: &TestTree,
        new: &TestTree,
        bounds: Rect,
    ) -> MultiphasePlanned {
        let req = generated_request(old, new, bounds);
        assert!(!overlaps(req.windows.iter().map(|window| window.from)));
        assert!(!overlaps(req.windows.iter().map(|window| window.to)));
        let first = plan_no_overlap_explained(&req).unwrap();
        let second = plan_no_overlap_explained(&req).unwrap();
        assert_eq!(first, second);
        assert_eq!(first.plan.phases.len(), first.explanation.phases.len());
        assert_eq!(
            first.explanation.validation,
            ValidationExplanation::passed()
        );
        for phase in &first.explanation.phases {
            assert!(phase.nodes.windows(2).all(|pair| pair[0].0 < pair[1].0));
        }
        assert!(validate_plan_continuous(&req, &first.plan));
        first
    }

    fn bounds_for_axis(axis: PhaseAxis) -> Rect {
        match axis {
            PhaseAxis::Horizontal => rect(0, 0, 400, 100),
            PhaseAxis::Vertical => rect(0, 0, 100, 400),
        }
    }

    fn push_generated_case_bidirectional(
        cases: &mut Vec<(TestTree, TestTree, Rect)>,
        old: TestTree,
        new: TestTree,
        bounds: Rect,
    ) {
        cases.push((old.clone(), new.clone(), bounds));
        cases.push((new, old, bounds));
    }

    fn request(windows: Vec<MultiphaseWindow>) -> MultiphaseRequest {
        let bounds = windows
            .iter()
            .map(|window| window.from.union(window.to))
            .reduce(|bounds, rect| bounds.union(rect))
            .unwrap_or_else(|| rect(0, 0, 1, 1));
        MultiphaseRequest {
            bounds,
            windows,
            clearance: 0,
        }
    }

    fn actions(plan: &MultiphasePlan) -> Vec<PhaseAction> {
        plan.phases
            .iter()
            .map(|phase| phase.action.as_uniform().unwrap())
            .collect()
    }

    fn step_to(plan: &MultiphasePlan, phase: usize, node_id: NodeId) -> Rect {
        plan.phases[phase]
            .steps
            .iter()
            .find(|step| step.node_id == node_id)
            .unwrap()
            .to
    }

    fn no_pattern_attempts(direction: PlanDirection) -> Vec<RejectedStrategy> {
        vec![
            RejectedStrategy {
                direction,
                strategy: PlanStrategy::SingleAction,
                reason: MultiphasePlanFailure::NoPattern,
            },
            RejectedStrategy {
                direction,
                strategy: PlanStrategy::SpaceThenOrthogonalGrowth {
                    axis: PhaseAxis::Horizontal,
                },
                reason: MultiphasePlanFailure::NoPattern,
            },
            RejectedStrategy {
                direction,
                strategy: PlanStrategy::SpaceThenOrthogonalGrowth {
                    axis: PhaseAxis::Vertical,
                },
                reason: MultiphasePlanFailure::NoPattern,
            },
            RejectedStrategy {
                direction,
                strategy: PlanStrategy::HierarchyOrderedScales,
                reason: MultiphasePlanFailure::NoPattern,
            },
            RejectedStrategy {
                direction,
                strategy: PlanStrategy::OrientationChange {
                    from_axis: PhaseAxis::Horizontal,
                },
                reason: MultiphasePlanFailure::NoPattern,
            },
            RejectedStrategy {
                direction,
                strategy: PlanStrategy::OrientationChange {
                    from_axis: PhaseAxis::Vertical,
                },
                reason: MultiphasePlanFailure::NoPattern,
            },
            RejectedStrategy {
                direction,
                strategy: PlanStrategy::SwapLanes {
                    axis: PhaseAxis::Horizontal,
                },
                reason: MultiphasePlanFailure::NoPattern,
            },
            RejectedStrategy {
                direction,
                strategy: PlanStrategy::SwapLanes {
                    axis: PhaseAxis::Vertical,
                },
                reason: MultiphasePlanFailure::NoPattern,
            },
        ]
    }

    #[test]
    fn horizontal_swap_shrinks_moves_then_grows_without_overlap() {
        let req = request(vec![
            window(1, rect(0, 0, 100, 100), rect(100, 0, 200, 100)),
            MultiphaseWindow {
                node_id: id(2),
                from: rect(100, 0, 200, 100),
                to: rect(0, 0, 100, 100),
                hierarchy: Default::default(),
            },
        ]);
        let planned = plan_no_overlap_explained(&req).unwrap();
        let plan = &planned.plan;
        assert_eq!(
            actions(plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical
                },
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical
                },
            ]
        );
        assert_eq!(plan.phases[0].steps[0].to, rect(0, 0, 100, 50));
        assert_eq!(plan.phases[0].steps[1].to, rect(100, 50, 200, 100));
        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::SwapLanes {
                axis: PhaseAxis::Horizontal
            }
        );
        assert_eq!(
            planned
                .explanation
                .phases
                .iter()
                .map(|phase| phase.reason)
                .collect::<Vec<_>>(),
            vec![
                PhaseReason::ShrinkIntoLanes {
                    lane_axis: PhaseAxis::Vertical
                },
                PhaseReason::MoveThroughFreedSpace,
                PhaseReason::GrowOutOfLanes,
            ]
        );
        assert_eq!(planned.explanation.phases[0].nodes, vec![id(1), id(2)]);
        assert!(validate_plan_continuous(&req, plan));
    }

    #[test]
    fn horizontal_swap_reverse_uses_equivalent_lanes() {
        let req = request(vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(100, 0, 200, 100),
                to: rect(0, 0, 100, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(0, 0, 100, 100),
                to: rect(100, 0, 200, 100),
                hierarchy: Default::default(),
            },
        ]);
        let plan = plan_no_overlap(&req).unwrap();
        assert_eq!(step_to(&plan, 0, id(2)), rect(0, 0, 100, 50));
        assert_eq!(step_to(&plan, 0, id(1)), rect(100, 50, 200, 100));
        assert!(validate_plan_continuous(&req, &plan));
    }

    #[test]
    fn horizontal_swap_lanes_follow_motion_direction_not_node_id() {
        let req = request(vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(100, 0, 200, 100),
                to: rect(0, 0, 100, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(0, 0, 100, 100),
                to: rect(100, 0, 200, 100),
                hierarchy: Default::default(),
            },
        ]);
        let plan = plan_no_overlap(&req).unwrap();
        assert_eq!(step_to(&plan, 0, id(2)), rect(0, 0, 100, 50));
        assert_eq!(step_to(&plan, 0, id(1)), rect(100, 50, 200, 100));
        assert!(validate_plan_continuous(&req, &plan));
    }

    #[test]
    fn swap_lanes_respect_requested_clearance() {
        let mut req = request(vec![
            window(1, rect(0, 0, 100, 100), rect(100, 0, 200, 100)),
            window(2, rect(100, 0, 200, 100), rect(0, 0, 100, 100)),
        ]);
        req.clearance = 10;

        let plan = plan_no_overlap(&req).unwrap();
        assert_eq!(step_to(&plan, 0, id(1)), rect(0, 0, 100, 45));
        assert_eq!(step_to(&plan, 0, id(2)), rect(100, 55, 200, 100));
        assert!(validate_plan_continuous(&req, &plan));
    }

    #[test]
    fn swap_lanes_tolerate_stationary_siblings_in_request() {
        let req = request(vec![
            window(1, rect(0, 0, 100, 100), rect(100, 0, 200, 100)),
            window(2, rect(100, 0, 200, 100), rect(0, 0, 100, 100)),
            window(3, rect(200, 0, 300, 100), rect(200, 0, 300, 100)),
        ]);

        let planned = plan_no_overlap_explained(&req).unwrap();
        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::SwapLanes {
                axis: PhaseAxis::Horizontal,
            }
        );
        assert!(validate_plan_continuous(&req, &planned.plan));
    }

    #[test]
    fn uneven_swap_lanes_split_move_and_same_axis_scale() {
        let req = request(vec![
            window(1, rect(0, 0, 101, 100), rect(101, 0, 201, 100)),
            window(2, rect(101, 0, 201, 100), rect(0, 0, 101, 100)),
        ]);
        let planned = plan_no_overlap_explained(&req).unwrap();
        let plan = &planned.plan;

        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::SwapLanes {
                axis: PhaseAxis::Horizontal,
            }
        );
        assert_eq!(
            actions(plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
            ]
        );
        assert_eq!(step_to(plan, 1, id(1)), rect(100, 0, 201, 50));
        assert_eq!(step_to(plan, 1, id(2)), rect(0, 50, 100, 100));
        assert_eq!(step_to(plan, 2, id(1)), rect(101, 0, 201, 50));
        assert_eq!(step_to(plan, 2, id(2)), rect(0, 50, 101, 100));
        assert!(validate_plan_continuous(&req, plan));
    }

    #[test]
    fn uneven_swap_lanes_tolerate_stationary_sibling_in_odd_layout() {
        let req = request(vec![
            window(1, rect(0, 0, 101, 100), rect(101, 0, 201, 100)),
            window(2, rect(101, 0, 201, 100), rect(0, 0, 101, 100)),
            window(3, rect(201, 0, 300, 100), rect(201, 0, 300, 100)),
        ]);
        let planned = plan_no_overlap_explained(&req).unwrap();

        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::SwapLanes {
                axis: PhaseAxis::Horizontal,
            }
        );
        assert!(validate_plan_continuous(&req, &planned.plan));
    }

    #[test]
    fn horizontal_rotation_uses_crossing_lanes() {
        let req = request(vec![
            window(1, rect(0, 0, 100, 100), rect(100, 0, 200, 100)),
            window(2, rect(100, 0, 200, 100), rect(200, 0, 300, 100)),
            window(3, rect(200, 0, 300, 100), rect(0, 0, 100, 100)),
        ]);
        let planned = plan_no_overlap_explained(&req).unwrap();

        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::SwapLanes {
                axis: PhaseAxis::Horizontal,
            }
        );
        assert_eq!(
            actions(&planned.plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
            ]
        );
        assert!(validate_plan_continuous(&req, &planned.plan));
    }

    #[test]
    fn vertical_swap_lanes_follow_motion_direction_not_node_id() {
        let req = request(vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 100, 100, 200),
                to: rect(0, 0, 100, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(0, 0, 100, 100),
                to: rect(0, 100, 100, 200),
                hierarchy: Default::default(),
            },
        ]);
        let plan = plan_no_overlap(&req).unwrap();
        assert_eq!(step_to(&plan, 0, id(2)), rect(0, 0, 50, 100));
        assert_eq!(step_to(&plan, 0, id(1)), rect(50, 100, 100, 200));
        assert!(validate_plan_continuous(&req, &plan));
    }

    #[test]
    fn generated_sibling_swaps_plan_for_both_axes() {
        let bounds = rect(0, 0, 240, 240);
        for axis in [PhaseAxis::Horizontal, PhaseAxis::Vertical] {
            let old = split(10, axis, &[1, 1], vec![leaf(1), leaf(2)]);
            let new = split(10, axis, &[1, 1], vec![leaf(2), leaf(1)]);
            assert_generated_case_plans(&old, &new, bounds);
        }
    }

    #[test]
    fn generated_size_redistributions_plan_as_single_axis_scale() {
        let horizontal_old = split(
            10,
            PhaseAxis::Horizontal,
            &[1, 1, 1],
            vec![leaf(1), leaf(2), leaf(3)],
        );
        let horizontal_new = split(
            10,
            PhaseAxis::Horizontal,
            &[1, 2, 1],
            vec![leaf(1), leaf(2), leaf(3)],
        );
        let horizontal_req =
            generated_request(&horizontal_old, &horizontal_new, rect(0, 0, 400, 100));
        let horizontal_plan = plan_no_overlap(&horizontal_req).unwrap();
        assert_eq!(
            actions(&horizontal_plan),
            vec![PhaseAction {
                kind: PhaseKind::Scale,
                axis: PhaseAxis::Horizontal,
            }]
        );
        assert!(validate_plan_continuous(&horizontal_req, &horizontal_plan));

        let vertical_old = split(
            10,
            PhaseAxis::Vertical,
            &[1, 1, 1],
            vec![leaf(1), leaf(2), leaf(3)],
        );
        let vertical_new = split(
            10,
            PhaseAxis::Vertical,
            &[1, 2, 1],
            vec![leaf(1), leaf(2), leaf(3)],
        );
        let vertical_req = generated_request(&vertical_old, &vertical_new, rect(0, 0, 100, 400));
        let vertical_plan = plan_no_overlap(&vertical_req).unwrap();
        assert_eq!(
            actions(&vertical_plan),
            vec![PhaseAction {
                kind: PhaseKind::Scale,
                axis: PhaseAxis::Vertical,
            }]
        );
        assert!(validate_plan_continuous(&vertical_req, &vertical_plan));
    }

    #[test]
    fn mixed_single_phase_accepts_distinct_axis_actions_when_proven() {
        let req = request(vec![
            window(1, rect(0, 0, 100, 100), rect(0, 0, 150, 100)),
            window(2, rect(200, 0, 300, 100), rect(200, 0, 300, 150)),
        ]);
        let planned = plan_no_overlap_explained(&req).unwrap();

        assert_eq!(planned.explanation.strategy, PlanStrategy::MixedSinglePhase);
        assert_eq!(planned.plan.phases.len(), 1);
        assert_eq!(
            planned.plan.phases[0].action,
            MultiphasePhaseAction::Mixed(vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
            ])
        );
        assert_eq!(
            planned.explanation.phases[0].reason,
            PhaseReason::MixedAxisActions
        );
        assert_eq!(planned.explanation.phases[0].nodes, vec![id(1), id(2)]);
        assert!(validate_plan_continuous(&req, &planned.plan));
    }

    #[test]
    fn mixed_single_phase_accepts_move_and_scale_when_proven() {
        let req = request(vec![
            window(1, rect(0, 0, 80, 80), rect(40, 0, 120, 80)),
            window(2, rect(200, 0, 280, 80), rect(200, 0, 280, 120)),
        ]);
        let planned = plan_no_overlap_explained(&req).unwrap();

        assert_eq!(planned.explanation.strategy, PlanStrategy::MixedSinglePhase);
        assert_eq!(
            planned.plan.phases[0].action,
            MultiphasePhaseAction::Mixed(vec![
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
            ])
        );
        assert_eq!(
            planned.explanation.phases[0].reason,
            PhaseReason::MixedAxisActions
        );
        assert_eq!(planned.explanation.phases[0].nodes, vec![id(1), id(2)]);
        assert!(validate_plan_continuous(&req, &planned.plan));
    }

    #[test]
    fn single_window_one_axis_group_is_still_multiphase_plannable() {
        let req = request(vec![window(1, rect(0, 0, 100, 100), rect(40, 0, 140, 100))]);
        let planned = plan_no_overlap_explained(&req).unwrap();

        assert_eq!(planned.explanation.strategy, PlanStrategy::SingleAction);
        assert_eq!(
            planned.plan.phases[0].action,
            MultiphasePhaseAction::Uniform(PhaseAction {
                kind: PhaseKind::Move,
                axis: PhaseAxis::Horizontal,
            })
        );
        assert_eq!(planned.explanation.phases[0].nodes, vec![id(1)]);
        assert!(validate_plan_continuous(&req, &planned.plan));
    }

    #[test]
    fn mixed_single_phase_still_rejects_diagonal_per_window_motion() {
        let req = request(vec![
            window(1, rect(0, 0, 80, 80), rect(40, 40, 120, 120)),
            window(2, rect(200, 0, 280, 80), rect(200, 0, 280, 120)),
        ]);

        assert_eq!(plan_no_overlap(&req), Err(MultiphaseError::NoPlan));
        let diagnostic = plan_no_overlap_with_diagnostics(&req).unwrap_err();
        let rejection = MultiphasePlanFailure::InvalidPhaseStep {
            action: PhaseAction {
                kind: PhaseKind::Scale,
                axis: PhaseAxis::Horizontal,
            },
            node_id: id(1),
        };
        assert_eq!(diagnostic.forward, rejection);
        assert_eq!(diagnostic.reverse, Some(rejection));
    }

    #[test]
    fn generated_nested_size_redistribution_scales_parent_axis_first() {
        let old = split(
            10,
            PhaseAxis::Horizontal,
            &[1, 1],
            vec![
                leaf(1),
                split(11, PhaseAxis::Vertical, &[1, 1], vec![leaf(2), leaf(3)]),
            ],
        );
        let new = split(
            10,
            PhaseAxis::Horizontal,
            &[1, 3],
            vec![
                leaf(1),
                split(11, PhaseAxis::Vertical, &[3, 1], vec![leaf(2), leaf(3)]),
            ],
        );
        let req = generated_request(&old, &new, rect(0, 0, 400, 100));
        let planned = plan_no_overlap_explained(&req).unwrap();
        let plan = &planned.plan;
        assert_eq!(
            actions(plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
            ]
        );
        assert_eq!(step_to(plan, 0, id(1)), rect(0, 0, 100, 100));
        assert_eq!(step_to(plan, 0, id(2)), rect(100, 0, 400, 50));
        assert_eq!(step_to(plan, 0, id(3)), rect(100, 50, 400, 100));
        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::HierarchyOrderedScales
        );
        assert_eq!(
            planned.explanation.phases[0].reason,
            PhaseReason::ParentAxisBeforeChildAxis {
                parent_axis: PhaseAxis::Horizontal,
                parent_depth: 1,
                child_axis: PhaseAxis::Vertical,
                child_depth: 2,
            }
        );
        assert_eq!(
            planned.explanation.phases[0].nodes,
            vec![id(1), id(2), id(3)]
        );
        assert_eq!(planned.explanation.phases[1].nodes, vec![id(2), id(3)]);
        assert_eq!(
            planned.explanation.validation,
            ValidationExplanation::passed()
        );
        assert!(validate_plan_continuous(&req, plan));
    }

    #[test]
    fn orientation_change_shrinks_moves_then_grows() {
        let req = request(vec![
            window(1, rect(0, 0, 200, 400), rect(0, 0, 400, 200)),
            window(2, rect(200, 0, 400, 400), rect(0, 200, 400, 400)),
        ]);
        let planned = plan_no_overlap_explained(&req).unwrap();
        let plan = &planned.plan;

        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::OrientationChange {
                from_axis: PhaseAxis::Horizontal,
            }
        );
        assert_eq!(
            actions(plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Horizontal,
                },
            ]
        );
        assert_eq!(step_to(plan, 0, id(1)), rect(0, 0, 200, 200));
        assert_eq!(step_to(plan, 0, id(2)), rect(200, 200, 400, 400));
        assert_eq!(step_to(plan, 1, id(2)), rect(0, 200, 200, 400));
        assert_eq!(step_to(plan, 2, id(1)), rect(0, 0, 400, 200));
        assert_eq!(step_to(plan, 2, id(2)), rect(0, 200, 400, 400));
        assert!(validate_plan_continuous(&req, plan));
    }

    #[test]
    fn two_axis_redistribution_without_hierarchy_still_falls_back() {
        let req = request(vec![
            window(1, rect(0, 0, 200, 100), rect(0, 0, 100, 100)),
            window(2, rect(200, 0, 400, 50), rect(100, 0, 400, 75)),
            window(3, rect(200, 50, 400, 100), rect(100, 75, 400, 100)),
        ]);
        assert_eq!(plan_no_overlap(&req), Err(MultiphaseError::NoPlan));
    }

    #[test]
    fn generated_stack_extractions_plan_for_both_axes_and_directions() {
        let horizontal_old = split(
            10,
            PhaseAxis::Horizontal,
            &[1, 1],
            vec![
                leaf(1),
                split(11, PhaseAxis::Vertical, &[1, 1], vec![leaf(2), leaf(3)]),
            ],
        );
        let horizontal_new = split(
            10,
            PhaseAxis::Horizontal,
            &[1, 2, 1],
            vec![leaf(1), leaf(2), leaf(3)],
        );
        assert_generated_case_plans(&horizontal_old, &horizontal_new, rect(0, 0, 400, 100));
        assert_generated_case_plans(&horizontal_new, &horizontal_old, rect(0, 0, 400, 100));

        let vertical_old = split(
            20,
            PhaseAxis::Vertical,
            &[1, 1],
            vec![
                leaf(1),
                split(21, PhaseAxis::Horizontal, &[1, 1], vec![leaf(2), leaf(3)]),
            ],
        );
        let vertical_new = split(
            20,
            PhaseAxis::Vertical,
            &[1, 2, 1],
            vec![leaf(1), leaf(2), leaf(3)],
        );
        assert_generated_case_plans(&vertical_old, &vertical_new, rect(0, 0, 100, 400));
        assert_generated_case_plans(&vertical_new, &vertical_old, rect(0, 0, 100, 400));
    }

    #[test]
    fn stack_extraction_with_resized_moving_child_still_moves_before_growth() {
        let old = split(
            10,
            PhaseAxis::Horizontal,
            &[1, 1],
            vec![
                leaf(1),
                split(11, PhaseAxis::Vertical, &[1, 1], vec![leaf(2), leaf(3)]),
            ],
        );
        let new = split(
            10,
            PhaseAxis::Horizontal,
            &[1, 1, 1],
            vec![leaf(1), leaf(2), leaf(3)],
        );
        let req = generated_request(&old, &new, rect(0, 0, 300, 120));
        let planned = plan_no_overlap_explained(&req).unwrap();
        let plan = &planned.plan;

        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::SpaceThenOrthogonalGrowth {
                axis: PhaseAxis::Horizontal,
            }
        );
        assert_eq!(
            actions(plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
            ]
        );
        assert_eq!(step_to(plan, 0, id(1)), rect(0, 0, 100, 120));
        assert_eq!(step_to(plan, 0, id(2)), rect(200, 0, 300, 60));
        assert_eq!(step_to(plan, 0, id(3)), rect(200, 60, 300, 120));
        assert_eq!(step_to(plan, 1, id(2)), rect(100, 0, 200, 60));
        assert_eq!(step_to(plan, 2, id(2)), rect(100, 0, 200, 120));
        assert_eq!(step_to(plan, 2, id(3)), rect(200, 0, 300, 120));
        assert!(validate_plan_continuous(&req, plan));
    }

    #[test]
    fn three_child_stack_extraction_plans_without_linear_fallback() {
        let old = split(
            10,
            PhaseAxis::Horizontal,
            &[1, 1],
            vec![
                leaf(1),
                split(
                    11,
                    PhaseAxis::Vertical,
                    &[1, 1, 1],
                    vec![leaf(2), leaf(3), leaf(4)],
                ),
            ],
        );
        let new = split(
            10,
            PhaseAxis::Horizontal,
            &[1, 1, 1],
            vec![
                leaf(1),
                leaf(3),
                split(11, PhaseAxis::Vertical, &[1, 1], vec![leaf(2), leaf(4)]),
            ],
        );
        let req = generated_request(&old, &new, rect(0, 0, 600, 300));
        let planned = plan_no_overlap_explained(&req).unwrap();

        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::SpaceThenOrthogonalGrowth {
                axis: PhaseAxis::Horizontal,
            }
        );
        assert_eq!(
            actions(&planned.plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
            ]
        );
        assert!(validate_plan_continuous(&req, &planned.plan));
    }

    #[test]
    fn validated_phase_paths_accept_interrupted_reverse_route() {
        let a_current = rect(50, 0, 150, 50);
        let b_current = rect(50, 50, 150, 100);
        let req = request(vec![
            window(1, a_current, rect(0, 0, 100, 100)),
            window(2, b_current, rect(100, 0, 200, 100)),
        ]);
        let paths = vec![
            vec![
                (a_current, rect(0, 0, 100, 50)),
                (rect(0, 0, 100, 50), rect(0, 0, 100, 100)),
            ],
            vec![
                (b_current, rect(100, 50, 200, 100)),
                (rect(100, 50, 200, 100), rect(100, 0, 200, 100)),
            ],
        ];

        let plan = validate_phase_paths(&req, &paths).unwrap();
        assert_eq!(
            actions(&plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical,
                },
            ]
        );
        assert!(validate_plan_continuous(&req, &plan));
    }

    #[test]
    fn bounded_generated_supported_split_tree_corpus_is_deterministic() {
        let mut cases = vec![];
        for axis in [PhaseAxis::Horizontal, PhaseAxis::Vertical] {
            let child_axis = axis.other();
            let bounds = bounds_for_axis(axis);

            push_generated_case_bidirectional(
                &mut cases,
                split(10, axis, &[1, 1], vec![leaf(1), leaf(2)]),
                split(10, axis, &[1, 1], vec![leaf(2), leaf(1)]),
                bounds,
            );
            push_generated_case_bidirectional(
                &mut cases,
                split(10, axis, &[1, 1, 1], vec![leaf(1), leaf(2), leaf(3)]),
                split(10, axis, &[1, 2, 1], vec![leaf(1), leaf(2), leaf(3)]),
                bounds,
            );
            push_generated_case_bidirectional(
                &mut cases,
                split(
                    10,
                    axis,
                    &[1, 1],
                    vec![
                        leaf(1),
                        split(11, child_axis, &[1, 1], vec![leaf(2), leaf(3)]),
                    ],
                ),
                split(10, axis, &[1, 2, 1], vec![leaf(1), leaf(2), leaf(3)]),
                bounds,
            );
            push_generated_case_bidirectional(
                &mut cases,
                split(
                    10,
                    axis,
                    &[1, 1],
                    vec![
                        split(11, child_axis, &[1, 1], vec![leaf(1), leaf(2)]),
                        leaf(3),
                    ],
                ),
                split(10, axis, &[1, 2, 1], vec![leaf(1), leaf(2), leaf(3)]),
                bounds,
            );
            push_generated_case_bidirectional(
                &mut cases,
                split(
                    10,
                    axis,
                    &[1, 1],
                    vec![
                        leaf(1),
                        split(11, child_axis, &[1, 1], vec![leaf(2), leaf(3)]),
                    ],
                ),
                split(
                    10,
                    axis,
                    &[1, 3],
                    vec![
                        leaf(1),
                        split(11, child_axis, &[3, 1], vec![leaf(2), leaf(3)]),
                    ],
                ),
                bounds,
            );
            push_generated_case_bidirectional(
                &mut cases,
                split(
                    10,
                    axis,
                    &[1, 1],
                    vec![
                        split(11, child_axis, &[1, 1], vec![leaf(1), leaf(2)]),
                        leaf(3),
                    ],
                ),
                split(
                    10,
                    axis,
                    &[3, 1],
                    vec![
                        split(11, child_axis, &[1, 3], vec![leaf(1), leaf(2)]),
                        leaf(3),
                    ],
                ),
                bounds,
            );
        }

        assert_eq!(cases.len(), 24);
        for (old, new, bounds) in cases {
            assert_generated_case_plans_deterministically(&old, &new, bounds);
        }
    }

    #[test]
    fn stack_extraction_creates_space_before_moving_child() {
        let req = request(vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 0, 200, 100),
                to: rect(0, 0, 100, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(200, 0, 400, 50),
                to: rect(100, 0, 300, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(3),
                from: rect(200, 50, 400, 100),
                to: rect(300, 0, 400, 100),
                hierarchy: Default::default(),
            },
        ]);
        let plan = plan_no_overlap(&req).unwrap();
        assert_eq!(
            actions(&plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Horizontal
                },
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical
                },
            ]
        );
        assert_eq!(plan.phases[0].steps[0].to, rect(0, 0, 100, 100));
        assert_eq!(plan.phases[0].steps[1].to, rect(300, 50, 400, 100));
        assert_eq!(plan.phases[1].steps[0].to, rect(100, 0, 300, 50));
        assert_eq!(plan.phases[2].steps[0].to, rect(100, 0, 300, 100));
        assert_eq!(plan.phases[2].steps[1].to, rect(300, 0, 400, 100));
        assert!(validate_plan_continuous(&req, &plan));
    }

    #[test]
    fn stack_extraction_reverse_replays_phases_in_reverse() {
        let req = request(vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 0, 100, 100),
                to: rect(0, 0, 200, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(100, 0, 300, 100),
                to: rect(200, 0, 400, 50),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(3),
                from: rect(300, 0, 400, 100),
                to: rect(200, 50, 400, 100),
                hierarchy: Default::default(),
            },
        ]);
        let planned = plan_no_overlap_explained(&req).unwrap();
        let plan = &planned.plan;
        assert_eq!(
            actions(plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical
                },
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Horizontal
                },
            ]
        );
        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::ReversedForwardPlan {
                original: Box::new(PlanStrategy::SpaceThenOrthogonalGrowth {
                    axis: PhaseAxis::Horizontal
                })
            }
        );
        assert!(validate_plan_continuous(&req, plan));
    }

    #[test]
    fn vertical_stack_extraction_creates_space_before_moving_child() {
        let req = request(vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 0, 100, 200),
                to: rect(0, 0, 100, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(0, 200, 50, 400),
                to: rect(0, 100, 100, 300),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(3),
                from: rect(50, 200, 100, 400),
                to: rect(0, 300, 100, 400),
                hierarchy: Default::default(),
            },
        ]);
        let plan = plan_no_overlap(&req).unwrap();
        assert_eq!(
            actions(&plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical
                },
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Vertical
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Horizontal
                },
            ]
        );
        assert_eq!(plan.phases[0].steps[0].to, rect(0, 0, 100, 100));
        assert_eq!(plan.phases[0].steps[1].to, rect(50, 300, 100, 400));
        assert_eq!(plan.phases[1].steps[0].to, rect(0, 100, 50, 300));
        assert_eq!(plan.phases[2].steps[0].to, rect(0, 100, 100, 300));
        assert_eq!(plan.phases[2].steps[1].to, rect(0, 300, 100, 400));
        assert!(validate_plan_continuous(&req, &plan));
    }

    #[test]
    fn vertical_stack_extraction_with_clearance_still_plans() {
        let old = split(
            20,
            PhaseAxis::Vertical,
            &[1, 1],
            vec![
                leaf(1),
                split(21, PhaseAxis::Horizontal, &[1, 1], vec![leaf(2), leaf(3)]),
            ],
        );
        let new = split(
            20,
            PhaseAxis::Vertical,
            &[1, 2, 1],
            vec![leaf(1), leaf(2), leaf(3)],
        );
        let mut req = generated_request(&old, &new, rect(0, 0, 100, 400));
        req.clearance = 10;
        let planned = plan_no_overlap_explained(&req).unwrap();

        assert_eq!(
            planned.explanation.strategy,
            PlanStrategy::SpaceThenOrthogonalGrowth {
                axis: PhaseAxis::Vertical,
            }
        );
        assert!(validate_plan_continuous(&req, &planned.plan));
    }

    #[test]
    fn vertical_stack_extraction_reverse_replays_phases_in_reverse() {
        let req = request(vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 0, 100, 100),
                to: rect(0, 0, 100, 200),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(0, 100, 100, 300),
                to: rect(0, 200, 50, 400),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(3),
                from: rect(0, 300, 100, 400),
                to: rect(50, 200, 100, 400),
                hierarchy: Default::default(),
            },
        ]);
        let plan = plan_no_overlap(&req).unwrap();
        assert_eq!(
            actions(&plan),
            vec![
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Horizontal
                },
                PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Vertical
                },
                PhaseAction {
                    kind: PhaseKind::Scale,
                    axis: PhaseAxis::Vertical
                },
            ]
        );
        assert!(validate_plan_continuous(&req, &plan));
    }

    #[test]
    fn unsupported_diagonal_motion_falls_back_to_linear() {
        let req = request(vec![MultiphaseWindow {
            node_id: id(1),
            from: rect(0, 0, 100, 100),
            to: rect(100, 100, 200, 200),
            hierarchy: Default::default(),
        }]);
        assert_eq!(plan_no_overlap(&req), Err(MultiphaseError::NoPlan));
        let diagnostic = plan_no_overlap_with_diagnostics(&req).unwrap_err();
        assert_eq!(diagnostic.forward, MultiphasePlanFailure::NoPattern);
        assert_eq!(diagnostic.reverse, Some(MultiphasePlanFailure::NoPattern));
        let mut expected = no_pattern_attempts(PlanDirection::Forward);
        expected.extend(no_pattern_attempts(PlanDirection::Reverse));
        assert_eq!(diagnostic.attempted, expected);
    }

    #[test]
    fn diagnostics_report_shrink_bound_rejections() {
        let req = MultiphaseRequest {
            bounds: rect(0, 0, 400, 100),
            clearance: 0,
            windows: vec![
                MultiphaseWindow {
                    node_id: id(1),
                    from: rect(0, 0, 200, 100),
                    to: rect(0, 0, 10, 100),
                    hierarchy: Default::default(),
                },
                MultiphaseWindow {
                    node_id: id(2),
                    from: rect(200, 0, 400, 100),
                    to: rect(10, 0, 400, 100),
                    hierarchy: Default::default(),
                },
            ],
        };

        assert!(matches!(
            plan_no_overlap_with_diagnostics(&req).unwrap_err().forward,
            MultiphasePlanFailure::ShrinkBound {
                axis: PhaseAxis::Horizontal,
                available: 10,
                required: 50,
            }
        ));
    }

    #[test]
    fn diagnostics_report_candidate_validation_rejections() {
        let req = request(vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 0, 60, 60),
                to: rect(180, 0, 240, 60),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(90, 0, 150, 60),
                to: rect(90, 0, 150, 60),
                hierarchy: Default::default(),
            },
        ]);
        let rejection =
            MultiphasePlanFailure::Validation(MultiphaseValidationError::PhaseOverlap {
                phase: 0,
                a: id(1),
                b: id(2),
            });
        let diagnostic = plan_no_overlap_with_diagnostics(&req).unwrap_err();

        assert_eq!(diagnostic.forward, rejection);
        assert_eq!(diagnostic.reverse, Some(rejection));
        assert_eq!(
            diagnostic.attempted[0],
            RejectedStrategy {
                direction: PlanDirection::Forward,
                strategy: PlanStrategy::SingleAction,
                reason: rejection,
            }
        );
        assert!(diagnostic.attempted.iter().any(|attempt| *attempt
            == RejectedStrategy {
                direction: PlanDirection::Reverse,
                strategy: PlanStrategy::SingleAction,
                reason: rejection,
            }));
    }

    #[test]
    fn hierarchy_metadata_classifies_depth_and_mono_transitions() {
        let source = MultiphaseHierarchyPosition {
            parent: Some(id(10)),
            depth: 2,
            sibling_index: Some(0),
            split_axis: Some(PhaseAxis::Vertical),
            nearest_horizontal_split_depth: Some(1),
            nearest_vertical_split_depth: Some(2),
            ..Default::default()
        };
        let target = MultiphaseHierarchyPosition {
            parent: Some(id(11)),
            depth: 1,
            sibling_index: Some(2),
            split_axis: Some(PhaseAxis::Horizontal),
            nearest_horizontal_split_depth: Some(1),
            ..Default::default()
        };
        assert_eq!(
            MultiphaseWindowHierarchy::new(source, target).transition,
            MultiphaseHierarchyTransition::Ascending
        );
        assert_eq!(source.nearest_vertical_split_depth, Some(2));

        let entering_mono = MultiphaseWindowHierarchy::new(
            source,
            MultiphaseHierarchyPosition {
                parent_is_mono: true,
                mono_active: true,
                ..target
            },
        );
        assert_eq!(
            entering_mono.transition,
            MultiphaseHierarchyTransition::EnteringMono
        );
        assert_eq!(
            entering_mono.reversed().transition,
            MultiphaseHierarchyTransition::ExitingMono
        );
    }

    #[test]
    fn continuous_validation_rejects_narrow_mid_phase_overlap() {
        let req = request(vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 0, 10, 10),
                to: rect(100, 0, 110, 10),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(13, 0, 14, 10),
                to: rect(13, 0, 14, 10),
                hierarchy: Default::default(),
            },
        ]);
        let plan = MultiphasePlan {
            phases: vec![MultiphasePhase {
                action: MultiphasePhaseAction::Uniform(PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                }),
                steps: vec![MultiphaseStep {
                    node_id: id(1),
                    from: rect(0, 0, 10, 10),
                    to: rect(100, 0, 110, 10),
                }],
            }],
        };

        assert_eq!(
            validate_plan_continuous_diagnostic(&req, &plan),
            Err(MultiphaseValidationError::PhaseOverlap {
                phase: 0,
                a: id(1),
                b: id(2),
            })
        );
    }

    #[test]
    fn continuous_validation_allows_edge_touching_motion() {
        let req = request(vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 0, 10, 10),
                to: rect(10, 0, 20, 10),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(20, 0, 30, 10),
                to: rect(20, 0, 30, 10),
                hierarchy: Default::default(),
            },
        ]);
        let plan = MultiphasePlan {
            phases: vec![MultiphasePhase {
                action: MultiphasePhaseAction::Uniform(PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                }),
                steps: vec![MultiphaseStep {
                    node_id: id(1),
                    from: rect(0, 0, 10, 10),
                    to: rect(10, 0, 20, 10),
                }],
            }],
        };

        assert!(validate_plan_continuous(&req, &plan));
    }

    #[test]
    fn continuous_validation_rejects_mixed_phase_action_count_mismatch() {
        let req = request(vec![
            window(1, rect(0, 0, 40, 40), rect(40, 0, 80, 40)),
            window(2, rect(100, 0, 140, 40), rect(100, 0, 140, 80)),
        ]);
        let plan = MultiphasePlan {
            phases: vec![MultiphasePhase {
                action: MultiphasePhaseAction::Mixed(vec![PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                }]),
                steps: vec![
                    MultiphaseStep {
                        node_id: id(1),
                        from: rect(0, 0, 40, 40),
                        to: rect(40, 0, 80, 40),
                    },
                    MultiphaseStep {
                        node_id: id(2),
                        from: rect(100, 0, 140, 40),
                        to: rect(100, 0, 140, 80),
                    },
                ],
            }],
        };

        assert_eq!(
            validate_plan_continuous_diagnostic(&req, &plan),
            Err(MultiphaseValidationError::PhaseActionCount {
                phase: 0,
                actions: 1,
                steps: 2,
            })
        );
    }

    #[test]
    fn continuous_validation_rejects_stale_step_start_rect() {
        let req = request(vec![MultiphaseWindow {
            node_id: id(1),
            from: rect(0, 0, 10, 10),
            to: rect(20, 0, 30, 10),
            hierarchy: Default::default(),
        }]);
        let plan = MultiphasePlan {
            phases: vec![MultiphasePhase {
                action: MultiphasePhaseAction::Uniform(PhaseAction {
                    kind: PhaseKind::Move,
                    axis: PhaseAxis::Horizontal,
                }),
                steps: vec![MultiphaseStep {
                    node_id: id(1),
                    from: rect(5, 0, 15, 10),
                    to: rect(20, 0, 30, 10),
                }],
            }],
        };

        assert_eq!(
            validate_plan_continuous_diagnostic(&req, &plan),
            Err(MultiphaseValidationError::StaleStepStart {
                phase: 0,
                node_id: id(1),
            })
        );
    }

    #[test]
    fn motion_groups_split_disjoint_layout_changes() {
        let windows = vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 0, 100, 100),
                to: rect(100, 0, 200, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(100, 0, 200, 100),
                to: rect(0, 0, 100, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(3),
                from: rect(300, 0, 400, 100),
                to: rect(400, 0, 500, 100),
                hierarchy: Default::default(),
            },
        ];
        assert_eq!(partition_motion_groups(&windows, 0), vec![vec![0, 1], vec![2]]);
    }

    #[test]
    fn motion_groups_are_transitive() {
        let windows = vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 0, 100, 100),
                to: rect(80, 0, 180, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(170, 0, 270, 100),
                to: rect(250, 0, 350, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(3),
                from: rect(90, 0, 180, 100),
                to: rect(180, 0, 260, 100),
                hierarchy: Default::default(),
            },
        ];
        assert_eq!(partition_motion_groups(&windows, 0), vec![vec![0, 1, 2]]);
    }

    #[test]
    fn motion_groups_join_across_animation_clearance() {
        let windows = vec![
            MultiphaseWindow {
                node_id: id(1),
                from: rect(0, 0, 100, 100),
                to: rect(0, 0, 80, 100),
                hierarchy: Default::default(),
            },
            MultiphaseWindow {
                node_id: id(2),
                from: rect(120, 0, 220, 100),
                to: rect(110, 0, 210, 100),
                hierarchy: Default::default(),
            },
        ];
        assert_eq!(partition_motion_groups(&windows, 0), vec![vec![0], vec![1]]);
        assert_eq!(partition_motion_groups(&windows, 10), vec![vec![0, 1]]);
    }
}