Fallback layout animations by motion group

src/animation/multiphase.rs

@@ -81,6 +81,33 @@ pub fn plan_no_overlap(request: &MultiphaseRequest) -> Result<MultiphasePlan, Mu
     Err(MultiphaseError::NoPlan)
 }
 
+pub(crate) fn partition_motion_groups(windows: &[MultiphaseWindow]) -> Vec<Vec<usize>> {
+    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(windows[idx]);
+            for other in 0..windows.len() {
+                if seen[other] || !bounds.intersects(&motion_bounds(windows[other])) {
+                    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);
@@ -380,6 +407,10 @@ fn overlaps(rects: impl IntoIterator<Item = Rect>) -> bool {
     false
 }
 
+fn motion_bounds(window: MultiphaseWindow) -> Rect {
+    window.from.union(window.to)
+}
+
 fn push_step(steps: &mut Vec<MultiphaseStep>, node_id: NodeId, from: Rect, to: Rect) {
     if from != to {
         steps.push(MultiphaseStep { node_id, from, to });
@@ -697,4 +728,48 @@ mod tests {
         }]);
         assert_eq!(plan_no_overlap(&req), Err(MultiphaseError::NoPlan));
     }
+
+    #[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),
+            },
+            MultiphaseWindow {
+                node_id: id(2),
+                from: rect(100, 0, 200, 100),
+                to: rect(0, 0, 100, 100),
+            },
+            MultiphaseWindow {
+                node_id: id(3),
+                from: rect(300, 0, 400, 100),
+                to: rect(400, 0, 500, 100),
+            },
+        ];
+        assert_eq!(partition_motion_groups(&windows), 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),
+            },
+            MultiphaseWindow {
+                node_id: id(2),
+                from: rect(170, 0, 270, 100),
+                to: rect(250, 0, 350, 100),
+            },
+            MultiphaseWindow {
+                node_id: id(3),
+                from: rect(90, 0, 180, 100),
+                to: rect(180, 0, 260, 100),
+            },
+        ];
+        assert_eq!(partition_motion_groups(&windows), vec![vec![0, 1, 2]]);
+    }
 }