util: add GeometricDecay util

src/utils/geometric_decay.rs

@@ -0,0 +1,41 @@
+use std::cell::Cell;
+
+pub struct GeometricDecay {
+    p1: f64,
+    p2: f64,
+    v: Cell<f64>,
+}
+
+impl GeometricDecay {
+    #[expect(dead_code)]
+    pub fn new(mut p1: f64, reset: u64) -> Self {
+        if p1.is_nan() || p1 < 0.01 {
+            p1 = 0.01;
+        }
+        if p1 > 0.99 {
+            p1 = 0.99;
+        }
+        let p2 = 1.0 - p1;
+        Self {
+            p1,
+            p2,
+            v: Cell::new(reset as f64 / p1),
+        }
+    }
+
+    #[expect(dead_code)]
+    pub fn reset(&self, v: u64) {
+        self.v.set(v as f64 / self.p1);
+    }
+
+    #[expect(dead_code)]
+    pub fn get(&self) -> u64 {
+        (self.p1 * self.v.get()) as u64
+    }
+
+    #[expect(dead_code)]
+    pub fn add(&self, n: u64) {
+        let v = n as f64 + self.p2 * self.v.get();
+        self.v.set(v);
+    }
+}