feat(supervisor): exponential backoff calculator

crates/xy-supervisor/src/backoff.rs

@@ -0,0 +1,70 @@
+use std::time::Duration;
+
+#[derive(Debug, Clone)]
+pub struct Backoff {
+    initial: Duration,
+    max: Duration,
+    current: Option<Duration>,
+}
+
+impl Backoff {
+    pub fn new(initial: Duration, max: Duration) -> Self {
+        Self {
+            initial,
+            max,
+            current: None,
+        }
+    }
+
+    pub fn next(&mut self) -> Duration {
+        let next = match self.current {
+            None => self.initial,
+            Some(d) => (d * 2).min(self.max),
+        };
+        self.current = Some(next);
+        next
+    }
+
+    pub fn reset(&mut self) {
+        self.current = None;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn starts_at_initial() {
+        let mut b = Backoff::new(Duration::from_secs(1), Duration::from_secs(30));
+        assert_eq!(b.next(), Duration::from_secs(1));
+    }
+
+    #[test]
+    fn doubles_each_call() {
+        let mut b = Backoff::new(Duration::from_secs(1), Duration::from_secs(30));
+        assert_eq!(b.next(), Duration::from_secs(1));
+        assert_eq!(b.next(), Duration::from_secs(2));
+        assert_eq!(b.next(), Duration::from_secs(4));
+        assert_eq!(b.next(), Duration::from_secs(8));
+    }
+
+    #[test]
+    fn caps_at_max() {
+        let mut b = Backoff::new(Duration::from_secs(1), Duration::from_secs(5));
+        for _ in 0..10 {
+            b.next();
+        }
+        assert_eq!(b.next(), Duration::from_secs(5));
+    }
+
+    #[test]
+    fn reset_starts_over() {
+        let mut b = Backoff::new(Duration::from_secs(1), Duration::from_secs(30));
+        b.next();
+        b.next();
+        b.next();
+        b.reset();
+        assert_eq!(b.next(), Duration::from_secs(1));
+    }
+}