1 files changed, 58 insertions, 0 deletions
diff --git a/src/backoff.rs b/src/backoff.rs
new file mode 100644
index 0000000..1f87b8d
--- /dev/null
+++ b/src/backoff.rs
@@ -0,0 +1,58 @@
+//! Exponential backoff: a delay that doubles, up to a limit, while something keeps
+//! failing, and starts over when it succeeds. Only the delay; each user keeps its own
+//! point in time (next attempt, deadline).
+
+use std::time::Duration;
+
+#[derive(Clone, Copy, Debug)]
+pub(crate) struct Backoff {
+    initial: Duration,
+    max: Duration,
+    current: Duration,
+}
+
+impl Backoff {
+    pub(crate) const fn new(initial: Duration, max: Duration) -> Self {
+        Self {
+            initial,
+            max,
+            current: initial,
+        }
+    }
+
+    /// The delay to use now.
+    pub(crate) fn current(&self) -> Duration {
+        self.current
+    }
+
+    /// Doubles the delay, up to the limit.
+    pub(crate) fn grow(&mut self) {
+        self.current = self.current.saturating_mul(2).min(self.max);
+    }
+
+    /// The delay to use now; the next call gives twice as much (up to the limit).
+    pub(crate) fn next(&mut self) -> Duration {
+        let delay = self.current;
+        self.grow();
+        delay
+    }
+
+    /// Back to the initial delay.
+    pub(crate) fn reset(&mut self) {
+        self.current = self.initial;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn doubles_up_to_the_limit_and_resets() {
+        let mut backoff = Backoff::new(Duration::from_secs(1), Duration::from_secs(5));
+        let delays: Vec<_> = (0..5).map(|_| backoff.next().as_secs()).collect();
+        assert_eq!(delays, [1, 2, 4, 5, 5]);
+        backoff.reset();
+        assert_eq!(backoff.current(), Duration::from_secs(1));
+    }
+}