feat: add metrics counter utility

Author: weihanglo

src/cargo/util/counter.rs

@@ -0,0 +1,61 @@
+use std::time::Instant;
+
+/// A metrics counter storing only latest `N` records.
+pub struct MetricsCounter<const N: usize> {
+    /// Slots to store metrics.
+    slots: [(usize, Instant); N],
+    /// The slot of the oldest record.
+    /// Also the next slot to store the new record.
+    index: usize,
+}
+
+impl<const N: usize> MetricsCounter<N> {
+    /// Creates a new counter with an initial value.
+    pub fn new(init: usize) -> Self {
+        debug_assert!(N > 0, "number of slots must be greater than zero");
+        Self {
+            slots: [(init, Instant::now()); N],
+            index: 0,
+        }
+    }
+
+    /// Adds record to the counter.
+    pub fn add(&mut self, data: usize) {
+        self.slots[self.index] = (data, Instant::now());
+        self.index = (self.index + 1) % N;
+    }
+
+    /// Calculates per-second average rate of all slots.
+    pub fn rate(&self) -> f32 {
+        let latest = self.slots[self.index.checked_sub(1).unwrap_or(N - 1)];
+        let oldest = self.slots[self.index];
+        let duration = (latest.1 - oldest.1).as_secs_f32();
+        let avg = (latest.0 - oldest.0) as f32 / duration;
+        if f32::is_nan(avg) {
+            0f32
+        } else {
+            avg
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::MetricsCounter;
+
+    #[test]
+    fn counter() {
+        let mut counter = MetricsCounter::<3>::new(0);
+        assert_eq!(counter.rate(), 0f32);
+        for i in 1..=5 {
+            counter.add(i);
+            assert!(counter.rate() > 0f32);
+        }
+    }
+
+    #[test]
+    #[should_panic(expected = "number of slots must be greater than zero")]
+    fn counter_zero_slot() {
+        let _counter = MetricsCounter::<0>::new(0);
+    }
+}

src/cargo/util/mod.rs

@@ -3,6 +3,7 @@ use std::time::Duration;
 
 pub use self::canonical_url::CanonicalUrl;
 pub use self::config::{homedir, Config, ConfigValue};
+pub(crate) use self::counter::MetricsCounter;
 pub use self::dependency_queue::DependencyQueue;
 pub use self::diagnostic_server::RustfixDiagnosticServer;
 pub use self::errors::{internal, CargoResult, CliResult, Test};
@@ -29,6 +30,7 @@ pub use self::workspace::{
 mod canonical_url;
 pub mod command_prelude;
 pub mod config;
+mod counter;
 pub mod cpu;
 mod dependency_queue;
 pub mod diagnostic_server;