Local counter: Don't re-allocate maps in Go 1.21+

local_counter.go

@@ -60,23 +60,6 @@ func (c *localCounter) Get(key string, currentWindow, previousWindow time.Time)
 	return 0, 0, nil
 }
 
-func (c *localCounter) evict(currentWindow time.Time) {
-	if c.latestWindow == currentWindow {
-		return
-	}
-
-	previousWindow := currentWindow.Add(-c.windowLength)
-	if c.latestWindow == previousWindow {
-		c.latestWindow = currentWindow
-		c.latestCounters, c.previousCounters = make(map[uint64]int), c.latestCounters
-		return
-	}
-
-	c.latestWindow = currentWindow
-	// NOTE: Don't use clear() to keep backward-compatibility.
-	c.previousCounters, c.latestCounters = make(map[uint64]int), make(map[uint64]int)
-}
-
 func limitCounterKey(key string) uint64 {
 	h := xxhash.New()
 	h.WriteString(key)

local_counter_go1.20.go

@@ -0,0 +1,21 @@
+//go:build !go1.21
+
+package httprate
+
+import "time"
+
+func (c *localCounter) evict(currentWindow time.Time) {
+	if c.latestWindow == currentWindow {
+		return
+	}
+
+	previousWindow := currentWindow.Add(-c.windowLength)
+	if c.latestWindow == previousWindow {
+		c.latestWindow = currentWindow
+		c.latestCounters, c.previousCounters = make(map[uint64]int), c.latestCounters
+		return
+	}
+
+	c.latestWindow = currentWindow
+	c.previousCounters, c.latestCounters = make(map[uint64]int), make(map[uint64]int)
+}

local_counter_go1.21.go

@@ -0,0 +1,25 @@
+//go:build go1.21
+
+package httprate
+
+import "time"
+
+func (c *localCounter) evict(currentWindow time.Time) {
+	if c.latestWindow == currentWindow {
+		return
+	}
+
+	previousWindow := currentWindow.Add(-c.windowLength)
+	if c.latestWindow == previousWindow {
+		c.latestWindow = currentWindow
+		// Shift the windows without map re-allocation.
+		clear(c.previousCounters)
+		c.latestCounters, c.previousCounters = c.previousCounters, c.latestCounters
+		return
+	}
+
+	c.latestWindow = currentWindow
+
+	clear(c.previousCounters)
+	clear(c.latestCounters)
+}