--- /dev/null
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package util
+
+import "time"
+
+// A Throttle permits throttling of a goroutine by
+// calling the Throttle method repeatedly.
+//
+type Throttle struct {
+ f float64 // f = (1-r)/r for 0 < r < 1
+ dt time.Duration // minimum run time slice; >= 0
+ tr time.Duration // accumulated time running
+ ts time.Duration // accumulated time stopped
+ tt time.Time // earliest throttle time (= time Throttle returned + tm)
+}
+
+// NewThrottle creates a new Throttle with a throttle value r and
+// a minimum allocated run time slice of dt:
+//
+// r == 0: "empty" throttle; the goroutine is always sleeping
+// r == 1: full throttle; the goroutine is never sleeping
+//
+// A value of r == 0.6 throttles a goroutine such that it runs
+// approx. 60% of the time, and sleeps approx. 40% of the time.
+// Values of r < 0 or r > 1 are clamped down to values between 0 and 1.
+// Values of dt < 0 are set to 0.
+//
+func NewThrottle(r float64, dt time.Duration) *Throttle {
+ var f float64
+ switch {
+ case r <= 0:
+ f = -1 // indicates always sleep
+ case r >= 1:
+ f = 0 // assume r == 1 (never sleep)
+ default:
+ // 0 < r < 1
+ f = (1 - r) / r
+ }
+ if dt < 0 {
+ dt = 0
+ }
+ return &Throttle{f: f, dt: dt, tt: time.Now().Add(dt)}
+}
+
+// Throttle calls time.Sleep such that over time the ratio tr/ts between
+// accumulated run (tr) and sleep times (ts) approximates the value 1/(1-r)
+// where r is the throttle value. Throttle returns immediately (w/o sleeping)
+// if less than tm ns have passed since the last call to Throttle.
+//
+func (p *Throttle) Throttle() {
+ if p.f < 0 {
+ select {} // always sleep
+ }
+
+ t0 := time.Now()
+ if t0.Before(p.tt) {
+ return // keep running (minimum time slice not exhausted yet)
+ }
+
+ // accumulate running time
+ p.tr += t0.Sub(p.tt) + p.dt
+
+ // compute sleep time
+ // Over time we want:
+ //
+ // tr/ts = r/(1-r)
+ //
+ // Thus:
+ //
+ // ts = tr*f with f = (1-r)/r
+ //
+ // After some incremental run time δr added to the total run time
+ // tr, the incremental sleep-time δs to get to the same ratio again
+ // after waking up from time.Sleep is:
+ if δs := time.Duration(float64(p.tr)*p.f) - p.ts; δs > 0 {
+ time.Sleep(δs)
+ }
+
+ // accumulate (actual) sleep time
+ t1 := time.Now()
+ p.ts += t1.Sub(t0)
+
+ // set earliest next throttle time
+ p.tt = t1.Add(p.dt)
+}