Imported Upstream version 2011.09.07upstream-weekly/2011.09.07

1 files changed, 88 insertions, 0 deletions

diff --git a/src/cmd/godoc/throttle.go b/src/cmd/godoc/throttle.go
new file mode 100644
index 000000000..193492802
--- /dev/null
+++ b/src/cmd/godoc/throttle.go
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+// 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 main
+
+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
+	tm int64   // minimum run time slice; >= 0
+	tr int64   // accumulated time running
+	ts int64   // accumulated time stopped
+	tt int64   // 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 tm nanoseconds:
+//
+//	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 tm < 0 are set to 0.
+//
+func NewThrottle(r float64, tm int64) *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 tm < 0 {
+		tm = 0
+	}
+	return &Throttle{f: f, tm: tm, tt: time.Nanoseconds() + tm}
+}
+
+// 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.Nanoseconds()
+	if t0 < p.tt {
+		return // keep running (minimum time slice not exhausted yet)
+	}
+
+	// accumulate running time
+	p.tr += t0 - (p.tt - p.tm)
+
+	// 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 := int64(float64(p.tr)*p.f) - p.ts; δs > 0 {
+		time.Sleep(δs)
+	}
+
+	// accumulate (actual) sleep time
+	t1 := time.Nanoseconds()
+	p.ts += t1 - t0
+
+	// set earliest next throttle time
+	p.tt = t1 + p.tm
+}