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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 sync

import "runtime"

// Cond implements a condition variable, a rendezvous point
// for goroutines waiting for or announcing the occurrence
// of an event.
//
// Each Cond has an associated Locker L (often a *Mutex or *RWMutex),
// which must be held when changing the condition and
// when calling the Wait method.
type Cond struct {
        L Locker // held while observing or changing the condition
        m Mutex  // held to avoid internal races

        // We must be careful to make sure that when Signal
        // releases a semaphore, the corresponding acquire is
        // executed by a goroutine that was already waiting at
        // the time of the call to Signal, not one that arrived later.
        // To ensure this, we segment waiting goroutines into
        // generations punctuated by calls to Signal.  Each call to
        // Signal begins another generation if there are no goroutines
        // left in older generations for it to wake.  Because of this
        // optimization (only begin another generation if there
        // are no older goroutines left), we only need to keep track
        // of the two most recent generations, which we call old
        // and new.
        oldWaiters int     // number of waiters in old generation...
        oldSema    *uint32 // ... waiting on this semaphore

        newWaiters int     // number of waiters in new generation...
        newSema    *uint32 // ... waiting on this semaphore
}

// NewCond returns a new Cond with Locker l.
func NewCond(l Locker) *Cond {
        return &Cond{L: l}
}

// Wait atomically unlocks c.L and suspends execution
// of the calling goroutine.  After later resuming execution,
// Wait locks c.L before returning.
//
// Because L is not locked when Wait first resumes, the caller
// typically cannot assume that the condition is true when
// Wait returns.  Instead, the caller should Wait in a loop:
//
//    c.L.Lock()
//    for !condition() {
//        c.Wait()
//    }
//    ... make use of condition ...
//    c.L.Unlock()
//
func (c *Cond) Wait() {
        c.m.Lock()
        if c.newSema == nil {
                c.newSema = new(uint32)
        }
        s := c.newSema
        c.newWaiters++
        c.m.Unlock()
        c.L.Unlock()
        runtime.Semacquire(s)
        c.L.Lock()
}

// Signal wakes one goroutine waiting on c, if there is any.
//
// It is allowed but not required for the caller to hold c.L
// during the call.
func (c *Cond) Signal() {
        c.m.Lock()
        if c.oldWaiters == 0 && c.newWaiters > 0 {
                // Retire old generation; rename new to old.
                c.oldWaiters = c.newWaiters
                c.oldSema = c.newSema
                c.newWaiters = 0
                c.newSema = nil
        }
        if c.oldWaiters > 0 {
                c.oldWaiters--
                runtime.Semrelease(c.oldSema)
        }
        c.m.Unlock()
}

// Broadcast wakes all goroutines waiting on c.
//
// It is allowed but not required for the caller to hold c.L
// during the call.
func (c *Cond) Broadcast() {
        c.m.Lock()
        // Wake both generations.
        if c.oldWaiters > 0 {
                for i := 0; i < c.oldWaiters; i++ {
                        runtime.Semrelease(c.oldSema)
                }
                c.oldWaiters = 0
        }
        if c.newWaiters > 0 {
                for i := 0; i < c.newWaiters; i++ {
                        runtime.Semrelease(c.newSema)
                }
                c.newWaiters = 0
                c.newSema = nil
        }
        c.m.Unlock()
}