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// Copyright 2020 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 par
import "fmt"
// Queue manages a set of work items to be executed in parallel. The number of
// active work items is limited, and excess items are queued sequentially.
type Queue struct {
maxActive int
st chan queueState
}
type queueState struct {
active int // number of goroutines processing work; always nonzero when len(backlog) > 0
backlog []func()
idle chan struct{} // if non-nil, closed when active becomes 0
}
// NewQueue returns a Queue that executes up to maxActive items in parallel.
//
// maxActive must be positive.
func NewQueue(maxActive int) *Queue {
if maxActive < 1 {
panic(fmt.Sprintf("par.NewQueue called with nonpositive limit (%d)", maxActive))
}
q := &Queue{
maxActive: maxActive,
st: make(chan queueState, 1),
}
q.st <- queueState{}
return q
}
// Add adds f as a work item in the queue.
//
// Add returns immediately, but the queue will be marked as non-idle until after
// f (and any subsequently-added work) has completed.
func (q *Queue) Add(f func()) {
st := <-q.st
if st.active == q.maxActive {
st.backlog = append(st.backlog, f)
q.st <- st
return
}
if st.active == 0 {
// Mark q as non-idle.
st.idle = nil
}
st.active++
q.st <- st
go func() {
for {
f()
st := <-q.st
if len(st.backlog) == 0 {
if st.active--; st.active == 0 && st.idle != nil {
close(st.idle)
}
q.st <- st
return
}
f, st.backlog = st.backlog[0], st.backlog[1:]
q.st <- st
}
}()
}
// Idle returns a channel that will be closed when q has no (active or enqueued)
// work outstanding.
func (q *Queue) Idle() <-chan struct{} {
st := <-q.st
defer func() { q.st <- st }()
if st.idle == nil {
st.idle = make(chan struct{})
if st.active == 0 {
close(st.idle)
}
}
return st.idle
}
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