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// Copyright 2018 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 implements parallel execution helpers.
package par
import (
// Work manages a set of work items to be executed in parallel, at most once each.
// The items in the set must all be valid map keys.
type Work struct {
f func(interface{}) // function to run for each item
running int // total number of runners
mu sync.Mutex
added map[interface{}]bool // items added to set
todo []interface{} // items yet to be run
wait sync.Cond // wait when todo is empty
waiting int // number of runners waiting for todo
func (w *Work) init() {
if w.added == nil {
w.added = make(map[interface{}]bool)
w.wait.L = &
// Add adds item to the work set, if it hasn't already been added.
func (w *Work) Add(item interface{}) {
if !w.added[item] {
w.added[item] = true
w.todo = append(w.todo, item)
if w.waiting > 0 {
// Do runs f in parallel on items from the work set,
// with at most n invocations of f running at a time.
// It returns when everything added to the work set has been processed.
// At least one item should have been added to the work set
// before calling Do (or else Do returns immediately),
// but it is allowed for f(item) to add new items to the set.
// Do should only be used once on a given Work.
func (w *Work) Do(n int, f func(item interface{})) {
if n < 1 {
panic("par.Work.Do: n < 1")
if w.running >= 1 {
panic("par.Work.Do: already called Do")
w.running = n
w.f = f
for i := 0; i < n-1; i++ {
go w.runner()
// runner executes work in w until both nothing is left to do
// and all the runners are waiting for work.
// (Then all the runners return.)
func (w *Work) runner() {
for {
// Wait for something to do.
for len(w.todo) == 0 {
if w.waiting == w.running {
// All done.
// Pick something to do at random,
// to eliminate pathological contention
// in case items added at about the same time
// are most likely to contend.
i := rand.Intn(len(w.todo))
item := w.todo[i]
w.todo[i] = w.todo[len(w.todo)-1]
w.todo = w.todo[:len(w.todo)-1]
// Cache runs an action once per key and caches the result.
type Cache struct {
m sync.Map
// Do calls the function f if and only if Do is being called for the first time with this key.
// No call to Do with a given key returns until the one call to f returns.
// Do returns the value returned by the one call to f.
func (c *Cache) Do(key interface{}, f func() interface{}) interface{} {
type entry struct {
once sync.Once
result interface{}
entryIface, ok := c.m.Load(key)
if !ok {
entryIface, _ = c.m.LoadOrStore(key, new(entry))
e := entryIface.(*entry)
e.once.Do(func() {
e.result = f()
return e.result