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- // Copyright 2017 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 semaphore_test
-
- import (
- "context"
- "fmt"
- "log"
- "runtime"
-
- "golang.org/x/sync/semaphore"
- )
-
- // Example_workerPool demonstrates how to use a semaphore to limit the number of
- // goroutines working on parallel tasks.
- //
- // This use of a semaphore mimics a typical “worker pool” pattern, but without
- // the need to explicitly shut down idle workers when the work is done.
- func Example_workerPool() {
- ctx := context.TODO()
-
- var (
- maxWorkers = runtime.GOMAXPROCS(0)
- sem = semaphore.NewWeighted(int64(maxWorkers))
- out = make([]int, 32)
- )
-
- // Compute the output using up to maxWorkers goroutines at a time.
- for i := range out {
- // When maxWorkers goroutines are in flight, Acquire blocks until one of the
- // workers finishes.
- if err := sem.Acquire(ctx, 1); err != nil {
- log.Printf("Failed to acquire semaphore: %v", err)
- break
- }
-
- go func(i int) {
- defer sem.Release(1)
- out[i] = collatzSteps(i + 1)
- }(i)
- }
-
- // Acquire all of the tokens to wait for any remaining workers to finish.
- //
- // If you are already waiting for the workers by some other means (such as an
- // errgroup.Group), you can omit this final Acquire call.
- if err := sem.Acquire(ctx, int64(maxWorkers)); err != nil {
- log.Printf("Failed to acquire semaphore: %v", err)
- }
-
- fmt.Println(out)
-
- // Output:
- // [0 1 7 2 5 8 16 3 19 6 14 9 9 17 17 4 12 20 20 7 7 15 15 10 23 10 111 18 18 18 106 5]
- }
-
- // collatzSteps computes the number of steps to reach 1 under the Collatz
- // conjecture. (See https://en.wikipedia.org/wiki/Collatz_conjecture.)
- func collatzSteps(n int) (steps int) {
- if n <= 0 {
- panic("nonpositive input")
- }
-
- for ; n > 1; steps++ {
- if steps < 0 {
- panic("too many steps")
- }
-
- if n%2 == 0 {
- n /= 2
- continue
- }
-
- const maxInt = int(^uint(0) >> 1)
- if n > (maxInt-1)/3 {
- panic("overflow")
- }
- n = 3*n + 1
- }
-
- return steps
- }
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