interface and internal (blackbox) implementation.

semaphore.go

// Package semaphore contains implementations of counting semaphores using both
// buffered channels and condition variables.
package semaphore

import "errors"

// Semaphore defines the protocol for any counting semaphore.
type Semaphore interface {
	// Acquire blocks until a slot can be acquired in the counting semaphore.
	Acquire()

	// Release releases a slot in the counting semaphore.
	Release()
}


const (
	// SemaAcquire signals slot acquired.
	SemaAcquire = iota << 1

	// SemaRelease signals slot available.
	SemaRelease
)

var (
	// ErrCountMustBePositive is generated when an invalid count is used to
	// initialize the semaphore.
	ErrCountMustBePositive = errors.New("count must be greater than 0")
)

// chanSema represents a counting semaphore.
type chanSema struct {
	sema chan int
}

// NewSemaphore generates a new counting semaphore. The count of the semaphore
// determines the rate at which it will throttle.
func NewSemaphore(count int) (Semaphore, error) {
	if count <= 0 {
		return nil, ErrCountMustBePositive
	}
	sema := make(chan int, count)
	for i := 0; i < count; i++ {
		sema <- SemaAcquire
	}
	return chanSema{sema}, nil
}

// Acquire blocks until a slot can be acquired in the counting semaphore.
func (sema chanSema) Acquire() {
	<-sema.sema
}

// Release releases a slot in the counting semaphore.
func (sema chanSema) Release() {
	sema.sema <- SemaRelease
}