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// Copyright 2014, Google Inc. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package pools
import (
"fmt"
"sync"
)
// IDPool is used to ensure that the set of IDs in use concurrently never
// contains any duplicates. The IDs start at 1 and increase without bound, but
// will never be larger than the peak number of concurrent uses.
//
// IDPool's Get() and Set() methods can be used concurrently.
type IDPool struct {
sync.Mutex
// used holds the set of values that have been returned to us with Put().
used map[uint32]bool
// maxUsed remembers the largest value we've given out.
maxUsed uint32
}
// NewIDPool creates and initializes an IDPool.
func NewIDPool() *IDPool {
return &IDPool{
used: make(map[uint32]bool),
}
}
// Get returns an ID that is unique among currently active users of this pool.
func (pool *IDPool) Get() (id uint32) {
pool.Lock()
defer pool.Unlock()
// Pick a value that's been returned, if any.
for key, _ := range pool.used {
delete(pool.used, key)
return key
}
// No recycled IDs are available, so increase the pool size.
pool.maxUsed += 1
return pool.maxUsed
}
// Put recycles an ID back into the pool for others to use. Putting back a value
// or 0, or a value that is not currently "checked out", will result in a panic
// because that should never happen except in the case of a programming error.
func (pool *IDPool) Put(id uint32) {
pool.Lock()
defer pool.Unlock()
if id < 1 || id > pool.maxUsed {
panic(fmt.Errorf("IDPool.Put(%v): invalid value, must be in the range [1,%v]", id, pool.maxUsed))
}
if pool.used[id] {
panic(fmt.Errorf("IDPool.Put(%v): can't put value that was already recycled", id))
}
// If we're recycling maxUsed, just shrink the pool.
if id == pool.maxUsed {
pool.maxUsed = id - 1
return
}
// Add it to the set of recycled IDs.
pool.used[id] = true
}