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// Copyright 2016 Google Inc. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package android
import (
"fmt"
"sync"
)
type OncePer struct {
values sync.Map
}
type onceValueWaiter chan bool
func (once *OncePer) maybeWaitFor(key OnceKey, value interface{}) interface{} {
if wait, isWaiter := value.(onceValueWaiter); isWaiter {
// The entry in the map is a placeholder waiter because something else is constructing the value
// wait until the waiter is signalled, then load the real value.
<-wait
value, _ = once.values.Load(key)
if _, isWaiter := value.(onceValueWaiter); isWaiter {
panic(fmt.Errorf("Once() waiter completed but key is still not valid"))
}
}
return value
}
// Once computes a value the first time it is called with a given key per OncePer, and returns the
// value without recomputing when called with the same key. key must be hashable. If value panics
// the panic will be propagated but the next call to Once with the same key will return nil.
func (once *OncePer) Once(key OnceKey, value func() interface{}) interface{} {
// Fast path: check if the key is already in the map
if v, ok := once.values.Load(key); ok {
return once.maybeWaitFor(key, v)
}
// Slow path: create a OnceValueWrapper and attempt to insert it
waiter := make(onceValueWaiter)
if v, loaded := once.values.LoadOrStore(key, waiter); loaded {
// Got a value, something else inserted its own waiter or a constructed value
return once.maybeWaitFor(key, v)
}
// The waiter is inserted, call the value constructor, store it, and signal the waiter. Use defer in case
// the function panics.
var v interface{}
defer func() {
once.values.Store(key, v)
close(waiter)
}()
v = value()
return v
}
// Get returns the value previously computed with Once for a given key. If Once has not been called for the given
// key Get will panic.
func (once *OncePer) Get(key OnceKey) interface{} {
v, ok := once.values.Load(key)
if !ok {
panic(fmt.Errorf("Get() called before Once()"))
}
return once.maybeWaitFor(key, v)
}
// OnceStringSlice is the same as Once, but returns the value cast to a []string
func (once *OncePer) OnceStringSlice(key OnceKey, value func() []string) []string {
return once.Once(key, func() interface{} { return value() }).([]string)
}
// OnceStringSlice is the same as Once, but returns two values cast to []string
func (once *OncePer) Once2StringSlice(key OnceKey, value func() ([]string, []string)) ([]string, []string) {
type twoStringSlice [2][]string
s := once.Once(key, func() interface{} {
var s twoStringSlice
s[0], s[1] = value()
return s
}).(twoStringSlice)
return s[0], s[1]
}
// OncePath is the same as Once, but returns the value cast to a Path
func (once *OncePer) OncePath(key OnceKey, value func() Path) Path {
return once.Once(key, func() interface{} { return value() }).(Path)
}
// OncePath is the same as Once, but returns the value cast to a SourcePath
func (once *OncePer) OnceSourcePath(key OnceKey, value func() SourcePath) SourcePath {
return once.Once(key, func() interface{} { return value() }).(SourcePath)
}
// OnceKey is an opaque type to be used as the key in calls to Once.
type OnceKey struct {
key interface{}
}
// NewOnceKey returns an opaque OnceKey object for the provided key. Two calls to NewOnceKey with the same key string
// DO NOT produce the same OnceKey object.
func NewOnceKey(key string) OnceKey {
return OnceKey{&key}
}
// NewCustomOnceKey returns an opaque OnceKey object for the provided key. The key can be any type that is valid as the
// key in a map, i.e. comparable. Two calls to NewCustomOnceKey with key values that compare equal will return OnceKey
// objects that access the same value stored with Once.
func NewCustomOnceKey(key interface{}) OnceKey {
return OnceKey{key}
}
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