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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
valuesLock sync.Mutex
}
type valueMap map[interface{}]interface{}
// 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.
func (once *OncePer) Once(key interface{}, value func() interface{}) interface{} {
// Fast path: check if the key is already in the map
if v, ok := once.values.Load(key); ok {
return v
}
// Slow path: lock so that we don't call the value function twice concurrently
once.valuesLock.Lock()
defer once.valuesLock.Unlock()
// Check again with the lock held
if v, ok := once.values.Load(key); ok {
return v
}
// Still not in the map, call the value function and store it
v := value()
once.values.Store(key, v)
return v
}
func (once *OncePer) Get(key interface{}) interface{} {
v, ok := once.values.Load(key)
if !ok {
panic(fmt.Errorf("Get() called before Once()"))
}
return v
}
func (once *OncePer) OnceStringSlice(key interface{}, value func() []string) []string {
return once.Once(key, func() interface{} { return value() }).([]string)
}
func (once *OncePer) Once2StringSlice(key interface{}, 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]
}
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