76 lines
2.1 KiB
Go
76 lines
2.1 KiB
Go
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// Copyright 2016 Google Inc. All rights reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package android
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import (
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"sync"
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"sync/atomic"
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)
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type OncePer struct {
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values atomic.Value
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valuesLock sync.Mutex
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}
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type valueMap map[interface{}]interface{}
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// Once computes a value the first time it is called with a given key per OncePer, and returns the
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// value without recomputing when called with the same key. key must be hashable.
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func (once *OncePer) Once(key interface{}, value func() interface{}) interface{} {
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// Atomically load the map without locking. If this is the first call Load() will return nil
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// and the type assertion will fail, leaving a nil map in m, but that's OK since m is only used
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// for reads.
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m, _ := once.values.Load().(valueMap)
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if v, ok := m[key]; ok {
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return v
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}
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once.valuesLock.Lock()
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defer once.valuesLock.Unlock()
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// Check again with the lock held
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m, _ = once.values.Load().(valueMap)
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if v, ok := m[key]; ok {
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return v
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}
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// Copy the existing map
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newMap := make(valueMap, len(m))
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for k, v := range m {
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newMap[k] = v
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}
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v := value()
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newMap[key] = v
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once.values.Store(newMap)
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return v
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}
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func (once *OncePer) OnceStringSlice(key interface{}, value func() []string) []string {
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return once.Once(key, func() interface{} { return value() }).([]string)
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}
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func (once *OncePer) Once2StringSlice(key interface{}, value func() ([]string, []string)) ([]string, []string) {
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type twoStringSlice [2][]string
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s := once.Once(key, func() interface{} {
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var s twoStringSlice
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s[0], s[1] = value()
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return s
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}).(twoStringSlice)
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return s[0], s[1]
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}
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