1// Copyright 2016 Google Inc. All rights reserved.
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7//     http://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14
15package android
16
17import (
18	"sync"
19	"sync/atomic"
20)
21
22type OncePer struct {
23	values     atomic.Value
24	valuesLock sync.Mutex
25}
26
27type valueMap map[interface{}]interface{}
28
29// Once computes a value the first time it is called with a given key per OncePer, and returns the
30// value without recomputing when called with the same key.  key must be hashable.
31func (once *OncePer) Once(key interface{}, value func() interface{}) interface{} {
32	// Atomically load the map without locking.  If this is the first call Load() will return nil
33	// and the type assertion will fail, leaving a nil map in m, but that's OK since m is only used
34	// for reads.
35	m, _ := once.values.Load().(valueMap)
36	if v, ok := m[key]; ok {
37		return v
38	}
39
40	once.valuesLock.Lock()
41	defer once.valuesLock.Unlock()
42
43	// Check again with the lock held
44	m, _ = once.values.Load().(valueMap)
45	if v, ok := m[key]; ok {
46		return v
47	}
48
49	// Copy the existing map
50	newMap := make(valueMap, len(m))
51	for k, v := range m {
52		newMap[k] = v
53	}
54
55	v := value()
56
57	newMap[key] = v
58	once.values.Store(newMap)
59
60	return v
61}
62
63func (once *OncePer) OnceStringSlice(key interface{}, value func() []string) []string {
64	return once.Once(key, func() interface{} { return value() }).([]string)
65}
66
67func (once *OncePer) Once2StringSlice(key interface{}, value func() ([]string, []string)) ([]string, []string) {
68	type twoStringSlice [2][]string
69	s := once.Once(key, func() interface{} {
70		var s twoStringSlice
71		s[0], s[1] = value()
72		return s
73	}).(twoStringSlice)
74	return s[0], s[1]
75}
76