github.com/westcoastroms/westcoastroms-build@v0.0.0-20190928114312-2350e5a73030/build/soong/android/onceper.go (about)

     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  
    15  package android
    16  
    17  import (
    18  	"fmt"
    19  	"sync"
    20  )
    21  
    22  type OncePer struct {
    23  	values     sync.Map
    24  	valuesLock sync.Mutex
    25  }
    26  
    27  type 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.
    31  func (once *OncePer) Once(key interface{}, value func() interface{}) interface{} {
    32  	// Fast path: check if the key is already in the map
    33  	if v, ok := once.values.Load(key); ok {
    34  		return v
    35  	}
    36  
    37  	// Slow path: lock so that we don't call the value function twice concurrently
    38  	once.valuesLock.Lock()
    39  	defer once.valuesLock.Unlock()
    40  
    41  	// Check again with the lock held
    42  	if v, ok := once.values.Load(key); ok {
    43  		return v
    44  	}
    45  
    46  	// Still not in the map, call the value function and store it
    47  	v := value()
    48  	once.values.Store(key, v)
    49  
    50  	return v
    51  }
    52  
    53  func (once *OncePer) Get(key interface{}) interface{} {
    54  	v, ok := once.values.Load(key)
    55  	if !ok {
    56  		panic(fmt.Errorf("Get() called before Once()"))
    57  	}
    58  
    59  	return v
    60  }
    61  
    62  func (once *OncePer) OnceStringSlice(key interface{}, value func() []string) []string {
    63  	return once.Once(key, func() interface{} { return value() }).([]string)
    64  }
    65  
    66  func (once *OncePer) Once2StringSlice(key interface{}, value func() ([]string, []string)) ([]string, []string) {
    67  	type twoStringSlice [2][]string
    68  	s := once.Once(key, func() interface{} {
    69  		var s twoStringSlice
    70  		s[0], s[1] = value()
    71  		return s
    72  	}).(twoStringSlice)
    73  	return s[0], s[1]
    74  }