forked from TrueCloudLab/restic
58 lines
1.7 KiB
Go
58 lines
1.7 KiB
Go
// 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 atomiccache provides a map-based cache that supports very fast
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// reads.
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package atomiccache
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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 mapType map[interface{}]interface{}
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// Cache is a map-based cache that supports fast reads via use of atomics.
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// Writes are slow, requiring a copy of the entire cache.
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// The zero Cache is an empty cache, ready for use.
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type Cache struct {
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val atomic.Value // mapType
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mu sync.Mutex // used only by writers
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}
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// Get returns the value of the cache at key. If there is no value,
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// getter is called to provide one, and the cache is updated.
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// The getter function may be called concurrently. It should be pure,
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// returning the same value for every call.
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func (c *Cache) Get(key interface{}, getter func() interface{}) interface{} {
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mp, _ := c.val.Load().(mapType)
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if v, ok := mp[key]; ok {
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return v
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}
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// Compute value without lock.
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// Might duplicate effort but won't hold other computations back.
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newV := getter()
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c.mu.Lock()
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mp, _ = c.val.Load().(mapType)
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newM := make(mapType, len(mp)+1)
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for k, v := range mp {
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newM[k] = v
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}
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newM[key] = newV
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c.val.Store(newM)
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c.mu.Unlock()
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return newV
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}
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