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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan. | |
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/ | |
// See page 276. | |
// Package memo provides a concurrency-safe memoization a function of | |
// a function. Requests for different keys proceed in parallel. | |
// Concurrent requests for the same key block until the first completes. | |
// This implementation uses a Mutex. | |
package memo | |
import "sync" | |
// Func is the type of the function to memoize. | |
type Func func(string) (interface{}, error) | |
type result struct { | |
value interface{} | |
err error | |
} | |
//!+ | |
type entry struct { | |
res result | |
ready chan struct{} // closed when res is ready | |
} | |
func New(f Func) *Memo { | |
return &Memo{f: f, cache: make(map[string]*entry)} | |
} | |
type Memo struct { | |
f Func | |
mu sync.Mutex // guards cache | |
cache map[string]*entry | |
} | |
func (memo *Memo) Get(key string) (value interface{}, err error) { | |
memo.mu.Lock() | |
e := memo.cache[key] | |
if e == nil { | |
// This is the first request for this key. | |
// This goroutine becomes responsible for computing | |
// the value and broadcasting the ready condition. | |
e = &entry{ready: make(chan struct{})} | |
memo.cache[key] = e | |
memo.mu.Unlock() | |
e.res.value, e.res.err = memo.f(key) | |
close(e.ready) // broadcast ready condition | |
} else { | |
// This is a repeat request for this key. | |
memo.mu.Unlock() | |
<-e.ready // wait for ready condition | |
} | |
return e.res.value, e.res.err | |
} | |
//!- |