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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
// type Func.  Requests for different keys run concurrently.
// Concurrent requests for the same key result in duplicate work.
package memo

import "sync"

type Memo struct {
    f     Func
    mu    sync.Mutex // guards cache
    cache map[string]result

type Func func(string) (interface{}, error)

type result struct {
    value interface{}
    err   error

func New(f Func) *Memo {
    return &Memo{f: f, cache: make(map[string]result)}


func (memo *Memo) Get(key string) (value interface{}, err error) {
    res, ok := memo.cache[key]
    if !ok {
        res.value, res.err = memo.f(key)

        // Between the two critical sections, several goroutines
        // may race to compute f(key) and update the map.
        memo.cache[key] = res
    return res.value, res.err