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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
}

//!-