memo.go
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memo.go
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	package memo

	import (
	"sync"
	"time"
	)

	type (
	Memo struct {
	f Func
	mu sync.Mutex //guards cache
	cache *entry

	cacheDur time.Duration
	lastCache time.Time
	}

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

	type (
	entry struct {
	res result
	ready chan struct{} // close when res is read
	}

	result struct {
	value interface{}
	err error
	}
	)

	func New(f Func, cd time.Duration) *Memo {
	return &Memo{
	f: f,
	cacheDur: cd,
	}
	}

	// Invalidate resets the cache to nil if the memo's given cache duration has
	// elapsed, and returns true if the cache was actually invalidated.
	func (memo *Memo) Invalidate() bool {
	defer memo.mu.Unlock()
	memo.mu.Lock()

	if memo.cache != nil && time.Now().Sub(memo.lastCache) > memo.cacheDur {
	memo.cache = nil
	memo.lastCache = time.Now()
	return true
	}
	return false
	}

	func (memo *Memo) Get() (value interface{}, err error) {
	memo.mu.Lock()
	if memo.cache == nil {
	memo.cache = &entry{ready: make(chan struct{})}
	memo.mu.Unlock()

	memo.cache.res.value, memo.cache.res.err = memo.f()

	close(memo.cache.ready)
	} else {
	memo.mu.Unlock()

	<-memo.cache.ready
	}
	return memo.cache.res.value, memo.cache.res.err
	}

	// Reset forcibly resets the cache to nil without checking if the cache
	// duration has elapsed.
	func (memo *Memo) Reset() {
	defer memo.mu.Unlock()
	memo.mu.Lock()

	memo.cache = nil
	memo.lastCache = time.Now()
	}