package singleflight

import (
	"sync"
)

var WaitCount int
var DirectCount int

type Caller struct {
	val interface{}
	err error

	wg sync.WaitGroup
}

type Group struct {
	mu sync.RWMutex
	m  map[string]*Caller
}

func (g *Group) Do(key string, fn func() (interface{}, error)) (interface{}, error) {
	g.mu.Lock()
	if g.m == nil {
		g.m = make(map[string]*Caller)
	}

	c, ok := g.m[key]
	if ok {
                //阻塞等待其它已經執行此操作的返回結果
		g.mu.Unlock()
		c.wg.Wait()
		WaitCount++
		return c.val, c.err
	}

	//直接請求獲取數據
	c = &Caller{}
	g.m[key] = c
	c.wg.Add(1)
	g.mu.Unlock()

	c.val, c.err = fn()
	c.wg.Done()

	g.mu.Lock()
	delete(g.m, key)
	g.mu.Unlock()

	DirectCount++
	return c.val, c.err
}

func TestGroup_Do(t *testing.T) {
	sg := &Group{}
	wg := sync.WaitGroup{}

	for i := 0; i < 10000; i++ {
		fn := func() (interface{}, error) {
			return i, nil
		}
		wg.Add(1)
		go func() {
			defer wg.Done()
			got, err := sg.Do("test-key", fn)
			_, _ = got, err
			//t.Log("got:", i)
		}()
	}

	wg.Wait()
	fmt.Println("waitCount:", WaitCount)
	fmt.Println("DirectCount:", DirectCount)
}