package main

import (
	"fmt"
	"sync"
	"time"
)

// Dedupe will combine a bunch of concurrent calls with the same key (string) into just one.
// Example:

func slowlyAdd(a, b int) int {
	key := fmt.Sprintf("%d+%d", a, b)
	v, _ := Dedupe(key, func() (interface{}, error) {
		// Here we put the slow code that we don't mind only running once.
		time.Sleep(2 * time.Second)
		return a + b, nil
	})
	return v.(int)
}

func main() {
	fmt.Println("Please wait while we slowly add the same numbers together a few times!")
	start := time.Now()
	var wg sync.WaitGroup
	for i := 0; i < 5; i++ {
		// Run five slowlyAdd calls in parallel (separate goroutines).
		wg.Add(1)
		go func() {
			fmt.Printf("Adding 1 + 2: %d\n", slowlyAdd(1, 2))
			wg.Done()
		}()
	}
	wg.Wait()
	fmt.Printf("Total time taken: %s\n", time.Since(start))
}

// Dedupe implementation:

type result struct {
	value interface{}
	err   error
}

var (
	dedupeMap = make(map[string][]chan<- result)
	dedupeMu  sync.Mutex
)

func Dedupe(key string, perform func() (interface{}, error)) (interface{}, error) {
	// Check if there's already an ongoing call.
	dedupeMu.Lock()
	if calls, ok := dedupeMap[key]; ok {
		// There is an ongoing call, join the list of waiting requests.
		// Buffered to allow the result to be written before it is read.
		ch := make(chan result, 1)
		dedupeMap[key] = append(calls, ch)
		dedupeMu.Unlock()
		r := <-ch
		return r.value, r.err
	}
	// There is no other call ongoing for the provided key.
	dedupeMap[key] = []chan<- result{}
	dedupeMu.Unlock()
	value, err := perform()
	// Get all waiting requests.
	dedupeMu.Lock()
	calls := dedupeMap[key]
	delete(dedupeMap, key)
	dedupeMu.Unlock()
	// Fulfill the waiting requests.
	for _, ch := range calls {
		ch <- result{value, err}
	}
	// Return the result.
	return value, err
}