// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package errgroup_test

import (
	"context"
	"crypto/md5"
	"fmt"
	"io/ioutil"
	"log"
	"os"
	"path/filepath"

	"golang.org/x/sync/errgroup"
)

// Pipeline demonstrates the use of a Group to implement a multi-stage
// pipeline: a version of the MD5All function with bounded parallelism from
// https://blog.golang.org/pipelines.
func ExampleGroup_pipeline() {
	m, err := MD5All(context.Background(), ".")
	if err != nil {
		log.Fatal(err)
	}

	for k, sum := range m {
		fmt.Printf("%s:\t%x\n", k, sum)
	}
}

type result struct {
	path string
	sum  [md5.Size]byte
}

// MD5All reads all the files in the file tree rooted at root and returns a map
// from file path to the MD5 sum of the file's contents. If the directory walk
// fails or any read operation fails, MD5All returns an error.
func MD5All(ctx context.Context, root string) (map[string][md5.Size]byte, error) {
	// ctx is canceled when g.Wait() returns. When this version of MD5All returns
	// - even in case of error! - we know that all of the goroutines have finished
	// and the memory they were using can be garbage-collected.
	g, ctx := errgroup.WithContext(ctx)
	paths := make(chan string)

	g.Go(func() error {
		defer close(paths)
		return filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
			if err != nil {
				return err
			}
			if !info.Mode().IsRegular() {
				return nil
			}
			select {
			case paths <- path:
			case <-ctx.Done():
				return ctx.Err()
			}
			return nil
		})
	})

	// Start a fixed number of goroutines to read and digest files.
	c := make(chan result)
	const numDigesters = 20
	for i := 0; i < numDigesters; i++ {
		g.Go(func() error {
			for path := range paths {
				data, err := ioutil.ReadFile(path)
				if err != nil {
					return err
				}
				select {
				case c <- result{path, md5.Sum(data)}:
				case <-ctx.Done():
					return ctx.Err()
				}
			}
			return nil
		})
	}
	go func() {
		g.Wait()
		close(c)
	}()

	m := make(map[string][md5.Size]byte)
	for r := range c {
		m[r.path] = r.sum
	}
	// Check whether any of the goroutines failed. Since g is accumulating the
	// errors, we don't need to send them (or check for them) in the individual
	// results sent on the channel.
	if err := g.Wait(); err != nil {
		return nil, err
	}
	return m, nil
}