via goroutine

/*
 Hash - Guillermo Estrada

  Simple utility to obtain the MD5 and/or SHA-1 
  of a file from the command line.

  2011

 Edited: Marko Mikulicic 2011
*/

package main

import (
	"io"
	"os"
	"fmt"
	"flag"
	"hash"
	"crypto/md5"
	"crypto/sha1"
	"crypto/sha256"
	"crypto/sha512"
	"crypto/ripemd160"
	"time"
)

type ParallelWriter struct {
	writer io.Writer
	ch chan []byte
}

func NewParallelWriter(writer io.Writer) ParallelWriter {
	ch := make(chan []byte, 100)

	go func() {
		for chunk := range ch {
			writer.Write(chunk)
		}
	}()

	return ParallelWriter{writer, ch}
}

func (self ParallelWriter) Write(p []byte) (n int, err os.Error) {

	cp := make([]byte, len(p), len(p))
	copy(cp, p)
	self.ch <- cp

	return len(p), nil
}

func main() {

	parallel := flag.Bool("parallel", false, "enable parallel calculation of multiple hash algorithms")

	algos := [...]string{"md5", "sha1", "sha256", "sha512", "ripemd160"}
	impls := [...]hash.Hash{md5.New(), sha1.New(), sha256.New(), sha512.New(), ripemd160.New()}
	flags := make([]*bool, len(algos))
	for i, a := range algos {
		flags[i] = flag.Bool(a, false, fmt.Sprintf("-%s calculate %s hash of file", a, a))
	}

	flag.Parse()


	any := false
	for _, f := range flags {
		any = any || *f
	}
	if any == false {
		fmt.Println("err: No hash specified. Please run with --help to see list of supported hash algos")
		os.Exit(1)
	}


	infile, err := os.Open(flag.Arg(0))
	if err != nil {
    fmt.Println(err)
    os.Exit(1)
  }

	writers := make([]io.Writer, 0, len(impls))
	for i, flag := range flags {
		if *flag {
			if *parallel {
				writers = append(writers, NewParallelWriter(impls[i]))
			} else {
				writers = append(writers, impls[i])
			}
		}
	}

	dest := io.MultiWriter(writers...)

	io.Copy(dest, infile)

	if *parallel {
		time.Sleep(2e9)
	}

	for i, flag := range flags {
		if *flag {
			fmt.Printf("%s: %x\n", algos[i], impls[i].Sum())
		}
	}

}