Simple utility to obtain multiple hashes of a file from the command line.

hash.go

/*
  Hash - Guillermo Estrada
         Marko Mikulicic
         Kyle Lemons

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

  2011
*/

package main

// Typically, imports are sorted in alphabetical order
import (
	"crypto/md5"
	"crypto/sha1"
	"crypto/sha256"
	"crypto/sha512"
	"crypto/ripemd160"
	"flag"
	"fmt"
	"hash"
	"io"
	"os"
)

func main() {
	
	// Instead of defining each flag, drying up code
	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()

	// Blocks of var/const work just as well inside a function call
	var (
		writers []io.Writer
		hashes  []hash.Hash
		names   []string
	)

	// Closures can be used to collect common operations into a nice, clean function call
	push := func(name string, h hash.Hash) {
		writers = append(writers, h) // a Hash is a writer, so this is easy
		hashes = append(hashes, h)
		names = append(names, name)
	}

	// Now any number of hashes can be added here with very little code
	for i, flag := range flags {
		if *flag {
			push(algos[i],impls[i])
		}
	}

	if len(names) == 0 {
		flag.Usage()
		os.Exit(1)
	}

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

	// The variadic expansion of a slice is really convenient.
	io.Copy(io.MultiWriter(writers...), in)

	for i, name := range names {
		fmt.Printf("%9s: %x\n", name, hashes[i].Sum())
	}
}