xorfiles.go

gistfile1.txt

package main

import (
	"fmt"
	"io"
	"os"
)

func main() {
	if len(os.Args) != 4 {
		fmt.Println("Usage: xorfiles file1 file2 out")
	}
	in1, err := os.Open(os.Args[1])
	if err != nil {
		fmt.Println(err)
		return
	}
	in2, err := os.Open(os.Args[2])
	if err != nil {
		fmt.Println(err)
		return
	}
	out, err := os.Create(os.Args[3])
	if err != nil {
		fmt.Println(err)
		return
	}
	xor(out, in1, in2)
}

var buf1 [1 << 20]byte
var buf2 [1 << 20]byte

func xor(w io.Writer, r1, r2 io.Reader) {
	var n1, n2, nn int
	var err1, err2 error
	for {
		n1, err1 = r1.Read(buf1[:])
		if n1 == 0 {
			break
		}
		n2, err2 = r2.Read(buf2[:n1])
		for n2 < n1 && err2 == nil {
			nn, err2 = r2.Read(buf2[n2:n1])
			n2 += nn
		}
		n := n1
		if n1 > n2 {
			n = n2
		}
		for i := range buf1[:n] {
			buf1[i] ^= buf2[i]
		}
		_, err := w.Write(buf1[:n])
		if err != nil {
			fmt.Fprintln(os.Stderr, err)
			return
		}
		if n1 > n {
			_, err = w.Write(buf1[n:n1])
			if err != nil {
				fmt.Fprintln(os.Stderr, err)
				return
			}
		}
		if err1 != nil || err2 != nil {
			break
		}
	}
	if err1 != nil && err1 != io.EOF {
		fmt.Fprintln(os.Stderr, err1)
	}
	if err2 != nil && err2 != io.EOF {
		fmt.Fprintln(os.Stderr, err2)
	}
	io.Copy(w, r1)
	io.Copy(w, r2)
}

xorfiles_test.go

package main

import (
	"bytes"
	"testing"
	"testing/iotest"
)

func TestXor(t *testing.T) {
	b1 := []byte("hello hello hello")
	b2 := []byte("hello there")
	want := append([]byte{}, b1...)
	for i := range b2 {
		want[i] ^= b2[i]
	}
	var got bytes.Buffer
	xor(&got, bytes.NewReader(b1), bytes.NewReader(b2))
	if !bytes.Equal(got.Bytes(), want) {
		t.Errorf("xor(b1, b2): expected %v, got %v", want, got.Bytes())
	}
	got.Reset()
	xor(&got, bytes.NewReader(b2), bytes.NewReader(b1))
	if !bytes.Equal(got.Bytes(), want) {
		t.Errorf("xor(b2, b1): expected %v, got %v", want, got.Bytes())
	}
	got.Reset()
	xor(&got, iotest.HalfReader(bytes.NewReader(b1)),
		iotest.HalfReader(bytes.NewReader(b2)))
	if !bytes.Equal(got.Bytes(), want) {
		t.Errorf("xor(HalfReader(b1), HalfReader(b2)): expected %v, got %v", want, got.Bytes())
	}
}