rot13.go

package main

import (
	"io"
	"os"
	"strings"
)

type rot13Reader struct {
	rot io.Reader
}

func rot13(b byte) byte {
	var a, z byte
	if 'a' <= b && b <= 'z' {
		a, z = 'a', 'z'
	}	else if 'A' <= b && b <= 'Z' {
		a, z = 'A', 'Z'
	}

	return (b-a+13)%(z-a+1) + a
}

func (rot *rot13Reader) Read(bytes []byte) (num int, err error) {
	num, err = rot.rot.Read(bytes)
	for i := 0; i < num; i++ {
		bytes[i] = rot13(bytes[i])
	}
	return
}
func main() {
	s := strings.NewReader("Lbh penpxrq gur pbqr!")
	r := rot13Reader{s}
	io.Copy(os.Stdout, &r)
}