rot13.go

package main

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

type rot13Reader struct {
	r io.Reader
}

func rot(b byte) byte {
	if 97 <= b && b <= 122 {
		return 97 + ((b - 97 + 13) % 26)
	} else if 65 <= b && b <= 90 {		
		return 65 + ((b - 65 + 13) % 26)
	} else {
		return b
	}
}

func (r rot13Reader) Read(b []byte) (int, error) {
	amount := 0
	for amount < len(b) {
		s := b[amount:]
		n, err := r.Read(s)
		for i := 0; i < n; i++ {
			s[i] = rot(s[i])
		}
		amount += n
		if err != nil {
			return amount, err
		}
	}
	return amount, nil
}

func main() {
	s := strings.NewReader("Lbh penpxrq gur pbqr!")
	r := rot13Reader{s}
	io.Copy(os.Stdout, &r)
}