package main

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

type rot13Reader struct {
	r io.Reader
}

func (r rot13Reader) Read(p []byte) (n int, err error) {
	n, err = r.r.Read(p)
	for i := range p[:n] {
		p[i] = rot13(p[i])
	}
	return
}

func rot13(b byte) byte {
	var a, z, A, Z byte = 'a', 'z', 'A', 'Z'
    if a < b && b < z {
        b = a + ((b - a + 13) % 26)
    } else if A < b && b < Z {
        b = A + ((b - A + 13) % 26)
    }
	return b
}

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