package main

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

type rot13Reader struct {
	r io.Reader
}

const capitalA = 65
const capitalZ = 89
const capitalMid = 77
const A = 97
const Z = 122
const Mid = 109

func (rot *rot13Reader) Read(b []byte) (n int, err error) {
	n, err = rot.r.Read(b)

	for i := 0; i < n; i++ {

		if b[i] <= capitalZ && b[i] >= capitalA {
			if b[i] <= capitalMid {
				b[i] = b[i] + 13
			} else {
				b[i] = b[i] - 13
			}
		} else if b[i] <= Z && b[i] >= A {
			if b[i] <= Mid {
				b[i] = b[i] + 13
			} else {
				b[i] = b[i] - 13
			}
		}
	}
	return
}

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