package main

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

type rot13Reader struct {
    r io.Reader
}

var rot13Input  []byte = []byte("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz")
var rot13Output []byte = []byte("NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm")

func indexOf(s []byte, b byte) int {
    for i, v := range s {
        if v == b {
            return i
        }
    }
    return -1
}

func (r rot13Reader) Read(b []byte) (n int, err error) {
    n, err = r.r.Read(b)
    if err == nil {
        for i := 0; i < n; i++ {
            c := b[i]
            if j := indexOf(rot13Input, c); j > -1 {
                b[i] = rot13Output[j]
            }
        }
    }
    return n, err
}

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