http://go-tour-jp.appspot.com/#61

go-tour-61.go

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 := 0; i < len(p); i++ {
		t := p[i]
		if p[i] >= 'a' {
			t = t - 'a'
		} else {
			t = t - 'A'
		}

		if t < 13 {
			p[i] += 13
		} else {
			p[i] -= 13
		}
	}

	return
}

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