rot13.go

package main

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

type rot13Reader struct {
	r io.Reader
}

func (r13 rot13Reader) Read(p []byte) (n int, e error) {
	n, e = r13.r.Read(p)
	if e != nil {
		return
	}

	for i, v := range p {
		if 'A' <= v && v <= 'Z' {
			p[i] = ((v - 'A' + 13) % 26) + 'A'
		} else if 'a' <= v && v <= 'z' {
			p[i] = ((v - 'a' + 13) % 26) + 'a'
		}
	}

	return
}

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