reader.go

package main

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

type rot13Reader struct {
	r io.Reader
}

func (r rot13Reader) Read(b []byte) (int, error) { 
	n, error := r.r.Read(b)
	for i := 0; i < n; i++ {
		if (b[i] >= 'A') && (b[i] <= 'Z') {
			b[i] += 13
			if b[i] > 'Z' {
				b[i] = 'A' + (b[i] - 'Z')  - 1
			}
		} else if (b[i] >= 'a') && (b[i] <= 'z') {
			b[i] += 13
			if b[i] > 'z' {
				b[i] = 'a' + (b[i] - 'z') - 1
			}
		}
	}
	return n, error
}

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