Implement a rot13Reader that implements io.Reader and reads from an io.Reader, modifying the stream by applying the ROT13 substitution cipher to all alphabetical characters.

go_tour_ex_rot13.go

package main

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

type rot13Reader struct {
    r io.Reader
}

func (rot13 *rot13Reader) Read(p []byte) (n int, err error) {
    n, err = rot13.r.Read(p)
    var b byte
    for i := 0; i < n; i++ {
        if c :=p[i]; c >= 'A' && c <= 'z' {
            if c >= 'a' {
                b = byte('a')
            } else {
                b = byte('A')
            }
            // rotate by 13 places
            p[i] = (c - b + 13) % 26 + b
        }
    }
    return
}

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