package main

import (
    "fmt"
    "strings"
)

type ckey struct {
    enc, dec func(rune) rune
}

func newCaesar(k int) (*ckey, bool) {
    if k < 1 || k > 25 {
        return nil, false
    }
    rk := rune(k)
    return &ckey{
        enc: func(c rune) rune {
            if c >= 'a' && c <= 'z'-rk || c >= 'A' && c <= 'Z'-rk {
                return c + rk
            } else if c > 'z'-rk && c <= 'z' || c > 'Z'-rk && c <= 'Z' {
                return c + rk - 26
            }
            return c
        },
        dec: func(c rune) rune {
            if c >= 'a'+rk && c <= 'z' || c >= 'A'+rk && c <= 'Z' {
                return c - rk
            } else if c >= 'a' && c < 'a'+rk || c >= 'A' && c < 'A'+rk {
                return c - rk + 26
            }
            return c
        },
    }, true
}

func (ck ckey) encipher(pt string) string {
    return strings.Map(ck.enc, pt)
}

func (ck ckey) decipher(ct string) string {
    return strings.Map(ck.dec, ct)
}

func main() {
    pt := "The five boxing wizards jump quickly"
    fmt.Println("Plaintext:", pt)
    for _, key := range []int{0, 1, 7, 25, 26} {
        ck, ok := newCaesar(key)
        if !ok {
            fmt.Println("Key", key, "invalid")
            continue
        }
        ct := ck.encipher(pt)
        fmt.Println("Key", key)
        fmt.Println("  Enciphered:", ct)
        fmt.Println("  Deciphered:", ck.decipher(ct))
    }
}