package main

import "fmt"

// encoding scheme:
// encode to byte array
// byte value < 26 means single character: byte value + 'A'
// byte value 26..255 means (byte value - 24) copies of next byte
func rllEncode(s string) (r []byte) {
    if s == "" {
        return
    }
    c := s[0]
    if c < 'A' || c > 'Z' {
        panic("invalid")
    }
    nc := byte(1)
    for i := 1; i < len(s); i++ {
        d := s[i]
        switch {
        case d != c:
        case nc < (255 - 24):
            nc++
            continue
        }
        if nc > 1 {
            r = append(r, nc+24)
        }
        r = append(r, c-'A')
        if d < 'A' || d > 'Z' {
            panic("invalid")
        }
        c = d
        nc = 1
    }
    if nc > 1 {
        r = append(r, nc+24)
    }
    r = append(r, c-'A')
    return
}

func main() {
    s := "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW"
    fmt.Println("source: ", len(s), "bytes:", s)
    e := rllEncode(s)
    fmt.Println("encoded:", len(e), "bytes:", e)
    d := rllDecode(e)
    fmt.Println("decoded:", len(d), "bytes:", d)
    fmt.Println("decoded = source:", d == s)
}

func rllDecode(e []byte) string {
    var c byte
    var d []byte
    for i := 0; i < len(e); i++ {
        b := e[i]
        if b < 26 {
            c = 1
        } else {
            c = b - 24
            i++
            b = e[i]
        }
        for c > 0 {
            d = append(d, b+'A')
            c--
        }
    }
    return string(d)
}