move-to-front_transform.sf

#!/usr/bin/ruby

# Author: Trizen
# Date: 13 June 2023
# https://github.com/trizen

# The Move to Front transform (MTF).

# Reference:
#   COMP526 Unit 7-6 2020-03-24 Compression - Move-to-front transform
#   https://youtube.com/watch?v=Q2pinaj3i9Y

func mtf_encode (bytes, alphabet = @(^256)) {

    var C = []

    for c in bytes {
        C << alphabet.first_index(c)
        alphabet.prepend(alphabet.delete_index(C.tail))
    }

    return C
}

func mtf_decode (encoded, alphabet = @(^256)) {

    var S = []

    for p in encoded {
        S << alphabet[p]
        alphabet.prepend(alphabet.delete_index(p))
    }

    return S
}

var str = "INEFICIENCIES"

var encoded = mtf_encode(str.bytes, @(ord('A') .. ord('Z')))
var decoded = mtf_decode(encoded, @(ord('A') .. ord('Z')))

say "Encoded: #{encoded}"               #=> Encoded: 8 13 6 7 3 6 1 3 4 3 3 3 18
say "Decoded: #{decoded.join_bytes}"    #=> Decoded: INEFICIENCIES

assert_eq(decoded.pack('C*'), str)

do {
    var str     = File(__FILE__).read(:raw)
    var encoded = mtf_encode(str.bytes)
    var decoded = mtf_decode(encoded)
    assert_eq(str, decoded.pack('C*'))
}