#!/usr/bin/env nomsu -V3.6.5.6
#
    This file defines actions for encoding/decoding base 64, as specified in:
    https://tools.ietf.org/html/rfc4648

%b64_str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
%reverse_b64 = (%b64_str.%i = (%i - 1) for %i in 1 to (length of %b64_str))
%reverse_b64."=" = 0

test:
    %cases = ["", "Zg==", "Zm8=", "Zm9v", "Zm9vYg==", "Zm9vYmE=", "Zm9vYmFy"]
    for %len = %encoded in %cases:
        %plain = "foobar".[1, %len - 1]
        assume ((base64 %plain) == %encoded) or barf ".."
            \(quote %plain) base64 encoded to \(quote (base64 %plain)) \
            ..instead of \(quote %encoded)
        assume ((base64 decode %encoded) == %plain) or barf ".."
            \(quote %encoded) base64 decoded to \(quote (base64 decode %encoded)) \
            ..instead of \(quote %plain)
action [base64 %str, base64 encode %str, %str base64]:
    %chars = []
    for %i in 1 to (length of %str) via 3:
        %bytes = [=lua "\%str:byte(\%i, \(%i + 2))"]
        add %b64_str.(((%bytes.1 & 252) >> 2) + 1) to %chars
        if (length of %bytes) is:
            3:
                add %b64_str.(((%bytes.1 & 3) << 4) + ((%bytes.2 & 240) >> 4) + 1) to %chars
                add %b64_str.(((%bytes.2 & 15) << 2) + ((%bytes.3 & 192) >> 6) + 1) to %chars
                add %b64_str.((%bytes.3 & 63) + 1) to %chars
            
            2:
                add %b64_str.(((%bytes.1 & 3) << 4) + ((%bytes.2 & 240) >> 4) + 1) to %chars
                add %b64_str.(((%bytes.2 & 15) << 2) + 1) to %chars
                add "=" to %chars
            
            1:
                add %b64_str.(((%bytes.1 & 3) << 4) + 1) to %chars
                add "=" to %chars
                add "=" to %chars
    
    return (%chars joined)

action [chr %] (=lua "string.char(\%)")
action [decode base64 %str, %str base64 decoded, base64 decode %str]:
    %chars = []
    for %i in 1 to (length of %str) via 4:
        %indices = (%reverse_b64.(%str.%) for % in %i to (%i + 3))
        add (chr ((%indices.1 << 2) + ((%indices.2 & 48) >> 4))) to %chars
        if (%str.(%i + 2) == "="): stop
        add (chr (((%indices.2 & 15) << 4) + ((%indices.3 & 60) >> 2))) to %chars
        if (%str.(%i + 3) == "="): stop
        add (chr (((%indices.3 & 3) << 6) + %indices.4)) to %chars
    
    return (%chars joined)