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strBase64:
let
inherit (builtins)
concatStringsSep genList stringLength substring trace ;
base64 = {
# n=0; for l in {A..Z} {a..z} {0..9} + /; do printf '"%s" = %2s; ' $l $n; (( n++ )); (( n % 8 )) || echo; done
"A" = 0; "B" = 1; "C" = 2; "D" = 3; "E" = 4; "F" = 5; "G" = 6; "H" = 7;
"I" = 8; "J" = 9; "K" = 10; "L" = 11; "M" = 12; "N" = 13; "O" = 14; "P" = 15;
"Q" = 16; "R" = 17; "S" = 18; "T" = 19; "U" = 20; "V" = 21; "W" = 22; "X" = 23;
"Y" = 24; "Z" = 25; "a" = 26; "b" = 27; "c" = 28; "d" = 29; "e" = 30; "f" = 31;
"g" = 32; "h" = 33; "i" = 34; "j" = 35; "k" = 36; "l" = 37; "m" = 38; "n" = 39;
"o" = 40; "p" = 41; "q" = 42; "r" = 43; "s" = 44; "t" = 45; "u" = 46; "v" = 47;
"w" = 48; "x" = 49; "y" = 50; "z" = 51; "0" = 52; "1" = 53; "2" = 54; "3" = 55;
"4" = 56; "5" = 57; "6" = 58; "7" = 59; "8" = 60; "9" = 61; "+" = 62; "/" = 63;
};
quartet_to_int24 = q:
# https://en.wikipedia.org/wiki/Base64
let
s = n: assert (stringLength q == 4); substring (3 - n) 1 q;
d = n: base64.${s n};
in if s 0 != "=" then
64 * (64 * (64 * (d 3) + (d 2)) + (d 1)) + (d 0)
else if s 1 != "=" then
64 * (64 * (64 * (d 3) + (d 2)) + (d 1)) / 256 # right shift by 8 bits
else
64 * (64 * (64 * (d 3) + (d 2))) / 65536 # right shift by 16 bits
;
int24_to_hex = i: # 16777215 (0xFFFFFF, 2^24-1) max
let
hex = "0123456789abcdef";
toHex = n:
let
d = n / 16;
r = n - 16 * d;
in "${if d != 0 then toHex d else ""}${substring r 1 hex}";
in assert (0 <= i && i <= 16777215); toHex i;
quartets = s:
let
l = stringLength s;
h = substring 0 4 s;
t = substring 4 (l - 4) s;
in [h] ++ (if t != "" then quartets t else []);
quartet_to_hex = q: # base64 quartet into hex with padding
let
i = quartet_to_int24 q;
h = int24_to_hex i;
s = if substring 2 1 q == "=" then 1
else if substring 3 1 q == "=" then 2
else 3; # number of bytes
w = s * 2; # number of hexadecimal digits
filler = concatStringsSep "" (genList (_: "0") (w - stringLength h));
in "${filler}${h}";
/*
FIXME: usage of library functions like concatMapString
causes very cryptic errors:
# nix-instantiate --eval --expr 'import ./fromBase64.nix "kjOzmCPxyw0bPciMsGSh5q+bT9g="' --show-trace
error: while evaluating anonymous function at .../fromBase64.nix:1:1, called from (string):1:18:
value is a function while a set was expected, at .../fromBase64.nix:3:4
*/
in concatStringsSep "" (map quartet_to_hex (quartets strBase64))
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