Encode to base 64 in pure Nix

base64.nix

{lib, ...}: {
  toBase64 = text: let
    inherit (lib) sublist mod stringToCharacters concatMapStrings;
    inherit (lib.strings) charToInt;
    inherit (builtins) substring foldl' genList elemAt length concatStringsSep stringLength;
    lookup = stringToCharacters "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    sliceN = size: list: n: sublist (n * size) size list;
    pows = [(64 * 64 * 64) (64 * 64) 64 1];
    intSextets = i: map (j: mod (i / j) 64) pows;
    compose = f: g: x: f (g x);
    intToChar = elemAt lookup;
    convertTripletInt = sliceInt: concatMapStrings intToChar (intSextets sliceInt);
    sliceToInt = foldl' (acc: val: acc * 256 + val) 0;
    convertTriplet = compose convertTripletInt sliceToInt;
    join = concatStringsSep "";
    convertLastSlice = slice: let
      len = length slice;
    in
      if len == 1
      then (substring 0 2 (convertTripletInt ((sliceToInt slice) * 256 * 256))) + "=="
      else if len == 2
      then (substring 0 3 (convertTripletInt ((sliceToInt slice) * 256))) + "="
      else "";
    len = stringLength text;
    nFullSlices = len / 3;
    bytes = map charToInt (stringToCharacters text);
    tripletAt = sliceN 3 bytes;
    head = genList (compose convertTriplet tripletAt) nFullSlices;
    tail = convertLastSlice (tripletAt nFullSlices);
  in
    join (head ++ [tail]);
}