Simple text parser for 4x6 characters created in AOC

main.dart

    /// Parse an AoC output to find the 6x4 letters encoded in it.
    /// Takes a list of bools and produces the resulting string.
    readLetters(List<bool> l) {
      var hexes = l
          // Split into 5-element chunks.
          .chunked(5)
          // Remove the last element of each chunk.
          .map((e) => e.take(4))
          // Convert to the equivalent binary, then convert that to a hex digit.
          .map((e) => e.fold(0, (s, t) => s * 2 + (t ? 1 : 0)).toRadixString(16))
          .toList();
      // Count how many letters there must be.
      var ll = hexes.length ~/ 6;
      // Helper function to skip through the list taking every ll'th hex digit,
      // starting at the given index and build a hex string as a key.
      hex(x) => 0.to(hexes.length, step: ll).map((e) => hexes[x + e]).join('');
      // Call the function for each letter start index, then join the results.
      // Look it up in the table, and return '?' for unrecognised character.
      return 0.to(ll).map((e) => letters[hex(e)] ?? '?').join('');
    }

    // Each char is 4 wide by 6 high, so can conveniently be encoded
    // as a string of 6 hex digits. Some letters have not been
    // seen yet.
    var letters = {
      '699f99': 'A',
      'e9e99e': 'B',
      '698896': 'C',
      'e9999e': 'D',
      'f8e88f': 'E',
      'f8e888': 'F',
      '698b97': 'G',
      '99f999': 'H',
      'e4444e': 'I',
      '311196': 'J',
      '9acaa9': 'K',
      '88888f': 'L',
      '9ff999': 'M',
      '9db999': 'N',
      '699996': 'O',
      'e99e88': 'P',
      '6999a5': 'Q',
      'e99ea9': 'R',
      '78861e': 'S',
      'f22222': 'T', //speculative!
      '999996': 'U',
      // no V
      // no W
      // no X
      '885222': 'Y', //right arm actually drifts into 5th column!
      'f1248f': 'Z',
      '000000': ' ',
    };

    var textDisplay = '''
    .##..###...##..###..####.####..##..#..#.###....##.#..#.#....#..#.#..#..##..###...##..###...###.######..#................#...#####.
    #..#.#..#.#..#.#..#.#....#....#..#.#..#..#......#.#.#..#....####.##.#.#..#.#..#.#..#.#..#.#......#..#..#................#...#...#.
    #..#.###..#....#..#.###..###..#....####..#......#.##...#....####.#.##.#..#.#..#.#..#.#..#.#......#..#..#.................#.#...#..
    ####.#..#.#....#..#.#....#....#.##.#..#..#......#.#.#..#....#..#.#..#.#..#.###..#..#.###...##....#..#..#..................#...#...
    #..#.#..#.#..#.#..#.#....#....#..#.#..#..#...#..#.#.#..#....#..#.#..#.#..#.#....#.#..#.#.....#...#..#..#..................#..#....
    #..#.###...##..###..####.#.....###.#..#.###...##..#..#.####.#..#.#..#..##..#.....#.#.#..#.###....#...##...................#..####.''';
    var textAnswer = 'ABCDEFGHIJKLMNOPQRSTU   YZ';

    void main(List<String> args) {
      var input =
          textDisplay.replaceAll('\n', '').split('').map((e) => e == '#').toList();
      print(readLetters(input));
      print(textAnswer);
      if (readLetters(input) != textAnswer) print('Ooops!');
    }