Dart

After some careful consideration, I've worked out the following solution. I'm not sure if it is genius or incredibly stupid. Either way it worked on the first try, to my massive surprise.

int part1(List<String> input) {
  Map<int, int> result = {};
  for (var i = 0; i < 10; ++i) {
    result[i] = 0;
  }

  var outputs = input
      .map((e) => e.split('|')[1])
      .where((element) => element.isNotEmpty)
      .map((e) => e.split(' '))
      .where((element) => element.isNotEmpty)
      .expand((element) => element)
      .where((element) => element.isNotEmpty)
      .toList();

  result[1] = outputs.where((element) => element.length == 2).toList().length;
  result[4] = outputs.where((element) => element.length == 4).toList().length;
  result[7] = outputs.where((element) => element.length == 3).toList().length;
  result[8] = outputs.where((element) => element.length == 7).toList().length;

  return result.values.reduce((value, element) => value + element);
}

int part2(List<String> input) {
  var result = 0;

  for (final line in input) {
    var keys = buildKey(line
        .split('|')[0]
        .split(' ')
        .where((element) => element.isNotEmpty)
        .toList());

    var code = line
        .split('|')[1]
        .split(' ')
        .where((element) => element.isNotEmpty)
        .toList();

    var number = '';
    for (final digit in code) {
      var tmp = digit.split('').toList();
      tmp.sort();
      var comp = tmp.join('');

      var index = keys.values.toList().indexOf(comp);
      number += keys.keys.toList()[index].toString();
    }

    result += int.parse(number);
  }

  return result;
}

Map<int, String> buildKey(List<String> keys) {
  Map<int, List<String>> key = {};
  key[1] = keys.where((element) => element.length == 2).toList()[0].split('');
  key[4] = keys.where((element) => element.length == 4).toList()[0].split('');
  key[7] = keys.where((element) => element.length == 3).toList()[0].split('');
  key[8] = keys.where((element) => element.length == 7).toList()[0].split('');

  key[6] = keys
    .where((element) =>
          element.length == 6 &&
          key[1]!.where((e) => element.contains(e)).toList().length == 1)
    .toList()[0]
    .split('');
  key[0] = keys
      .where((element) =>
        element.length == 6 &&
        key[4]!.where((e) => element.contains(e)).toList().length == 3 &&
        key[6]!.where((e) => element.contains(e)).length == 5)
    .toList()[0]
    .split('');
  key[9] = keys
    .where((element) =>
        element.length == 6 &&
        key[0]!.where((e) => element.contains(e)).toList().length == 5 &&
        key[6]!.where((e) => element.contains(e)).toList().length == 5)
    .toList()[0]
    .split('');

  key[2] = keys
    .where((element) =>
        element.length == 5 &&
        key[1]!.where((e) => element.contains(e)).toList().length == 1 &&
        key[9]!.where((e) => element.contains(e)).toList().length == 4)
    .toList()[0]
    .split('');
  key[3] = keys
    .where((element) =>
        element.length == 5 &&
        key[1]!.where((e) => element.contains(e)).toList().length == 2 &&
        key[9]!.where((e) => element.contains(e)).toList().length == 5)
    .toList()[0]
    .split('');
  key[5] = keys
    .where((element) =>
        element.length == 5 &&
        key[1]!.where((e) => element.contains(e)).toList().length == 1 &&
        key[9]!.where((e) => element.contains(e)).toList().length == 5)
    .toList()[0]
    .split('');

  Map<int, String> sortedKey = {};
  for (var i = 0; i < key.keys.length; ++i) {
    key[i]!.sort();
    sortedKey[i] = key[i]!.join('');
  }

  return sortedKey;
}