AOC2022 day20

readme.md

AOC2022 day20

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main.dart

// AOC 2022, Day 20: Grove Positioning System
// Mix a list according to some rules
// 1) Once.
// 2) Ten times with a key.
// https://dartpad.dev/?id=2ae60c3e3a6ae2edf57a5eff9c298d8d

import 'package:collection/collection.dart';

extension IntegerRangeExtension on int {
  List<int> to(int end, {int step = 1}) =>
      List.generate((end - this + (step - 1)) ~/ step, (i) => i * step + this);
}

parseLines(List<String> lines, {int key = 1}) =>
    lines.map((e) => int.parse(e) * key).indexed.toList();

process(List<(int, int)> ns) {
  for (var n in ns.indexed) {
    // "move each number forward or backward in the file a number of positions
    // equal to the value of the number being moved"
    var ix = ns.indexWhere((e) => e.$1 == n.$1);
    var el = ns.removeAt(ix);
    ns.insert((ix + el.$2) % ns.length, el);
  }
  return ns;
}

int answer(List<(int, int)> ns) {
  // "sum of the 1000th, 2000th, and 3000th numbers after the value 0"
  var ix = ns.indexWhere((e) => e.$2 == 0);
  return [1000, 2000, 3000].map((e) => ns[(ix + e) % ns.length].$2).sum;
}

part1(List<String> lines) => answer(process(parseLines(lines)));

// "you need to apply the decryption key, 811589153"
part2(List<String> lines) => answer(
    0.to(10).fold(parseLines(lines, key: 811589153), (s, _) => process(s)));

var testdata = ["1", "2", "-3", "3", "-2", "0", "4"];

void main(List<String> args) {
  var dt = DateTime.now();
  assert(part1(testdata) == 3);
  assert(part2(testdata) == 1623178306);
  var rt = DateTime.now().difference(dt).inMilliseconds;
  print('tests succeeded in $rt ms');
}