AOC day23

main.dart

// AOC 2022, Day 23: Unstable Diffusion
// Move some elves around according to odd rules
// 1) See how much they spread out after 10 moves.
// 2) How long does it take for them to all get stuck?
// https://dartpad.dev/?id=5c5cc4b78c1b541159db97f34a5b389a

import 'dart:math';

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);
}

const headings = {
  'NW': Point(-1, -1),
  'N': Point(0, -1),
  'NE': Point(1, -1),
  'W': Point(-1, 0),
  'E': Point(1, 0),
  'SW': Point(-1, 1),
  'S': Point(0, 1),
  'SE': Point(1, 1)
};
var n8 = headings.values;

const rules = {
  'N': ['N', 'NE', 'NW'],
  'S': ['S', 'SE', 'SW'],
  'W': ['W', 'NW', 'SW'],
  'E': ['E', 'NE', 'SE'],
};
var looks = rules.keys.toList();

late Set<Point<int>> elfPositions;
Set<Point<int>> positionElves(List<String> lines) => lines.indexed
    .map((r) => r.$2
        .split('')
        .indexed
        .where((c) => c.$2 == '#')
        .map((c) => Point(c.$1, r.$1)))
    .flattened
    .toSet();

bool moveElves(int dir) {
  // planned location is the key, old location is the value.
  var moves = <Point<int>, Point<int>>{};
  var badMoves = <Point<int>>{};
  for (var pos in elfPositions
      .where((e) => n8.any((n) => elfPositions.contains(n + e)))) {
    for (var offset in 0.to(4)) {
      var look = looks[(dir + offset) % 4];
      if (rules[look]!.any((e) => elfPositions.contains(pos + headings[e]!))) {
        continue;
      }
      var np = pos + headings[look]!;
      if (moves.containsKey(np)) {
        badMoves.add(np);
      } else {
        moves[np] = pos;
      }
      break;
    }
  }

  var goodMoves = moves.entries.where((m) => !badMoves.contains(m.key));
  for (var m in goodMoves) {
    elfPositions
      ..remove(m.value)
      ..add(m.key);
  }
  return goodMoves.isNotEmpty;
}

part1(List<String> lines) {
  elfPositions = positionElves(lines);

  0.to(10).forEach((dir) => moveElves(dir % 4));
  var xs = elfPositions.map((e) => e.x);
  var ys = elfPositions.map((e) => e.y);
  return (1 + xs.max - xs.min) * (1 + ys.max - ys.min) - elfPositions.length;
}

part2(List<String> lines) {
  elfPositions = positionElves(lines);
  for (var round in 0.to(9999)) {
    if (!moveElves(round % 4)) return round + 1;
  }
  return -1;
}

const testdata = [
  "..............",
  "..............",
  ".......#......",
  ".....###.#....",
  "...#...#.#....",
  "....#...##....",
  "...#.###......",
  "...##.#.##....",
  "....#..#......",
  "..............",
  "..............",
  ".............."
];

void main(List<String> args) {
  var dt = DateTime.now();
  assert(part1(testdata) == 110);
  assert(part2(testdata) == 20);
  var rt = DateTime.now().difference(dt).inMilliseconds;
  print('tests succeeded in $rt ms');
}