Dart

I dug out the hill-climbing trail-walking code from 2022 Day 12, put it up on blocks, and stripped all the weirdness out of it.

Ended up with just a simple flood-fill from each trailhead, so it turns out I only actually used the Graph and Node classes which I then also stripped out…

import 'dart:math';
import 'package:collection/collection.dart';
import 'package:more/more.dart';

const dirs = [Point(0, 1), Point(0, -1), Point(1, 0), Point(-1, 0)];
late Map<Point<int>, int> nodes;
late Map<Point<int>, List<Point<int>>> nexts;

List<Point<int>> parse(List<String> lines) {
  nodes = {
    for (var y in 0.to(lines.length))
      for (var x in 0.to(lines.first.length))
        Point(x, y): int.parse(lines[y][x])
  };
  nexts = Map.fromEntries(nodes.keys.map((n) => MapEntry(
      n,
      dirs
          .map((d) => n + d)
          .where((d) => (nodes[d] ?? -1) - nodes[n]! == 1)
          .toList())));
  return nodes.keys.where((e) => nodes[e] == 0).toList();
}

Set<Point> countEnds(Point here, [sofar = const <Point>{}]) {
  if (nodes[here]! == 9) return {here};
  var frontier = nexts[here]!.where((e) => !sofar.contains(e));
  return frontier
      .map((e) => countEnds(e, sofar.toSet()..add(e)))
      .flattened
      .toSet();
}

int countPaths(Point here, [sofar = const <Point>{}]) {
  if (nodes[here]! == 9) return 1;
  var frontier = nexts[here]!.where((e) => !sofar.contains(e));
  return frontier.map((e) => countPaths(e, sofar.toSet()..add(e))).sum;
}

part1(List<String> lines) => parse(lines).map((s) => countEnds(s).length).sum;
part2(List<String> lines) => parse(lines).map((s) => countPaths(s)).sum;