This rope bridge creaks as you walk along it. You aren't sure how old it is, or whether it can even support your weight.

It seems to support the Elves just fine, though. The bridge spans a gorge which was carved out by the massive river far below you.

Read the full puzzle.

using System;
using System.Collections.Generic;
using System.Linq;

namespace AdventOfCode.Y2022.Day09;

[ProblemName("Rope Bridge")]
class Solution : Solver {

    public object PartOne(string input) => Tails(input, 2).ToHashSet().Count;
    public object PartTwo(string input) => Tails(input, 10).ToHashSet().Count;

    // simulates a rope with the given length as its head moves
    // according to the input and returns the position of its 
    // tail knot in each step.
    private IEnumerable<Knot> Tails(string input, int ropeLength) {
        var rope = Enumerable.Repeat(new Knot(0, 0), ropeLength).ToArray();
        yield return rope.Last();

        foreach (var line in input.Split("\n")) {
            var parts = line.Split(' ');
            var dir = parts[0];
            var dist = int.Parse(parts[1]);

            for (var i = 0; i < dist; i++) {
                MoveHead(rope, dir);
                yield return rope.Last();
            }
        }
    }

    record struct Knot(int irow, int icol);

    // moves the head in the given direction, inplace update 
    // of the rope
    void MoveHead(Knot[] rope, string dir) {
        rope[0] = dir switch {
            "U" => rope[0] with { irow = rope[0].irow - 1 },
            "D" => rope[0] with { irow = rope[0].irow + 1 },
            "L" => rope[0] with { icol = rope[0].icol - 1 },
            "R" => rope[0] with { icol = rope[0].icol + 1 },
            _ => throw new ArgumentException(dir)
        };

        // knots move when become disconnected from the previous 
        // sibling in the rope:
        for (var i = 1; i < rope.Length; i++) {
            var drow = rope[i - 1].irow - rope[i].irow; 
            var dcol = rope[i - 1].icol - rope[i].icol;

            if (Math.Abs(drow) > 1 || Math.Abs(dcol) > 1) {
                rope[i] = new Knot(
                    rope[i].irow + Math.Sign(drow), 
                    rope[i].icol + Math.Sign(dcol)
                );
            }
        }
    }
}

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