You arrive in the year 18. If it weren't for the coat you got in 1018, you would be very cold: the North Pole base hasn't even been constructed.

Rather, it hasn't been constructed yet. The Elves are making a little progress, but there's not a lot of liquid water in this climate, so they're getting very dehydrated. Maybe there's more underground?

Read the full puzzle.

using System;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

namespace AdventOfCode.Y2018.Day17;

[ProblemName("Reservoir Research")]
class Solution : Solver {

    public object PartOne(string input) => Regex.Matches(Fill(input), "[~|]").Count;
    public object PartTwo(string input) => Regex.Matches(Fill(input), "[~]").Count;

    string Fill(string input) {
        var (width, height) = (2000, 2000);
        var mtx = new char[width, height];

        for (var y = 0; y < height; y++) {
            for (var x = 0; x < width; x++) {
                mtx[x, y] = '.';
            }
        }

        foreach (var line in input.Split("\n")) {
            var nums = Regex.Matches(line, @"\d+").Select(g => int.Parse(g.Value)).ToArray();
            for (var i = nums[1]; i <= nums[2]; i++) {
                if (line.StartsWith("x")) {
                    mtx[nums[0], i] = '#';
                } else {
                    mtx[i, nums[0]] = '#';
                }
            }
        }
        FillRecursive(mtx, 500, 0);

        var (minY, maxY) = (int.MaxValue, int.MinValue);
        for (var y = 0; y < height; y++) {
            for (var x = 0; x < width; x++) {
                if (mtx[x, y] == '#') {
                    minY = Math.Min(minY, y);
                    maxY = Math.Max(maxY, y);
                }
            }
        }
        var sb = new StringBuilder();
        for (var y = minY; y <= maxY; y++) {
            for (var x = 0; x < width; x++) {
                sb.Append(mtx[x, y]);
            }
            sb.AppendLine();
        }
        return sb.ToString();
    }

    void FillRecursive(char[,] mtx, int x, int y) {
        var width = mtx.GetLength(0);
        var height = mtx.GetLength(1);
        if (mtx[x, y] != '.') {
            return;
        }
        mtx[x, y] = '|';
        if (y == height - 1) {
            return ;
        }
        FillRecursive(mtx, x, y + 1);

        if (mtx[x, y + 1] == '#' || mtx[x, y + 1] == '~') {
            if (x > 0) {
                FillRecursive(mtx, x - 1, y);
            }
            if (x < width - 1) {
                FillRecursive(mtx, x + 1, y);
            }
        }

        if (IsStill(mtx, x, y)) {
            foreach (var dx in new[] { -1, 1 }) {
                for (var xT = x; xT >= 0 && xT < width && mtx[xT, y] == '|'; xT += dx) {
                    mtx[xT, y] = '~';
                }
            }
        }
    }

    bool IsStill(char[,] mtx, int x, int y) {
        var width = mtx.GetLength(0);
        foreach (var dx in new[] { -1, 1 }) {
            for (var xT = x; xT >= 0 && xT < width && mtx[xT, y] != '#'; xT += dx) {
                if (mtx[xT, y] == '.' || mtx[xT, y + 1] == '|') {
                    return false;
                }
            }
        }
        return true;
    }
}

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