You arrive at the first floor of this new building to discover a much less welcoming environment than the shiny atrium of the last one. Instead, you are in a maze of twisty little cubicles, all alike.

Every location in this area is addressed by a pair of non-negative integers (x,y). Each such coordinate is either a wall or an open space. You can't move diagonally. The cube maze starts at 0,0 and seems to extend infinitely toward positive x and y; negative values are invalid, as they represent a location outside the building. You are in a small waiting area at 1,1.

Read the full puzzle.

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

namespace AdventOfCode.Y2016.Day13;

[ProblemName("A Maze of Twisty Little Cubicles")]
class Solution : Solver {

    public object PartOne(string input) => 
        Steps(int.Parse(input))
        .First(s => s.icol == 31 && s.irow == 39)
        .steps;

    public object PartTwo(string input) =>
        Steps(int.Parse(input))
        .TakeWhile(s => s.steps <= 50)
        .Count();

    IEnumerable<(int steps, int irow, int icol)> Steps(int input) {
        var q = new Queue<(int steps, int irow, int icol)>();
        q.Enqueue((0, 1, 1));
        var seen = new HashSet<(int, int)>();
        seen.Add((1, 1));
        var n = (
                from drow in new[] { -1, 0, 1 }
                from dcol in new[] { -1, 0, 1 }
                where Math.Abs(drow) + Math.Abs(dcol) == 1
                select (drow, dcol)
            ).ToArray();

        while (q.Any()) {
            var (steps, irow, icol) = q.Dequeue();

            yield return (steps, irow, icol);

            foreach (var (drow, dcol) in n) {
                var (irowT, icolT) = (irow + drow, icol + dcol);
                if (irowT >= 0 && icolT >= 0) {
                    var w = icolT * icolT + 3 * icolT + 2 * icolT * irowT + irowT + irowT * irowT + input;
                    if (Convert.ToString(w, 2).Count(ch => ch == '1') % 2 == 0) {
                        if (!seen.Contains((irowT, icolT))) {
                            q.Enqueue((steps + 1, irowT, icolT));
                            seen.Add((irowT, icolT));
                        }
                    }
                }
            }
        }
    }
}

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