AdventOfCode/Days/Day20.cs

using System.Collections.Frozen;
using System.Numerics;
using System.Runtime.Intrinsics.X86;
using Spectre.Console;

namespace AdventOfCode.Days;

public class Day20 : Day
{
    public override int Number => 20;
    public override string Name => "Race Condition";

    private const int Size = 141;
    private const char Wall = '#';
    private const char Empty = '.';

    private readonly List<Point> _cheatOffsets =
    [
        (2, 0),
        (1, 1),
        (0, 2),
        (-1, 1),
        (-2, 0),
        (-1, -1),
        (0, -2),
        (1, -1)
    ];

    public override void RunPart1(bool display = true)
    {
        var (map, start, end) = ParseMap();

        // Find track path
        var trackPath = new List<Point>();
        var distanceFromStart = new Dictionary<Point, int>();

        var position = start;

        var distance = 0;
        while (position != end)
        {
           trackPath.Add(position);
           distanceFromStart[position] = distance;

           // Find next track position
           distance++;
           var up = position + new Point(0, -1);
           if (up is { X: >= 0 and < Size, Y: >= 0 and < Size } && map[up.X, up.Y] is not Wall && !distanceFromStart.ContainsKey(up))
           {
               position = up;

               continue;
           }

           var down = position + new Point(0, 1);
           if (down is { X: >= 0 and < Size, Y: >= 0 and < Size } && map[down.X, down.Y] is not Wall && !distanceFromStart.ContainsKey(down))
           {
               position = down;

               continue;
           }

           var left = position + new Point(-1, 0);
           if (left is { X: >= 0 and < Size, Y: >= 0 and < Size } && map[left.X, left.Y] is not Wall && !distanceFromStart.ContainsKey(left))
           {
               position = left;

               continue;
           }

           var right = position + new Point(1, 0);
           if (right is { X: >= 0 and < Size, Y: >= 0 and < Size } && map[right.X, right.Y] is not Wall && !distanceFromStart.ContainsKey(right))
           {
               position = right;
           }
        }

        // Also add end
        trackPath.Add(position);
        distanceFromStart[position] = distance;

        var cheats = new Dictionary<(Point StartPosition, Point EndPosition), int>();

        // Now return to start and try all possible combination of cheats for each track position
        foreach (var trackPosition in trackPath)
        {
            var originDistance = distanceFromStart[trackPosition];

            foreach (var cheatOffset in _cheatOffsets)
            {
                var cheatDestination = trackPosition + cheatOffset;

                if (cheatDestination.X is < 0 or >= Size || cheatDestination.Y is < 0 or >= Size)
                {
                    continue;
                }

                if (map[cheatDestination.X, cheatDestination.Y] is Wall)
                {
                    continue;
                }

                var destinationDistance = distanceFromStart[cheatDestination];
                var savedDistance = destinationDistance - originDistance - 2;

                // Only take into account real shortcuts, not those that do not save time
                if (savedDistance < 1)
                {
                    continue;
                }

                cheats.Add((trackPosition, cheatDestination), savedDistance);
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Number of cheats that save at least 100 picoseconds: [yellow]{cheats.Count(c => c.Value >= 100)}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        var (map, start, end) = ParseMap();

        // Find track path
        var trackPath = new List<Point>();
        var distanceFromStart = new Dictionary<Point, int>();

        var position = start;

        var distance = 0;
        while (position != end)
        {
           trackPath.Add(position);
           distanceFromStart[position] = distance;

           // Find next track position
           distance++;
           var up = position + new Point(0, -1);
           if (up is { X: >= 0 and < Size, Y: >= 0 and < Size } && map[up.X, up.Y] is not Wall && !distanceFromStart.ContainsKey(up))
           {
               position = up;

               continue;
           }

           var down = position + new Point(0, 1);
           if (down is { X: >= 0 and < Size, Y: >= 0 and < Size } && map[down.X, down.Y] is not Wall && !distanceFromStart.ContainsKey(down))
           {
               position = down;

               continue;
           }

           var left = position + new Point(-1, 0);
           if (left is { X: >= 0 and < Size, Y: >= 0 and < Size } && map[left.X, left.Y] is not Wall && !distanceFromStart.ContainsKey(left))
           {
               position = left;

               continue;
           }

           var right = position + new Point(1, 0);
           if (right is { X: >= 0 and < Size, Y: >= 0 and < Size } && map[right.X, right.Y] is not Wall && !distanceFromStart.ContainsKey(right))
           {
               position = right;
           }
        }

        // Also add end
        trackPath.Add(position);
        distanceFromStart[position] = distance;

        var cheats = new Dictionary<(Point StartPosition, Point EndPosition), int>();

        // Now return to start and try all possible combination of cheats for each track position
        foreach (var trackPosition in trackPath)
        {
            var originDistance = distanceFromStart[trackPosition];

            foreach (var cheatDestination in trackPath.Where(destination => trackPosition.DistanceTo(destination) <= 20))
            {
                var cheatLength = trackPosition.DistanceTo(cheatDestination);

                var destinationDistance = distanceFromStart[cheatDestination];
                var savedDistance = destinationDistance - originDistance - cheatLength;

                // Only take into account real shortcuts, not those that do not save time
                if (savedDistance < 1)
                {
                    continue;
                }

                if (cheats.TryGetValue((trackPosition, cheatDestination), out var otherSavedDistance))
                {
                    if (savedDistance <= otherSavedDistance)
                    {
                        continue;
                    }
                }

                cheats[(trackPosition, cheatDestination)] = savedDistance;
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Number of cheats that save at least 100 picoseconds: [yellow]{cheats.Count(c => c.Value >= 100)}[/][/]");
        }
    }

    private (char[,] Map, Point Start, Point End) ParseMap()
    {
        var map = new char[Size, Size];
        Point start = default;
        Point end = default;

        var y = 0;
        foreach (var line in Input.AsSpan().EnumerateLines())
        {
            var x = 0;

            foreach (var symbol in line)
            {
                if (symbol == 'S')
                {
                    start = new Point(x, y);
                }
                else if (symbol == 'E')
                {
                    end = new Point(x, y);
                }

                if (symbol is Wall)
                {
                    map[x, y] = Wall;
                }
                else
                {
                    map[x, y] = Empty;
                }

                x++;
            }

            y++;
        }

        return (map, start, end);
    }

    private readonly record struct Point(int X, int Y)
        : IAdditionOperators<Point, Point, Point>,
            ISubtractionOperators<Point, Point, Point>,
            IMultiplyOperators<Point, int, Point>
    {
        public static Point operator +(Point left, Point right)
        {
            return new Point(left.X + right.X, left.Y + right.Y);
        }

        public static Point operator -(Point left, Point right)
        {
            return new Point(left.X - right.X, left.Y - right.Y);
        }

        public static Point operator *(Point left, int right)
        {
            return new Point(left.X * right, left.Y * right);
        }

        public static Point operator *(int left, Point right)
        {
            return new Point(right.X * left, right.Y * left);
        }

        public static implicit operator Point((int X, int Y) point)
            => new(point.X, point.Y);

        public int DistanceTo(Point other)
        {
            var distance = other - this;

            return Math.Abs(distance.X) + Math.Abs(distance.Y);
        }
    }
}