AdventOfCode/Days/Day7.cs

using System.Buffers;
using System.Numerics;
using Spectre.Console;

namespace AdventOfCode.Days;

public class Day7 : Day
{
    public override int Number => 7;
    public override string Name => "Laboratories";

    public const int Width = 141;
    public const int Height = 142;

    public override void RunPart1(bool display = true)
    {
        var splitCount = 0;

        var splittersMap = ParseSplittersMap(Input, out var startPosition);

        var beams = new HashSet<Position>();
        var nextLineBeams = new List<Position>();
        var usedSplitters = new HashSet<Position>();
        beams.Add(startPosition with { I = startPosition.I + 1 });

        // Simulate beams going down
        var row = 1;
        while (row < Height - 1)
        {
            foreach (var beamPosition in beams)
            {
                var targetPosition = beamPosition with { I = beamPosition.I + 1 };

                // There is nothing below
                if (!splittersMap[targetPosition.I, targetPosition.J])
                {
                    nextLineBeams.Add(targetPosition);
                }
                // There is a splitter, only counts it if not already used
                else if (usedSplitters.Add(targetPosition))
                {
                    splitCount++;

                    // Left and right beam
                    if (targetPosition.J - 1 >= 0)
                    {
                        nextLineBeams.Add(targetPosition with { J = targetPosition.J - 1 });
                    }

                    if (targetPosition.J + 1 < Width)
                    {
                        nextLineBeams.Add(targetPosition with { J = targetPosition.J + 1 });
                    }
                }
            }

            // Always delete current line beams since we iterate from top to bottom
            beams.Clear();
            foreach (var nextLineBeam in nextLineBeams)
            {
                beams.Add(nextLineBeam);
            }
            nextLineBeams.Clear();

            // Also clear the used splitters list for next line
            usedSplitters.Clear();

            row++;
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Beam split count: [yellow]{splitCount}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        var splittersMap = ParseSplittersMap(Input, out var startPosition);

        var quantumPositions = new Dictionary<Position, long>();
        var nextQuantumPositions = new Dictionary<Position, long>();
        quantumPositions.Add(startPosition with { I = startPosition.I + 1 }, 1L);

        // Simulate beams going down for each timeline
        var row = 1;
        while (row < Height - 1)
        {
            // Simulation step
            foreach (var (beamPosition, timelinesCount) in quantumPositions)
            {
                var targetPosition = beamPosition with { I = beamPosition.I + 1 };

                // There is nothing below, keep going down
                if (!splittersMap[targetPosition.I, targetPosition.J])
                {
                    UpdateTimelines(nextQuantumPositions, targetPosition, timelinesCount);
                }
                else
                {
                    // Left and right beams
                    var leftBeamPosition = targetPosition with { J = targetPosition.J - 1 };
                    var rightBeamPosition = targetPosition with { J = targetPosition.J + 1 };

                    UpdateTimelines(nextQuantumPositions, leftBeamPosition, timelinesCount);
                    UpdateTimelines(nextQuantumPositions, rightBeamPosition, timelinesCount);
                }
            }

            // Always delete current line beams since we iterate from top to bottom
            quantumPositions.Clear();
            foreach (var nextQuantumPosition in nextQuantumPositions)
            {
                quantumPositions.Add(nextQuantumPosition.Key, nextQuantumPosition.Value);
            }
            nextQuantumPositions.Clear();

            row++;
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Timelines count: [yellow]{quantumPositions.Sum(p => p.Value)}[/][/]");
        }

        return;

        // Update and merge timelines
        static void UpdateTimelines(Dictionary<Position, long> nextQuantumPositions, Position targetPosition, long timelinesCount)
        {
            // Merge timelines if there is already one for this target position
            if (nextQuantumPositions.TryGetValue(targetPosition, out var otherTimelinesCount))
            {
                nextQuantumPositions[targetPosition] = otherTimelinesCount + timelinesCount;
            }
            // No timelines to merge, no new timeline
            else
            {
                nextQuantumPositions[targetPosition] = timelinesCount;
            }
        }
    }

    private static bool[,] ParseSplittersMap(string input, out Position startPosition)
    {
        var map = new bool[Height, Width];
        startPosition = new Position(0, 0);

        var i = 0;
        foreach (var line in input.EnumerateLines())
        {
            var j = 0;
            foreach (var symbol in line)
            {
                // Find start
                if (symbol is 'S')
                {
                    startPosition = new Position(i, j);

                    continue;
                }

                map[i, j] = symbol is '^';

                j++;
            }

            i++;
        }

        return map;
    }

    private record Position(int I, int J);
}