AdventOfCode/Days/Day9.cs

using System.Collections.Frozen;
using System.Numerics;
using Spectre.Console;

namespace AdventOfCode.Days;

public class Day9 : Day
{
    public override int Number => 9;
    public override string Name => "Disk Fragmenter";

    public override void RunPart1(bool display = true)
    {
        var (blocks, freeBlocksStarts) = ParseDiskBlocks();

        var nextFreeIndex = freeBlocksStarts.Dequeue();

        for (var i = blocks.Count - 1; i >= 0; i--)
        {
            // There is no more available space to move blocks into on the left
            if (nextFreeIndex >= i)
            {
                break;
            }

            if (blocks[i] is -1)
            {
                continue;
            }

            blocks[nextFreeIndex] = blocks[i];
            blocks[i] = -1;

            nextFreeIndex++;

            // Go to next free blocks if current free block is full
            if (blocks[nextFreeIndex] is not -1)
            {
                nextFreeIndex = freeBlocksStarts.Dequeue();
            }
        }

        var checksum = blocks
            .Select((block, index) => block is -1 ? 0 : (long)block * index)
            .Sum();

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Filesystem checksum: [yellow]{checksum}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        var (blocks, freeBlocks) = ParseDiskBlocks2();

        var mapStart = string.Join(" ", blocks);

        for (var i = blocks.Count - 1; i >= 0; i--)
        {
            if (blocks[i] is -1)
            {
                continue;
            }

            // Get file length
            var fileId = blocks[i];
            var endPosition = i;
            var startPosition = i;

            while (startPosition > 0 && blocks[startPosition - 1] == fileId)
            {
                startPosition--;
            }

            var length = (endPosition - startPosition) + 1;

            // Find first available free block on the left
            var freeBlockIndex = freeBlocks.FindIndex(b => b.StartIndex < startPosition && b.Length >= length);

            if (freeBlockIndex is not -1)
            {
                var freeBlock = freeBlocks[freeBlockIndex];

                for (var offset = 0; offset < length; offset++)
                {
                    blocks[freeBlock.StartIndex + offset] = blocks[startPosition + offset];
                    blocks[startPosition + offset] = -1;
                }

                // Update free block start and length
                freeBlocks[freeBlockIndex] = (freeBlock.StartIndex + length, freeBlock.Length - length);
            }

            // Move to next file or free block
            i = startPosition;
        }

        var mapEnd = string.Join(" ", blocks).Replace("-1", ".");

        var checksum = blocks
            .Select((block, index) => block is -1 ? 0 : (long)block * index)
            .Sum();

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Filesystem checksum: [yellow]{checksum}[/][/]");
        }
    }

    private (List<int> Blocks, Queue<int> FreeBlocksStarts) ParseDiskBlocks()
    {
        var blocks = new List<int>(20_000 * 10);
        var freeBlocksStarts = new Queue<int>();

        var readingBlock = true;
        var blockId = 0;

        foreach (var length in Input.Select(encoded => encoded - '0'))
        {
            if (readingBlock)
            {
                for (var i = 0; i < length; i++)
                {
                    blocks.Add(blockId);
                }

                blockId++;
            }
            else if (length > 0)
            {
                freeBlocksStarts.Enqueue(blocks.Count);

                for (var i = 0; i < length; i++)
                {
                    blocks.Add(-1);
                }
            }

            readingBlock = !readingBlock;
        }

        return (blocks, freeBlocksStarts);
    }

    private (List<int> Blocks, List<(int StartIndex, int Length)> FreeBlocks) ParseDiskBlocks2()
    {
        var blocks = new List<int>(20_000 * 10);
        var freeBlocksStarts = new List<(int StartIndex, int Length)>();

        var readingBlock = true;
        var blockId = 0;

        foreach (var length in Input.Select(encoded => encoded - '0'))
        {
            if (readingBlock)
            {
                for (var i = 0; i < length; i++)
                {
                    blocks.Add(blockId);
                }

                blockId++;
            }
            else if (length > 0)
            {
                freeBlocksStarts.Add((blocks.Count, length));

                for (var i = 0; i < length; i++)
                {
                    blocks.Add(-1);
                }
            }

            readingBlock = !readingBlock;
        }

        return (blocks, freeBlocksStarts);
    }

}