Advent of Code - 2024 - Day 9

var input = await File.ReadAllTextAsync("input.txt");

var decompressed = new List<int>();
var filesReversed = new List<(int Id, int Index, int Length)>();
var freeBlocks = new List<(int Index, int Length)>();

for (var i = 1; i <= input.Length; i += 2)
{
    var fileLength = int.Parse(input[i - 1].ToString());
    if (fileLength > 0)
    {
        filesReversed.Insert(0, (i / 2, decompressed.Count, fileLength));
    }

    var freeSpace = i == input.Length ? 0 : int.Parse(input[i].ToString());
    if (freeSpace > 0)
    {
        freeBlocks.Add((decompressed.Count + fileLength, freeSpace));
    }

    decompressed.AddRange(Enumerable.Repeat(i / 2, fileLength));
    decompressed.AddRange(Enumerable.Repeat(-1, freeSpace));
}

List<int> compressed = [..decompressed];

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

    var freeBlock = compressed.IndexOf(-1);
    if (freeBlock >= i)
    {
        break;
    }

    compressed[freeBlock] = block;
    compressed[i] = -1;
}

var part1 = compressed.Select((c, i) => c == -1 ? 0L : c * i).Sum();

Console.WriteLine($"Part 1: {part1}");

compressed = [..decompressed];

foreach (var file in filesReversed)
{
    var freeSpace = freeBlocks.FirstOrDefault(f => f.Index < file.Index && f.Length >= file.Length);
    if (freeSpace == default)
    {
        continue;
    }

    for (var j = 0; j < file.Length; j++)
    {
        compressed[freeSpace.Index + j] = file.Id;
        compressed[file.Index + j] = -1;
    }

    if (freeSpace.Length == file.Length)
    {
        freeBlocks.Remove(freeSpace);
    }
    else
    {
        freeBlocks[freeBlocks.IndexOf(freeSpace)] = (freeSpace.Index + file.Length, freeSpace.Length - file.Length);
    }

    if (freeBlocks[0].Index > file.Index)
    {
        break;
    }
}

var part2 = compressed.Select((c, i) => c == -1 ? 0L : c * i).Sum();

Console.WriteLine($"Part 2: {part2}");