Day 9: Disk Fragmenter

Megathread guidelines

  • Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
  • You can send code in code blocks by using three backticks, the code, and then three backticks or use something such as https://topaz.github.io/paste/ if you prefer sending it through a URL

FAQ

  • Ananace
    3•3 days ago

    Was really blanking on how to do this one nicely, so a bunch of stacked loops it is…
    Also ended up writing two separate solutions for the first and second part, since I couldn’t get acceptable performance otherwise. Still takes half a second on my machine, mainly on the second part.

    This is technically the second implementation, the first one took minutes to calculate, so I wasn’t really okay with stamping it as my solution-of-choice.

    Can definitely still be improved, but I’ve been poking and prodding at this code for hours on end now, so it’s long past time to let it sit for a while and see if I get any better ideas later.

    C# 
    int[] layout = new int[0];
public void Input(IEnumerable<string> lines)
{
  layout = string.Join("", lines).ToCharArray().Select(c => int.Parse(c.ToString())).ToArray();
}

public void Part1()
{
  ushort?[] blocks = BuildBlockmap().ToArray();

  var it = 0;
  for (var i = blocks.Length - 1; i > it; i--)
  {
    if (blocks[i] == null)
      continue;

    while (it < blocks.Length && blocks[it] != null)
      ++it;

    if (it >= blocks.Length)
      break;

    (blocks[it], blocks[i]) = (blocks[i], null);
  }

  long checksum = 0;
  foreach (var part in blocks.OfType<ushort>().Select((b, i) => i * b))
    checksum += part;

  Console.WriteLine($"Checksum: {checksum}");
}

public void Part2()
{
  var sparse = BuildSparsemap().ToList();

  for (var i = sparse.Count - 1; i >= 0; i--)
  {
    if (sparse[i].Item1 == null)
      continue;

    for (var j = 0; j < i; ++j)
    {
      if (sparse[j].Item1 != null)
        continue;

      if (sparse[i].Item2 > sparse[j].Item2)
        continue;

      var size = sparse[j].Item2;
      size -= sparse[i].Item2;

      (sparse[j], sparse[i]) = (sparse[i], (null, sparse[i].Item2));

      if (i + 1 < sparse.Count && sparse[i + 1].Item1 == null)
      {
        sparse[i] = (null, (ushort)(sparse[i].Item2 + sparse[i + 1].Item2));
        sparse.RemoveAt(i + 1);
      }

      if (sparse[i - 1].Item1 == null)
      {
        sparse[i - 1] = (null, (ushort)(sparse[i - 1].Item2 + sparse[i].Item2));
        sparse.RemoveAt(i);
      }

      if (size > 0)
        sparse.Insert(j + 1, (null, size));

      j = i + 1;
    }
  }

  int ind = 0;
  long checksum = 0;
  foreach (var (val, cnt) in sparse)
    for (var i = 0; i < cnt; ++i)
    {
      checksum += (val ?? 0) * ind;
      ++ind;
    }

  Console.WriteLine($"Checksum: {checksum}");
}

IEnumerable<ushort?> BuildBlockmap()
{
  ushort blockit = 0;
  bool block = true;
  foreach (var value in layout)
  {
    for (int i = 0; i < value; ++i)
      yield return block ? blockit : null;
    if (block)
      blockit++;
    block = !block;
  }
}

IEnumerable<(ushort?, ushort)> BuildSparsemap()
{
  ushort blockit = 0;
  bool block = true;
  foreach (var value in layout)
  {
    if (block)
      yield return (blockit++, (ushort)value);
    else
      yield return (null, (ushort)value);
    block = !block;
  }
}