2024 Day 9 part 2 (inefficient)

2024/day9.exs

@@ -27,11 +27,50 @@ defmodule Day9 do
     |> elem(0)
   end
 
-  # Wrote a crazy long implementation, but was fatally flawed because it didn't consider that
-  # moving a block needs to leave contiguous spaces behind if there were spaces on either side.
-  # Maybe can store the spaces as ranges, then merge the ranges?
-  def part2(_input) do
-    :ok
+  # Super inefficient, but this is the 2nd attempt after trying to be more efficient
+  # worked on the sample but not the actual input
+  def part2(input) do
+    {_, _, _, disk, files} =
+      Enum.reduce(input, {0, 0, true, %{}, []}, fn len, {idx, id, file?, disk, files} ->
+        if file? do
+          disk = Map.put(disk, idx, {:file, id, len})
+          files = [{idx, id, len} | files]
+          {idx + len, id + 1, not file?, disk, files}
+        else
+          disk = Map.put(disk, idx, {:space, len})
+          {idx + len, id, not file?, disk, files}
+        end
+      end)
+
+    Enum.reduce(files, disk, fn {idx, id, len}, disk ->
+      # This scans the disk every time. Could possibly optimise by storing the known
+      # eligible spaces separately
+      case Enum.find(0..(idx - 1), &match?({:space, gap} when gap >= len, disk[&1])) do
+        nil ->
+          disk
+
+        space_idx ->
+          {:space, spaces} = Map.fetch!(disk, space_idx)
+          disk = disk |> Map.delete(idx) |> Map.put(space_idx, {:file, id, len})
+
+          remainder_spaces = spaces - len
+
+          if remainder_spaces > 0 do
+            Map.put(disk, space_idx + len, {:space, remainder_spaces})
+          else
+            disk
+          end
+      end
+    end)
+    |> Enum.reduce(0, fn
+      {_idx, {:space, _}}, checksum ->
+        checksum
+
+      {idx, {:file, id, len}}, checksum ->
+        for idx <- idx..(idx + len - 1), reduce: checksum do
+          checksum -> checksum + idx * id
+        end
+    end)
   end
 
   def input do