#!/usr/bin/env elixir

argv = System.argv()

if length(argv) != 1 do
  IO.puts(:stderr, "Usage: 2024/day-09/main.exs <file>")
  System.stop(64)
end

disk_map =
  argv |> hd() |> File.read!() |> String.trim() |> String.to_charlist() |> Enum.map(&(&1 - ?0))

file_blocks = disk_map |> Enum.take_every(2)
free_blocks = disk_map |> tl() |> Enum.take_every(2)

file_count = file_blocks |> Enum.sum()

blocks =
  Enum.zip(file_blocks, free_blocks)
  |> Enum.with_index()
  |> Enum.map(fn {{file_block, free_block}, i} ->
    List.duplicate(i, file_block) ++ List.duplicate(-1, free_block)
  end)
  |> Kernel.++([List.duplicate(length(file_blocks) - 1, Enum.at(file_blocks, -1))])
  |> List.flatten()

neg_block_length = -length(blocks)

checksum =
  blocks
  |> Enum.reduce_while({-1, []}, fn block, {last, list} ->
    if length(list) == file_count do
      {:halt, {last, list}}
    else
      cond do
        Enum.at(blocks, last) == -1 ->
          last =
            Enum.reduce_while(last..neg_block_length//-1, last, fn _, acc ->
              if Enum.at(blocks, acc) == -1 do
                {:cont, acc - 1}
              else
                {:halt, acc}
              end
            end)

          if block == -1 do
            {:cont, {last - 1, Enum.concat(list, [Enum.at(blocks, last)])}}
          else
            {:cont, {last, Enum.concat(list, [block])}}
          end

        block == -1 ->
          {:cont, {last - 1, Enum.concat(list, [Enum.at(blocks, last)])}}

        true ->
          {:cont, {last, Enum.concat(list, [block])}}
      end
    end
  end)
  |> elem(1)
  |> Enum.with_index()
  |> Enum.sum_by(fn {block, i} -> block * i end)