2024 Day 9 part 1

2024/day9.exs

@@ -0,0 +1,77 @@
+#!/usr/bin/env elixir
+defmodule Day9 do
+  def part1(input) do
+    {size, _, spaces, _, disk, files} =
+      Enum.reduce(input, {0, 0, 0, true, %{}, []}, fn len,
+                                                      {idx, id, spaces, file?, disk, files} ->
+        if file? do
+          disk = Enum.into(idx..(idx + len - 1), disk, fn i -> {i, id} end)
+          files = if file?, do: for(_ <- 1..len, do: id) ++ files, else: files
+          {idx + len, id + 1, spaces, not file?, disk, files}
+        else
+          {idx + len, id, spaces + len, not file?, disk, files}
+        end
+      end)
+
+    for i <- 0..(size - spaces - 1), reduce: {0, files} do
+      {checksum, files} ->
+        case Map.fetch(disk, i) do
+          {:ok, block} ->
+            {checksum + block * i, files}
+
+          :error ->
+            [block | files] = files
+            {checksum + block * i, files}
+        end
+    end
+    |> 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
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      for <<c::binary-1 <- String.trim_trailing(input, "\n")>>, do: String.to_integer(c)
+    else
+      _ -> :error
+    end
+  end
+
+  #######################
+  # HERE BE BOILERPLATE #
+  #######################
+
+  def run do
+    case input() do
+      :error -> print_usage()
+      input -> run_parts_with_timer(input)
+    end
+  end
+
+  defp run_parts_with_timer(input) do
+    run_with_timer(1, fn -> part1(input) end)
+    run_with_timer(2, fn -> part2(input) end)
+  end
+
+  defp run_with_timer(part, fun) do
+    {time, result} = :timer.tc(fun)
+    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
+    IO.puts("#{inspect(result)}\n")
+  end
+
+  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
+  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
+  defp format_time(μsec), do: "#{μsec / 1_000_000}s"
+
+  defp print_usage do
+    IO.puts("Usage: elixir day9.exs input_filename")
+  end
+end
+
+Day9.run()