2021 day 13

2021/day13.exs

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+defmodule Day13 do
+  defmodule Paper do
+    defstruct dots: MapSet.new(), instructions: []
+  end
+
+  def part1(paper) do
+    folded = fold(paper)
+    MapSet.size(folded.dots)
+  end
+
+  defp fold(%Paper{dots: dots, instructions: [{axis, pos} | rest]}) do
+    dots =
+      MapSet.new(dots, fn {x, y} ->
+        cond do
+          axis == "x" and x > pos -> {pos - (x - pos), y}
+          axis == "y" and y > pos -> {x, pos - (y - pos)}
+          true -> {x, y}
+        end
+      end)
+
+    %Paper{dots: dots, instructions: rest}
+  end
+
+  def part2(paper) do
+    paper
+    |> fold_all
+    |> draw
+  end
+
+  defp fold_all(%Paper{instructions: []} = paper), do: paper
+  defp fold_all(%Paper{} = paper), do: paper |> fold |> fold_all
+
+  defp draw(paper) do
+    {max_x, max_y} =
+      for {x, y} <- paper.dots, reduce: {0, 0} do
+        {max_x, max_y} -> {max(x, max_x), max(y, max_y)}
+      end
+
+    for y <- 0..max_y do
+      row =
+        for x <- 0..max_x do
+          if MapSet.member?(paper.dots, {x, y}), do: ?#, else: ?\s
+        end
+
+      # could Enum.join("\n") here, but I want to play with iolists
+      [row | "\n"]
+    end
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      [dots, instructions] = String.split(input, "\n\n")
+
+      dots =
+        dots
+        |> String.split([",", "\n"])
+        |> Enum.map(&String.to_integer/1)
+        |> Enum.chunk_every(2)
+        |> MapSet.new(fn [x, y] -> {x, y} end)
+
+      instructions =
+        instructions
+        |> String.split("\n", trim: true)
+        |> Enum.map(fn <<"fold along ", axis::binary-1, "=", pos::binary>> ->
+          {axis, String.to_integer(pos)}
+        end)
+
+      %Paper{dots: dots, instructions: instructions}
+    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("#{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 day13.exs input_filename")
+  end
+end
+
+Day13.run()