2021 day 5

2021/day5.exs

@@ -0,0 +1,83 @@
+defmodule Day5 do
+  def part1(input) do
+    input
+    |> Enum.reduce(_points = %{}, fn
+      {{x, y1}, {x, y2}}, points ->
+        for y <- y1..y2, reduce: points, do: (points -> plot(points, {x, y}))
+
+      {{x1, y}, {x2, y}}, points ->
+        for x <- x1..x2, reduce: points, do: (points -> plot(points, {x, y}))
+
+      _point, points ->
+        points
+    end)
+    |> Enum.count(fn {_point, count} -> count >= 2 end)
+  end
+
+  def part2(input) do
+    input
+    |> Enum.reduce(_points = %{}, fn
+      {{x, y1}, {x, y2}}, points ->
+        for y <- y1..y2, reduce: points, do: (points -> plot(points, {x, y}))
+
+      {{x1, y}, {x2, y}}, points ->
+        for x <- x1..x2, reduce: points, do: (points -> plot(points, {x, y}))
+
+      {{x1, x1}, {x2, x2}}, points ->
+        for x <- x1..x2, reduce: points, do: (points -> plot(points, {x, x}))
+
+      {{x1, y1}, {x2, y2}}, points ->
+        for point <- Enum.zip(x1..x2, y1..y2), reduce: points, do: (points -> plot(points, point))
+    end)
+    |> Enum.count(fn {_point, count} -> count >= 2 end)
+  end
+
+  defp plot(points, point) do
+    Map.put(points, point, (points[point] || 0) + 1)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split(["\n", " -> ", ","], trim: true)
+      |> Enum.map(&String.to_integer/1)
+      |> Enum.chunk_every(4)
+      |> Enum.map(fn [x1, y1, x2, y2] -> {{x1, y1}, {x2, y2}} end)
+    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 day5.exs input_filename")
+  end
+end
+
+Day5.run()