2022 Day 9

2022/day9.exs

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+defmodule Day9 do
+  defmodule Rope do
+    defstruct head: {0, 0},
+              tail: {0, 0},
+              knots: for(_ <- 1..8, do: {0, 0}),
+              visited: MapSet.new([{0, 0}])
+  end
+
+  def part1(input) do
+    input
+    |> Enum.reduce(%Rope{}, &motion/2)
+    |> then(fn %Rope{visited: visited} -> MapSet.size(visited) end)
+  end
+
+  defp motion({_dir, 0}, rope), do: rope
+
+  defp motion({dir, amount}, rope) do
+    head = step_head(rope.head, dir)
+    tail = step_tail(head, rope.tail)
+    visited = MapSet.put(rope.visited, tail)
+    motion({dir, amount - 1}, %Rope{head: head, tail: tail, visited: visited})
+  end
+
+  defp step_head({x, y}, "R"), do: {x + 1, y}
+  defp step_head({x, y}, "L"), do: {x - 1, y}
+  defp step_head({x, y}, "U"), do: {x, y + 1}
+  defp step_head({x, y}, "D"), do: {x, y - 1}
+
+  defp step_tail({hx, hy}, {tx, ty}) when abs(hx - tx) <= 1 and abs(hy - ty) <= 1, do: {tx, ty}
+  defp step_tail({tx, hy}, {tx, ty}) when hy - ty > 0, do: {tx, ty + 1}
+  defp step_tail({tx, hy}, {tx, ty}) when hy - ty < 0, do: {tx, ty - 1}
+  defp step_tail({hx, ty}, {tx, ty}) when hx - tx > 0, do: {tx + 1, ty}
+  defp step_tail({hx, ty}, {tx, ty}) when hx - tx < 0, do: {tx - 1, ty}
+
+  defp step_tail({hx, hy}, {tx, ty}) do
+    dx = if hx - tx > 0, do: 1, else: -1
+    dy = if hy - ty > 0, do: 1, else: -1
+    {tx + dx, ty + dy}
+  end
+
+  def part2(input) do
+    input
+    |> Enum.reduce(%Rope{}, &motion2/2)
+    |> then(fn %Rope{visited: visited} -> MapSet.size(visited) end)
+  end
+
+  defp motion2({_dir, 0}, rope), do: rope
+
+  defp motion2({dir, amount}, rope) do
+    head = step_head(rope.head, dir)
+
+    {knots, leader} =
+      Enum.map_reduce(rope.knots, head, fn knot, leader ->
+        new_knot = step_tail(leader, knot)
+        {new_knot, new_knot}
+      end)
+
+    tail = step_tail(leader, rope.tail)
+
+    visited = MapSet.put(rope.visited, tail)
+    motion2({dir, amount - 1}, %Rope{head: head, tail: tail, knots: knots, visited: visited})
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.trim()
+      |> String.split("\n")
+      |> Enum.map(fn <<dir::binary-1, " ", amount::binary>> ->
+        {dir, String.to_integer(amount)}
+      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 day9.exs input_filename")
+  end
+end
+
+Day9.run()