From 7589011fa658ca74ccf154c84344a1d8178cb7ff Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Fri, 6 Dec 2024 15:17:38 +0900
Subject: [PATCH] 2024 Day6 part 1

---
 2024/day6.exs | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100755 2024/day6.exs

diff --git a/2024/day6.exs b/2024/day6.exs
new file mode 100755
index 0000000..8008426
--- /dev/null
+++ b/2024/day6.exs
@@ -0,0 +1,84 @@
+#!/usr/bin/env elixir
+defmodule Day6 do
+  def part1({max_x, max_y, {x, y}, obstacles}) do
+    walk(x, y, max_x, max_y, obstacles, MapSet.new(), 0, -1)
+    |> MapSet.size()
+  end
+
+  def walk(x, y, max_x, max_y, _obstacles, visited, _x_dir, _) when x == max_x or y == max_y do
+    visited
+  end
+
+  def walk(x, y, max_x, max_y, obstacles, visited, x_dir, y_dir) do
+    next_x = x + x_dir
+    next_y = y + y_dir
+
+    if MapSet.member?(obstacles, {next_x, next_y}) do
+      {x_dir, y_dir} = turn_right(x_dir, y_dir)
+      walk(x, y, max_x, max_y, obstacles, visited, x_dir, y_dir)
+    else
+      visited = MapSet.put(visited, {x, y})
+      walk(next_x, next_y, max_x, max_y, obstacles, visited, x_dir, y_dir)
+    end
+  end
+
+  def turn_right(0, -1), do: {1, 0}
+  def turn_right(1, 0), do: {0, 1}
+  def turn_right(0, 1), do: {-1, 0}
+  def turn_right(-1, 0), do: {0, -1}
+
+  def part2(_input) do
+    # hmmmmmmm probably need completely different solution
+    :ok
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      for <<char::binary-1 <- input>>, reduce: {0, 0, 0, nil, MapSet.new()} do
+        {x, max_x, y, guard, obstacles} ->
+          case char do
+            "." -> {x + 1, max_x, y, guard, obstacles}
+            "#" -> {x + 1, max_x, y, guard, MapSet.put(obstacles, {x, y})}
+            "^" -> {x + 1, max_x, y, {x, y}, obstacles}
+            "\n" -> {0, x, y + 1, guard, obstacles}
+          end
+      end
+      |> Tuple.delete_at(0)
+    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 day6.exs input_filename")
+  end
+end
+
+Day6.run()