From 6c003ed3c22c65bea8c2b6c427fccca12eacf7bc Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Sun, 8 Dec 2024 21:41:22 +0900
Subject: [PATCH] 2024 Day 8

---
 2024/day8.exs | 115 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 115 insertions(+)
 create mode 100755 2024/day8.exs

diff --git a/2024/day8.exs b/2024/day8.exs
new file mode 100755
index 0000000..3fbc042
--- /dev/null
+++ b/2024/day8.exs
@@ -0,0 +1,115 @@
+#!/usr/bin/env elixir
+defmodule Day8 do
+  def part1({x, y, antennas}) do
+    Enum.reduce(antennas, MapSet.new(), fn coords, antinodes ->
+      MapSet.union(antinodes, find_antinodes(coords, x, y))
+    end)
+    |> MapSet.size()
+  end
+
+  def find_antinodes(coords, x, y) do
+    coords
+    |> find_pairs()
+    |> Enum.flat_map(fn pair -> calc_antinodes(pair, x, y) end)
+    |> MapSet.new()
+  end
+
+  def find_pairs([_]), do: []
+  def find_pairs([a | rest]), do: for(b <- rest, do: {a, b}) ++ find_pairs(rest)
+
+  def calc_antinodes({{x_a, y_a}, {x_b, y_b}}, max_x, max_y) do
+    dx = x_a - x_b
+    dy = y_a - y_b
+    antinode_1 = {x_a + dx, y_a + dy}
+    antinode_2 = {x_b - dx, y_b - dy}
+
+    Enum.filter([antinode_1, antinode_2], fn
+      {x, y} -> 0 <= x and x < max_x and 0 <= y and y < max_y
+    end)
+  end
+
+  def part2({x, y, antennas}) do
+    Enum.reduce(antennas, MapSet.new(), fn coords, antinodes ->
+      MapSet.union(antinodes, find_resonant_harmonics(coords, x, y))
+    end)
+    |> MapSet.size()
+  end
+
+  def find_resonant_harmonics(coords, x, y) do
+    coords
+    |> find_pairs
+    |> Enum.flat_map(fn pair -> calc_resonant_harmonics(pair, x, y) end)
+    |> MapSet.new()
+  end
+
+  def calc_resonant_harmonics({{x_a, y_a}, {x_b, y_b}}, max_x, max_y) do
+    dx = x_a - x_b
+    dy = y_a - y_b
+
+    [{x_a, y_a}, {x_b, y_b}] ++
+      resonate(x_a, y_a, dx, dy, max_x, max_y) ++
+      resonate(x_b, y_b, dx * -1, dy * -1, max_x, max_y)
+  end
+
+  def resonate(x, y, dx, dy, max_x, max_y, multiplier \\ 1) do
+    next_x = x + dx * multiplier
+    next_y = y + dy * multiplier
+
+    if next_x < 0 or next_x >= max_x or next_y < 0 or next_y >= max_y do
+      []
+    else
+      [{next_x, next_y} | resonate(x, y, dx, dy, max_x, max_y, multiplier + 1)]
+    end
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      {_, x, y, antennas} =
+        for <<char::binary-1 <- input>>, reduce: {0, 0, 0, []} do
+          {x, max_x, y, antennas} ->
+            case char do
+              "." -> {x + 1, max_x, y, antennas}
+              "\n" -> {0, x, y + 1, antennas}
+              freq -> {x + 1, max_x, y, [{freq, {x, y}} | antennas]}
+            end
+        end
+
+      {x, y, antennas |> Enum.group_by(&elem(&1, 0), &elem(&1, 1)) |> Map.values()}
+    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 day8.exs input_filename")
+  end
+end
+
+Day8.run()