From d60e640e466cfb83d930d9cdcbedcf4a96bee56e Mon Sep 17 00:00:00 2001
From: Caleb Webber <caleb@codingthemsoftly.com>
Date: Fri, 18 Apr 2025 12:32:22 -0400
Subject: [PATCH] add 2021 day 2

---
 elixir/lib/Y2021/day2.exs | 78 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 elixir/lib/Y2021/day2.exs

diff --git a/elixir/lib/Y2021/day2.exs b/elixir/lib/Y2021/day2.exs
new file mode 100644
index 0000000..aa0da25
--- /dev/null
+++ b/elixir/lib/Y2021/day2.exs
@@ -0,0 +1,78 @@
+defmodule Submarine1 do
+  import AoC.Util.Tuple
+
+  @sub_key :sub
+  @default {0,0}
+
+  def reset(), do: Process.delete(@sub_key)
+  defp update(key, f, default), do: Process.put(key, f.(Process.get(key, default)))
+  def forward(n), do: update(@sub_key, &pairwise_sum(&1, {n, 0}), @default)
+  def down(n), do: update(@sub_key, &pairwise_sum(&1, {0, n}), @default)
+  def up(n), do: update(@sub_key, &pairwise_sum(&1, {0, -n}), @default)
+  
+  def checksum() do
+    {x, y} = Process.get(@sub_key, @default)
+    x * y
+  end
+end
+
+defmodule Submarine2 do
+  import AoC.Util.Tuple
+
+  @sub_key :sub
+  @default {0, 0, 0}
+
+  def reset(), do: Process.delete(@sub_key)
+  defp update(key, f, default), do: Process.put(key, f.(Process.get(key, default)))
+  def forward(n), do: update(@sub_key, fn {_,_,a} = p -> pairwise_sum(p, {n, a*n, 0}) end, @default)
+  def down(n), do: update(@sub_key, &pairwise_sum(&1, {0, 0, n}), @default)
+  def up(n), do: update(@sub_key, &pairwise_sum(&1, {0, 0, -n}), @default)
+  
+  def checksum() do
+    {x, y, _} = Process.get(@sub_key, @default)
+    x * y
+  end
+end
+
+gen_code = fn instructions, part ->
+  instructions = Code.string_to_quoted(instructions)
+
+  quote do
+    unquote(
+      case part do
+        :part1 -> quote do: import Submarine1
+        :part2 -> quote do: import Submarine2
+      end
+    )
+
+    reset()
+
+    unquote(instructions)
+
+    checksum()
+  end
+end
+
+solve = fn instructions, part ->
+  {soln, _} = Code.eval_quoted(gen_code.(instructions, part))
+
+  soln
+end
+
+test_input = """
+forward 5
+down 5
+forward 8
+up 3
+down 8
+forward 2
+"""
+
+if solve.(test_input, :part1) != 150 or solve.(test_input, :part2) != 900 do
+  raise "unable to solve example input"
+end
+
+input = File.read!("./lib/Y2021/day2.txt")
+
+solve.(input, :part2)
+|> IO.puts()