From b0b4a3f040eca4fde0aaec77e9030ba2aa56141b Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Fri, 2 Dec 2022 23:45:27 +0900
Subject: [PATCH] 2021 Day3 just for kicks. Lost my part 1 answer...

---
 2021/day3.exs | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)
 create mode 100644 2021/day3.exs

diff --git a/2021/day3.exs b/2021/day3.exs
new file mode 100644
index 0000000..72700c5
--- /dev/null
+++ b/2021/day3.exs
@@ -0,0 +1,77 @@
+defmodule Day3 do
+  def part2(input) do
+    oxygen_generator_rating = find_rating(input, 0, :oxygen_generator)
+    co2_scrubber_rating = find_rating(input, 0, :co2_scrubber)
+    oxygen_generator_rating * co2_scrubber_rating
+  end
+
+  def find_rating([bits], _bits_seen, _mode) do
+    <<value::size(bit_size(bits))>> = bits
+    value
+  end
+
+  def find_rating(values, bits_seen, mode) do
+    bit = find_common(values, bits_seen, mode)
+
+    values
+    |> filter_by_bit(bits_seen, bit)
+    |> find_rating(bits_seen + 1, mode)
+  end
+
+  def find_common(values, bits_seen, mode) do
+    {zeros, ones} =
+      values
+      |> Enum.map(fn <<_::size(bits_seen), bit::1, _rest::bits>> -> bit end)
+      |> Enum.reduce({_zeros = 0, _ones = 0}, fn
+        0, {zeros, ones} -> {zeros + 1, ones}
+        1, {zeros, ones} -> {zeros, ones + 1}
+      end)
+
+    case mode do
+      :oxygen_generator -> if zeros <= ones, do: 1, else: 0
+      :co2_scrubber -> if zeros <= ones, do: 0, else: 1
+    end
+  end
+
+  def filter_by_bit(values, num_prev_bits, bit) do
+    Enum.filter(values, &match?(<<_::size(num_prev_bits), ^bit::1, _rest::bits>>, &1))
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      strings = String.split(input, "\n", trim: true)
+      length = byte_size(hd(strings))
+      Enum.map(strings, &<<String.to_integer(&1, 2)::size(length)>>)
+    else
+      _ -> :error
+    end
+  end
+
+  #######################
+  # HERE BE BOILERPLATE #
+  #######################
+
+  def run do
+    case input() do
+      :error -> print_usage()
+      input -> run_with_timer(fn -> part2(input) end)
+    end
+  end
+
+  defp run_with_timer(fun) do
+    {time, result} = :timer.tc(fun)
+    IO.puts("Part 2 (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 day3.exs input_filename")
+  end
+end
+
+Day3.run()