2023 Day7 Part 1

Will use comments if I want to document thoughts.

.gitignore

@@ -0,0 +1,3 @@
+input
+sample
+arena

2015/day1.exs

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+defmodule Day1 do
+  def part1(<<>>), do: 0
+  def part1(<<?(, rest::binary>>), do: 1 + part1(rest)
+  def part1(<<?), rest::binary>>), do: -1 + part1(rest)
+
+  def part2(input), do: find_basement(input, _floor = 0, _pos = 1)
+
+  def find_basement(<<?), _::binary>>, 0, pos), do: pos
+  def find_basement(<<?), rest::binary>>, floor, pos), do: find_basement(rest, floor - 1, pos + 1)
+  def find_basement(<<?(, rest::binary>>, floor, pos), do: find_basement(rest, floor + 1, pos + 1)
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+    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 day1.exs input_filename")
+  end
+end
+
+Day1.run()

2015/day2.exs

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+defmodule Day2 do
+  def part1(input) do
+    input
+    |> Enum.map(fn [x, y, z] ->
+      a = x * y
+      b = x * z
+      c = y * z
+      min = Enum.min([a, b, c])
+      2 * (a + b + c) + min
+    end)
+    |> Enum.sum()
+  end
+
+  def part2(input) do
+    input
+    |> Enum.map(fn box ->
+      [x, y, z] = Enum.sort(box)
+      2 * (x + y) + x * y * z
+    end)
+    |> Enum.sum()
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split(["\n", "x"], trim: true)
+      |> Enum.map(&String.to_integer/1)
+      |> Enum.chunk_every(3)
+    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 day2.exs input_filename")
+  end
+end
+
+Day2.run()

2015/day3.exs

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+defmodule Day3 do
+  def part1(input), do: move(input, MapSet.new([{0, 0}]), {0, 0})
+
+  def move(<<?<, rest::binary>>, houses, {x, y}), do: deliver(rest, houses, {x - 1, y})
+  def move(<<?>, rest::binary>>, houses, {x, y}), do: deliver(rest, houses, {x + 1, y})
+  def move(<<?^, rest::binary>>, houses, {x, y}), do: deliver(rest, houses, {x, y - 1})
+  def move(<<?v, rest::binary>>, houses, {x, y}), do: deliver(rest, houses, {x, y + 1})
+  def move(<<>>, houses, _addr), do: MapSet.size(houses)
+
+  def deliver(moves, houses, addr), do: move(moves, MapSet.put(houses, addr), addr)
+
+  def part2(input), do: move2(input, MapSet.new([{0, 0}]), {0, 0}, {0, 0})
+
+  def move2(<<ms, mb, rest::binary>>, houses, santa, bot) do
+    santa = next(ms, santa)
+    bot = next(mb, bot)
+    move2(rest, MapSet.union(houses, MapSet.new([santa, bot])), santa, bot)
+  end
+
+  def move2(<<>>, houses, _santa, _bot), do: MapSet.size(houses)
+
+  def next(?<, {x, y}), do: {x - 1, y}
+  def next(?>, {x, y}), do: {x + 1, y}
+  def next(?^, {x, y}), do: {x, y - 1}
+  def next(?v, {x, y}), do: {x, y + 1}
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+    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 day3.exs input_filename")
+  end
+end
+
+Day3.run()

2015/day4.exs

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+defmodule Day4 do
+  def part1(input), do: mine(input, 1, "00000")
+
+  def mine(input, nonce, prefix) do
+    hash = :crypto.hash(:md5, input <> Integer.to_string(nonce)) |> Base.encode16()
+    if String.starts_with?(hash, prefix), do: nonce, else: mine(input, nonce + 1, prefix)
+  end
+
+  def part2(input), do: mine(input, 1, "000000")
+
+  def input do
+    with [input] <- System.argv() do
+      input
+    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 day4.exs input")
+  end
+end
+
+Day4.run()

2015/day5.exs

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+defmodule Day5 do
+  def part1(input) do
+    Enum.count(input, &nice?/1)
+  end
+
+  def nice?(str) do
+    at_least_three_vowels(str) and
+      twice_in_a_row(str) and
+      not String.contains?(str, ["ab", "cd", "pq", "xy"])
+  end
+
+  def at_least_three_vowels(str) do
+    str
+    |> String.graphemes()
+    |> Enum.frequencies()
+    |> Map.take(["a", "e", "i", "o", "u"])
+    |> Map.values()
+    |> Enum.sum()
+    |> Kernel.>=(3)
+  end
+
+  def twice_in_a_row(<<x, x, _::binary>>), do: true
+  def twice_in_a_row(<<_x, rest::binary>>), do: twice_in_a_row(rest)
+  def twice_in_a_row(<<>>), do: false
+
+  def part2(input) do
+   Enum.count(input, &nice2?/1)
+  end
+
+  def nice2?(str) do
+    pairs = pairs(str)
+    uniq = MapSet.new(pairs)
+    length(pairs) > MapSet.size(uniq) and sandwich(str)
+  end
+
+  def pairs(str, prev \\ nil)
+  def pairs(<<x, x, rest::binary>>, <<x, x>>), do: pairs(<<x, rest::binary>>)
+  def pairs(<<x, y, rest::binary>>, _prev), do: [<<x, y>> | pairs(<<y, rest::binary>>, <<x, y>>)]
+  def pairs(<<_>>, _prev), do: []
+
+  def sandwich(<<x, y, x, _rest::binary>>), do: true
+  def sandwich(<<_x, y, z, rest::binary>>), do: sandwich(<<y, z, rest::binary>>)
+  def sandwich(_), do: false
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split("\n", trim: true)
+    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 day5.exs input_filename")
+  end
+end
+
+Day5.run()

2021/README.md

@@ -1,5 +0,0 @@
-# Advent of Code 2021 in Zig!
-
-I just started learning Zig on Christmas Day 2021, so after running through a few tutorials I'm planning on running through a few of the 2021 puzzles as a way to get to know the language, probably starting sometime in January 2022.
-
-<img width="943" alt="image" src="https://user-images.githubusercontent.com/498229/147409293-6a855c9c-7d89-4440-9192-240eb47b3d85.png">

2021/day10.exs

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+defmodule Day10 do
+  def part1(input) do
+    input
+    |> Enum.map(&parse_line(&1, []))
+    |> Enum.filter(&match?({:corrupted, _}, &1))
+    |> Enum.map(fn {:corrupted, char} -> score(char) end)
+    |> Enum.sum()
+  end
+
+  defp parse_line(_, {:corrupted, char}), do: {:corrupted, char}
+  defp parse_line("", stack), do: {:incomplete, stack}
+  defp parse_line(<<char::utf8, rest::binary>>, []), do: parse_line(rest, [char])
+
+  defp parse_line(<<char::utf8, next::binary>>, [prev | popped] = stack) do
+    stack =
+      case char do
+        char when char in '([{<' -> [char | stack]
+        ?) when prev == ?( -> popped
+        ?] when prev == ?[ -> popped
+        ?} when prev == ?{ -> popped
+        ?> when prev == ?< -> popped
+        char -> {:corrupted, char}
+      end
+
+    parse_line(next, stack)
+  end
+
+  defp score(?)), do: 3
+  defp score(?]), do: 57
+  defp score(?}), do: 1197
+  defp score(?>), do: 25137
+
+  def part2(input) do
+    scores =
+      input
+      |> Enum.map(&parse_line(&1, []))
+      |> Enum.filter(&match?({:incomplete, _}, &1))
+      |> Enum.map(fn {_, chars} -> score2(chars) end)
+      |> Enum.sort()
+
+    Enum.at(scores, trunc(length(scores) / 2))
+  end
+
+  defp score2(chars) do
+    for char <- chars, reduce: 0 do
+      score -> score * 5 + points(char)
+    end
+  end
+
+  defp points(?(), do: 1
+  defp points(?[), do: 2
+  defp points(?{), do: 3
+  defp points(?<), do: 4
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      String.split(input, "\n", trim: true)
+    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 day10.exs input_filename")
+  end
+end
+
+Day10.run()

2021/day13.exs

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+defmodule Day13 do
+  defmodule Paper do
+    defstruct dots: MapSet.new(), instructions: []
+  end
+
+  def part1(paper) do
+    folded = fold(paper)
+    MapSet.size(folded.dots)
+  end
+
+  defp fold(%Paper{dots: dots, instructions: [{axis, pos} | rest]}) do
+    dots =
+      MapSet.new(dots, fn {x, y} ->
+        cond do
+          axis == "x" and x > pos -> {pos - (x - pos), y}
+          axis == "y" and y > pos -> {x, pos - (y - pos)}
+          true -> {x, y}
+        end
+      end)
+
+    %Paper{dots: dots, instructions: rest}
+  end
+
+  def part2(paper) do
+    paper
+    |> fold_all
+    |> draw
+  end
+
+  defp fold_all(%Paper{instructions: []} = paper), do: paper
+  defp fold_all(%Paper{} = paper), do: paper |> fold |> fold_all
+
+  defp draw(paper) do
+    {max_x, max_y} =
+      for {x, y} <- paper.dots, reduce: {0, 0} do
+        {max_x, max_y} -> {max(x, max_x), max(y, max_y)}
+      end
+
+    for y <- 0..max_y do
+      row =
+        for x <- 0..max_x do
+          if MapSet.member?(paper.dots, {x, y}), do: ?#, else: ?\s
+        end
+
+      # could Enum.join("\n") here, but I want to play with iolists
+      [row | "\n"]
+    end
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      [dots, instructions] = String.split(input, "\n\n")
+
+      dots =
+        dots
+        |> String.split([",", "\n"])
+        |> Enum.map(&String.to_integer/1)
+        |> Enum.chunk_every(2)
+        |> MapSet.new(fn [x, y] -> {x, y} end)
+
+      instructions =
+        instructions
+        |> String.split("\n", trim: true)
+        |> Enum.map(fn <<"fold along ", axis::binary-1, "=", pos::binary>> ->
+          {axis, String.to_integer(pos)}
+        end)
+
+      %Paper{dots: dots, instructions: instructions}
+    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 day13.exs input_filename")
+  end
+end
+
+Day13.run()

2021/day16.exs

@@ -0,0 +1,140 @@
+defmodule Day16 do
+  def part1(input) do
+    {_type, _rest, _count, version_sum} = decode_packet(input, 0, 0)
+    version_sum
+  end
+
+  @literal_type 4
+  @literal_part 1
+  @literal_end 0
+  @total_length_type 0
+  @sub_packets_length_type 1
+
+  defp decode_packet(<<version::3, @literal_type::3, rest::bits>>, count, version_sum) do
+    decode_literal(rest, <<>>, count + 6, version_sum + version)
+  end
+
+  defp decode_packet(<<version::3, operator_type::3, rest::bits>>, count, version_sum) do
+    {sub_packets, rest, count, version_sum} =
+      case rest do
+        <<@total_length_type::1, length::15, rest::bits>> ->
+          decode_sub_packets_by(
+            :length,
+            length,
+            [],
+            rest,
+            count + 3 + 3 + 1 + 15,
+            version_sum + version
+          )
+
+        <<@sub_packets_length_type::1, num_sub_packets::11, rest::bits>> ->
+          decode_sub_packets_by(
+            :quantity,
+            num_sub_packets,
+            [],
+            rest,
+            count + 3 + 3 + 1 + 11,
+            version_sum + version
+          )
+      end
+
+    {{:operator, operator_type, sub_packets}, rest, count, version_sum}
+  end
+
+  defp decode_literal(<<@literal_part::1, group::bits-4, rest::bits>>, acc, count, version_sum) do
+    decode_literal(rest, <<acc::bits, group::bits>>, count + 5, version_sum)
+  end
+
+  defp decode_literal(<<@literal_end::1, group::bits-4, rest::bits>>, acc, count, version_sum) do
+    literal_binary = <<acc::bits, group::bits>>
+    <<literal::size(bit_size(literal_binary))>> = literal_binary
+    {{:literal, literal}, rest, count + 5, version_sum}
+  end
+
+  defp decode_sub_packets_by(_method, 0, decoded_packets, bits, count, version_sum) do
+    {Enum.reverse(decoded_packets), bits, count, version_sum}
+  end
+
+  defp decode_sub_packets_by(method, remaining, decoded_packets, bits, count, version_sum) do
+    {decoded_packet, rest, packet_size, version_sum} = decode_packet(bits, 0, version_sum)
+
+    remaining =
+      case method do
+        :length -> remaining - packet_size
+        :quantity -> remaining - 1
+      end
+
+    decode_sub_packets_by(
+      method,
+      remaining,
+      [decoded_packet | decoded_packets],
+      rest,
+      count + packet_size,
+      version_sum
+    )
+  end
+
+  def part2(input) do
+    {packet, _rest, _count, _version_sum} = decode_packet(input, 0, 0)
+    evaluate(packet)
+  end
+
+  defp evaluate({:literal, literal}), do: literal
+
+  defp evaluate({:operator, op, args}) do
+    args
+    |> Enum.map(&evaluate/1)
+    |> calculate(op)
+  end
+
+  defp calculate(args, 0), do: Enum.sum(args)
+  defp calculate(args, 1), do: Enum.reduce(args, &Kernel.*/2)
+  defp calculate(args, 2), do: Enum.min(args)
+  defp calculate(args, 3), do: Enum.max(args)
+  defp calculate([a, b], 5), do: if(a > b, do: 1, else: 0)
+  defp calculate([a, b], 6), do: if(a < b, do: 1, else: 0)
+  defp calculate([a, b], 7), do: if(a == b, do: 1, else: 0)
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.trim()
+      |> Base.decode16!()
+    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 day16.exs input_filename")
+  end
+end
+
+Day16.run()

2021/day3.exs

@@ -0,0 +1,98 @@
+defmodule Day3 do
+  def part1(input) do
+    gamma_rating = find_rating(input, 0, :most, <<>>)
+    epsillon_rating = find_rating(input, 0, :least, <<>>)
+    gamma_rating * epsillon_rating
+  end
+
+  defp find_rating([value | _], _, _, bits) when bit_size(value) == bit_size(bits) do
+    <<rating::size(bit_size(bits))>> = bits
+    rating
+  end
+
+  defp find_rating(values, bits_seen, mode, acc) do
+    bit = find_common(values, bits_seen, mode)
+    find_rating(values, bits_seen + 1, mode, <<acc::bits-size(bits_seen), bit::1>>)
+  end
+
+  def part2(input) do
+    oxygen_generator_rating = find_rating2(input, 0, :most)
+    co2_scrubber_rating = find_rating2(input, 0, :least)
+    oxygen_generator_rating * co2_scrubber_rating
+  end
+
+  defp find_rating2([bits], _bits_seen, _mode) do
+    <<value::size(bit_size(bits))>> = bits
+    value
+  end
+
+  defp find_rating2(values, bits_seen, mode) do
+    bit = find_common(values, bits_seen, mode)
+
+    values
+    |> filter_by_bit(bits_seen, bit)
+    |> find_rating2(bits_seen + 1, mode)
+  end
+
+  defp 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
+      :most -> if zeros <= ones, do: 1, else: 0
+      :least -> if zeros <= ones, do: 0, else: 1
+    end
+  end
+
+  defp 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_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 dayREPLACE_ME.exs input_filename")
+  end
+end
+
+Day3.run()

2021/day4.exs

@@ -0,0 +1,133 @@
+defmodule Day4 do
+  @rows for y <- 0..4, do: for(x <- 0..4, do: {x, y})
+  @cols for x <- 0..4, do: for(y <- 0..4, do: {x, y})
+
+  def part1(input) do
+    {current_number, winning_board} = play(input)
+    sum_unmarked(winning_board) * current_number
+  end
+
+  def part2(input) do
+    {current_number, last_board} = play_until_last(input)
+    sum_unmarked(last_board) * current_number
+  end
+
+  def play({[current_number | next_numbers], boards}) do
+    boards = for board <- boards, do: mark(board, current_number)
+
+    case check_for_winner(boards) do
+      nil -> play({next_numbers, boards})
+      board -> {current_number, board}
+    end
+  end
+
+  def play_until_last({_numbers, [_last_board]} = input), do: play(input)
+
+  def play_until_last({numbers, boards} = input) do
+    {_current_number, current_winner} = play(input)
+
+    current_winner_no_markers = Map.new(current_winner, fn {k, {v, _marker}} -> {k, v} end)
+
+    remaining_boards =
+      Enum.reject(boards, fn board ->
+        current_winner_no_markers == Map.new(board, fn {k, {v, _marker}} -> {k, v} end)
+      end)
+
+    play_until_last({numbers, remaining_boards})
+  end
+
+  def sum_unmarked(board) do
+    board
+    |> Enum.filter(&match?({_key, {_number, false}}, &1))
+    |> Enum.map(fn {_key, {number, _marked}} -> number end)
+    |> Enum.sum()
+  end
+
+  def mark(board, current_number) do
+    case Enum.find(board, &match?({_key, {^current_number, false}}, &1)) do
+      nil -> board
+      {key, _value} -> Map.put(board, key, {current_number, true})
+    end
+  end
+
+  def check_for_winner([]), do: nil
+
+  def check_for_winner([board | boards]) do
+    row_complete? =
+      Enum.any?(@rows, fn row ->
+        values = for key <- row, do: board[key]
+        Enum.all?(values, &match?({_number, true}, &1))
+      end)
+
+    col_complete? =
+      Enum.any?(@cols, fn col ->
+        values = for key <- col, do: board[key]
+        Enum.all?(values, &match?({_number, true}, &1))
+      end)
+
+    if row_complete? or col_complete? do
+      board
+    else
+      check_for_winner(boards)
+    end
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      [numbers | boards] = String.split(input, "\n", trim: true)
+      numbers = numbers |> String.split(",") |> Enum.map(&String.to_integer/1)
+      boards = parse_boards(boards)
+      {numbers, boards}
+    else
+      _ -> :error
+    end
+  end
+
+  defp parse_boards([]), do: []
+
+  defp parse_boards(boards) do
+    {board, boards} = Enum.split(boards, 5)
+
+    board = Enum.map(board, &String.split/1)
+
+    board =
+      for {row, y} <- Enum.with_index(board), {value, x} <- Enum.with_index(row), into: %{} do
+        {{x, y}, {String.to_integer(value), false}}
+      end
+
+    [board | parse_boards(boards)]
+  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 day4.exs input_filename")
+  end
+end
+
+Day4.run()

2021/day5.exs

@@ -0,0 +1,83 @@
+defmodule Day5 do
+  def part1(input) do
+    input
+    |> Enum.reduce(_points = %{}, fn
+      {{x, y1}, {x, y2}}, points ->
+        for y <- y1..y2, reduce: points, do: (points -> plot(points, {x, y}))
+
+      {{x1, y}, {x2, y}}, points ->
+        for x <- x1..x2, reduce: points, do: (points -> plot(points, {x, y}))
+
+      _point, points ->
+        points
+    end)
+    |> Enum.count(fn {_point, count} -> count >= 2 end)
+  end
+
+  def part2(input) do
+    input
+    |> Enum.reduce(_points = %{}, fn
+      {{x, y1}, {x, y2}}, points ->
+        for y <- y1..y2, reduce: points, do: (points -> plot(points, {x, y}))
+
+      {{x1, y}, {x2, y}}, points ->
+        for x <- x1..x2, reduce: points, do: (points -> plot(points, {x, y}))
+
+      {{x1, x1}, {x2, x2}}, points ->
+        for x <- x1..x2, reduce: points, do: (points -> plot(points, {x, x}))
+
+      {{x1, y1}, {x2, y2}}, points ->
+        for point <- Enum.zip(x1..x2, y1..y2), reduce: points, do: (points -> plot(points, point))
+    end)
+    |> Enum.count(fn {_point, count} -> count >= 2 end)
+  end
+
+  defp plot(points, point) do
+    Map.put(points, point, (points[point] || 0) + 1)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split(["\n", " -> ", ","], trim: true)
+      |> Enum.map(&String.to_integer/1)
+      |> Enum.chunk_every(4)
+      |> Enum.map(fn [x1, y1, x2, y2] -> {{x1, y1}, {x2, y2}} 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 day5.exs input_filename")
+  end
+end
+
+Day5.run()

2021/day6.exs

@@ -0,0 +1,84 @@
+defmodule Day6 do
+  def part1(input) do
+    input
+    |> run_days(80)
+    |> length()
+  end
+
+  defp run_days(school, 0), do: school
+  defp run_days(school, days), do: run_days(day(school), days - 1)
+
+  defp day(school), do: Enum.flat_map(school, &fish/1)
+
+  defp fish(0), do: [6, 8]
+  defp fish(age), do: [age - 1]
+
+  # Implementing part 2 completely differently to avoid exponential growth
+
+  def part2(input) do
+    counts = Map.new(0..8, fn age -> {age, 0} end)
+
+    input
+    |> Enum.frequencies()
+    |> Enum.into(counts)
+    |> run_days2(256)
+    |> Map.values()
+    |> Enum.sum()
+  end
+
+  defp run_days2(counts, 0), do: counts
+  defp run_days2(counts, days), do: run_days2(day2(counts), days - 1)
+
+  defp day2(counts) do
+    {breeders, counts} = Map.pop(counts, 0)
+
+    counts
+    |> Map.new(fn {age, count} -> {age - 1, count} end)
+    |> Map.update(6, 0, &(&1 + breeders))
+    |> Map.put(8, breeders)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.trim()
+      |> String.split(",")
+      |> Enum.map(&String.to_integer/1)
+    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 day6.exs input_filename")
+  end
+end
+
+Day6.run()

2021/day7.exs

@@ -0,0 +1,81 @@
+defmodule Day7 do
+  def part1(input) do
+    input
+    |> Enum.frequencies()
+    |> Enum.into(Map.new(Enum.min(input)..Enum.max(input), &{&1, 0}))
+    |> calculate_costs()
+    |> Enum.min()
+  end
+
+  defp calculate_costs(crabs) do
+    for here <- Map.keys(crabs) do
+      for {there, crabs_there} <- Map.delete(crabs, here), reduce: 0 do
+        cost -> cost + abs(here - there) * crabs_there
+      end
+    end
+  end
+
+  def part2(input) do
+    input
+    |> Enum.frequencies()
+    |> Enum.into(Map.new(Enum.min(input)..Enum.max(input), &{&1, 0}))
+    |> calculate_sum_costs()
+    |> Enum.min()
+  end
+
+  defp calculate_sum_costs(crabs) do
+    for here <- Map.keys(crabs) do
+      for {there, crabs_there} <- Map.delete(crabs, here), reduce: 0 do
+        cost ->
+          distance = abs(here - there)
+          # Mmm maths. Can get the sum of 1..n by doing (n * n+1)/2. Guess you have to know it.
+          sum = div(distance * (distance + 1), 2)
+          cost + sum * crabs_there
+      end
+    end
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.trim()
+      |> String.split(",")
+      |> Enum.map(&String.to_integer/1)
+    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 day7.exs input_filename")
+  end
+end
+
+Day7.run()

2021/day9.exs

@@ -0,0 +1,111 @@
+defmodule Day9 do
+  defmodule Cave do
+    defstruct floor: %{}, max_x: 0, max_y: 0
+  end
+
+  def part1(cave) do
+    for x <- 0..cave.max_x, y <- 0..cave.max_y, lowest_neighbour?(cave, x, y), reduce: 0 do
+      risk_level -> risk_level + cave.floor[{x, y}] + 1
+    end
+  end
+
+  defp neighbours(cave, x, y) do
+    [{x - 1, y}, {x + 1, y}, {x, y - 1}, {x, y + 1}]
+    |> Enum.reject(fn {x, y} -> x < 0 or x > cave.max_x or y < 0 or y > cave.max_y end)
+  end
+
+  defp lowest_neighbour?(cave, x, y) do
+    here = cave.floor[{x, y}]
+    Enum.all?(neighbours(cave, x, y), fn {xn, yn} -> here < cave.floor[{xn, yn}] end)
+  end
+
+  def part2(cave) do
+    cave
+    |> remove_nines()
+    |> find_basins()
+    |> Enum.sort(:desc)
+    |> Enum.take(3)
+    |> Enum.reduce(&Kernel.*/2)
+  end
+
+  defp find_basins(%Cave{floor: floor}) when floor == %{}, do: []
+
+  defp find_basins(cave) do
+    {cave, location} = get_start_location(cave)
+    {cave, basin} = find_basin(cave, [location], [location])
+    [basin | find_basins(cave)]
+  end
+
+  defp find_basin(cave, [], basin), do: {cave, length(basin)}
+
+  defp find_basin(cave, [{x, y} | locations], basin) do
+    neighbours = cave.floor |> Map.take(neighbours(cave, x, y)) |> Map.keys()
+    cave = %Cave{cave | floor: Map.drop(cave.floor, neighbours)}
+    find_basin(cave, locations ++ neighbours, basin ++ neighbours)
+  end
+
+  defp remove_nines(cave) do
+    floor = Map.reject(cave.floor, &match?({_k, 9}, &1))
+    %Cave{cave | floor: floor}
+  end
+
+  defp get_start_location(cave) do
+    [{location, _v}] = Enum.take(cave.floor, 1)
+    cave = %Cave{cave | floor: Map.delete(cave.floor, location)}
+    {cave, location}
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      rows =
+        input
+        |> String.split("\n", trim: true)
+        |> Enum.map(&String.split(&1, "", trim: true))
+
+      floor =
+        for {row, y} <- Enum.with_index(rows), {height, x} <- Enum.with_index(row), into: %{} do
+          {{x, y}, String.to_integer(height)}
+        end
+
+      max_x = length(hd(rows)) - 1
+      max_y = length(rows) - 1
+
+      %Cave{floor: floor, max_x: max_x, max_y: max_y}
+    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()

2022/README.md

@@ -0,0 +1,11 @@
+# AdventOfCode2022
+
+My (attempted) solutions to [Advent of Code 2022](https://adventofcode.com/2022) in Elixir.
+
+<img width="869" alt="image" src="https://user-images.githubusercontent.com/498229/207063420-9e176055-d40c-4b33-a826-368c04a7b75d.png">
+
+## Running:
+
+```sh
+elixir dayN.exs input_filename
+```

2022/day1.exs

@@ -0,0 +1,58 @@
+defmodule Day1 do
+  def part1(input) do
+    input
+    |> Enum.map(&Enum.sum/1)
+    |> Enum.max()
+  end
+
+  def part2(input) do
+    input
+    |> Enum.map(&Enum.sum/1)
+    |> Enum.sort(:desc)
+    |> Enum.take(3)
+    |> Enum.sum()
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split("\n\n")
+      |> Enum.map(&for elf <- String.split(&1, "\n", trim: true), do: String.to_integer(elf))
+    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 day1.exs input_filename")
+  end
+end
+
+Day1.run()

2022/day10.exs

@@ -0,0 +1,109 @@
+defmodule Day10 do
+  defmodule Instr do
+    defstruct cmd: nil, cycles: 0, amt: nil
+  end
+
+  def part1(input) do
+    process(input, _cycle = 1, _x = 1, _sample_at = 20, _sig = 0)
+  end
+
+  defp process([], _cycle, _x, _sample_at, sig), do: sig
+
+  defp process(instrs, cycle, x, sample_at, sig) when cycle == sample_at do
+    process(instrs, cycle, x, sample_at + 40, sig + cycle * x)
+  end
+
+  defp process([%Instr{cycles: 0} | rest], cycle, x, sample_at, sig) do
+    process(rest, cycle + 1, x, sample_at, sig)
+  end
+
+  defp process([instr = %Instr{cycles: 1, cmd: :noop} | rest], cycle, x, sample_at, sig) do
+    process([%Instr{instr | cycles: 0} | rest], cycle, x, sample_at, sig)
+  end
+
+  defp process([instr = %Instr{cycles: 1, cmd: :addx, amt: amt} | rest], cycle, x, sample_at, sig) do
+    process([%Instr{instr | cycles: 0} | rest], cycle, x + amt, sample_at, sig)
+  end
+
+  defp process([instr = %Instr{cycles: cycles} | rest], cycle, x, sample_at, sig) do
+    process([%Instr{instr | cycles: cycles - 1} | rest], cycle + 1, x, sample_at, sig)
+  end
+
+  def part2(input) do
+    input
+    |> scan(_x = 1, _pixel = 0, _pixels = [[]])
+    |> Enum.map(&Enum.reverse/1)
+    |> Enum.reverse()
+    |> Enum.join("\n")
+  end
+
+  defp scan([], _x, _pixel, pixels), do: pixels
+
+  defp scan(instrs, x, _pixel = 40, pixels) do
+    scan(instrs, x, _pixel = 0, [[] | pixels])
+  end
+
+  defp scan([%Instr{cmd: :noop} | rest], x, pixel, [row | pixels]) do
+    scan(rest, x, pixel + 1, [draw_pixel(pixel, x, row) | pixels])
+  end
+
+  defp scan([%Instr{cycles: 1, cmd: :addx, amt: amt} | rest], x, pixel, [row | pixels]) do
+    scan(rest, x + amt, pixel + 1, [draw_pixel(pixel, x, row) | pixels])
+  end
+
+  defp scan([instr = %Instr{cycles: cycles} | rest], x, pixel, [row | pixels]) do
+    scan([%Instr{instr | cycles: cycles - 1} | rest], x, pixel + 1, [
+      draw_pixel(pixel, x, row) | pixels
+    ])
+  end
+
+  defp draw_pixel(pixel, x, row) when pixel in (x - 1)..(x + 1), do: ["#" | row]
+  defp draw_pixel(_pixel, _x, row), do: [" " | row]
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.trim()
+      |> String.split("\n")
+      |> Enum.map(fn
+        "noop" -> %Instr{cmd: :noop, cycles: 1}
+        "addx " <> x -> %Instr{cmd: :addx, cycles: 2, amt: String.to_integer(x)}
+      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 day10.exs input_filename")
+  end
+end
+
+Day10.run()

2022/day11.exs

@@ -0,0 +1,188 @@
+defmodule Monkey do
+  use GenServer
+
+  defstruct [
+    :num,
+    :items_caught,
+    :operation,
+    :divisor,
+    :true_monkey,
+    :false_monkey,
+    :inspected_count
+  ]
+
+  # Server
+  #########
+
+  def init(%Monkey{} = monkey) do
+    {:ok, {monkey, _monkeys = %{}}}
+  end
+
+  def handle_cast({:catch, item}, {monkey, monkies}) do
+    monkey = catch_item(monkey, item)
+    {:noreply, {monkey, monkies}}
+  end
+
+  def handle_call({:register, monkies}, _from, {monkey, _monkies}) do
+    {:reply, :ok, {monkey, monkies}}
+  end
+
+  def handle_call(:turn, _from, {monkey, monkies}) do
+    monkey = do_business(monkey, monkies)
+    {:reply, :done, {%Monkey{monkey | items_caught: []}, monkies}}
+  end
+
+  def handle_call(:get_count, _from, {monkey, monkies}) do
+    {:reply, monkey.inspected_count, {monkey, monkies}}
+  end
+
+  # Client
+  #########
+  def throw(monkey, item), do: GenServer.cast(monkey, {:catch, item})
+  def register(monkey, monkeys), do: GenServer.call(monkey, {:register, monkeys})
+  def take_turn(monkey), do: GenServer.call(monkey, :turn)
+  def report_count(monkey), do: GenServer.call(monkey, :get_count)
+
+  # Monkey business
+  ##################
+
+  defp catch_item(monkey, item), do: %Monkey{monkey | items_caught: [item | monkey.items_caught]}
+
+  defp do_business(monkey, monkies) do
+    count =
+      for item <- Enum.reverse(monkey.items_caught), reduce: 0 do
+        count ->
+          item = monkey.operation.(item) |> div(3)
+
+          to_monkey =
+            if rem(item, monkey.divisor) == 0 do
+              monkey.true_monkey
+            else
+              monkey.false_monkey
+            end
+
+          throw(monkies[to_monkey], item)
+          count + 1
+      end
+
+    %Monkey{monkey | items_caught: [], inspected_count: monkey.inspected_count + count}
+  end
+end
+
+defmodule Day11 do
+  def part1(input) do
+    monkies = setup_monkies(input)
+    order = monkies |> Map.keys() |> Enum.sort()
+
+    run_rounds(20, monkies, order)
+
+    order
+    |> Enum.map(&Monkey.report_count(monkies[&1]))
+    |> Enum.sort(:desc)
+    |> Enum.take(2)
+    |> Enum.reduce(&Kernel.*/2)
+  end
+
+  def setup_monkies(input) do
+    monkies =
+      input
+      |> Enum.with_index()
+      |> Map.new(fn {monkey, idx} ->
+        {:ok, pid} = GenServer.start_link(Monkey, monkey)
+        {idx, pid}
+      end)
+
+    Enum.each(monkies, fn {_k, monkey} -> Monkey.register(monkey, monkies) end)
+
+    monkies
+  end
+
+  defp run_rounds(0, _monkies, _order), do: :ok
+
+  defp run_rounds(rounds, monkies, order) do
+    Enum.each(order, &Monkey.take_turn(monkies[&1]))
+    run_rounds(rounds - 1, monkies, order)
+  end
+
+  def part2(_input) do
+    "not implemented"
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split("\n\n")
+      |> Enum.map(fn monkey ->
+        [
+          <<"Monkey ", monkey_num::binary-1, ":">>,
+          "Starting items: " <> starting_items,
+          <<"Operation: new = old ", op::binary-1, " ", operand::binary>>,
+          "Test: divisible by " <> divisor,
+          "If true: throw to monkey " <> true_monkey,
+          "If false: throw to monkey " <> false_monkey
+        ] = monkey |> String.trim() |> String.split("\n") |> Enum.map(&String.trim/1)
+
+        starting_items = String.split(starting_items, ", ")
+
+        [monkey_num, divisor, true_monkey, false_monkey | starting_items] =
+          Enum.map(
+            [monkey_num, divisor, true_monkey, false_monkey] ++ starting_items,
+            &String.to_integer/1
+          )
+
+        operation =
+          case {op, operand} do
+            {"+", "old"} -> fn old -> old + old end
+            {"*", "old"} -> fn old -> old * old end
+            {"+", operand} -> fn old -> old + String.to_integer(operand) end
+            {"*", operand} -> fn old -> old * String.to_integer(operand) end
+          end
+
+        %Monkey{
+          num: monkey_num,
+          items_caught: Enum.reverse(starting_items),
+          operation: operation,
+          divisor: divisor,
+          true_monkey: true_monkey,
+          false_monkey: false_monkey,
+          inspected_count: 0
+        }
+      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 day11.exs input_filename")
+  end
+end
+
+Day11.run()

2022/day2.exs

@@ -0,0 +1,100 @@
+defmodule Day2 do
+  def part1(input) do
+    input
+    |> Enum.map(fn
+      {opponent, "X"} -> {opponent, :rock}
+      {opponent, "Y"} -> {opponent, :paper}
+      {opponent, "Z"} -> {opponent, :scissors}
+    end)
+    |> Enum.map(&play/1)
+    |> Enum.sum()
+  end
+
+  @rock 1
+  @paper 2
+  @scissors 3
+  @lose 0
+  @draw 3
+  @win 6
+
+  defp play({:rock, :rock}), do: @rock + @draw
+  defp play({:paper, :rock}), do: @rock + @lose
+  defp play({:scissors, :rock}), do: @rock + @win
+  defp play({:rock, :paper}), do: @paper + @win
+  defp play({:paper, :paper}), do: @paper + @draw
+  defp play({:scissors, :paper}), do: @paper + @lose
+  defp play({:rock, :scissors}), do: @scissors + @lose
+  defp play({:paper, :scissors}), do: @scissors + @win
+  defp play({:scissors, :scissors}), do: @scissors + @draw
+
+  def part2(input) do
+    input
+    |> Enum.map(fn
+      {opponent, "X"} -> {opponent, :lose}
+      {opponent, "Y"} -> {opponent, :draw}
+      {opponent, "Z"} -> {opponent, :win}
+    end)
+    |> Enum.map(&play2/1)
+    |> Enum.sum()
+  end
+
+  defp play2({:rock, :win}), do: @win + @paper
+  defp play2({:paper, :win}), do: @win + @scissors
+  defp play2({:scissors, :win}), do: @win + @rock
+  defp play2({:rock, :draw}), do: @draw + @rock
+  defp play2({:paper, :draw}), do: @draw + @paper
+  defp play2({:scissors, :draw}), do: @draw + @scissors
+  defp play2({:rock, :lose}), do: @lose + @scissors
+  defp play2({:paper, :lose}), do: @lose + @rock
+  defp play2({:scissors, :lose}), do: @lose + @paper
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split([" ", "\n"], trim: true)
+      |> Enum.map(fn
+        "A" -> :rock
+        "B" -> :paper
+        "C" -> :scissors
+        me -> me
+      end)
+      |> Enum.chunk_every(2)
+      |> Enum.map(&List.to_tuple/1)
+    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 day2.exs input_filename")
+  end
+end
+
+Day2.run()

2022/day3.exs

@@ -0,0 +1,71 @@
+defmodule Day3 do
+  def part1(input) do
+    input
+    |> Enum.map(fn str ->
+      size = byte_size(str)
+
+      str
+      |> String.split("", trim: true)
+      |> Enum.split(div(size, 2))
+    end)
+    |> Enum.map(fn {a, b} -> hd(a -- a -- b) end)
+    |> Enum.map(&score/1)
+    |> Enum.sum()
+  end
+
+  defp score(<<lower::utf8>>) when lower in ?a..?z, do: lower - ?a + 1
+  defp score(<<upper::utf8>>) when upper in ?A..?Z, do: upper - ?A + 27
+
+  def part2(input) do
+    input
+    |> Enum.map(&String.split(&1, "", trim: true))
+    |> Enum.chunk_every(3)
+    |> Enum.map(fn [a, b, c] ->
+      d = a -- a -- b
+      hd(d -- d -- c)
+    end)
+    |> Enum.map(&score/1)
+    |> Enum.sum()
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      String.split(input, "\n", trim: true)
+    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 day3.exs input_filename")
+  end
+end
+
+Day3.run()

2022/day4.exs

@@ -0,0 +1,55 @@
+defmodule Day4 do
+  def part1(input) do
+    Enum.count(input, fn [a, b] -> MapSet.subset?(a, b) or MapSet.subset?(b, a) end)
+  end
+
+  def part2(input) do
+    Enum.count(input, fn [a, b] -> not MapSet.disjoint?(a, b) end)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split(["-", ",", "\n"], trim: true)
+      |> Enum.map(&String.to_integer/1)
+      |> Enum.chunk_every(2)
+      |> Enum.map(fn [a, b] -> MapSet.new(a..b) end)
+      |> Enum.chunk_every(2)
+    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 day4.exs input_filename")
+  end
+end
+
+Day4.run()

2022/day5.exs

@@ -0,0 +1,92 @@
+defmodule Day5 do
+  def part1(input), do: input |> move_with(&move_9000/3) |> top_crates()
+  def part2(input), do: input |> move_with(&move_9001/3) |> top_crates()
+
+  defp move_with({stacks, procedure}, move) do
+    for [number, from_index, to_index] <- procedure, reduce: stacks do
+      stacks ->
+        {moved_from, moved_to} = move.(number, stacks[from_index], stacks[to_index])
+        %{stacks | from_index => moved_from, to_index => moved_to}
+    end
+  end
+
+  defp move_9000(0, from, to), do: {from, to}
+  defp move_9000(number, [crate | from], to), do: move_9000(number - 1, from, [crate | to])
+
+  defp move_9001(number, from, to) do
+    {moving, moved_from} = Enum.split(from, number)
+    {moved_from, moving ++ to}
+  end
+
+  defp top_crates(stacks) do
+    stacks
+    |> Enum.sort_by(fn {label, _stack} -> label end)
+    |> Enum.reduce("", fn {_label, [top | _stack]}, acc -> acc <> top end)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      [raw_stacks, raw_procedure] = String.split(input, "\n\n")
+
+      stacks =
+        raw_stacks
+        |> String.split("\n")
+        |> parse_stacks()
+        |> Enum.zip()
+        |> Enum.map(fn stack -> stack |> Tuple.to_list() |> Enum.drop_while(&is_nil/1) end)
+        |> Enum.with_index()
+        |> Map.new(fn {stack, index} -> {index + 1, stack} end)
+
+      procedure =
+        raw_procedure
+        |> String.split(["move ", " from ", " to ", "\n"], trim: true)
+        |> Enum.map(&String.to_integer/1)
+        |> Enum.chunk_every(3)
+
+      {stacks, procedure}
+    else
+      _ -> :error
+    end
+  end
+
+  defp parse_stacks([_labels_]), do: []
+  defp parse_stacks([line | rest]), do: [parse_line(line) | parse_stacks(rest)]
+
+  defp parse_line(<<"">>), do: []
+  defp parse_line(<<"    ", rest::binary>>), do: [nil | parse_line(rest)]
+  defp parse_line(<<"[", crate::binary-1, "]", rest::binary>>), do: [crate | parse_line(rest)]
+  defp parse_line(<<" ", rest::binary>>), do: parse_line(rest)
+
+  #######################
+  # 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 day5.exs input_filename")
+  end
+end
+
+Day5.run()

2022/day6.exs

@@ -0,0 +1,72 @@
+defmodule Day6 do
+  @packet_marker_size 4
+  @message_marker_size 14
+
+  def part1(input) do
+    find_packet_marker(input)
+  end
+
+  defguardp all_different(a, b, c, d)
+            when a != b and a != c and a != d and b != c and b != d and c != d
+
+  defp find_packet_marker(buffer, seen \\ @packet_marker_size)
+  defp find_packet_marker([a, b, c, d | _rest], seen) when all_different(a, b, c, d), do: seen
+  defp find_packet_marker([_skip | rest], seen), do: find_packet_marker(rest, seen + 1)
+
+  def part2(input) do
+    find_message_marker(input)
+  end
+
+  defp find_message_marker([_skip | rest] = buffer, seen \\ @message_marker_size) do
+    window = Enum.take(buffer, @message_marker_size)
+
+    if Enum.uniq(window) == window do
+      seen
+    else
+      find_message_marker(rest, seen + 1)
+    end
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.trim()
+      |> String.codepoints()
+    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 day6.exs input_filename")
+  end
+end
+
+Day6.run()

2022/day7.exs

@@ -0,0 +1,145 @@
+defmodule Day7 do
+  @root ["/"]
+
+  def part1(input) do
+    input
+    |> filetree_with_dir_sizes()
+    |> Enum.filter(fn
+      {_k, {:dir, size, _paths}} -> size <= 100_000
+      {_k, {:file, _size, _paths}} -> false
+    end)
+    |> Enum.map(fn {_k, {_type, size, _paths}} -> size end)
+    |> Enum.sum()
+  end
+
+  defp filetree_with_dir_sizes(input) do
+    tree = build_tree(input)
+
+    tree
+    |> dirs_fewest_descendents_first()
+    |> Enum.reduce(tree, &put_dir_size/2)
+  end
+
+  defp put_dir_size(path, tree) do
+    {:dir, nil, paths} = tree[path]
+    size = tree |> Map.take(paths) |> Enum.map(fn {_, {_, size, _}} -> size end) |> Enum.sum()
+    Map.put(tree, path, {:dir, size, paths})
+  end
+
+  defp dirs_fewest_descendents_first(tree, to_check \\ [@root], found_dirs \\ [])
+  defp dirs_fewest_descendents_first(_tree, [], found_dirs), do: found_dirs
+
+  defp dirs_fewest_descendents_first(tree, [node | rest], dirs) do
+    case tree[node] do
+      {:file, _size, nil} -> dirs_fewest_descendents_first(tree, rest, dirs)
+      {:dir, _size, paths} -> dirs_fewest_descendents_first(tree, paths ++ rest, [node | dirs])
+    end
+  end
+
+  defp build_tree(input, path \\ [], tree \\ %{})
+  defp build_tree([], _path, tree), do: tree
+  defp build_tree([{:cd, "/"} | rest], _path, tree), do: build_tree(rest, @root, tree)
+  defp build_tree([{:cd, ".."} | rest], [_here | path], tree), do: build_tree(rest, path, tree)
+  defp build_tree([{:cd, dir} | rest], path, tree), do: build_tree(rest, [dir | path], tree)
+
+  defp build_tree([{:ls, items} | rest], path, tree) do
+    {paths, tree} =
+      Enum.map_reduce(items, tree, fn
+        # Create an empty directory for each dir listed
+        # (needed if we we forget to ls inside a dir)
+        {:dir, nil, name}, tree ->
+          item_path = [name | path]
+          {item_path, Map.put(tree, item_path, {:dir, nil, []})}
+
+        # Create a file for each file listed
+        {:file, size, name}, tree ->
+          item_path = [name | path]
+          {item_path, Map.put(tree, item_path, {:file, size, nil})}
+      end)
+
+    # Create current directory with listed children
+    tree = Map.put(tree, path, {:dir, nil, paths})
+
+    build_tree(rest, path, tree)
+  end
+
+  @total_size 70_000_000
+  @space_needed 30_000_000
+
+  def part2(input) do
+    tree = filetree_with_dir_sizes(input)
+
+    {:dir, space_used, _} = tree[@root]
+    unused_space = @total_size - space_used
+    space_to_free = @space_needed - unused_space
+
+    tree
+    |> Enum.filter(&match?({_path, {:dir, _size, _paths}}, &1))
+    |> Enum.map(fn {_path, {:dir, size, _paths}} -> size end)
+    |> Enum.sort()
+    |> Enum.find(fn size -> size >= space_to_free end)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.trim()
+      |> String.split("\n")
+      |> parse_input()
+    else
+      _ -> :error
+    end
+  end
+
+  defp parse_input([]), do: []
+  defp parse_input(["$ cd " <> dir | rest]), do: [{:cd, dir} | parse_input(rest)]
+
+  defp parse_input(["$ ls" | rest]) do
+    {listings, rest} = Enum.split_while(rest, &(!match?("$" <> _, &1)))
+
+    listings =
+      Enum.map(listings, fn
+        "dir " <> dir ->
+          {:dir, nil, dir}
+
+        file ->
+          [size, name] = String.split(file)
+          {:file, String.to_integer(size), name}
+      end)
+
+    [{:ls, listings} | parse_input(rest)]
+  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 day7.exs input_filename")
+  end
+end
+
+Day7.run()

2022/day8.exs

@@ -0,0 +1,130 @@
+defmodule Day8 do
+  defmodule Tree do
+    defstruct height: nil, visible: false, score: []
+  end
+
+  def part1({{max_x, max_y}, trees}) do
+    trees
+    |> look_in_direction(0..max_y, 0..max_x, false)
+    |> look_in_direction(0..max_x, 0..max_y, true)
+    |> look_in_direction(0..max_y, max_x..0, false)
+    |> look_in_direction(0..max_x, max_y..0, true)
+    |> Enum.count(&match?({_coord, %Tree{visible: true}}, &1))
+  end
+
+  defp look_in_direction(trees, outer_range, inner_range, flip_xy?) do
+    Enum.reduce(outer_range, trees, fn y, trees ->
+      {_height, trees} =
+        Enum.reduce_while(inner_range, {_height = -1, trees}, fn
+          _x, {9, trees} ->
+            {:halt, {9, trees}}
+
+          x, {height, trees} ->
+            coord = if flip_xy?, do: {y, x}, else: {x, y}
+            tree = trees[coord]
+
+            if tree.height > height do
+              {:cont, {tree.height, Map.replace(trees, coord, %Tree{tree | visible: true})}}
+            else
+              {:cont, {height, trees}}
+            end
+        end)
+
+      trees
+    end)
+  end
+
+  def part2({{max_x, max_y}, trees}) do
+    trees
+    |> count_viewing_distance(0..max_y, 0..max_x, false)
+    |> count_viewing_distance(0..max_x, 0..max_y, true)
+    |> count_viewing_distance(0..max_y, max_x..0, false)
+    |> count_viewing_distance(0..max_x, max_y..0, true)
+    |> Enum.map(fn {_coord, %Tree{score: score}} -> Enum.reduce(score, &Kernel.*/2) end)
+    |> Enum.max()
+  end
+
+  defp count_viewing_distance(trees, outer_range, inner_range, flip_xy?) do
+    Enum.reduce(outer_range, trees, fn y, trees ->
+      {_scoring, trees} =
+        Enum.reduce(inner_range, {_scoring = [], trees}, fn
+          x, {scoring, trees} ->
+            coord = if flip_xy?, do: {y, x}, else: {x, y}
+            tree = trees[coord]
+
+            {scoring, trees} =
+              Enum.reduce(scoring, {[], trees}, fn {coord, scoring_tree}, {scoring, trees} ->
+                %Tree{score: [score | scores]} = scoring_tree
+                scoring_tree = %Tree{scoring_tree | score: [score + 1 | scores]}
+                trees = Map.replace(trees, coord, scoring_tree)
+
+                if scoring_tree.height <= tree.height do
+                  {scoring, trees}
+                else
+                  {[{coord, scoring_tree} | scoring], trees}
+                end
+              end)
+
+            tree = %Tree{tree | score: [0 | tree.score]}
+            {[{coord, tree} | scoring], Map.replace(trees, coord, tree)}
+        end)
+
+      trees
+    end)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      grid =
+        input
+        |> String.trim()
+        |> String.split("\n")
+        |> Enum.map(&String.graphemes/1)
+
+      max_x = length(hd(grid)) - 1
+      max_y = length(grid) - 1
+
+      trees =
+        for {row, y} <- Enum.with_index(grid), {height, x} <- Enum.with_index(row), into: %{} do
+          {{x, y}, %Tree{height: String.to_integer(height)}}
+        end
+
+      {{max_x, max_y}, trees}
+    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 day8.exs input_filename")
+  end
+end
+
+Day8.run()

2022/day9.exs

@@ -0,0 +1,109 @@
+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()

2023/day1.exs

@@ -0,0 +1,77 @@
+defmodule Day1 do
+  def part1(input) do
+    input
+    |> Enum.map(fn line ->
+      numbers = filter_digits(line)
+      String.to_integer(String.first(numbers) <> String.last(numbers))
+    end)
+    |> Enum.sum()
+  end
+
+  def filter_digits(<<>>), do: <<>>
+  def filter_digits(<<x, rest::binary>>) when x in ?1..?9, do: <<x>> <> filter_digits(rest)
+  def filter_digits(<<_, rest::binary>>), do: filter_digits(rest)
+
+  def part2(input) do
+    input
+    |> Enum.map(fn line ->
+      numbers = filter_digits2(line)
+      String.to_integer(String.first(numbers) <> String.last(numbers))
+    end)
+    |> Enum.sum()
+  end
+
+  def filter_digits2(<<>>), do: <<>>
+  def filter_digits2(<<x, rest::binary>>) when x in ?1..?9, do: <<x>> <> filter_digits2(rest)
+  def filter_digits2(<<"one", rest::binary>>), do: "1" <> filter_digits2("e" <> rest)
+  def filter_digits2(<<"two", rest::binary>>), do: "2" <> filter_digits2("o" <> rest)
+  def filter_digits2(<<"three", rest::binary>>), do: "3" <> filter_digits2("e" <> rest)
+  def filter_digits2(<<"four", rest::binary>>), do: "4" <> filter_digits2(rest)
+  def filter_digits2(<<"five", rest::binary>>), do: "5" <> filter_digits2("e" <> rest)
+  def filter_digits2(<<"six", rest::binary>>), do: "6" <> filter_digits2(rest)
+  def filter_digits2(<<"seven", rest::binary>>), do: "7" <> filter_digits2("n" <> rest)
+  def filter_digits2(<<"eight", rest::binary>>), do: "8" <> filter_digits2("t" <> rest)
+  def filter_digits2(<<"nine", rest::binary>>), do: "9" <> filter_digits2("e" <> rest)
+  def filter_digits2(<<_, rest::binary>>), do: filter_digits2(rest)
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      String.split(input, "\n", trim: true)
+    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 day1.exs input_filename")
+  end
+end
+
+Day1.run()

2023/day2.exs

@@ -0,0 +1,79 @@
+#!/usr/bin/env elixir
+defmodule Day2 do
+  def part1(input) do
+    input
+    |> Map.filter(fn {_id, cubes} ->
+      cubes["red"] <= 12 and cubes["green"] <= 13 and cubes["blue"] <= 14
+    end)
+    |> Map.keys()
+    |> Enum.sum()
+  end
+
+  def part2(input) do
+    input
+    |> Enum.map(fn {_id, cubes} -> cubes |> Map.values() |> Enum.product() end)
+    |> Enum.sum()
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split("\n", trim: true)
+      |> Map.new(fn line ->
+        ["Game " <> id | rounds] = String.split(line, [": ", "; "])
+
+        rounds =
+          rounds
+          |> Enum.map(&parse_round/1)
+          |> Enum.reduce(
+            %{"red" => 0, "green" => 0, "blue" => 0},
+            &Map.merge(&1, &2, fn _colour, count1, count2 -> max(count1, count2) end)
+          )
+
+        {String.to_integer(id), rounds}
+      end)
+    else
+      _ -> :error
+    end
+  end
+
+  def parse_round(round) do
+    round
+    |> String.split([", ", " "])
+    |> Enum.chunk_every(2)
+    |> Map.new(fn [number, colour] -> {colour, String.to_integer(number)} 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 day2.exs input_filename")
+  end
+end
+
+Day2.run()

2023/day3.exs

@@ -0,0 +1,90 @@
+#!/usr/bin/env elixir
+defmodule Day3 do
+  def part1(_input) do
+    # for each number, check around its edges to see if there's a symbol
+    fn {{x, y}, number, length}, acc ->
+      top = for x <- (x - 1)..(x + length), do: {x, y - 1}
+      bottom = for x <- (x - 1)..(x + length), do: {x, y + 1}
+      box = [{x - 1, y}, {x + length, y}] ++ top ++ bottom
+      if part_number?(box), do: [number | acc], else: acc
+    end
+    |> :ets.foldl(_acc = [], :numbers)
+    |> Enum.sum()
+  end
+
+  def part_number?([]), do: false
+
+  def part_number?([coord | rest]) do
+    case :ets.lookup(:symbols, coord) do
+      [_symbol] -> true
+      [] -> part_number?(rest)
+    end
+  end
+
+  def part2(_input) do
+    :ok
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      :ets.new(:numbers, [:named_table])
+      :ets.new(:symbols, [:named_table])
+      parse_schematic(input, _x = 0, _y = 0)
+    else
+      _ -> :error
+    end
+  end
+
+  def parse_schematic("", _x, _y), do: :ok
+  def parse_schematic("." <> rest, x, y), do: parse_schematic(rest, x + 1, y)
+  def parse_schematic("\n" <> rest, _x, y), do: parse_schematic(rest, 0, y + 1)
+
+  def parse_schematic(<<digit::utf8>> <> rest, x, y) when digit in ?0..?9 do
+    {number, length, rest} = take_digits(<<digit::utf8>> <> rest, "", 0)
+    :ets.insert(:numbers, {{x, y}, number, length})
+    parse_schematic(rest, x + length, y)
+  end
+
+  def parse_schematic(<<symbol::utf8>> <> rest, x, y) do
+    :ets.insert(:symbols, {{x, y}, <<symbol::utf8>>})
+    parse_schematic(rest, x + 1, y)
+  end
+
+  def take_digits(<<digit::utf8>> <> rest, digits, count) when digit in ?0..?9,
+    do: take_digits(rest, digits <> <<digit::utf8>>, count + 1)
+
+  def take_digits(rest, digits, count), do: {String.to_integer(digits), count, rest}
+
+  #######################
+  # 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 day3.exs input_filename")
+  end
+end
+
+# Day3.run()

2023/day4.exs

@@ -0,0 +1,78 @@
+#!/usr/bin/env elixir
+defmodule Day4 do
+  def part1(input) do
+    input
+    |> Enum.map(fn 0 -> 0; num_winners -> 2 ** (num_winners - 1) end)
+    |> Enum.sum()
+  end
+
+  def part2(input) do
+    input
+    |> Enum.map(fn num_winners -> {_num_copies = 1, num_winners} end)
+    |> copy()
+    |> Enum.sum()
+  end
+
+  def copy([]), do: []
+
+  def copy([{num_copies, num_winners} | rest]) do
+    {to_copy, left_alone} = Enum.split(rest, num_winners)
+
+    copied =
+      Enum.map(to_copy, fn {child_num_copies, num_winners} ->
+        {num_copies + child_num_copies, num_winners}
+      end)
+
+    [num_copies | copy(copied ++ left_alone)]
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split("\n", trim: true)
+      |> Enum.map(fn line ->
+        [_card_id, winning, have] = String.split(line, [": ", " | "])
+
+        winning = winning |> String.split(" ", trim: true) |> MapSet.new()
+        have = have |> String.split(" ", trim: true) |> MapSet.new()
+
+        MapSet.intersection(winning, have) |> MapSet.size()
+      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("#{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 day4.exs input_filename")
+  end
+end
+
+Day4.run()

2023/day5.exs

@@ -0,0 +1,110 @@
+#!/usr/bin/env elixir
+defmodule Day5 do
+  def part1({seeds, maps}) do
+    seeds
+    |> Enum.map(fn seed -> Enum.reduce(maps, seed, &lookup/2) end)
+    |> Enum.min()
+  end
+
+  def lookup([], item), do: item
+
+  def lookup([{source, offset} | rest], item) do
+    if item in source do
+      item + offset
+    else
+      lookup(rest, item)
+    end
+  end
+
+  def part2({seeds, maps}) do
+    seed_ranges =
+      seeds
+      |> Enum.chunk_every(2)
+      |> Enum.map(fn [start, len] -> start..(start + len - 1) end)
+
+    maps
+    |> Enum.reduce(seed_ranges, &process_ranges/2)
+    |> Enum.map(& &1.first)
+    |> Enum.min()
+  end
+
+  def process_ranges(_map_ranges, []), do: []
+
+  def process_ranges(map_ranges, [item_range | rest]) do
+    overlap =
+      Enum.find(map_ranges, fn {source, _offset} -> not Range.disjoint?(source, item_range) end)
+
+    case overlap do
+      {source, offset} ->
+        to_shift = max(source.first, item_range.first)..min(source.last, item_range.last)
+
+        remainders =
+          [
+            if(item_range.first < source.first, do: item_range.first..(source.first - 1)),
+            if(item_range.last > source.last, do: (source.last + 1)..item_range.last)
+          ]
+          |> Enum.reject(&is_nil/1)
+
+        [Range.shift(to_shift, offset) | process_ranges(map_ranges, remainders ++ rest)]
+
+      nil ->
+        [item_range | process_ranges(map_ranges, rest)]
+    end
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      ["seeds: " <> seeds | maps] = String.split(input, "\n\n")
+      seeds = seeds |> String.split() |> Enum.map(&String.to_integer/1)
+
+      maps =
+        Enum.map(maps, fn line ->
+          [_header, _map | items] = String.split(line)
+
+          items
+          |> Enum.map(&String.to_integer/1)
+          |> Enum.chunk_every(3)
+          |> Enum.map(fn [dest, source, offset] ->
+            {source..(source + offset), dest - source}
+          end)
+        end)
+
+      {seeds, maps}
+    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, charlists: :as_lists)}\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 day5.exs input_filename")
+  end
+end
+
+Day5.run()

2023/day6.exs

@@ -0,0 +1,69 @@
+#!/usr/bin/env elixir
+defmodule Day6 do
+  def part1(input) do
+    input
+    |> Enum.map(fn line -> Enum.map(line, &String.to_integer/1) end)
+    |> Enum.zip()
+    |> Enum.map(fn {race, record} -> count_winning(race, record) end)
+    |> Enum.product()
+  end
+
+  def count_winning(race, record) do
+    scores = for n <- 1..div(race, 2), do: n * (race - n)
+    [_middle | rev_no_middle] = reversed = Enum.reverse(scores)
+    scores = scores ++ if rem(race, 2) == 1, do: reversed, else: rev_no_middle
+    Enum.count(scores, fn score -> score > record end)
+  end
+
+  def part2(input) do
+    [race, record] = Enum.map(input, fn line -> line |> Enum.join() |> String.to_integer() end)
+    count_winning(race, record)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split("\n", trim: true)
+      |> Enum.map(fn line ->
+        line
+        |> String.split()
+        |> Enum.drop(1)
+      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("#{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()

2023/day7.exs

@@ -0,0 +1,123 @@
+#!/usr/bin/env elixir
+defmodule Day7 do
+  def part1(input) do
+    input
+    |> Enum.sort_by(&detect_type/1, &rank/2)
+    |> Enum.with_index()
+    |> Enum.map(fn {{_hand, bid}, rank} -> bid * (rank + 1) end)
+    |> Enum.sum()
+  end
+
+  def detect_type({hand, _bid}) do
+    frequencies =
+      hand
+      |> Enum.frequencies()
+      |> Enum.group_by(fn {_card, freq} -> freq end)
+      |> Map.new(fn {freq, cards} -> {freq, length(cards)} end)
+
+    type =
+      cond do
+        frequencies[5] -> :five_kind
+        frequencies[4] -> :four_kind
+        frequencies[3] && frequencies[2] -> :full_house
+        frequencies[3] -> :three_kind
+        frequencies[2] == 2 -> :two_pair
+        frequencies[2] -> :one_pair
+        true -> :high_card
+      end
+
+    {type, hand}
+  end
+
+  def rank({same_type, hand1}, {same_type, hand2}), do: tiebreak(hand1, hand2)
+  def rank({:high_card, _}, _), do: true
+  def rank({:one_pair, _}, {:high_card, _}), do: false
+  def rank({:one_pair, _}, _), do: true
+  def rank({:two_pair, _}, {b, _}) when b in [:high_card, :one_pair], do: false
+  def rank({:two_pair, _}, _), do: true
+  def rank({:three_kind, _}, {b, _}) when b in [:high_card, :one_pair, :two_pair], do: false
+  def rank({:three_kind, _}, _), do: true
+  def rank({:full_house, _}, {b, _}) when b in [:five_kind, :four_kind], do: true
+  def rank({:full_house, _}, _), do: false
+  def rank({:four_kind, _}, {:five_kind, _}), do: true
+  def rank({:four_kind, _}, _), do: false
+  def rank({:five_kind, _}, _), do: false
+
+  def tiebreak([first | resta], [first | restb]), do: tiebreak(resta, restb)
+  def tiebreak(["1" | _], _), do: true
+  def tiebreak(["2" | _], ["1" | _]), do: false
+  def tiebreak(["2" | _], _), do: true
+  def tiebreak(["3" | _], [b | _]) when b in ["1", "2"], do: false
+  def tiebreak(["3" | _], _), do: true
+  def tiebreak(["4" | _], [b | _]) when b in ["1", "2", "3"], do: false
+  def tiebreak(["4" | _], _), do: true
+  def tiebreak(["5" | _], [b | _]) when b in ["1", "2", "3", "4"], do: false
+  def tiebreak(["5" | _], _), do: true
+  def tiebreak(["6" | _], [b | _]) when b in ["1", "2", "3", "4", "5"], do: false
+  def tiebreak(["6" | _], _), do: true
+  def tiebreak(["7" | _], [b | _]) when b in ["1", "2", "3", "4", "5", "6"], do: false
+  def tiebreak(["7" | _], _), do: true
+  def tiebreak(["8" | _], [b | _]) when b in ["A", "K", "Q", "J", "T", "9"], do: true
+  def tiebreak(["8" | _], _), do: false
+  def tiebreak(["9" | _], [b | _]) when b in ["A", "K", "Q", "J", "T"], do: true
+  def tiebreak(["9" | _], _), do: false
+  def tiebreak(["T" | _], [b | _]) when b in ["A", "K", "Q", "J"], do: true
+  def tiebreak(["T" | _], _), do: false
+  def tiebreak(["J" | _], [b | _]) when b in ["A", "K", "Q"], do: true
+  def tiebreak(["J" | _], _), do: false
+  def tiebreak(["Q" | _], [b | _]) when b in ["A", "K"], do: true
+  def tiebreak(["Q" | _], _), do: false
+  def tiebreak(["K" | _], ["A" | _]), do: true
+  def tiebreak(["K" | _], _), do: false
+  def tiebreak(["A" | _], _), do: false
+
+  def part2(_input) do
+    :ok
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split("\n", trim: true)
+      |> Enum.map(fn line ->
+        [hands, bid] = String.split(line)
+        {String.graphemes(hands), String.to_integer(bid)}
+      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("#{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 day7.exs input_filename")
+  end
+end
+
+# Day7.run()

README.md

@@ -2,6 +2,6 @@
 
 My (attempted) solutions to [Advent of Code](https://adventofcode.com/).
 
-- [2021: Zig](https://github.com/adamu/AdventOfCode/tree/main/2021)
+- [2022: Elixir](https://git.adamu.jp/adam/AdventOfCode/src/branch/main/2022)
-- [2020: Elixir](https://github.com/adamu/AdventOfCode/tree/main/2020)
+- [2020: Elixir](https://git.adamu.jp/adam/AdventOfCode/src/branch/main/2020)
-- [2018: Elixir](https://github.com/adamu/AdventOfCode/tree/main/2018)
+- [2018: Elixir](https://git.adamu.jp/adam/AdventOfCode/src/branch/main/2018)

dayN.exs

@@ -0,0 +1,51 @@
+#!/usr/bin/env elixir
+defmodule DayREPLACE_ME do
+  def part1(input) do
+    input
+  end
+
+  def part2(_input) do
+    :ok
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+    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 dayREPLACE_ME.exs input_filename")
+  end
+end
+
+# DayREPLACE_ME.run()

template/README

@@ -1,25 +0,0 @@
-Day REPLACE_ME Notes
-
-+--------+
-| Part 1 |
-+--------+
-
-$ elixir dayREPLACE_MEpart1.exs
-
-Thoughts:
-
-
-+--------+
-| Part 2 |
-+--------+
-
-$ elixir dayREPLACE_MEpart2.exs
-
-Thoughts:
-
-
-+------------------+
-| Overall Thoughts |
-+------------------+
-
-

template/dayNpart1.exs

@@ -1,9 +0,0 @@
-defmodule DayREPLACE_MEPart1 do
-  def run do
-    File.read!("input")
-    |> String.split("\n", trim: true)
-    |> IO.inspect()
-  end
-end
-
-DayREPLACE_MEPart1.run()