aoc/23/elixir/lib/days/day7.ex

defmodule AOC.Day7 do
  # use AOC.Day, day: 7, input: "example"
  use AOC.Day, day: 7

  @type_to_power %{
    five_of_a_kind: 6,
    four_of_a_kind: 5,
    full_house: 4,
    three_of_a_kind: 3,
    two_pair: 2,
    one_pair: 1,
    high_card: 0
  }

  @card_to_power %{
    ?A => 14,
    ?K => 13,
    ?Q => 12,
    ?J => 11,
    ?T => 10,
    ?9 => 9,
    ?8 => 8,
    ?7 => 7,
    ?6 => 6,
    ?5 => 5,
    ?4 => 4,
    ?3 => 3,
    ?2 => 2
  }

  def parse_input(lines) do
    lines
    |> Enum.map(fn line ->
      [hand, bid] = String.split(line, " ")

      {hand, String.to_integer(bid)}
    end)
  end

  def replace_jokers(hand, card) do
    Enum.map(hand, fn
      ?J -> card
      c -> c
    end)
  end

  def best_joker_replacement(hand) do
    freqs =
      Enum.frequencies(hand)
      |> Enum.reject(fn {card, _count} -> card == ?J end)

    cond do
      Enum.any?(freqs, fn {_card, count} -> count == 4 end) ->
        {card, _} = Enum.find(freqs, fn {_card, count} -> count == 4 end)
        card

      Enum.any?(freqs, fn {_card, count} -> count == 3 end) ->
        {card, _} = Enum.find(freqs, fn {_card, count} -> count == 3 end)
        card

      Enum.any?(freqs, fn {_card, count} -> count == 2 end) ->
        doubles = Enum.filter(freqs, fn {_card, count} -> count == 2 end)

        if length(doubles) == 1 do
          hd(doubles) |> elem(0)
        else
          [{double1, _}, {double2, _}] = doubles

          if Map.fetch!(@card_to_power, double1) >= Map.fetch!(@card_to_power, double2) do
            double1
          else
            double2
          end
        end

      true ->
        freqs
        |> Enum.map(fn {card, _count} -> card end)
        |> Enum.sort(fn card1, card2 ->
          Map.fetch!(@card_to_power, card1) >= Map.fetch!(@card_to_power, card2)
        end)
        |> hd()
    end
  end

  def find_best_joker_configuration(hand) do
    freqs = Enum.frequencies(hand)
    joker_count = Map.get(freqs, ?J, 0)

    case joker_count do
      0 ->
        hand

      5 ->
        ~c"AAAAA"

      _ ->
        joker_replacement = best_joker_replacement(hand)

        replace_jokers(hand, joker_replacement)
    end
  end

  def hand_to_type(hand, joker_rule) do
    freqs =
      hand
      |> String.to_charlist()
      |> then(fn hand ->
        if joker_rule do
          find_best_joker_configuration(hand)
        else
          hand
        end
      end)
      |> Enum.frequencies()
      |> Enum.map(fn {_k, v} -> v end)

    cond do
      Enum.member?(freqs, 5) -> :five_of_a_kind
      Enum.member?(freqs, 4) -> :four_of_a_kind
      Enum.member?(freqs, 3) and Enum.member?(freqs, 2) -> :full_house
      Enum.member?(freqs, 3) -> :three_of_a_kind
      Enum.count(freqs, &(&1 == 2)) == 2 -> :two_pair
      Enum.member?(freqs, 2) -> :one_pair
      true -> :high_card
    end
  end

  def compare_same_type_hands(hand1, hand2, joker_rule) do
    hand1 = String.to_charlist(hand1)
    hand2 = String.to_charlist(hand2)

    Enum.zip(hand1, hand2)
    |> compare_same_type_hands2(joker_rule)
  end

  def compare_same_type_hands2([], _), do: true

  def compare_same_type_hands2([{card1, card2} | tl], joker_rule) do
    if card1 == card2 do
      compare_same_type_hands2(tl, joker_rule)
    else
      cond do
        joker_rule and card1 == ?J -> false
        joker_rule and card2 == ?J -> true
        true -> Map.fetch!(@card_to_power, card1) >= Map.fetch!(@card_to_power, card2)
      end
    end
  end

  def sort_hands(hands, joker_rule \\ false) do
    Enum.sort(hands, fn {hand1, _bid1}, {hand2, _bid2} ->
      type1 = hand_to_type(hand1, joker_rule)
      type2 = hand_to_type(hand2, joker_rule)

      if type1 == type2 do
        compare_same_type_hands(hand1, hand2, joker_rule)
      else
        Map.fetch!(@type_to_power, type1) >= Map.fetch!(@type_to_power, type2)
      end
    end)
  end

  def part1(input) do
    input
    |> sort_hands()
    |> Enum.reverse()
    |> Enum.with_index(1)
    |> Enum.map(fn {{_card, bid}, rank} -> rank * bid end)
    |> Enum.sum()
  end

  def part2(input) do
    input
    |> sort_hands(true)
    |> Enum.reverse()
    |> Enum.with_index(1)
    |> Enum.map(fn {{_card, bid}, rank} -> rank * bid end)
    |> Enum.sum()
  end
end