my-solutions/advent-of-code-2023/elixir/day_07/lib/solution.ex

defmodule Solution do
  @spec get_total_winnings(String.t()) :: integer()
  def get_total_winnings(filename, with_jokers \\ false) do
    hands =
      filename
      |> File.stream!()
      |> Enum.map(&parse_line/1)
      |> Map.new()

    sorted_cards =
      hands
      |> Map.keys()
      |> Enum.sort_by(fn hand -> get_sorting_order(hand, with_jokers) end)

    sorted_cards
    |> Enum.with_index(1)
    |> Enum.reduce(0, fn {el, idx}, acc -> acc + idx * hands[el] end)
  end

  defp parse_line(line) do
    [hand, bid] =
      line
      |> String.trim()
      |> String.split(" ")

    {hand, String.to_integer(bid)}
  end

  # Map a hand to a list that may be used for sorting.
  defp get_sorting_order(hand, with_jokers) do
    # Map cards to corresponding power,
    # e.g. "57TA" -> [5, 7, 10, 14]
    powers =
      hand
      |> String.graphemes()
      |> Enum.map(fn card -> get_card_power(card, with_jokers) end)

    # Prepend hand rank.
    [get_rank(hand, with_jokers) | powers]
  end

  @doc """
  Converts a card symbol to a number.

  ## Examples

    iex> Solution.get_card_power("J")
    11

    iex> Solution.get_card_power("J", true)
    1

    iex> Solution.get_card_power("5")
    5
  """
  def get_card_power(card, with_jokers \\ false) do
    case card do
      "A" ->
        14

      "K" ->
        13

      "Q" ->
        12

      "J" ->
        if with_jokers, do: 1, else: 11

      "T" ->
        10

      num ->
        String.to_integer(num)
    end
  end

  @doc """
  Returns the rank of a hand based on the number of identical cards.

  ## Examples

      iex> Solution.get_rank("AAAAA")
      7

      iex> Solution.get_rank("AA8AA")
      6

      iex> Solution.get_rank("23332")
      5

      iex> Solution.get_rank("TTT98")
      4

      iex> Solution.get_rank("23432")
      3
      
      iex> Solution.get_rank("A23A4")
      2
      
      iex> Solution.get_rank("23456")
      1
  """
  def get_rank(hand, with_jokers \\ false) do
    hand = if with_jokers, do: replace_joker(hand), else: hand

    freq =
      hand
      |> String.graphemes()
      |> Enum.frequencies()
      |> Map.values()
      |> Enum.sort()

    case freq do
      # five of a kind
      [5] -> 7
      # four of a kind
      [1, 4] -> 6
      # full house
      [2, 3] -> 5
      # three of a kind
      [1, 1, 3] -> 4
      # two pair
      [1, 2, 2] -> 3
      # one pair
      [1, 1, 1, 2] -> 2
      # high card
      _ -> 1
    end
  end

  @doc """
  Replaces all J cards in hand with the card that will result in the best rank.

  ## Examples

      iex> Solution.replace_joker("T55J5")
      "T5555"

      iex> Solution.replace_joker("KTJJT")
      "KTTTT"

      iex> Solution.replace_joker("JJJJJ")
      "AAAAA"
  """
  def replace_joker(hand) do
    freq =
      hand
      |> String.replace("J", "")
      |> String.graphemes()
      |> Enum.frequencies()

    if Enum.empty?(Map.keys(freq)) do
      String.replace(hand, "J", "A")
    else
      max_count =
        freq
        |> Map.values()
        |> Enum.max()

      {most_used_symbol, _} =
        freq
        |> Map.to_list()
        |> Enum.find(fn {_, count} -> count == max_count end)

      String.replace(hand, "J", most_used_symbol)
    end
  end
end