chess/lib/chess/moves/piece.ex

defmodule Chess.Moves.Piece do
  @moduledoc false

  alias Chess.Board
  alias Chess.Moves.Generator
  alias Chess.Moves.Pieces.Knight
  alias Chess.Moves.Pieces.Pawn

  def attacked?(board, {file, rank}) do
    attacked_by_rook_or_queen?(board, {file, rank}) ||
      attacked_by_bishop_or_queen?(board, {file, rank}) ||
      attacked_by_knight?(board, {file, rank}) ||
      attacked_by_pawn?(board, {file, rank})
  end

  defp attacked_by_rook_or_queen?(board, {file, rank}) do
    _attacked?(board, {file, rank}, {0, 1}, ["rook", "queen"]) ||
      _attacked?(board, {file, rank}, {0, -1}, ["rook", "queen"]) ||
      _attacked?(board, {file, rank}, {1, 0}, ["rook", "queen"]) ||
      _attacked?(board, {file, rank}, {-1, 0}, ["rook", "queen"])
  end

  defp attacked_by_bishop_or_queen?(board, {file, rank}) do
    _attacked?(board, {file, rank}, {1, 1}, ["bishop", "queen"]) ||
      _attacked?(board, {file, rank}, {1, -1}, ["bishop", "queen"]) ||
      _attacked?(board, {file, rank}, {-1, 1}, ["bishop", "queen"]) ||
      _attacked?(board, {file, rank}, {-1, -1}, ["bishop", "queen"])
  end

  defp attacked_by_knight?(board, {file, rank}) do
    _attacked?(board, {file, rank}, Knight.pattern, "knight")
  end

  defp attacked_by_pawn?(board, {file, rank}) do
    colour =
      board
      |> Board.piece({file, rank})
      |> Map.get("colour")

    board
    |> _attacked_by_pawn?({file, rank}, Pawn.pattern(colour))
  end

  defp _attacked_by_pawn?(board, {file, rank}, pattern) do
    moves =
      board
      |> Generator.moves({file, rank}, pattern)

    Enum.any?(moves, &(match_piece(board, &1, "pawn")))
  end

  defp _attacked?(_board, {0, _rank}, {-1, _}, _), do: false
  defp _attacked?(_board, {_file, 0}, {_, -1}, _), do: false
  defp _attacked?(_board, {7, _rank}, {1, _}, _), do: false
  defp _attacked?(_board, {_file, 7}, {_, 1}, _), do: false
  defp _attacked?(board, {file, rank}, {fv, rv}, pieces) do
    board
    |> Generator.moves({file, rank}, {fv, rv})
    |> List.last
    |> case do
      {file, rank} ->
        piece = board["#{file},#{rank}"]

        Enum.any?(pieces, &(match?(%{"type" => ^&1}, piece)))
      nil ->
        false
    end
  end

  defp _attacked?(board, {file, rank}, pattern, piece_type) do
    moves =
      board
      |> Generator.moves({file, rank}, pattern)

    Enum.any?(moves, &(match_piece(board, &1, piece_type)))
  end

  defp match_piece(board, {file, rank}, piece_type) do
    piece =
      board
      |> Board.piece({file, rank})

    piece["type"] == piece_type
  end
end