-- Write a function which takes a description of a chess position and
-- returns a list of all legal captures from that position.
--
-- The position will be given as a table, containing the keys "turn",
-- indicating which player it is to move, "board", which will contain
-- a rank-major grid of pieces (that is, a list of ranks/rows).  Each
-- piece will be indicated by its unicode symbol.  The lack of a piece
-- will be indicated by nil.  If the previous move was a pawn moving
-- two places, the key "en_passant_target" will be set to a table with
-- "rank" and "file" keys set to the (1-based) rank and file indices
-- of the square passed over by the pawn.
--
-- For example, the starting position would be given as:
--
-- {
--     turn = "white",
--     board = {
--         {"♖", "♘", "♗", "♕", "♔", "♗", "♘", "♖"},
--         {"♙", "♙", "♙", "♙", "♙", "♙", "♙", "♙"},
--         {nil, nil, nil, nil, nil, nil, nil, nil},
--         {nil, nil, nil, nil, nil, nil, nil, nil},
--         {nil, nil, nil, nil, nil, nil, nil, nil},
--         {nil, nil, nil, nil, nil, nil, nil, nil},
--         {"♟︎", "♟︎", "♟︎", "♟︎", "♟︎", "♟︎", "♟︎", "♟︎"},
--         {"♜", "♞", "♝", "♛", "♚", "♝", "♞", "♜"},
--     },
-- }
--
-- Moves should be returned in figurine algebraic notation, without
-- indicators for en passant, check or checkmate (i.e. no "e.p.", "+"
-- or "#").  For example, "♞xc6" for a black night capturing on c6 and
-- "exd7" for an e-file pawn capturing on d7.  For the sake of
-- simplicity, there's no requirement to disambiguate between two
-- pieces of the same type when the notation for the capture would
-- otherwise be the same (e.g. if two rooks can capture on the same
-- square).  However, there should be duplicate entries in the results
-- list for each.
--
-- Solution --------------------------------------------------------------------
function legal_captures(position)
	-- Your implementation here
end

-- Tests -----------------------------------------------------------------------

local luaunit = require("luaunit.luaunit")

function test_starting_position()
	local position = {
		turn = "white",
		board = {
			{"♖", "♘", "♗", "♕", "♔", "♗", "♘", "♖"},
			{"♙", "♙", "♙", "♙", "♙", "♙", "♙", "♙"},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{"♟︎", "♟︎", "♟︎", "♟︎", "♟︎", "♟︎", "♟︎", "♟︎"},
			{"♜", "♞", "♝", "♛", "♚", "♝", "♞", "♜"},
		},
	}
	local expected = {}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_queens_gambit()
	local position = {
		turn = "black",
		board = {
			{"♖", "♘", "♗", "♕", "♔", "♗", "♘", "♖"},
			{"♙", "♙", nil, nil, "♙", "♙", "♙", "♙"},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, "♙", "♙", nil, nil, nil, nil},
			{nil, nil, nil, "♟︎", nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{"♟︎", "♟︎", "♟︎", nil, "♟︎", "♟︎", "♟︎", "♟︎"},
			{"♜", "♞", "♝", "♛", "♚", "♝", "♞", "♜"},
		},
	}
	local expected = {"dxc4"}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_scandinavian_defense()
	local position = {
		turn = "white",
		board = {
			{"♖", "♘", "♗", "♕", "♔", "♗", "♘", "♖"},
			{"♙", "♙", "♙", "♙", nil, "♙", "♙", "♙"},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, "♙", nil, nil, nil},
			{nil, nil, nil, "♟︎", nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{"♟︎", "♟︎", "♟︎", nil, "♟︎", "♟︎", "♟︎", "♟︎"},
			{"♜", "♞", "♝", "♛", "♚", "♝", "♞", "♜"},
		},
	}
	local expected = {"exd5"}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_knight_captures()
	local position = {
		turn = "black",
		board = {
			{"♙", "♙", "♙", "♙", "♙", "♙", "♙", "♙"},
			{"♙", "♙", "♙", nil, "♙", "♙", "♙", "♙"},
			{"♙", "♙", nil, nil, nil, "♙", "♙", "♙"},
			{"♙", nil, nil, "♞", nil, nil, "♙", "♙"},
			{"♙", "♙", nil, nil, nil, "♙", "♙", "♙"},
			{"♙", "♙", "♙", nil, "♙", "♙", "♙", "♙"},
			{"♙", "♙", "♙", "♙", "♙", "♙", "♙", "♙"},
			{"♙", "♙", "♙", "♙", "♙", "♙", "♙", "♙"},
		},
	}
	local expected = {
		"♞xb3", "♞xb5", "♞xc2", "♞xc6", "♞xe2", "♞xe6", "♞xf3", "♞xf5",
	}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_knight_not_blocked()
	local position = {
		turn = "white",
		board = {
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, "♟︎", "♟︎", "♟︎", nil, nil},
			{nil, nil, nil, nil, "♟︎", nil, nil, nil},
			{nil, nil, "♟︎", "♟︎", "♘", nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
		},
	}
	local expected = {"♘xd3", "♘xf3"}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_bishop_captures()
	local position = {
		turn = "black",
		board = {
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{"♙", nil, nil, nil, nil, nil, "♙", nil},
			{nil, nil, nil, "♙", nil, nil, "♙", nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, "♙", "♝", nil, nil, nil, "♙"},
			{nil, nil, nil, nil, "♙", nil, nil, nil},
			{nil, "♙", nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, "♙", nil, nil, nil, nil},
		},
	}
	local expected = {"♝xa2", "♝xb7", "♝xe6", "♝xg2"}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_bishop_blocked()
	local position = {
		turn = "black",
		board = {
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, "♙", nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, "♞", nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, "♝", nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
		},
	}
	local expected = {}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_rook_captures()
	local position = {
		turn = "white",
		board = {
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{"♟︎", nil, nil, nil, nil, nil, "♟︎", nil},
			{nil, nil, nil, "♟︎", nil, nil, "♟︎", nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, "♟︎", "♖", nil, nil, nil, "♟︎"},
			{nil, nil, nil, nil, "♟︎", nil, nil, nil},
			{nil, "♟︎", nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, "♟︎", nil, nil, nil, nil},
		},
	}
	local expected = {"♖xc5", "♖xd3", "♖xd8", "♖xh5"}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_rook_blocked()
	local position = {
		turn = "white",
		board = {
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, "♖", "♙", nil, nil, nil, "♟︎", nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
		},
	}
	local expected = {}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_queen_captures()
	local position = {
		turn = "black",
		board = {
			{nil, "♙", "♙", "♙", "♙", "♙", nil, "♙"},
			{"♙", "♙", "♙", nil, "♙", "♙", "♙", "♙"},
			{"♙", "♙", nil, nil, nil, "♙", "♙", "♙"},
			{"♙", nil, nil, "♛", nil, nil, "♙", nil},
			{"♙", "♙", nil, nil, nil, "♙", "♙", "♙"},
			{"♙", "♙", "♙", nil, "♙", "♙", "♙", "♙"},
			{nil, "♙", "♙", "♙", "♙", "♙", nil, "♙"},
			{"♙", "♙", "♙", nil, "♙", "♙", "♙", nil},
		},
	}
	local expected = {
		"♛xa4", "♛xb2", "♛xb6", "♛xd1", "♛xd7", "♛xf2", "♛xf6", "♛xg4",
	}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_queen_blocked()
	local position = {
		turn = "white",
		board = {
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, "♟︎", nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, "♙", nil, nil, nil, nil, nil},
			{nil, "♕", "♙", nil, nil, nil, "♟︎", nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
		},
	}
	local expected = {}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_king_captures()
	local position = {
		turn = "black",
		board = {
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, "♙", "♙", "♙", nil, nil, nil},
			{nil, nil, "♙", "♚", "♙", nil, nil, nil},
			{nil, nil, "♙", "♙", "♙", nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
		},
	}
	local expected = {"♚xc3","♚xc4","♚xc5","♚xd3","♚xd5","♚xe3","♚xe4","♚xe5",}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_en_passant()
	local position = {
		turn = "white",
		board = {
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, "♟︎", "♙", nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
		},
		en_passant_target = {rank = 6, file = 4},
	}
	local expected = {"exd6"}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_cannot_capture_if_leads_to_check()
	local position = {
		turn = "white",
		board = {
			{nil, nil, nil, nil, "♔", nil, nil, nil},
			{nil, nil, nil, nil, "♗", nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, "♟︎", nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, "♜", nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
		},
		en_passant_target = {rank = 6, file = 4},
	}
	local expected = {}
	luaunit.assertItemsEquals(legal_captures(position), expected)
end

function test_position_with_lots_of_captures()
	local position = {
		turn = "white",
		board = {
			{"♚", nil, nil, "♔", "♝", nil, nil, "♖"},
			{nil, "♖", "♜", "♕", "♞", "♕", "♘", nil},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{"♗", "♛", "♕", "♝", "♕", "♛", "♕", "♞"},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, "♕", "♝", "♕", "♛", "♕", "♘", "♗"},
			{nil, nil, nil, nil, nil, nil, nil, nil},
			{nil, nil, nil, "♜", "♕", "♛", nil, nil},
		},
		en_passant_target = {rank = 6, file = 4},
	}
	local expected = {
		"♔xe1", "♔xe2", "♔xc2", "♖xh4", "♖xe1",
		"♖xb4", "♖xc2", "♕xf4", "♕xb4", "♕xe1",
		"♕xd4", "♕xe2", "♕xc2", "♕xh4", "♕xd4",
		"♕xe1", "♕xf4", "♕xe2", "♘xh4", "♘xf4",
		"♘xe1", "♗xc6", "♗xc2", "♕xe6", "♕xe2",
		"♕xc6", "♕xc2", "♕xd4", "♕xb4", "♕xc6",
		"♕xc2", "♕xe6", "♕xe2", "♕xf4", "♕xd4",
		"♕xe6", "♕xe2", "♕xh4", "♕xf4", "♕xd8",
		"♕xd4", "♕xb4", "♕xc6", "♕xf8", "♕xf4",
		"♕xb4", "♕xd8", "♕xd4", "♕xe6", "♕xc6",
		"♕xd8", "♕xh4", "♕xd4", "♕xf8", "♕xf4",
		"♕xe6", "♘xf8", "♘xh4", "♘xf4", "♗xf8",
		"♗xf4", "♕xc6", "♕xe6", "♕xf8", "♕xd8",
	}
	local captures = legal_captures(position)
	luaunit.assertItemsEquals(captures, expected)
end

os.exit(luaunit.LuaUnit.run())