MaybePieceByCoordinates.hs

{-
	Copyright (C) 2018 Dr. Alistair Ward

	This file is part of BishBosh.

	BishBosh is free software: you can redistribute it and/or modify
	it under the terms of the GNU General Public License as published by
	the Free Software Foundation, either version 3 of the License, or
	(at your option) any later version.

	BishBosh is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
	GNU General Public License for more details.

	You should have received a copy of the GNU General Public License
	along with BishBosh.  If not, see <http://www.gnu.org/licenses/>.
-}
{- |
 [@AUTHOR@]	Dr. Alistair Ward

 [@DESCRIPTION@]

	* Models the /board/ as a sparse array, each element of which might contain a /piece/.

	* N.B.: while this could be represented as @Data.Map.Map Coordinates Piece@, replacing 'Data.Array.IArray.!' with 'Data.Map.lookup',
	it actually required more space (despite having at most half the elements) & runs slower (because of 'compare').

	* cf. the piece-centric model of the board defined in "BishBosh.State.CoordinatesByRankByLogicalColour".
-}

module BishBosh.State.MaybePieceByCoordinates(
-- * Types
-- ** Type-synonyms
--	Transformation,
-- ** Data-types
	MaybePieceByCoordinates(),
-- * Functions
	inferMoveType,
	findBlockingPiece,
	findAttackerInDirection,
	listDestinationsFor,
--	listToRaster,
--	shows2D,
	show2D,
-- ** Accessors
	dereference,
--	getPieces,
-- ** Mutators
	movePiece,
-- ** Predicates
	isVacant,
	isOccupied,
	isClear,
	isObstructed,
	isEnPassantMove
) where

import			Control.Applicative((<|>))
import			Control.Arrow((&&&), (***))
import			Control.Category((>>>))
import			Data.Array.IArray((!), (//))
import qualified	BishBosh.Attribute.ANSIColourCode			as Attribute.ANSIColourCode
import qualified	BishBosh.Attribute.ColourScheme				as Attribute.ColourScheme
import qualified	BishBosh.Attribute.Direction				as Attribute.Direction
import qualified	BishBosh.Attribute.LogicalColour			as Attribute.LogicalColour
import qualified	BishBosh.Attribute.LogicalColourOfSquare		as Attribute.LogicalColourOfSquare
import qualified	BishBosh.Attribute.MoveType				as Attribute.MoveType
import qualified	BishBosh.Attribute.PhysicalColour			as Attribute.PhysicalColour
import qualified	BishBosh.Attribute.Rank					as Attribute.Rank
import qualified	BishBosh.Cartesian.Abscissa				as Cartesian.Abscissa
import qualified	BishBosh.Cartesian.Coordinates				as Cartesian.Coordinates
import qualified	BishBosh.Cartesian.Ordinate				as Cartesian.Ordinate
import qualified	BishBosh.Component.Accountant				as Component.Accountant
import qualified	BishBosh.Component.CastlingMove				as Component.CastlingMove
import qualified	BishBosh.Component.Move					as Component.Move
import qualified	BishBosh.Component.Piece				as Component.Piece
import qualified	BishBosh.Component.PieceSquareByCoordinatesByRank	as Component.PieceSquareByCoordinatesByRank
import qualified	BishBosh.Component.Zobrist				as Component.Zobrist
import qualified	BishBosh.Data.Exception					as Data.Exception
import qualified	BishBosh.Notation.Figurine				as Notation.Figurine
import qualified	BishBosh.Property.Empty					as Property.Empty
import qualified	BishBosh.Property.ExtendedPositionDescription		as Property.ExtendedPositionDescription
import qualified	BishBosh.Property.FixedMembership			as Property.FixedMembership
import qualified	BishBosh.Property.ForsythEdwards			as Property.ForsythEdwards
import qualified	BishBosh.Property.Opposable				as Property.Opposable
import qualified	BishBosh.Property.Orientated				as Property.Orientated
import qualified	BishBosh.Property.Reflectable				as Property.Reflectable
import qualified	BishBosh.StateProperty.Censor				as StateProperty.Censor
import qualified	BishBosh.StateProperty.Hashable				as StateProperty.Hashable
import qualified	BishBosh.StateProperty.Mutator				as StateProperty.Mutator
import qualified	BishBosh.StateProperty.Seeker				as StateProperty.Seeker
import qualified	BishBosh.Text.ShowList					as Text.ShowList
import qualified	BishBosh.Type.Length					as Type.Length
import qualified	Control.Arrow
import qualified	Control.DeepSeq
import qualified	Control.Exception
import qualified	Data.Array.IArray
import qualified	Data.Char
import qualified	Data.Default
import qualified	Data.Foldable
import qualified	Data.List
import qualified	Data.List.Extra
import qualified	Data.Maybe
import qualified	ToolShed.Data.List.Runlength

{- |
	* This structure allows one to determine what /piece/ (if any) is located at specific /coordinates/.

	* N.B.: this could be implemented using 'Data.Vector.Vector', which being indexed by 'Int' is no longer polymorphic & permits many unsafe operations; but the result is no faster.
-}
newtype MaybePieceByCoordinates	= MkMaybePieceByCoordinates {
	deconstruct	:: Cartesian.Coordinates.ArrayByCoordinates (
		Maybe Component.Piece.Piece	-- Each square optionally contains a piece.
	)
} deriving (Eq, Ord)

-- | Used to separate the /ranks/ of the /board/ as represented by the IO-format <https://en.wikipedia.org/wiki/Forsyth%E2%80%93Edwards_Notation>.
-- | Chops a list into a 2-D list.
listToRaster :: [a] -> [[a]]
listToRaster	= Data.List.Extra.chunksOf $ fromIntegral Cartesian.Abscissa.xLength {-CAVEAT: this also depends on the raster-order-}

instance Read MaybePieceByCoordinates where
	readsPrec _	= Property.ForsythEdwards.readsFEN

instance Show MaybePieceByCoordinates where
	showsPrec _	= Property.ForsythEdwards.showsFEN

instance Property.ExtendedPositionDescription.ReadsEPD MaybePieceByCoordinates where
	readsEPD s
		| length rows /= fromIntegral Cartesian.Ordinate.yLength || any (
			(/= fromIntegral Cartesian.Abscissa.xLength) . length
		) rows		= []	-- No parse.
		| otherwise	= [(MkMaybePieceByCoordinates . Cartesian.Coordinates.listArrayByCoordinates . concat $ reverse rows, remainder)]
		where
			(rows, remainder)	= Control.Arrow.first (
				map (
					concatMap (
						\c -> case reads [c] of
							[(i, "")]	-> replicate i Nothing	-- Expand the runlength-code so that each row has the same length.
							_		-> [Just piece | (piece, []) <- Property.ExtendedPositionDescription.readsEPD [c]] -- List-comprehension.
					)
				) . Text.ShowList.splitOn (== Property.ExtendedPositionDescription.rankSeparator)
			 ) . span (
				`elem` (
					Property.ExtendedPositionDescription.rankSeparator : Component.Piece.epdCharacterSet ++ concatMap show [1 .. Cartesian.Abscissa.xLength]
				)
			 ) $ Data.List.Extra.trimStart s

instance Property.ExtendedPositionDescription.ShowsEPD MaybePieceByCoordinates where
	showsEPD MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= foldr1 (
		>>>	-- Render the line with the highest y-coordinate first.
	 ) . Data.List.intersperse (
		showChar Property.ExtendedPositionDescription.rankSeparator	-- Separate the lines.
	 ) . map (
		foldr1 (.) . concatMap (
			\(runLength, maybePiece) -> Data.Maybe.maybe [
				shows runLength	-- Represent empty squares.
			] (
				replicate runLength . Property.ExtendedPositionDescription.showsEPD	-- Render each piece.
			) maybePiece
		) . ToolShed.Data.List.Runlength.encode
	 ) . listToRaster $ Data.Foldable.toList byCoordinates

instance Property.ForsythEdwards.ReadsFEN MaybePieceByCoordinates

instance Property.ForsythEdwards.ShowsFEN MaybePieceByCoordinates

instance Data.Default.Default MaybePieceByCoordinates where
	def = Property.ForsythEdwards.readFEN . Data.List.intercalate [Property.ExtendedPositionDescription.rankSeparator] $ map ($ Attribute.LogicalColour.Black) [
		showNobility,
		showPawnRow
	 ] ++ replicate 4 "8" ++ map ($ Attribute.LogicalColour.White) [
		showPawnRow,
		showNobility
	 ] where
		showPieces :: [Component.Piece.Piece] -> String
		showPieces	= concatMap Property.ForsythEdwards.showFEN

		showPawnRow, showNobility :: Attribute.LogicalColour.LogicalColour -> String
		showPawnRow logicalColour	= showPieces . replicate (fromIntegral Cartesian.Abscissa.xLength) $ Component.Piece.mkPawn logicalColour
		showNobility logicalColour	= showPieces $ map (Component.Piece.mkPiece logicalColour) Attribute.Rank.nobility

instance Property.Reflectable.ReflectableOnX MaybePieceByCoordinates where
	reflectOnX MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= MkMaybePieceByCoordinates . Cartesian.Coordinates.arrayByCoordinates . map (
		Property.Reflectable.reflectOnX *** fmap Property.Opposable.getOpposite
	 ) $ Data.Array.IArray.assocs byCoordinates

instance Property.Reflectable.ReflectableOnY MaybePieceByCoordinates where
	reflectOnY MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= MkMaybePieceByCoordinates $ Data.Array.IArray.ixmap (minBound, maxBound) Property.Reflectable.reflectOnY byCoordinates

instance Property.Empty.Empty MaybePieceByCoordinates where
	empty	= MkMaybePieceByCoordinates . Cartesian.Coordinates.listArrayByCoordinates $ repeat Property.Empty.empty

instance Control.DeepSeq.NFData MaybePieceByCoordinates where
	rnf MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= Control.DeepSeq.rnf byCoordinates

instance StateProperty.Censor.Censor MaybePieceByCoordinates where
	countPiecesByLogicalColour	= Data.List.foldl' (
		\acc piece -> let
			acc'@(nBlack, nWhite)	= (
				if Component.Piece.isBlack piece
					then Control.Arrow.first
					else Control.Arrow.second
			 ) succ acc
		in nBlack `seq` nWhite `seq` acc'
	 ) (0, 0) . getPieces

	countPieces	= fromIntegral . length . getPieces

	countPieceDifferenceByRank	= Data.Array.IArray.accumArray (+) 0 (minBound, maxBound) . map (
		Component.Piece.getRank &&& (
			\piece -> (
				if Component.Piece.isBlack piece
					then negate
					else id
			) 1
		)
	 ) . getPieces

	hasInsufficientMaterial maybePieceByCoordinates	= all (
		(`notElem` Attribute.Rank.individuallySufficientMaterial) . Component.Piece.getRank . snd {-piece-}
	 ) locatedPieces && case blackKnights ++ whiteKnights of
		[]	-> Cartesian.Coordinates.areSquaresIsochromatic bishops
		[_]	-> null bishops
		_	-> False
		where
			locatedPieces	= StateProperty.Seeker.findAllPieces maybePieceByCoordinates

			[blackKnights, blackBishops, whiteKnights, whiteBishops]	= [
				[
					coordinates |
						(coordinates, piece)	<- locatedPieces,
						piece == Component.Piece.mkPiece logicalColour rank
				] |
					logicalColour	<- Property.FixedMembership.members,
					rank		<- [Attribute.Rank.Knight, Attribute.Rank.Bishop]
			 ] -- List-comprehension.

			bishops	= blackBishops ++ whiteBishops

	hasBothKings maybePieceByCoordinates	= case Data.List.partition Component.Piece.isBlack . filter Component.Piece.isKing $ getPieces maybePieceByCoordinates of
		([_], [_])	-> True
		_		-> False

instance StateProperty.Hashable.Hashable MaybePieceByCoordinates where
	listRandoms MkMaybePieceByCoordinates { deconstruct = byCoordinates } zobrist	= [
		Component.Zobrist.dereferenceRandomByCoordinatesByRankByLogicalColour (Component.Piece.getLogicalColour piece, Component.Piece.getRank piece, coordinates) zobrist |
			(coordinates, Just piece)	<- Data.Array.IArray.assocs byCoordinates
	 ] -- List-comprehension.

instance StateProperty.Mutator.Mutator MaybePieceByCoordinates where
	defineCoordinates maybePiece coordinates MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= Control.Exception.assert (
		Data.Maybe.isJust maybePiece || Data.Maybe.isJust (byCoordinates ! coordinates)
	 ) . MkMaybePieceByCoordinates $ byCoordinates // [(coordinates, maybePiece)]

{- |
	* Find any @Knight@s of the specified /logical colour/, in attack-range around the specified /coordinates/.

	* CAVEAT: nothing is said about whether any /piece/ at the specified /coordinates/ belongs to the opponent, as one might expect.

	* CAVEAT: less efficient than 'State.CoordinatesByRankByLogicalColour.findProximateKnights'.
-}
instance StateProperty.Seeker.Seeker MaybePieceByCoordinates where
	findProximateKnights logicalColour destination MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= filter (
		(== Just knight) . (byCoordinates !)
	 ) $ Component.Piece.findAttackDestinations destination knight where
		knight	= Component.Piece.mkKnight logicalColour

	findPieces predicate MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= [
		(coordinates, piece) |
			(coordinates, Just piece)	<- Data.Array.IArray.assocs byCoordinates,
			predicate piece
	 ] -- List-comprehension.

instance Component.Accountant.Accountant MaybePieceByCoordinates where
	sumPieceSquareValueByLogicalColour pieceSquareByCoordinatesByRank nPieces = (
		\(b, w) -> [b, w]
	 ) . Data.List.foldl' (
		\(b, w) (coordinates, piece) -> let
			logicalColour		= Component.Piece.getLogicalColour piece
			pieceSquareValue	= Component.PieceSquareByCoordinatesByRank.findPieceSquareValue pieceSquareByCoordinatesByRank nPieces logicalColour (Component.Piece.getRank piece) coordinates
		in if Attribute.LogicalColour.isBlack logicalColour
			then let b' = b + pieceSquareValue in b' `seq` (b', w)
			else let w' = w + pieceSquareValue in w' `seq` (b, w')
	 ) (0, 0) . StateProperty.Seeker.findAllPieces

-- | Dereference the array.
dereference
	:: Cartesian.Coordinates.Coordinates
	-> MaybePieceByCoordinates
	-> Maybe Component.Piece.Piece
{-# INLINE dereference #-}
dereference coordinates MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= byCoordinates ! coordinates

-- | Infer the type of the specified /move/.
inferMoveType
	:: Component.Move.Move
	-> Maybe Attribute.Rank.Rank	-- ^ The /rank/ to which a @Pawn@ should be promoted; defaulting to @Queen@.
	-> MaybePieceByCoordinates
	-> Attribute.MoveType.MoveType
inferMoveType move maybePromotionRank maybePieceByCoordinates@MkMaybePieceByCoordinates { deconstruct = byCoordinates }
	| Just sourcePiece <- byCoordinates ! Component.Move.getSource move	= Data.Maybe.maybe (
		if isEnPassantMove move maybePieceByCoordinates
			then Attribute.MoveType.enPassant	-- N.B.: if this move is valid, then one's opponent must have just double advanced an adjacent Pawn.
			else let
				destination	= Component.Move.getDestination move
			in Attribute.MoveType.mkNormalMoveType (
				fmap Component.Piece.getRank $ byCoordinates ! destination	-- Record the rank of any piece which was taken; the logical colour is inferred to be the opposite of 'sourcePiece'.
			) $ if Component.Piece.isPawnPromotion destination sourcePiece
				then maybePromotionRank <|> Just Attribute.Rank.defaultPromotionRank
				else Nothing
	) Component.CastlingMove.getMoveType $ if Component.Piece.isKing sourcePiece
		then Data.List.find (
			(== move) . Component.CastlingMove.getKingsMove
		) . Component.CastlingMove.getCastlingMoves $ Component.Piece.getLogicalColour sourcePiece
		else Nothing
	| otherwise	= Control.Exception.throw . Data.Exception.mkSearchFailure . showString "BishBosh.State.MaybePieceByCoordinates.inferMoveType:\tno piece exists at " . shows (Component.Move.getSource move) . showString "; " $ shows maybePieceByCoordinates "."

{- |
	* Lists the destination-/coordinates/ to which the referenced /piece/ can move, & the /rank/ of any /piece/ taken.

	* N.B.: one can reference either player's /piece/, regardless of whose turn it is to move.

	* CAVEAT: doesn't include either /Castling/ or /En-passant/, because this function doesn't know the history of the game.

	* CAVEAT: doesn't check whether any proposed /move/ exposes one's @King@, because this function doesn't assume the existence of a @King@.

	* CAVEAT: the opponent's @King@ may be one of the destinations returned, but only if it was actually their move next.

	* CAVEAT: doesn't typically check whether anything (let alone the specified /piece/) exists at the specified source-/coordinates/.
-}
listDestinationsFor
	:: Cartesian.Coordinates.Coordinates					-- ^ The source for which destinations are required.
	-> Component.Piece.Piece						-- ^ The /piece/ at the specified source.
	-> MaybePieceByCoordinates
	-> [(Cartesian.Coordinates.Coordinates, Maybe Attribute.Rank.Rank)]	-- ^ The destination & the rank of any piece taken.
listDestinationsFor source piece maybePieceByCoordinates@MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= Control.Exception.assert (
	byCoordinates ! source == Just piece
 ) $ if Component.Piece.getRank piece `elem` Attribute.Rank.fixedAttackRange
	then {-P,N,K-} let
		findAttackDestinations predicate	= [
			(destination, fmap Component.Piece.getRank maybeDestinationPiece) |
				destination	<- Component.Piece.findAttackDestinations source piece,
				let maybeDestinationPiece	= byCoordinates ! destination,
				predicate maybeDestinationPiece
		 ] -- List-comprehension.
	in if Component.Piece.isPawn piece
		then findAttackDestinations (
			Data.Maybe.maybe False {-unoccupied-} $ (/= logicalColour) . Component.Piece.getLogicalColour
		) ++ let
			advance	:: Cartesian.Coordinates.Coordinates -> Cartesian.Coordinates.Coordinates
			advance	= Cartesian.Coordinates.advance logicalColour

			advancedLocation	= advance source
		in if isVacant advancedLocation maybePieceByCoordinates
			then map (
				flip (,) Nothing	-- N.B.: a Pawn can only take diagonally.
			) $ advancedLocation : [
				doubleAdvancedLocation |
					Cartesian.Coordinates.isPawnsFirstRank logicalColour source,
					let doubleAdvancedLocation	= advance advancedLocation,
					isVacant doubleAdvancedLocation maybePieceByCoordinates
			] -- List-comprehension.
			else []	-- The path immediately ahead is blocked.
		else {-N,K-} findAttackDestinations . Data.Maybe.maybe True {-unoccupied-} $ (/= logicalColour) . Component.Piece.getLogicalColour
	else {-R,B,Q-} let
		takeUntil (destination : remainder)
			| Just blockingPiece <- byCoordinates ! destination	= [
				(
					destination,
					Just $ Component.Piece.getRank blockingPiece
				) | Component.Piece.getLogicalColour blockingPiece /= logicalColour
			] -- List-comprehension.
			| otherwise	= (destination, Nothing) : takeUntil remainder	-- Recurse.
		takeUntil _	= []
	in [
		pairs |
			direction	<- Component.Piece.getAttackDirections piece,
			pairs		<- takeUntil $ Cartesian.Coordinates.extrapolate direction source
	] -- List-comprehension.
	where
		logicalColour	= Component.Piece.getLogicalColour piece

-- | Show the /board/ in two dimensions, with /x/ & /y/ indexes.
shows2D
	:: Type.Length.Column			-- ^ The column-magnification.
	-> Attribute.ColourScheme.ColourScheme
	-> Bool					-- ^ Whether to depict pieces as Unicode figurines.
	-> (Type.Length.X, Type.Length.Y)	-- ^ The origin from which axes are labelled.
	-> MaybePieceByCoordinates
	-> ShowS		-- ^ Output suitable for display on a terminal.
shows2D boardColumnMagnification colourScheme depictFigurine (xOrigin, yOrigin) MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= (
	foldr (
		\(y, pairs) showsRow -> showsRow . showString axisGraphicsRendition . showChar y . foldr (
			\(coordinates, c) acc' -> showString (
				Attribute.ANSIColourCode.selectGraphicsRendition False {-isBold-} . Attribute.ANSIColourCode.mkBgColourCode $ (
					if Attribute.LogicalColourOfSquare.isBlack $ Cartesian.Coordinates.getLogicalColourOfSquare coordinates
						then Attribute.ColourScheme.getDarkSquareColour
						else Attribute.ColourScheme.getLightSquareColour
				) colourScheme
			) . showString (
				Attribute.ANSIColourCode.selectGraphicsRendition True {-isBold-} . Attribute.ANSIColourCode.mkFgColourCode $ (
					if Data.Char.isLower c {-Black-}
						then Attribute.ColourScheme.getDarkPieceColour
						else Attribute.ColourScheme.getLightPieceColour
				) colourScheme
			) . let
				showPadding	= showString (fromIntegral (pred boardColumnMagnification) `replicate` ' ')
			in showPadding . showChar c . showPadding . acc'
		) showsReset pairs . showChar '\n'
	) id . zip (
		take (fromIntegral Cartesian.Ordinate.yLength) . enumFrom . Data.Char.chr $ fromIntegral yOrigin
	) . listToRaster . map (
		Control.Arrow.second . Data.Maybe.maybe ' ' $ if depictFigurine
			then Notation.Figurine.toFigurine	-- Represent each piece as a Unicode figurine.
			else head . show			-- Represent each piece as an ASCII character.
	) $ Data.Array.IArray.assocs byCoordinates
 ) . showString (
	replicate (fromIntegral boardColumnMagnification) ' '	-- Shift the line of x-axis labels right.
 ) . showString axisGraphicsRendition . foldr (.) showsReset (
	Data.List.intersperse (
		showString $ replicate (2 * fromIntegral (pred boardColumnMagnification)) ' '	-- Separate each of the x-axis labels.
	) . map showChar . take (
		fromIntegral Cartesian.Abscissa.xLength
	) . enumFrom . Data.Char.chr $ fromIntegral xOrigin
 ) where
	axisGraphicsRendition :: Attribute.ANSIColourCode.GraphicsRendition
	axisGraphicsRendition	= Attribute.ANSIColourCode.selectGraphicsRendition True {-isBold-} $ Attribute.ANSIColourCode.mkFgColourCode Attribute.PhysicalColour.green

	showsReset :: ShowS
	showsReset	= showString $ Attribute.ANSIColourCode.selectGraphicsRendition False Data.Default.def

-- | Show the board using a two-dimensional representation.
show2D
	:: Type.Length.Column			-- ^ The column-magnification.
	-> Attribute.ColourScheme.ColourScheme
	-> Bool					-- ^ Whether to depict figurines.
	-> (Type.Length.X, Type.Length.Y)	-- ^ The origin from which axes are labelled.
	-> MaybePieceByCoordinates
	-> String		-- ^ The output suitable for display on a terminal.
show2D boardColumnMagnification colourScheme depictFigurine (xOrigin, yOrigin) maybePieceByCoordinates	= shows2D boardColumnMagnification colourScheme depictFigurine (xOrigin, yOrigin) maybePieceByCoordinates ""

-- | Extract the pieces from the board, discarding their coordinates.
getPieces :: MaybePieceByCoordinates -> [Component.Piece.Piece]
getPieces MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= Data.Maybe.catMaybes $ Data.Foldable.toList byCoordinates

{- |
	* Find the first /piece/ of either /logical colour/, encountered along a straight line in the specified /direction/, from just after the specified /coordinates/.

	* CAVEAT: this is a performance-hotspot.
-}
findBlockingPiece
	:: Attribute.Direction.Direction	-- ^ The direction in which to search.
	-> Cartesian.Coordinates.Coordinates	-- ^ The starting point.
	-> MaybePieceByCoordinates
	-> Maybe Component.Piece.LocatedPiece
findBlockingPiece direction source MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= Data.Maybe.listToMaybe [
	(coordinates, piece) |
		(coordinates, Just piece)	<- map (id &&& (byCoordinates !)) $ Cartesian.Coordinates.extrapolate direction source
 ] -- List-comprehension.

{- |
	* Find the /coordinates/ of any attacker who can strike the specified /coordinates/, in a straight line along the specified /direction/ (as seen by the target).

	* N.B.: there no requirement for there to actually be a /piece/ to attack at the specified target.
-}
findAttackerInDirection
	:: Attribute.LogicalColour.LogicalColour				-- ^ The defender's /logical colour/.
	-> Attribute.Direction.Direction					-- ^ The /direction/ from the /coordinates/ of concern; the opposite /direction/ from which an attacker might strike.
	-> Cartesian.Coordinates.Coordinates					-- ^ The defender's square.
	-> MaybePieceByCoordinates
	-> Maybe (Cartesian.Coordinates.Coordinates, Attribute.Rank.Rank)	-- ^ Any opposing /piece/ which can attack the specified square from the specified /direction/.
findAttackerInDirection destinationLogicalColour direction destination	= (=<<) (
	\(source, sourcePiece) -> if Component.Piece.getLogicalColour sourcePiece /= destinationLogicalColour && Component.Piece.canAttackAlong source destination sourcePiece
		then Just (source, Component.Piece.getRank sourcePiece)
		else Nothing
 ) . findBlockingPiece direction destination

-- | Whether the specified /coordinates/ are unoccupied.
isVacant
	:: Cartesian.Coordinates.Coordinates
	-> MaybePieceByCoordinates
	-> Bool
{-# INLINE isVacant #-}
isVacant coordinates MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= Data.Maybe.isNothing $ byCoordinates ! coordinates

-- | Whether the specified /coordinates/ are occupied.
isOccupied
	:: Cartesian.Coordinates.Coordinates
	-> MaybePieceByCoordinates
	-> Bool
{-# INLINE isOccupied #-}
isOccupied coordinates	= not . isVacant coordinates

{- |
	* Whether the open interval (source, destination) is unobstructed.

	* CAVEAT: the move must be straight, so that all intermediate points lie on squares of the board.

	* N.B.: the specified end-points are uninspected.
-}
isClear
	:: Cartesian.Coordinates.Coordinates	-- ^ Source.
	-> Cartesian.Coordinates.Coordinates	-- ^ Destination.
	-> MaybePieceByCoordinates
	-> Bool
{-# INLINABLE isClear #-}	-- N.B.: required to ensure specialisation of 'Cartesian.Coordinates.interpolate'.
isClear source destination maybePieceByCoordinates	= Control.Exception.assert (
	source /= destination && Property.Orientated.isStraight (Component.Move.mkMove source destination)
 ) . all (`isVacant` maybePieceByCoordinates) . init {-discard the destination-} $ Cartesian.Coordinates.interpolate source destination

-- | Whether there's a blockage between a /piece/ presumed to exist at the specified source, & a /piece/ presumed to exist @ the specified destination.
isObstructed
	:: Cartesian.Coordinates.Coordinates	-- ^ Source.
	-> Cartesian.Coordinates.Coordinates	-- ^ Destination.
	-> MaybePieceByCoordinates
	-> Bool
isObstructed source destination	= not . isClear source destination

{- |
	* Whether the specified /move/ matches the rules for /en-passant/.

	* CAVEAT: assumes that the /move/ is valid;
	otherwise one would also need to confirm that the opponent's @Pawn@ had just double-advanced into the appropriate position.
-}
isEnPassantMove
	:: Component.Move.Move
	-> MaybePieceByCoordinates
	-> Bool
isEnPassantMove move maybePieceByCoordinates@MkMaybePieceByCoordinates { deconstruct = byCoordinates }
	| Just piece	<- byCoordinates ! source
	, let logicalColour	= Component.Piece.getLogicalColour piece
	= Cartesian.Coordinates.isEnPassantRank logicalColour source && Component.Piece.isPawn piece && destination `elem` Component.Piece.findAttackDestinations source piece && isVacant destination maybePieceByCoordinates	-- The move is either En-passant or invalid.
	| otherwise	= False	-- No piece.
	where
		(source, destination)	= Component.Move.getSource &&& Component.Move.getDestination $ move

-- | Self-documentation.
type Transformation	= MaybePieceByCoordinates -> MaybePieceByCoordinates

{- |
	* Adjust the array to reflect a move.

	* CAVEAT: regrettably this allocates an entire array.
-}
movePiece
	:: Component.Move.Move
	-> Component.Piece.Piece			-- ^ The (possibly promoted) piece to place at the destination.
	-> Maybe Cartesian.Coordinates.Coordinates	-- ^ Destination of any En-passant @Pawn@.
	-> Transformation
movePiece move destinationPiece maybeEnPassantDestination MkMaybePieceByCoordinates { deconstruct = byCoordinates }	= MkMaybePieceByCoordinates $ byCoordinates // Data.Maybe.maybe id (
	(:) . flip (,) Nothing	-- Take the Pawn en-passant.
 ) maybeEnPassantDestination [
	(
		Component.Move.getSource move,
		Nothing	-- Remove the piece from the source.
	), (
		Component.Move.getDestination move,
		Just destinationPiece	-- Place the piece at the destination, removing any opposing incumbent as a side-effect.
	)
 ]