{-# LANGUAGE PatternGuards #-}

-- | Represents an integral rectangular area of the 2D plane.
--   Using `Int`s (instead of `Float`s) for the bounds means we can safely
--   compare extents for equality.
module Graphics.Gloss.Data.Extent
	( Extent
	, Coord
	, makeExtent
	, takeExtent
	, squareExtent
	, sizeOfExtent
	, isUnitExtent
	, coordInExtent
	, pointInExtent
	, centerCoordOfExtent
	, cutQuadOfExtent
	, quadOfCoord
	, pathToCoord
	, intersectSegExtent
	, touchesSegExtent)
where
import Graphics.Gloss.Data.Point
import Graphics.Gloss.Data.Quad
import Graphics.Gloss.Geometry.Line
import Data.Maybe


-- | A rectangular area of the 2D plane.
--   We keep the type abstract to ensure that invalid extents cannot be
--   constructed.
data Extent
	= Extent Int Int Int Int
	deriving (Eq, Show)


-- | An integral coordinate.
type Coord
	= (Int, Int)


-- | Construct an extent.
--	The north value must be > south, and east > west, else `error`.
makeExtent 
	:: Int 	-- ^ y max (north)
	-> Int 	-- ^ y min (south)
	-> Int 	-- ^ x max (east)
	-> Int 	-- ^ x min (west)
	-> Extent

makeExtent n s e w
	| n >= s, e >= w
	= Extent n s e w
	
	| otherwise
	= error "Graphics.Gloss.Geometry.Extent.makeExtent: invalid extent"


-- | Take the NSEW components of an extent.
takeExtent :: Extent -> (Int, Int, Int, Int)
takeExtent (Extent n s e w)
	= (n, s, e, w)


-- | A square extent of a given size.
squareExtent :: Int -> Extent
squareExtent i
	= Extent i 0 i 0


-- | Get the width and height of an extent.
sizeOfExtent :: Extent -> (Int, Int)
sizeOfExtent (Extent n s e w)
	= (e - w, n - s)


-- | Check if an extent is a square with a width and height of 1.
isUnitExtent :: Extent -> Bool
isUnitExtent extent
	= sizeOfExtent extent == (1, 1)


-- | Check whether a coordinate lies inside an extent.
coordInExtent :: Extent -> Coord -> Bool
coordInExtent (Extent n s e w) (x, y)
	=  x >= w && x < e
	&& y >= s && y < n


-- | Check whether a point lies inside an extent.
pointInExtent :: Extent -> Point -> Bool
pointInExtent (Extent n s e w) (x, y)
 = let	n'	= fromIntegral n
	s'	= fromIntegral s
	e'	= fromIntegral e
	w'	= fromIntegral w
	
   in	x >= w' && x <= e'
     && y >= s' && y <= n'


-- | Get the coordinate that lies at the center of an extent.
centerCoordOfExtent :: Extent -> (Int, Int)
centerCoordOfExtent (Extent n s e w)
 = 	( w + (e - w) `div` 2
	, s + (n - s) `div` 2)


-- | Cut one quadrant out of an extent.
cutQuadOfExtent :: Quad -> Extent -> Extent
cutQuadOfExtent quad (Extent n s e w)  
 = let	hheight	= (n - s) `div` 2
	hwidth	= (e - w) `div` 2
   in	case quad of
	  	NW -> Extent n (s + hheight)  (e - hwidth) w
		NE -> Extent n (s + hheight)  e (w + hwidth)
		SW -> Extent (n - hheight) s  (e - hwidth) w
		SE -> Extent (n - hheight) s  e (w + hwidth)
	
	
-- | Get the quadrant that this coordinate lies in, if any.
quadOfCoord :: Extent -> Coord -> Maybe Quad
quadOfCoord extent coord
 	= listToMaybe 
	$ filter (\q -> coordInExtent (cutQuadOfExtent q extent) coord)
	$ allQuads

	
-- | Constuct a path to a particular coordinate in an extent.
pathToCoord :: Extent -> Coord -> Maybe [Quad]
pathToCoord extent coord
	| isUnitExtent extent	
	= Just []
	
	| otherwise
	= do	quad	<- quadOfCoord extent coord
		rest	<- pathToCoord (cutQuadOfExtent quad extent) coord
		return	$ quad : rest


-- | If a line segment (P1-P2) intersects the outer edge of an extent then
--   return the intersection point, that is closest to P1, if any.
--   If P1 is inside the extent then `Nothing`.
--
--   @
--                   P2
--                  /
--            ----/-
--            | /  |
--            +    |
--           /------
--         / 
--        P1
--   @
-- 
intersectSegExtent :: Point -> Point -> Extent -> Maybe Point
intersectSegExtent p1@(x1, y1) p2 (Extent n' s' e' w')
	-- starts below extent
	| y1 < s
	, Just pos	<- intersectSegHorzSeg p1 p2 s w e
	= Just pos
	
	-- starts above extent
	| y1 > n
	, Just pos	<- intersectSegHorzSeg p1 p2 n w e
	= Just pos

	-- starts left of extent
	| x1 < w
	, Just pos	<- intersectSegVertSeg p1 p2 w s n
	= Just pos
	
	-- starts right of extent
	| x1 > e
	, Just pos	<- intersectSegVertSeg p1 p2 e s n
	= Just pos

	-- must be starting inside extent.
	| otherwise
	= Nothing
	
	where	n	= fromIntegral n'
		s	= fromIntegral s'
		e	= fromIntegral e'
		w	= fromIntegral w'


-- | Check whether a line segment's endpoints are inside an extent, or if it
--   intersects with the boundary.
touchesSegExtent :: Point -> Point -> Extent -> Bool
touchesSegExtent p1 p2 extent
	=   pointInExtent extent p1
       	 || pointInExtent extent p2
	 || isJust (intersectSegExtent p1 p2 extent)