Segment.hs

module Geometry.Segment
	( Segment
	, translateSegment
	, splitSegmentsOnY
	, splitSegmentsOnX
	, chooseSplitX)
where
import Graphics.Gloss
import Graphics.Gloss.Geometry.Line
import Data.Maybe
import Data.Function
import qualified Data.Vector.Unboxed	as V

-- | A line segement in the 2D plane.
type Segment	= (Int, (Float, Float), (Float, Float))


-- | Translate both endpoints of a segment.
translateSegment :: Float -> Float -> Segment -> Segment
translateSegment tx ty (n, (x1, y1), (x2, y2))
	= (n, (x1 + tx, y1 + ty), (x2 + tx, y2 + ty))
	

-- | Split segments that cross the line y = y0, for some y0.
splitSegmentsOnY :: Float -> V.Vector Segment -> V.Vector Segment
splitSegmentsOnY y0 segs
 = let	
	-- TODO: we only need to know IF the seg crosse the line here,
	--       not the actual intersection point. Do a faster test.
	(segsCross, segsOther)
		= V.unstablePartition 
			(\(_, p1, p2) -> isJust $ intersectSegHorzLine p1 p2 y0)
			segs

	-- TODO: going via lists here is bad.
	splitCrossingSeg :: Segment -> V.Vector Segment
	splitCrossingSeg (n, p1, p2)
	 = let	Just pCross	= intersectSegHorzLine p1 p2 y0
	   in	V.fromList [(n, p1, pCross), (n, pCross, p2)]
	
	-- TODO: vector append requires a copy.	
   in	segsOther V.++ (V.concat $ map splitCrossingSeg $ V.toList segsCross)


-- | Split segments that cross the line x = x0, for some x0.
splitSegmentsOnX :: Float -> V.Vector Segment -> V.Vector Segment
splitSegmentsOnX x0 segs
 = let	
	-- TODO: we only need to know IF the seg crosse the line here,
	--       not the actual intersection point. Do a faster test.
	(segsCross, segsOther)
		= V.unstablePartition 
			(\(_, p1, p2) -> isJust $ intersectSegVertLine p1 p2 x0)
			segs

	-- TODO: going via lists here is bad.
	splitCrossingSeg :: Segment -> V.Vector Segment
	splitCrossingSeg (n, p1, p2)
	 = let	Just pCross	= intersectSegVertLine p1 p2 x0
	   in	V.fromList [(n, p1, pCross), (n, pCross, p2)]
	
	-- TODO: vector append requires a copy.	
   in	segsOther V.++ (V.concat $ map splitCrossingSeg $ V.toList segsCross)


-- | Decide where to split the plane.
--   TODO: We're just taking the first point of the segment in the middle of the vector.
--	It might be better to base the split on:
--	 - the closest segment
--	 - the widest sgement
--	 - the one closes to the middle of the field.
--       - some combination of above.
--
chooseSplitX :: V.Vector Segment -> Float
chooseSplitX segments
 = let	Just (_, (x1, _), _)	= segments V.!? (V.length segments `div` 2)
   in	x1