cga/Algorithms/PolygonIntersection/Core.hs
2014-10-25 20:39:34 +02:00

167 lines
5.7 KiB
Haskell

module Algorithms.PolygonIntersection.Core where
import Algebra.Vector
import Algebra.VectorTypes
import Control.Applicative
import Data.Dequeue (BankersDequeue)
import qualified Data.Dequeue as Q
import Data.List
import Data.Maybe
import Diagrams.TwoD.Types
import MyPrelude
import QueueEx
import Safe
-- |Describes a point on the convex hull of the polygon.
-- In addition to the point itself, both it's predecessor and
-- successor are saved for convenience.
data PolyPT =
PolyA {
id' :: PT
, pre :: PT
, suc :: PT
}
| PolyB {
id' :: PT
, pre :: PT
, suc :: PT
}
deriving (Show, Eq)
isPolyA :: PolyPT -> Bool
isPolyA PolyA {} = True
isPolyA _ = False
isPolyB :: PolyPT -> Bool
isPolyB = not . isPolyA
-- |Shift a list of sorted convex hull points of a polygon so that
-- the first element in the list is the one with the highest y-coordinate.
-- This is done in O(n).
sortLexPoly :: [PT] -> [PT]
sortLexPoly ps = maybe [] (`shiftM` ps) (elemIndex (yMax ps) ps)
where
yMax = foldl1 (\x y -> if ptCmpY x y == GT then x else y)
-- |Sort the points of two polygons according to their y-coordinates,
-- while saving the origin of that point. This is done in O(n).
sortLexPolys :: ([PT], [PT]) -> [PolyPT]
sortLexPolys (pA'@(_:_), pB'@(_:_)) =
queueToList $ go (Q.fromList . sortLexPoly $ pA')
(Q.fromList . sortLexPoly $ pB')
where
-- Start recursive algorithm, each polygon is represented by a Queue.
-- Traverse predecessor and successor and insert them in the right
-- order into the resulting queue.
-- We start at the max y-coordinates of both polygons.
go :: BankersDequeue PT -> BankersDequeue PT -> BankersDequeue PolyPT
go pA pB
-- Nothing to sort.
| Q.null pA && Q.null pB = Q.empty
-- Current point of polygon A is higher on the y-axis than the
-- current point of polygon B, so insert it into the resulting
-- queue and traverse the rest.
| ptCmpY (fromMaybe negInfPT . Q.first $ pA)
(fromMaybe posInfPT . Q.first $ pB) == GT
= Q.pushFront (go (maybeShift . snd . Q.popFront $ pA) pB)
(mkPolyPT PolyA pA' pA)
-- Same as above, except that the current point of polygon B
-- is higher.
| otherwise = Q.pushFront (go pA (maybeShift . snd . Q.popFront $ pB))
(mkPolyPT PolyB pB' pB)
mkPolyPT f xs qs = f (fromJust . Q.first $ qs)
(getPT' polySuccessor xs qs)
(getPT' polyPredecessor xs qs)
where
getPT' f' xs' = fromJust . f' xs' . uQfirst
-- Compare the first and the last element of the queue according
-- to their y-coordinate and shift the queue (if necessary) so that
-- the element with the highest value is at the front.
maybeShift :: BankersDequeue PT -> BankersDequeue PT
maybeShift q = if ptCmpY (fromMaybe posInfPT . Q.first $ q)
(fromMaybe negInfPT . Q.last $ q) == GT
then q
else shiftQueueRight q
sortLexPolys _ = []
-- |Get the successor of a point on a convex hull of a polygon.
-- Returns Nothing if the point is not on the convex hull. This
-- is done in O(n).
polySuccessor :: [PT] -> PT -> Maybe PT
polySuccessor pts = polyPreSucInternal (length pts - 1, 0, 1) pts
-- |Get the predecessor of a point on a convex hull of a polygon.
-- Returns Nothing if the point is not on the convex hull. This
-- is done in O(n).
polyPredecessor :: [PT] -> PT -> Maybe PT
polyPredecessor pts = polyPreSucInternal (0, length pts - 1, negate 1) pts
-- |Abstraction for polyPredecessor and polySuccessor.
polyPreSucInternal :: (Int, Int, Int) -> [PT] -> PT -> Maybe PT
polyPreSucInternal (i1, i2, i3) pts pt = case index of
Nothing -> Nothing
Just index' -> if index' == i1
then pts `atMay` i2
else pts `atMay` (index' + i3)
where
index = elemIndex pt pts
-- |Get all points that intersect between both polygons. This is done
-- in O(n).
intersectionPoints :: [PolyPT] -> [PT]
intersectionPoints [] = []
intersectionPoints xs' =
rmdups $
(++) (segIntersections . scanLine $ xs')
(intersectionPoints (tail xs'))
where
-- Get the scan line or in other words the
-- Segment pairs we are going to check for intersection.
scanLine :: [PolyPT] -> ([Segment], [Segment])
scanLine sp@(_:_) = (,) (getSegment isPolyA) (getSegment isPolyB)
where
getSegment f = fromMaybe []
((\x -> [(id' x, suc x), (id' x, pre x)])
<$> (listToMaybe . filter f $ sp))
scanLine _ = ([], [])
-- Gets the actual intersections between the segments of
-- both polygons we currently examine. This is done in O(1)
-- since we have max 4 segments.
segIntersections :: ([Segment], [Segment]) -> [PT]
segIntersections (a@(_:_), b@(_:_)) =
catMaybes
. fmap (\[x, y] -> intersectSeg' x y)
$ combinations a b
segIntersections _ = []
-- Gets all unique(!) combinations of two arrays. Both arrays
-- are max 2, so this is actually O(1) for this algorithm.
combinations :: [a] -> [a] -> [[a]]
combinations xs ys = concat . fmap (\y -> fmap (\x -> [y, x]) xs) $ ys
testArr :: ([PT], [PT])
testArr = ([p2 (200.0, 500.0),
p2 (0.0, 200.0),
p2 (200.0, 100.0),
p2 (400.0, 300.0)],
[p2 (350.0, 450.0),
p2 (275.0, 225.0),
p2 (350.0, 50.0),
p2 (500.0, 0.0),
p2 (450.0, 400.0)])