Fix bug in line zoning by using less ambitious algorithm

src/Geometry/Zone.hs

@@ -1,8 +1,9 @@
 --{-# LANGUAGE TupleSections #-}
 module Geometry.Zone
     ( ddaExt
-    ) 
-    where
+    , ddaExt'
+    , ddaSq
+    ) where
 import Geometry.Data
 
 import Data.Foldable
@@ -77,10 +78,49 @@ sizeZoneOfPoint' s = fmap (divTo s)
 --    | y > x = [x .. y]
 --    | otherwise = [y .. x]
 
+-- | Determines a "square" zone of points for a line
+ddaSq :: Float -> V2 Float -> V2 Float -> IM.IntMap IS.IntSet
+ddaSq s (V2 sx sy) (V2 ex ey) = IM.fromSet (const ys) xs
+  where
+    maxMin a b | a >= b = (a,b)
+        | otherwise = (b,a)
+    (maxx,minx) = maxMin (divTo s sx) (divTo s ex)
+    (maxy,miny) = maxMin (divTo s sy) (divTo s ey)
+    xs = IS.fromDistinctAscList [minx-1..maxx+1]
+    ys = IS.fromDistinctAscList [miny-1..maxy+1]
+
 -- | Determines which horizontal and vertical lines on a grid are crossed by a
 -- line. For each adds the x-y index of the square to the right or above the
 -- crossed grid line. Also adds the index of the square containing the start
 -- point.
+ddaExt' :: Float -> V2 Float -> V2 Float -> IM.IntMap IS.IntSet
+ddaExt' s sp@(V2 sx sy) ep@(V2 ex ey) 
+    | x1 <= x2 = addsp . addys . IM.fromDistinctAscList $ zip [x1 .. x2] 
+                    $ map (IS.singleton . divTo s) [x1y,x1y+ydx..]
+    | otherwise = ddaExt' s ep sp
+  where
+    addsp im = let V2 x y = sizeZoneOfPoint' s sp
+        in insertXY im (x,y)
+    x1 = divTo s sx
+    x2 = divTo s ex
+    x1y = fx' sp ep $ s * fromIntegral x1
+    ydx = s * ydx' sp ep
+    addys m = add2s m ypairs 
+    y1 = divTo s sy
+    y2 = divTo s ey
+    y1x = fy' sp ep $ s * fromIntegral y1
+    y2x = fy' sp ep $ s * fromIntegral y2
+    xdy = s * xdy' sp ep
+    ypairs 
+        | y1 <= y2  = zip (map (divTo s) [y1x,y1x+xdy..])
+                         [y1 .. y2] 
+        | otherwise = zip (map (divTo s) [y2x,y2x+xdy..])
+                         [y2-1 .. y1-1] 
+-- | Determines which horizontal and vertical lines on a grid are crossed by a
+-- line. For each adds the x-y index of the square to the right or above the
+-- crossed grid line. Also adds the index of the square containing the start
+-- point.
+-- Not correct, eg ddaExt 10 (V2 40 50) (V2 0 0)
 ddaExt :: Float -> V2 Float -> V2 Float -> IM.IntMap IS.IntSet
 ddaExt s sp@(V2 sx sy) ep@(V2 ex ey) 
     | x1 <= x2 = addsp . addys . IM.fromDistinctAscList $ zip [x1 .. x2] 
@@ -89,7 +129,7 @@ ddaExt s sp@(V2 sx sy) ep@(V2 ex ey)
                     $ map (IS.singleton . divTo s) [x2y,x2y+ydx..]
   where
     addsp im = let V2 x y = sizeZoneOfPoint' s sp
-            in insertXY im (x,y)
+        in insertXY im (x,y)
     x1 = divTo s sx
     x2 = divTo s ex
     x1y = fx' sp ep $ s * fromIntegral x1