Initial data commit

Task/Bitmap-Bresenhams-line-algorithm/Haskell/bitmap-bresenhams-line-algorithm.hs

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+module Bitmap.Line(line) where
+
+import Bitmap
+import Control.Monad
+import Control.Monad.ST
+import qualified Data.STRef
+
+var = Data.STRef.newSTRef
+get = Data.STRef.readSTRef
+mutate = Data.STRef.modifySTRef
+
+line :: Color c => Image s c -> Pixel -> Pixel -> c -> ST s ()
+line i (Pixel (xa, ya)) (Pixel (xb, yb)) c = do
+    yV <- var y1
+    errorV <- var $ deltax `div` 2
+    forM_ [x1 .. x2] (\x -> do
+        y <- get yV
+        setPix i (Pixel $ if steep then (y, x) else (x, y)) c
+        mutate errorV $ subtract deltay
+        error <- get errorV
+        when (error < 0) (do
+            mutate yV (+ ystep)
+            mutate errorV (+ deltax)))
+  where steep = abs (yb - ya) > abs (xb - xa)
+        (xa', ya', xb', yb') = if steep
+          then (ya, xa, yb, xb)
+          else (xa, ya, xb, yb)
+        (x1, y1, x2, y2) = if xa' > xb'
+          then (xb', yb', xa', ya')
+          else (xa', ya', xb', yb')
+        deltax = x2 - x1
+        deltay = abs $ y2 - y1
+        ystep = if y1 < y2 then 1 else -1