Drop Rasterific for non-filled circles

Rasterific appears to generate some pretty surprising, if  not
completely wrong, circles at especially low sizes - this was resulting
in unexpected behavior with vision calculation, including the character
never being able to see directly to the left of them, among other
things. This moves back to the old midpoint circle algorithm I pulled
off of rosetta code, but only for the non-filled circle. The filled
circle is still using the wonky algorithm for now, but at some point I'd
love to refactor it such that empty circles are eg always a subset of
non-filled circles.

src/Xanthous/Util/Graphics.hs

@@ -30,16 +30,45 @@ import           Linear.V2
 --------------------------------------------------------------------------------
 
 
-circle :: (Num i, Integral i, Ix i)
+-- | Generate a circle centered at the given point and with the given radius
+-- using the <midpoint circle algorithm
+-- https://en.wikipedia.org/wiki/Midpoint_circle_algorithm>.
+--
+-- Code taken from <https://rosettacode.org/wiki/Bitmap/Midpoint_circle_algorithm#Haskell>
+circle :: (Num i, Ord i)
        => (i, i) -- ^ center
        -> i      -- ^ radius
        -> [(i, i)]
-circle (ox, oy) radius
-  = pointsFromRaster (ox + radius) (oy + radius)
-  $ stroke 1 JoinRound (CapRound, CapRound)
-  $ Raster.circle (Raster.V2 (fromIntegral ox) (fromIntegral oy))
-  $ fromIntegral radius
+circle (x₀, y₀) radius
+  -- Four initial points, plus the generated points
+  = (x₀, y₀ + radius) : (x₀, y₀ - radius) : (x₀ + radius, y₀) : (x₀ - radius, y₀) : points
+    where
+      -- Creates the (x, y) octet offsets, then maps them to absolute points in all octets.
+      points = concatMap generatePoints $ unfoldr step initialValues
 
+      generatePoints (x, y)
+        = [ (x₀ `xop` x', y₀ `yop` y')
+          | (x', y') <- [(x, y), (y, x)]
+          , xop <- [(+), (-)]
+          , yop <- [(+), (-)]
+          ]
+
+      initialValues = (1 - radius, 1, (-2) * radius, 0, radius)
+
+      step (f, ddf_x, ddf_y, x, y)
+        | x >= y = Nothing
+        | otherwise = Just ((x', y'), (f', ddf_x', ddf_y', x', y'))
+        where
+          (f', ddf_y', y') | f >= 0 = (f + ddf_y' + ddf_x', ddf_y + 2, y - 1)
+                           | otherwise = (f + ddf_x, ddf_y, y)
+          ddf_x' = ddf_x + 2
+          x' = x + 1
+
+
+-- | Generate a *filled* circle centered at the given point and with the given
+-- radius using the Rasterific package. Note that since this uses a different
+-- implementation, this is not a strict superset of the 'circle' function
+-- (unfortunately - would like to make that not the case!)
 filledCircle :: (Num i, Integral i, Ix i)
              => (i, i) -- ^ center
              -> i      -- ^ radius
@@ -72,8 +101,6 @@ pointsFromRaster w h raster
            $ renderDrawing @Word8 (fromIntegral w) (fromIntegral h) 0
            $ withTexture (uniformTexture 1) raster
 
-
-
 -- | Draw a line between two points using Bresenham's line drawing algorithm
 --
 -- Code taken from <https://wiki.haskell.org/Bresenham%27s_line_drawing_algorithm>