patrl · January 17, 2023 16:17 · Jan 17, 2023
diff --git a/shadowcast.hs b/shadowcast.hs
@@ -0,0 +1,43 @@
+computeQ :: TileMap -> Transform -> Slope -> Slope -> Int -> Visible ()
+computeQ m t s e y = do
+    let xmin = roundTiesUp (fromIntegral y * s) -- compute the initial x coordinate bounded by the starting slope
+    let xmax = roundTiesDown (fromIntegral y * e) -- compute the ending x coordinate bounded by the ending slope
+    let xys = (coord . (,y)) <$> [xmin..xmax] -- create a list of coordinates to recurse through by pairing xs with the depth
+
+    -- a helper function to recurse through a list of coordinates, and potentially create branching row computations
+    -- the first argument is a Maybe Bool which tracks whether the previous coordinate was a wall
+    -- Nothing indicates that there was no previous coordinate (we're at the beginning of the list)
+    -- s indicates the starting slope; this can change as the recursion proceeds.
+    -- xys indicates the list of coordinates
+    go Nothing s xys where
+
+      -----------------------------------------
+      -- definition of row recursion follows --
+      -----------------------------------------
+
+      -- if the last tile in the row is visible, then continue onto the next row, otherwise break the recursion 
+      go final s' [] = unless (final == Just True) $ do
+        computeQ m t s' e (y + 1)
+
+      -- case 1: no previous tile.
+      -- this indicates that we're at the beginning of a row recursion
+      go Nothing s' (c:cs) = do
+        let w = isWall m (t c) -- w is a boolean which indicates whether the current focus is a wall (and therefore invisible)
+        when (w || isSymmetric s' e c) $ do -- when the current tile is a wall or symmetric...
+          addToVis (t c) -- ...add the tranformed coordinate it to the visible coordinates
+        go (Just w) s' cs -- now, recurse through the remainder of the wall, setting the Maybe Bool to depend on whether the focus is a wall
+
+      -- case 2: there is a previous tile, whether it was a wall or not is indicated by w'
+      go (Just w') s' (c:cs) = do 
+        let w = isWall m (t c) -- let w indicate the wallhood os te current focus
+        when (w || isSymmetric s' e c) $ do -- when the current tile is a wall or symmetric...
+          addToVis (t c) -- ...add the transformed coordinate to the visible coordinates
+        if -- the next action depends on wallhood of the current and previous tiles
+          | (w' && not w) -> -- if the previous tile is a wall, and the current tile isn't a wall...
+              go (Just w) (slope c) cs -- continue the recursion through the row, but adjust the start slope to brush the prev tile
+          | (not w' && w) -> do -- if the previous tile isn't a wall, but the current tile is a wall
+              computeQ m t s' (slope c) (y + 1) -- start a new branching computation on the next row, with an adjusted end slope
+              go (Just w) s' cs -- continue recursing through the row
+          | otherwise -> 
+              go (Just w) s' cs -- in other circumstances, continue recursing through the row
+
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