use the proprer line rendering algorithm

The proper Bresenham setup used by the fx-92 SC+.

drawing.py

@@ -76,11 +76,40 @@ class Window:
         SDL_SetRenderDrawColor(self.r, *color)
         SDL_RenderClear(self.r)
 
-    def line(self, x1, x2, y1, y2, color):
+    def line(self, x1, y1, x2, y2, color):
         """Draw a straight line."""
 
         SDL_SetRenderDrawColor(self.r, *color)
-        SDL_RenderDrawLine(self.r, x1, x2, y1, y2)
+
+        # Bresenham line drawing algorithm
+        sgn = lambda x: -1 if x < 0 else (1 if x > 0 else 0)
+
+        x, y = x1, y1
+        dx, dy = x2 - x1, y2 - y1
+        sx, sy = sgn(dx), sgn(dy)
+        dx, dy = abs(dx), abs(dy)
+        cumul = 0
+
+        SDL_RenderDrawPoint(self.r, x, y)
+
+        if dx >= dy:
+            cumul = dx // 2
+            for i in range(dx):
+                x += sx
+                cumul += dy
+                if cumul > dx:
+                    cumul -= dx
+                    y += sy
+                SDL_RenderDrawPoint(self.r, x, y)
+        else:
+            cumul = dy // 2
+            for i in range(dy):
+                y += sy
+                cumul += dx
+                if cumul > dy:
+                    cumul -= dy
+                    x += sx
+                SDL_RenderDrawPoint(self.r, x, y)
 
     def wait(self):
         """Wait for the window to be closed."""