Floodfill algorithm in Python

"ctx = canvas.getContext("2d") URL = "/blog/floodfill-algorithm-in-python/_python.txt" async def load_bitmap(url: str) -> list[list[int]]: # Fetch the text file from the URL response = await fetch(url) text = await response.text() bitmap: list[list[int]] = [] for line in text.splitlines(): line = line.strip() if not line: continue row = [int(ch) for ch in line if ch in "01"] if row: bitmap.append(row) return bitmap"

"def draw_bitmap(bitmap): rows = len(bitmap) cols = len(bitmap[0]) if rows > 0 else 0 if rows == 0 or cols == 0: return for y, row in enumerate(bitmap): for x, value in enumerate(row): if value == 1: ctx.fillStyle = "black" else: ctx.fillStyle = "white" ctx.fillRect(x * PIXEL_SIZE, y * PIXEL_SIZE, PIXEL_SIZE, PIXEL_SIZE) _neighbours = [(1, 0), (-1, 0), (0, 1), (0, -1)] async def fill_bitmap(bitmap, x, y): if bitmap[y][x] == 1: return ctx = canvas.getContext("2d") r, g, b = (random.randint(0, 255) for _ in range(3)) ctx.fillStyle = f"rgb({r}, {g}, {b})" def draw_pixel(x, y): ctx.fillRect(x * PIXEL_SIZE, y * PIXEL_SIZE, PIXEL_SIZE, PIXEL_SIZE) pixels = collections.deque([(x, y)]) seen = set((x, y)) while pixels: nx, ny = pixels.pop() draw_pixel(nx, ny) for dx, dy in _neighbours: x_, y_ = nx + dx, ny + dy if x_ < 0 or x_ >= IMG_WIDTH or y_ < 0 or y_ >= IMG_HEIGHT or (x_, y_) in seen: continue if bitmap[y_][x_] == 0: seen.add((x_, y_)) pixels.appendleft((x_, y_)) await asyncio.sleep(0.0001) is_running = False def get_event_coords(event): """Return (clientX, clientY) for mouse/pointer/touch events."""}], "

Bitmap data is loaded from a text source and parsed into a 2D integer grid of 0/1 values. The drawing routine maps 0 to white and 1 to black and renders each pixel scaled by PIXEL_SIZE. The flood fill routine uses an explicit frontier (collections.deque) and a seen set to traverse four-connected neighbours, performing boundary checks and skipping already-seen or out-of-bounds coordinates. Each newly visited pixel is colored with a random RGB value and drawn. An asynchronous sleep step yields visual animation and prevents blocking the event loop during traversal.

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