import pygame


def get_empty_cells(grid):
    """
    Returns a list of (x, y) tuples for cells in grid that are empty (value 0).
    """
    empty = []
    for y, row in enumerate(grid):
        for x, cell in enumerate(row):
            if cell == 0:
                empty.append((x, y))
    return empty


def get_cell_center(x, y, cell_size=80):
    """
    Converts cell coordinates (x, y) to pixel coordinates of the cell center.
    """
    return pygame.Vector2(x * cell_size + cell_size / 2, y * cell_size + cell_size / 2)


def draw_arrow(surface, color, start, end, width=3):
    pygame.draw.line(surface, color, start, end, width)
    direction = (end - start).normalize()
    arrow_size = 10
    perpendicular = pygame.Vector2(-direction.y, direction.x)
    point1 = end - direction * arrow_size + perpendicular * arrow_size / 2
    point2 = end - direction * arrow_size - perpendicular * arrow_size / 2
    pygame.draw.polygon(surface, color, [end, point1, point2])