import pygame


class Particle:
    def __init__(self,x,y):
        self.pos = (x,y)
        self.velocity = (0,0)
        self.accel = (0,0)
        self.surface = None

    def set_surface(self,surface):
        self.surface = surface

    def appear(self,screen):
        if self.surface is not None:
            pygame.draw.circle(screen, BLUE, self.pos, CIRCLE_RADIUS)
            blit_pos = (self.pos[0]-self.surface.get_width()/2, self.pos[1]-self.surface.get_height()/2)
            screen.blit(self.surface, blit_pos)

    def update(self):
        self.velocity = (self.velocity[0]+self.accel[0], self.velocity[1]+self.accel[1])
        self.pos = (self.pos[0] + self.velocity[0], self.pos[1] + self.velocity[1])
        self.accel = (0,0)

    def apply_force(self,force):
        self.accel = (self.accel[0]+force[0],self.accel[1]+force[1])

projectiles = {}
width = 640
height = 600
SIZE = (width, height)
FRICTION = 0.3
WHITE = (255, 255, 255)
RED = (255, 0, 0)
BLUE = (0, 0, 255)
CIRCLE_RADIUS = 2
BLACK = (0, 0, 0)

def init_circle():
    global particle
    particle = Particle(width // 2, height // 2)
    surface = pygame.Surface((CIRCLE_RADIUS * 2, CIRCLE_RADIUS * 2))
    pygame.draw.circle(surface, RED, (CIRCLE_RADIUS, CIRCLE_RADIUS), CIRCLE_RADIUS)
    particle.set_surface(surface.convert_alpha())

def main():
    screen = init_game()
    game_loop(screen)
    exit_game()

def init_game():
    global clock
    clock = pygame.time.Clock()
    pygame.init()
    screen = pygame.display.set_mode(SIZE)
    init_circle()
    return screen

def game_loop(screen):
    while True:
        if not event_handling():
            break
        if not update_game():
            break
        event_handling()
        update_game()
        draw_screen(screen)


def event_handling():
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            return False
    keys_pressed = pygame.key.get_pressed()
    if keys_pressed[pygame.K_UP]:
        particle.apply_force((0,-1))
    if keys_pressed[pygame.K_DOWN]:
        particle.apply_force((0,1))
    if keys_pressed[pygame.K_LEFT]:
        particle.apply_force((-1,0))
    if keys_pressed[pygame.K_RIGHT]:
        particle.apply_force((1,0))
    particle.apply_force((particle.velocity[0]*-FRICTION,particle.velocity[1]*-FRICTION))
    particle.update()
    return True

def update_game():
    clock.tick(60)
    return True

def draw_screen(screen):
    screen.fill(BLACK)
    particle.appear(screen)
    pygame.display.flip()

def exit_game():
    pygame.quit()

main()