import pygame
import random
import math

background_colour = (255,255,255)
(width, height) = (400, 400)
mass_of_air = 0.2
elasticity = 0.75
gravity = (math.pi, 0.002)

def addVectors((angle1, length1), (angle2, length2)):
    x  = math.sin(angle1) * length1 + math.sin(angle2) * length2
    y  = math.cos(angle1) * length1 + math.cos(angle2) * length2
    
    angle = 0.5 * math.pi - math.atan2(y, x)
    length  = math.hypot(x, y)

    return (angle, length)

def findParticle(particles, x, y):
    for p in particles:
        if math.hypot(p.x-x, p.y-y) <= p.size:
            return p
    return None

def collide(p1, p2):
    dx = p1.x - p2.x
    dy = p1.y - p2.y
    
    dist = math.hypot(dx, dy)
    if dist < p1.size + p2.size:
        angle = math.atan2(dy, dx) + 0.5 * math.pi
        total_mass = p1.mass + p2.mass

        (p1.angle, p1.speed) = addVectors((p1.angle, p1.speed*(p1.mass-p2.mass)/total_mass), (angle, 2*p2.speed*p2.mass/total_mass))
        (p2.angle, p2.speed) = addVectors((p2.angle, p2.speed*(p2.mass-p1.mass)/total_mass), (angle+math.pi, 2*p1.speed*p1.mass/total_mass))
	p1.speed *= elasticity
	p2.speed *= elasticity

        overlap = 0.5*(p1.size + p2.size - dist+1)
        p1.x += math.sin(angle)*overlap
        p1.y -= math.cos(angle)*overlap
        p2.x -= math.sin(angle)*overlap
        p2.y += math.cos(angle)*overlap

class Particle():
    def __init__(self, (x, y), size, mass=1):
        self.x = x
        self.y = y
        self.size = size
        self.colour = (0, 0, 255)
        self.thickness = 0
        self.speed = 0
        self.angle = 0
        self.mass = mass
        self.drag = (self.mass/(self.mass + mass_of_air)) ** self.size

    def display(self):
        pygame.draw.circle(screen, self.colour, (int(self.x), int(self.y)), self.size, self.thickness)

    def move(self):
        #(self.angle, self.speed) = addVectors((self.angle, self.speed), gravity)
        self.x += math.sin(self.angle) * self.speed
        self.y -= math.cos(self.angle) * self.speed
        self.speed *= self.drag

    def bounce(self):
        if self.x > width - self.size:
            self.x = 2*(width - self.size) - self.x
            self.angle = - self.angle
            self.speed *= elasticity

        elif self.x < self.size:
            self.x = 2*self.size - self.x
            self.angle = - self.angle
            self.speed *= elasticity

        if self.y > height - self.size:
            self.y = 2*(height - self.size) - self.y
            self.angle = math.pi - self.angle
            self.speed *= elasticity

        elif self.y < self.size:
            self.y = 2*self.size - self.y
            self.angle = math.pi - self.angle
            self.speed *= elasticity

screen = pygame.display.set_mode((width, height))
pygame.display.set_caption('Tutorial 9')

number_of_particles = 5
my_particles = []

for n in range(number_of_particles):
    size = random.randint(10, 20)
    density = random.randint(1, 20)
    x = random.randint(size, width-size)
    y = random.randint(size, height-size)

    particle = Particle((x, y), size, density*size**2)
    particle.colour = (200-density*10, 200-density*10, 255)
    particle.speed = random.random()
    particle.angle = random.uniform(0, math.pi*2)

    my_particles.append(particle)

selected_particle = None
running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == pygame.MOUSEBUTTONDOWN:
            (mouseX, mouseY) = pygame.mouse.get_pos()
            selected_particle = findParticle(my_particles, mouseX, mouseY)
        elif event.type == pygame.MOUSEBUTTONUP:
            selected_particle = None

    if selected_particle:
        (mouseX, mouseY) = pygame.mouse.get_pos()
        dx = mouseX - selected_particle.x
        dy = mouseY - selected_particle.y
        selected_particle.angle = 0.5*math.pi + math.atan2(dy, dx)
        selected_particle.speed = math.hypot(dx, dy) * 0.1

    screen.fill(background_colour)

    for i, particle in enumerate(my_particles):
        particle.move()
        particle.bounce()
        for particle2 in my_particles[i+1:]:
            collide(particle, particle2)
        particle.display()

    pygame.display.flip()