from numarray import *

class Ball:

	def __init__(self):
		self.pos = array([0.0, 1.0])
		self.vel = array([1.0, 0.0])
		self.acc = array([0.0, 0.0])
		self.forc = array([0.0, 0.0])
		self.mass = 1.0;

class Simulation:

	gravity = array([0.0, -9.8])
	drag = 0.03;
	elastic = 0.8

	# ~letter size paper
	scale = array([ 72 * 8.25, 72 * 10])

	def __init__(self):
		self.ball = Ball()
		pass

	def step(self, dt):
		ball = self.ball

		#print ball.pos[1]

		# calculate forces
		ball.forc = (-self.drag * ball.vel) + (ball.mass * self.gravity);

		#print ball.forc

		# calculate acceleration
		ball.acc = ball.forc / ball.mass;

		# integrate
		vel = ball.vel + ball.acc * dt;
		pos = ball.pos + ball.vel * dt;

		# check walls
		if (pos[1] < 0) | (pos[1] > 1):
			vel[1] = -ball.vel[1] * self.elastic
			pos[1] = ball.pos[1] + 0

		if (pos[0] < 0) | (pos[0] > 1):
			vel[0] = -ball.vel[0] * self.elastic
			pos[0] = ball.pos[0] + 0


		ball.vel = vel
		ball.pos = pos

		# scale to paper
		pos = pos * self.scale

		# draw circle
		print "%0.2f %0.2f %0.2f 0 360 arc" % (pos[0], pos[1], 6*abs(ball.vel[1])+8)
		print "stroke"

	def start(self, final, dt):
		pos = self.ball.pos * self.scale
		print "%d %d moveto" % (pos[0], pos[1])
		for i in range(0,int(final/dt)):
			self.step(dt*i)
		print 'stroke'


savedout = sys.stdout;

def main():
	sys.stdout = open('ball.ps', 'w')
	init()
	sim = Simulation()
	sim.start(0.0115,0.00001)
	fin()

def init():
	pass
	print  """
	%!PS-2.0
	/inch {72 mul} def
	0.3 setlinewidth
	0.85 0.5 0.2 setrgbcolor
	"""

def fin():
	print """
	showpage
	"""
	sys.stdout = savedout

if __name__ == "__main__": main()