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Task/Animate-a-pendulum/Python/animate-a-pendulum.py

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+import pygame, sys
+from pygame.locals import *
+from math import sin, cos, radians
+
+pygame.init()
+
+WINDOWSIZE = 250
+TIMETICK = 100
+BOBSIZE = 15
+
+window = pygame.display.set_mode((WINDOWSIZE, WINDOWSIZE))
+pygame.display.set_caption("Pendulum")
+
+screen = pygame.display.get_surface()
+screen.fill((255,255,255))
+
+PIVOT = (WINDOWSIZE/2, WINDOWSIZE/10)
+SWINGLENGTH = PIVOT[1]*4
+
+class BobMass(pygame.sprite.Sprite):
+    def __init__(self):
+        pygame.sprite.Sprite.__init__(self)
+        self.theta = 45
+        self.dtheta = 0
+        self.rect = pygame.Rect(PIVOT[0]-SWINGLENGTH*cos(radians(self.theta)),
+                                PIVOT[1]+SWINGLENGTH*sin(radians(self.theta)),
+                                1,1)
+        self.draw()
+
+    def recomputeAngle(self):
+        scaling = 3000.0/(SWINGLENGTH**2)
+
+        firstDDtheta = -sin(radians(self.theta))*scaling
+        midDtheta = self.dtheta + firstDDtheta
+        midtheta = self.theta + (self.dtheta + midDtheta)/2.0
+
+        midDDtheta = -sin(radians(midtheta))*scaling
+        midDtheta = self.dtheta + (firstDDtheta + midDDtheta)/2
+        midtheta = self.theta + (self.dtheta + midDtheta)/2
+
+        midDDtheta = -sin(radians(midtheta)) * scaling
+        lastDtheta = midDtheta + midDDtheta
+        lasttheta = midtheta + (midDtheta + lastDtheta)/2.0
+
+        lastDDtheta = -sin(radians(lasttheta)) * scaling
+        lastDtheta = midDtheta + (midDDtheta + lastDDtheta)/2.0
+        lasttheta = midtheta + (midDtheta + lastDtheta)/2.0
+
+        self.dtheta = lastDtheta
+        self.theta = lasttheta
+        self.rect = pygame.Rect(PIVOT[0]-
+                                SWINGLENGTH*sin(radians(self.theta)),
+                                PIVOT[1]+
+                                SWINGLENGTH*cos(radians(self.theta)),1,1)
+
+
+    def draw(self):
+        pygame.draw.circle(screen, (0,0,0), PIVOT, 5, 0)
+        pygame.draw.circle(screen, (0,0,0), self.rect.center, BOBSIZE, 0)
+        pygame.draw.aaline(screen, (0,0,0), PIVOT, self.rect.center)
+        pygame.draw.line(screen, (0,0,0), (0, PIVOT[1]), (WINDOWSIZE, PIVOT[1]))
+
+    def update(self):
+        self.recomputeAngle()
+        screen.fill((255,255,255))
+        self.draw()
+
+bob = BobMass()
+
+TICK = USEREVENT + 2
+pygame.time.set_timer(TICK, TIMETICK)
+
+def input(events):
+    for event in events:
+        if event.type == QUIT:
+            sys.exit(0)
+        elif event.type == TICK:
+            bob.update()
+
+while True:
+    input(pygame.event.get())
+    pygame.display.flip()