OpenGL program for Pendulum

// tree.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include
#include //Needed for "exit" function
#include "math.h"

//Include OpenGL header files, so that we can use OpenGL
#ifdef __APPLE__
#include
#include
#else
#include "glut.h"
#endif

using namespace std;


float rot = 0;
float angle = 135;
float inc = 1.0;

void drawCircle(float segments, float radius, float sx, float sy)
{
float theta = 2 * 3.1415926 / segments;
float tan_factor = tanf(theta);
float radial_factor = cosf(theta);
float x = radius;
float y = 0;

int cache_pt = 0;
double cache_x;
double cache_y;

glBegin(GL_LINES);
for (int ii = 0; ii < segments; ii++) {
if(!cache_pt) {
cache_x = x+sx;
cache_y = y+sy;
cache_pt = 1;
} else {
//glVertex2f(cache_x,cache_y);
glVertex2f(x + sx, y + sy);
cache_x = x+sx;
cache_y = y+sy;
}
float tx = -y;
float ty = x;
x += tx * tan_factor;
y += ty * tan_factor;
x *= radial_factor;
y *= radial_factor;
}
glEnd();
}


void drawCircles(float x1,float y1,double angle) {
float segments = 25;
float radius = 1.0;
// Draw the original circle
drawCircle(segments,radius,x1,y1);

// Draw the pendulum circle
double radian = angle*3.14/180;
float m = (double)tan((double)radian);
float y2 = 10 * cos((double) radian);
float x2 = 10 * sin((double) radian);
drawCircle(segments,radius,x2,y2);

glBegin(GL_LINES);
glVertex2f(x1,y1);
glVertex2f(x2,y2);
glEnd();

}

void display(void) {
glClearColor (0.0,0.0,0.0,1.0);
glClear (GL_COLOR_BUFFER_BIT);
glLoadIdentity();
glTranslatef(-10,0,-30);
glColor3f(1,1,1);

if (angle>225) {
angle = 225;
inc = -inc;
}
if (angle<135) {
angle = 135;
inc = -inc;
}
angle += inc;
drawCircles(0,0,angle);
glutSwapBuffers();

}

void reshape (int w, int h) {
glViewport (0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective (60, (GLfloat)w / (GLfloat)h, 0.1, 100.0);
glMatrixMode (GL_MODELVIEW);
}

void keyboard(unsigned char key, int x, int y)
{
switch (key) {
case 27:
exit(0);
break;
}
}

int main (int argc, char **argv) {
glutInit (&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE);
glutInitWindowSize (1200, 800);
glutInitWindowPosition (0, 0);
glutCreateWindow ("A Pendulum");

glutDisplayFunc (display);
glutIdleFunc (display);
glutReshapeFunc (reshape);
glutKeyboardFunc (keyboard);
glutMainLoop ();
return 0;
}