clock

#include <gl/glut.h>
#include <math.h>

struct Point {
	GLint x;
	GLint y;
};

void draw_needle(Point p1, Point p2) {
	GLfloat dx = p2.x - p1.x;
	GLfloat dy = p2.y - p1.y;

	GLfloat x1 = p1.x;
	GLfloat y1 = p1.y;

	GLfloat step = 0;

	if(abs(dx) > abs(dy)) {
		step = abs(dx);
	} else {
		step = abs(dy);
	}

	GLfloat xInc = dx/step;
	GLfloat yInc = dy/step;

	for(float i = 1; i <= step; i++) {
		glVertex2f(x1, y1);
		x1 += xInc;
		y1 += yInc;
	}
}

void draw_circle(Point circle, GLfloat radius) {
	GLfloat step = 1/radius;
	GLfloat x, y;

	for(GLfloat theta = 0; theta <= 360; theta += step) {
		x = circle.x + (radius * cos(theta));
		y = circle.y + (radius * sin(theta));
		glVertex2f(x, y);
	}
}

Point circle = {320, 250};
GLint radius = 150;

GLint nRadius = 140;
double nDegree = 0;

void display(void) {
	Point needle;

	needle.y = circle.y + (nRadius * sin(nDegree));
	needle.x = circle.x + (nRadius * cos(nDegree));

	glClear(GL_COLOR_BUFFER_BIT);
	glBegin(GL_POINTS);
	glColor3f(1.0, 1.0, 1.0);
	draw_circle(circle, radius);

	glColor3f(1.0, 1.0, 1.0);
	draw_needle(circle, needle);
	glEnd();
	glFlush();

	nDegree -= 0.004;
}

void Timer(int value) {
	glutTimerFunc(33, Timer, 0);
	glutPostRedisplay();
}

void glutInit() {
	glClearColor(0.0, 0.0, 0.0, 1.0);
    gluOrtho2D(0.0f, 640.0f, 0.0f, 480.0f);
}

int main(int argc, char **argv) {
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
	glutInitWindowPosition(150, 150);
	glutInitWindowSize(640, 480);
	glutCreateWindow("Clock");
	glutDisplayFunc(display);
	glutInit();
	Timer(0);
	glutMainLoop();

	return 0;
}