// the simplest opengl example

#include <GL/glut.h>
#include <math.h>
// compile:
// gcc -Wall -Wextra -Werror -lglut -lGL -lm -std=gnu99 -D_GNU_SOURCE opengl.c -o openglcircle

#define _U_    __attribute__((__unused__))
// exit by ctrl+q or escape
void keyPressed(unsigned char key, _U_ int x, _U_ int y){
	int mod = glutGetModifiers(); // window = glutGetWindow()
	if(mod == GLUT_ACTIVE_CTRL && key == 'q') exit(0);
	else if(key == 27) exit(0);
}

void DrawCircle(float cx, float cy, float r, int num_segments){
	float theta = 2. * M_PI / (float)num_segments;
	float c, s, t, x = r, y = 0.;
	sincosf(theta, &s, &c);
	glBegin(GL_LINE_LOOP);
	for(int ii = 0; ii < num_segments; ii++){
		glVertex2f(x + cx, y + cy);
		t = x;
		x = c * x - s * y;
		y = s * t + c * y;
	}
	glEnd();
}

void display(){
	glClear(GL_COLOR_BUFFER_BIT);
	DrawCircle(0.,0.,0.5,100);
	glFlush();
}

int main(int argc, char** argv){
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
	glutInitWindowSize(400, 400);
	glutCreateWindow("A Simple Circle");
	glutDisplayFunc(display);
	glutKeyboardFunc(keyPressed);
	glutMainLoop();
}