Simple_2D_Orbit

#include <allegro5/allegro.h>
#include <allegro5/allegro_primitives.h>
#include <math.h>

// By request on my C++ blog, here is the source for the simple 2D orbit.

const int SCREEN_X = 800;
const int SCREEN_Y = 600;
const float fps = 60.0f;

int main(int argc, char *argv[])
{
    const double grav_const = 6.6742e-11;   // The "G" in the formula
    const double earth_mass = 5.975e24;     // The "m" in the formula
    double radius = 6.37814e6;              // The "r" in the formula
    double angle = 0;
    double grav_accel = 0;
    double body_pos_x = 2.4e6;              // An X axis position of the body to be attracted
    double body_pos_y = 7.5e5;              // A Y axis position of the body to be attracted
    double body_vel_x = 0.0;                // Body velocity split into two components, X and Y
    double body_vel_y = 8.5e3;
    double scale = 10000.0;
    bool loop = true;
    // To get different orbit effects, make small changes to the body positions and velocities.
    // It is not advisable to alter the value of the gravitational constant.
    // The program runs at X 60 speed, as the screen update is 60 FPS, and the formulae are
    // for 1 second update. I have left it as such for clarity, though a time factor could
    // easily be added to the calculations to get real time update.

    // The following is just Allegro 5 initialization stuff.
    // Replace with your own screen output preferences, as required.
    ALLEGRO_DISPLAY *display = NULL;
    ALLEGRO_TIMER *timer = NULL;
    ALLEGRO_EVENT_QUEUE *event_queue = NULL;
    al_init();
    al_init_primitives_addon();
    al_set_new_display_option(ALLEGRO_VSYNC, 1, ALLEGRO_SUGGEST);
    display = al_create_display(SCREEN_X, SCREEN_Y);
    al_set_window_title(display, *argv);
    al_install_mouse();
    timer = al_create_timer(1.0f / fps);
    event_queue = al_create_event_queue();
    al_register_event_source(event_queue, al_get_display_event_source(display));
    al_register_event_source(event_queue, al_get_mouse_event_source());
    al_register_event_source(event_queue, al_get_timer_event_source(timer));
    al_start_timer(timer);
    // Allegro 5 initialization stuff ends here.
    // For practical readability purposes, I have omitted
    // error trapping routines and am taking it on faith
    // that Allegro will initialize properly, though in
    // "real world" applications they should be included.


    while (loop == true)
    {
        ALLEGRO_EVENT event;
        al_wait_for_event(event_queue, &event);

        switch(event.type)
        {
            case ALLEGRO_EVENT_DISPLAY_CLOSE:
                loop = false;
                break;

            case ALLEGRO_EVENT_TIMER:
                // The heart of the matter happens here.
                radius = sqrt(pow(body_pos_x, 2) + pow(body_pos_y, 2));
                grav_accel = (grav_const * (earth_mass / pow(radius, 2)));
                angle = atan2(body_pos_x, body_pos_y);
                body_vel_x += (sin(angle) * grav_accel);
                body_vel_y += (cos(angle) * grav_accel);
                body_pos_x -= body_vel_x;
                body_pos_y -= body_vel_y;
                // And that is all there is to it.

                // Now, draw calculations to screen.
                al_clear_to_color(al_map_rgb(0,0,0));
                al_draw_circle(float(SCREEN_X / 2), float(SCREEN_Y / 2),
                5, al_map_rgb(235,150,50), 1);
                al_draw_circle(float(body_pos_x / scale) + float(SCREEN_X / 2),
                               float(body_pos_y / scale) + float(SCREEN_Y / 2),
                               3, al_map_rgb(50, 150, 200), 1);
                al_flip_display();
                break;

            case ALLEGRO_EVENT_MOUSE_BUTTON_DOWN:
                loop = false;
                break;
        }

    }

    al_stop_timer(timer);
    al_destroy_timer(timer);
    al_destroy_display(display);
    return 0;
}