/*
 * square1.c
 * this program attempts to draw a square in a loop
 */

/* the width and height of the screen */
#define WIDTH 240
#define HEIGHT 160

/* these identifiers define different bit positions of the display control */
#define MODE3 0x0003
#define BG2 0x0400

/* the screen is simply a pointer into memory at a specific address this
 *  * pointer points to 16-bit colors of which there are 240x160 */
volatile unsigned short* screen = (volatile unsigned short*) 0x6000000;

/* the display control pointer points to the gba graphics register */
volatile unsigned long* display_control = (volatile unsigned long*) 0x4000000;

/* compute a 16-bit integer color based on the three components */
unsigned short make_color(unsigned char r, unsigned char g, unsigned char b) {
    unsigned short color = (b & 0x1f) << 10;
    color |= (g & 0x1f) << 5;
    color |= (r & 0x1f);
    return color;
}

/* a colored square */
struct square {
    unsigned short x, y, size, color;
};

/* place a pixel of a given color on the screen */
void put_pixel(int row, int col, unsigned short color) {
    /* set the screen location to this color */
    screen[row * WIDTH + col] = color;
}

/* draw a square onto the screen */
void draw_square(struct square* s) {
    /* for each row */
    for (unsigned short row = s->y; row < (s->y + s->size); row++) { 
        /* for each column */
        for (unsigned short col = s->x; col < (s->x + s->size); col++) {
            put_pixel(row, col, s->color);
        }
    }
}

/* clear the screen to black */
void clear_screen() {
    /* set each pixel black */
    for (unsigned short row = 0; row < HEIGHT; row++) { 
        for (unsigned short col = 0; col < WIDTH; col++) {
            put_pixel(row, col, 0);
        }
    }
}

/* the main function */
int main() { 
    /* we set the mode to mode 3 with background 2 on */
    *display_control = MODE3 | BG2;

    /* make a green square */
    struct square s = {10, 10, 15, make_color(0, 20, 2)};

    /* loop forever */
    while (1) {
        /* clear the screen */
        clear_screen();

        /* draw the square */
        draw_square(&s);
    }
}