/*
 * square2.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 MODE4 0x0004
#define BG2 0x0400

/* this bit indicates whether to display the front or the back buffer
 * this allows us to refer to bit 4 of the display_control register */
#define SHOW_BACK 0x10;

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

/* the address of the color palette used in graphics mode 4 */
volatile unsigned short* palette = (volatile unsigned short*) 0x5000000;

/* pointers to the front and back buffers - the front buffer is the start
 * of the screen array and the back buffer is a pointer to the second half */
volatile unsigned short* front_buffer = (volatile unsigned short*) 0x6000000;
volatile unsigned short* back_buffer = (volatile unsigned short*)  0x600A000;

/* keep track of the next palette index */
int next_palette_index = 0;

/*
 * function which adds a color to the palette and returns the
 * index to it
 */
unsigned char add_color(unsigned char r, unsigned char g, unsigned char b) {
    unsigned short color = b << 10;
    color += g << 5;
    color += r;

    /* add the color to the palette */
    palette[next_palette_index] = color;

    /* increment the index */
    next_palette_index++;

    /* return index of color just added */
    return next_palette_index - 1;
}

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

/* put a pixel on the screen in mode 4 */
void put_pixel(volatile unsigned short* buffer, int row, int col, unsigned char color) {
    /* find the offset which is the regular offset divided by two */
    unsigned short offset = (row * WIDTH + col) >> 1;

    /* read the existing pixel which is there */
    unsigned short pixel = buffer[offset];

    /* if it's an odd column */
    if (col & 1) {
        /* put it in the left half of the short */
        buffer[offset] = (color << 8) | (pixel & 0x00ff);
    } else {
        /* it's even, put it in the left half */
        buffer[offset] = (pixel & 0xff00) | color;
    }
}

/* draw a square onto the screen */
void draw_square(volatile unsigned short* buffer, struct square* s) {
    unsigned short row, col;
    /* for each row of the square */
    for (row = s->y; row < (s->y + s->size); row++) {
        /* loop through each column of the square */
        for (col = s->x; col < (s->x + s->size); col++) {
            /* set the screen location to this color */
            put_pixel(buffer, row, col, s->color);
        }
    }
}

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

/* this function takes a video buffer and returns to you the other one */
volatile unsigned short* flip_buffers(volatile unsigned short* buffer) {
    /* if the back buffer is up, return that */
    if(buffer == front_buffer) {
        /* clear back buffer bit and return back buffer pointer */
        *display_control &= ~SHOW_BACK;
        return back_buffer;
    } else {
        /* set back buffer bit and return front buffer */
        *display_control |= SHOW_BACK;
        return front_buffer;
    }
}

/* the main function */
int main() {
    /* we set the mode to mode 4 with bg2 on */
    *display_control = MODE4 | BG2;

    /* add black to the palette */
    unsigned char black = add_color(0, 0, 0);

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

    /* the buffer we start with */
    volatile unsigned short* buffer = back_buffer;

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

        /* draw the square */
        draw_square(buffer, &s);

        /* swap the buffers */
        buffer = flip_buffers(buffer);
    }
}