/*
 * gba.c
 */

#include "gba.h"

/* 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;

/* 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;

/* the button register holds the bits which indicate whether each button has
 * been pressed - this has got to be volatile as well
 */
volatile unsigned short* buttons = (volatile unsigned short*) 0x04000130;

/* the scanline counter is a memory cell which is updated to indicate how
 * much of the screen has been drawn */
volatile unsigned short* scanline_counter = (volatile unsigned short*) 0x4000006;

/* wait for the screen to be fully drawn so we can do something during vblank */
void wait_vblank() {
    /* wait until all 160 lines have been updated */
    while (*scanline_counter < 160) { }
}

/* this function checks whether a particular button has been pressed */
unsigned char button_pressed(unsigned short button) {
    /* and the button register with the button constant we want */
    unsigned short pressed = *buttons & button;

    /* if this value is zero, then it's not pressed */
    if (pressed == 0) {
        return 1;
    } else {
        return 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) {
    static int next_palette_index = 0;
    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;
}

/* 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;
    }
}

/* 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;
    }
}

/* 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);
        }
    }
}