#include <lwk/driver.h>
#include <lwk/console.h>
#include <lwk/string.h>

/** Base address of the VGA frame buffer. */
static volatile uint8_t * const vga_fb = (uint8_t *) 0xffffffff800b8000ul;

/** Current cursor row coordinate. */
static int row = 0;

/** Current cursor column coordinate. */
static int col = 0;

/** Number of rows on the screen. */
static int nrows = 25;

/** Number of columns on the screen. */
static int ncols = 80;

/** Set when vga console has been initialized. */
static int initialized = 0;

/** Calculates the offset in the vga_fb corresponding to (row, col). */
static inline int cursor(int row, int col)
{
	return (row * ncols * 2) + col * 2;
}

/**
 * Scrolls everything on the screen up by one row.
 */ 
static void vga_scroll(void)
{
	int i;

	// Move all existing lines up by one
	memmove(
		(void *) vga_fb,
		(void *) (vga_fb + cursor(1, 0)),
		(nrows - 1) * ncols * sizeof(uint16_t)
	);

	// Blank the new line at the bottom of the screen
	for (i = 0; i < ncols; i++)
		vga_fb[cursor(nrows-1, i)] = ' ';
}

/**
 * Moves cursor to the next line.
 */ 
static void vga_newline(void)
{
	row = row + 1;
	col = 0;

	if (row == nrows) {
		row = nrows - 1;
		vga_scroll();
	}
}

/**
 * Sets the VGA font color.
 */
static void vga_set_font_color(uint8_t color)
{
	int i, j;
	for (i = 0; i < nrows; i++)
		for (j = 0; j < ncols; j++)
			vga_fb[cursor(i, j) + 1] = color;
}

/**
 * Prints a single character to the screen.
 */
static void vga_putc(unsigned char c)
{
	// Print the character
	vga_fb[cursor(row, col)] = c;

	// Move cursor
	if (++col == ncols)
		vga_newline();
}

/**
 * Writes a string to the screen at the current cursor location.
 */
static void vga_write(struct console *con, const char *str)
{
	unsigned char c;

	while ((c = *str++) != '\0') {
		switch (c) {
			case '\n':
				vga_newline();
				break;

			case '\t':
				/* Emulate a TAB */
				vga_putc(' ');
				while ((col % 8) != 0)
					vga_putc(' ');
				break;

			default: 
				vga_putc(c);
		}
	}
}

/**
 * VGA console device.
 */
static struct console vga_console = {
	.name  = "VGA Console",
	.write = vga_write
};

/**
 * Initializes and registers the VGA console driver.
 */
void vga_console_init(void)
{
	if (initialized) {
		printk(KERN_ERR "VGA console already initialized.\n");
		return;
	}

	vga_set_font_color(0x0F /* White */);
	console_register(&vga_console);
	initialized = 1;
}

driver_init(vga_console_init);

/**
 * Sets the row on the screen to start printing at.
 * This is used to avoid overwriting BIOS/boot messages.
 * At least on x86-64, this is set automatically as part
 * of the bootstrap process.
 */
driver_param(row, int);