#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <strlib.h>

#if !defined(__i386__)
#error "This tutorial needs to be compiled with a ix86-elf compiler"
#endif

/* Hardware text mode color constants. */
enum vga_color {
    VGA_COLOR_BLACK = 0,
    VGA_COLOR_BLUE = 1,
    VGA_COLOR_GREEN = 2,
    VGA_COLOR_CYAN = 3,
    VGA_COLOR_RED = 4,
    VGA_COLOR_MAGENTA = 5,
    VGA_COLOR_BROWN = 6,
    VGA_COLOR_LIGHT_GREY = 7,
    VGA_COLOR_DARK_GREY = 8,
    VGA_COLOR_LIGHT_BLUE = 9,
    VGA_COLOR_LIGHT_GREEN = 10,
    VGA_COLOR_LIGHT_CYAN = 11,
    VGA_COLOR_LIGHT_RED = 12,
    VGA_COLOR_LIGHT_MAGENTA = 13,
    VGA_COLOR_LIGHT_BROWN = 14,
    VGA_COLOR_WHITE = 15,
};

static inline uint8_t vga_entry_color(enum vga_color fg, enum vga_color bg) 
{
    return fg | bg << 4;
}

static inline uint16_t vga_entry(unsigned char uc, uint8_t color) 
{
    return (uint16_t) uc | (uint16_t) color << 8;
}

static inline unsigned char vga_char(uint16_t entry) 
{
    return entry & 0xFF;
}

static inline uint8_t vga_color(uint16_t entry) 
{
    return (entry >> 8) & 0xFF;
}

static const size_t VGA_WIDTH = 80;
static const size_t VGA_HEIGHT = 25;

size_t terminal_row;
size_t terminal_column;
uint8_t terminal_color;
uint16_t* terminal_buffer;

void terminal_initialize(void) 
{
    terminal_row = 0;
    terminal_column = 0;
    terminal_color = vga_entry_color(VGA_COLOR_LIGHT_GREY, VGA_COLOR_BLACK);
    terminal_buffer = (uint16_t*) 0xC03FF000;
    for (size_t y = 0; y < VGA_HEIGHT; y++) {
        for (size_t x = 0; x < VGA_WIDTH; x++) {
            const size_t index = y * VGA_WIDTH + x;
            terminal_buffer[index] = vga_entry(' ', terminal_color);
        }
    }
}

void terminal_setcolor(uint8_t color) 
{
    terminal_color = color;
}

static void terminal_putentryat(char c, uint8_t color, size_t x, size_t y) 
{
    const size_t index = y * VGA_WIDTH + x;
    terminal_buffer[index] = vga_entry(c, color);
}

static uint16_t terminal_getentryat(size_t x, size_t y) 
{
    const size_t index = y * VGA_WIDTH + x;
    return terminal_buffer[index];
}

static void terminal_scroll() 
{
    for (size_t y = 0; y < VGA_HEIGHT; y++) {
        for (size_t x = 0; x < VGA_WIDTH; x++) {
            if (y == VGA_HEIGHT - 1) {
                terminal_putentryat(' ', terminal_color, x, y);
            }
            uint16_t entry = terminal_getentryat(x, y + 1);
            char scrolled = vga_char(entry);
            uint8_t color = vga_color(entry);
            terminal_putentryat(scrolled, color, x, y);
        }
    }
    terminal_row = VGA_HEIGHT - 1;
    terminal_column = 0;
}

static void terminal_newline() {
    if (++terminal_row == VGA_HEIGHT) {
        terminal_scroll();
    }
    terminal_column = 0;
}

void terminal_putchar(char c) 
{
    if (c == '\n') {
        terminal_newline();
        return;
    }
    terminal_putentryat(c, terminal_color, terminal_column, terminal_row);
    if (++terminal_column >= VGA_WIDTH) {
        terminal_column = 0;
        terminal_row++;
    }

    if (terminal_row >= VGA_HEIGHT)
        terminal_scroll();
}

static void terminal_write(const char* data, size_t size) 
{
    for (size_t i = 0; i < size; i++)
        terminal_putchar(data[i]);
}

void terminal_writestring(const char* data) 
{
    terminal_write(data, strlen(data));
}