#if !defined(__cplusplus)
#include <stdbool.h> /* C doesn't have booleans by default. */
#endif
#include <stddef.h>
#include <stdint.h>

/* Check if the compiler thinks we are targeting the wrong operating system. */
#if defined(__linux__)
/* TODO how does this check for a cross compiler? It was failing. */
/*#error "You are not using a cross-compiler, you will most certainly run into trouble"*/
#endif

/* This tutorial will only work for the 32-bit ix86 targets. */
#if !defined(__i386__)
#error "This tutorial needs to be compiled with a ix86-elf compiler"
#endif

/* Hardware text mode color constants. */
enum vga_color {
    COLOR_BLACK         = 0,
    COLOR_BLUE          = 1,
    COLOR_GREEN         = 2,
    COLOR_CYAN          = 3,
    COLOR_RED           = 4,
    COLOR_MAGENTA       = 5,
    COLOR_BROWN         = 6,
    COLOR_LIGHT_GREY    = 7,
    COLOR_DARK_GREY     = 8,
    COLOR_LIGHT_BLUE    = 9,
    COLOR_LIGHT_GREEN   = 10,
    COLOR_LIGHT_CYAN    = 11,
    COLOR_LIGHT_RED     = 12,
    COLOR_LIGHT_MAGENTA = 13,
    COLOR_LIGHT_BROWN   = 14,
    COLOR_WHITE         = 15,
};

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

static uint16_t make_vgaentry(char c, uint8_t color)
{
    uint16_t c16 = c;
    uint16_t color16 = color;
    return c16 | color16 << 8;
}

/* static size_t strlen(const char* str) */
/* { */
/*     size_t ret = 0; */
/*     for (const char* p = str; *p != 0; p++, ret++); */
/*     return ret; */
/* } */

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;

static void terminal_initialize()
{
    terminal_row = 0;
    terminal_column = 0;
    terminal_color = make_color(COLOR_LIGHT_GREY, COLOR_BLACK);
    terminal_buffer = (uint16_t*) 0xB8000;
    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] = make_vgaentry(' ', terminal_color);
        }
    }
}

/* static 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] = make_vgaentry(c, color);
}

static void terminal_putchar(char c)
{
    terminal_putentryat(c, terminal_color, terminal_column, terminal_row);
    if (++terminal_column == VGA_WIDTH) {
        terminal_column = 0;
        if (++terminal_row == VGA_HEIGHT) {
            terminal_row = 0;
        }
    }
}

static void terminal_writestring(const char* data)
{
    for (const char* p = data; *p != 0; p++)
        terminal_putchar(*p);
}

#if defined(__cplusplus)
extern "C" /* Use C linkage for kernel_main. */
#endif
void kernel_main()
{
    terminal_initialize();
    terminal_writestring("Hello, kernel World!\n");
}