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#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
 
/* Check if the compiler thinks you are targeting the wrong operating system. */
#if defined(__linux__)
	#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 {
	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;
}
 
size_t strlen(const char* str) 
{
	size_t len = 0;
	while (str[len])
		len++;
	return len;
}
 
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*) 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] = vga_entry(' ', terminal_color);
		}
	}
}
 
void terminal_setcolor(uint8_t color) 
{
	terminal_color = color;
}
 
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);
}
 
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;
	}
}
 
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));
}
 
void kernel_main(void) 
{
	/* Initialize terminal interface */
	terminal_initialize();
 
	/* Newline support is left as an exercise. */
	terminal_writestring("Hello, kernel World!");
}

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; constants for multiboot header
; See more tutorials for better comprehension
MALIGN      equ  1<<0               ; Align all modules on page boudaries, (need) (more details) Also 1 << 0 == 1 (fucking tutorial)
MEMINFO     equ  1<<1               ; provide memory map
FLAGS       equ  MALIGN | MEMINFO   ; I think FLAGS = 11 ? It's multiboot flag field
MAGIC       equ  0x1BADB002         ; magic number
CHECKSUM    equ -(MAGIC + FLAGS)    ; checksum of above, to prove we are multiboot

; set multiboot section
section .multiboot
    align 4
    dd MAGIC
    dd FLAGS
    dd CHECKSUM

section .data
    align 4096

; initial stack
section .initial_stack, nobits
    align 4

stack_bottom:
    ; 1 MB of uninitialized data for stack
    resb 104856
stack_top:

; kernel entry, main text section
section .text
    global _start


; define _start, aligned by linker.ld script
_start:
    mov esp, stack_top
    extern kernel_main
    push ebx
    call kernel_main
loop:
    jmp loop

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ASM_SRC		= boot.asm
NASM		= nasm
ASM_FLAGS	= -f elf32

CC			= i386-elf-gcc
C_SRC		= kernel.c
C_FLAGS		= -W -std=gnu99 -ffreestanding -O2 -Wall -Wextra

OBJ			= $(ASM_SRC:.asm=.o) $(C_SRC:.c=.o)
QEMU		= qemu-system-i386
QEMU_FLAGS	= -cdrom
LD			= ld/linker.ld
GRUB		= grub/grub.cfg

OS_NAME		= myos
OS_BIN		= $(OS_NAME).bin
OS_ISO		= $(OS_NAME).iso

all: build
	mkdir -p isodir/boot/grub
	cp $(OS_BIN) isodir/boot/$(OS_BIN)
	cp $(GRUB) isodir/boot/$(GRUB)
	grub-mkrescue -o $(OS_ISO) isodir

build: $(OBJ)
	$(CC) -T $(LD) -o $(OS_BIN) -ffreestanding -O2 -nostdlib $(OBJ) -lgcc

run:
	$(QEMU) $(QEMU_FLAGS) $(OS_ISO)

clean:
	$(RM) $(OBJ)
	$(RM) *.bin

fclean: clean
	$(RM) *.iso

re: fclean all

key:
	sudo dd if=$(OS_ISO) of=/dev/sda && sync

%.o: %.c
	$(CC) -c $(C_FLAGS) -o $@ $<

%.o: %.asm
	$(NASM) $(ASM_FLAGS) -o $@ $<