#include <kernel/tty.h>
#include <debugging.h>
#include <stdint.h>
#include <stddef.h>

#define NUM_ENTRIES 3

struct SegmentDescriptor {
    uint32_t limit;
    uint32_t base;
    uint8_t access_byte;
    uint8_t flags;
};

struct GDT {
    uint16_t size;
    uint64_t* base;
} __attribute__((packed));

static uint64_t entries[NUM_ENTRIES];
static size_t current_entry = 0;
struct GDT gdt;

static void encodeGdtEntry(uint8_t *target, struct SegmentDescriptor source)
{
    // Check the limit to make sure that it can be encoded
    if (source.limit > 0xFFFFF) {
        terminal_writestring("WARNING: GDT entry has a limit (");
        print_hex_bytes(&source.limit, 4);
        terminal_writestring(") that is larger than accepted.");
        return;
    }
    
    // Encode the limit
    target[0] = source.limit & 0xFF;
    target[1] = (source.limit >> 8) & 0xFF;
    target[6] = (source.limit >> 16) & 0x0F;
    
    // Encode the base
    target[2] = source.base & 0xFF;
    target[3] = (source.base >> 8) & 0xFF;
    target[4] = (source.base >> 16) & 0xFF;
    target[7] = (source.base >> 24) & 0xFF;
    
    // Encode the access byte
    target[5] = source.access_byte;
    
    // Encode the flags
    target[6] |= (source.flags << 4);

    print_hex_bytes(target, 8);
}

static void add_entry(struct SegmentDescriptor entry)
{
    if (current_entry == NUM_ENTRIES) {
        terminal_writestring("WARNING: More GDT entries than NUM_ENTRIES.\n");
        return;
    }

    uint64_t descriptor = 0;
    encodeGdtEntry((uint8_t*) &descriptor, entry);

    entries[current_entry] = descriptor;

    current_entry++;
}

static void load_gdt() 
{
    gdt.size = NUM_ENTRIES * 8 - 1;
    gdt.base = entries;

    for (int i = 0; i < NUM_ENTRIES; i++) {
        print_hex_bytes(gdt.base + i, 8);
        terminal_putchar('\n');
    }

    asm ( "lgdt %0" : : "m"(gdt) );
}

void gdt_init()
{
    terminal_initialize();
    struct SegmentDescriptor null;
    null.limit = 0;
    null.base = 0;
    null.access_byte = 0;
    null.flags = 0;

    terminal_writestring("Kernel Null: ");

    add_entry(null);

    terminal_putchar('\n');

    struct SegmentDescriptor kernel_code;
    kernel_code.limit = 0x000003FF;
    kernel_code.base =  0x00000000;
    kernel_code.access_byte = 0b10011010;
    kernel_code.flags = 0b1100;

    terminal_writestring("Kernel Code: ");

    add_entry(kernel_code);
    terminal_putchar('\n');

    struct SegmentDescriptor kernel_data;
    kernel_code.limit = 0x000003FF;
    kernel_data.base =  0x00400000;
    kernel_data.access_byte = 0b10010010;
    kernel_data.flags = 0b1100;

    terminal_writestring("Kernel Data: ");

    add_entry(kernel_data);
    terminal_putchar('\n');

    load_gdt();
}