#include <stdint.h>

#define SEG_DESCTYPE(x)  ((x) << 0x04) // Descriptor type (0 for system, 1 for code/data)
#define SEG_PRES(x)      ((x) << 0x07) // Present
#define SEG_SAVL(x)      ((x) << 0x0C) // Available for system use
#define SEG_LONG(x)      ((x) << 0x0D) // Long mode
#define SEG_SIZE(x)      ((x) << 0x0E) // Size (0 for 16-bit, 1 for 32)
#define SEG_GRAN(x)      ((x) << 0x0F) // Granularity (0 for 1B - 1MB, 1 for 4KB - 4GB)
#define SEG_PRIV(x)     (((x) &  0x03) << 0x05)   // Set privilege level (0 - 3)
 
#define SEG_DATA_RD        0x00 // Read-Only
#define SEG_DATA_RDA       0x01 // Read-Only, accessed
#define SEG_DATA_RDWR      0x02 // Read/Write
#define SEG_DATA_RDWRA     0x03 // Read/Write, accessed
#define SEG_DATA_RDEXPD    0x04 // Read-Only, expand-down
#define SEG_DATA_RDEXPDA   0x05 // Read-Only, expand-down, accessed
#define SEG_DATA_RDWREXPD  0x06 // Read/Write, expand-down
#define SEG_DATA_RDWREXPDA 0x07 // Read/Write, expand-down, accessed
#define SEG_CODE_EX        0x08 // Execute-Only
#define SEG_CODE_EXA       0x09 // Execute-Only, accessed
#define SEG_CODE_EXRD      0x0A // Execute/Read
#define SEG_CODE_EXRDA     0x0B // Execute/Read, accessed
#define SEG_CODE_EXC       0x0C // Execute-Only, conforming
#define SEG_CODE_EXCA      0x0D // Execute-Only, conforming, accessed
#define SEG_CODE_EXRDC     0x0E // Execute/Read, conforming
#define SEG_CODE_EXRDCA    0x0F // Execute/Read, conforming, accessed
 
#define GDT_CODE_PL0    (SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                         SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
                         SEG_PRIV(0)     | SEG_CODE_EXRD)
 
#define GDT_DATA_PL0    (SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                         SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
                         SEG_PRIV(0)     | SEG_DATA_RDWR)
 
#define GDT_CODE_PL3    (SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                         SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
                         SEG_PRIV(3)     | SEG_CODE_EXRD)
 
#define GDT_DATA_PL3    (SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                         SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
                         SEG_PRIV(3)     | SEG_DATA_RDWR)

struct gpt_ptr {
        uint16_t limit;
        uint32_t base;
}__attribute__((packed));

uint64_t gdt_desc[5];
struct gpt_ptr gp;

extern void flush_gdt(struct gpt_ptr *ptr);

uint64_t create_gdt_entry(uint32_t base, uint32_t limit, uint16_t flag) {
        uint64_t ret;

        ret =   limit           & 0x000F0000;
        ret |=  (flag << 8)     & 0x00F0FF00;
        ret |=  (base >> 16)    & 0x000000FF;
        ret |=  base            & 0xFF000000;

        ret <<= 32;

        ret |= base     << 16;
        ret |= limit    & 0x0000FFFF;

        return ret;
}

void gdt_install(void) {
        gp.limit = 64 * 5 - 1;
        gp.base = (uint32_t)&gdt_desc;

        gdt_desc[0] = create_gdt_entry(0, 0, 0);
        gdt_desc[1] = create_gdt_entry(0, 0x000FFFFF, GDT_CODE_PL0);
        gdt_desc[2] = create_gdt_entry(0, 0x000FFFFF, GDT_DATA_PL0);
        gdt_desc[3] = create_gdt_entry(0, 0x000FFFFF, GDT_DATA_PL3);
        gdt_desc[4] = create_gdt_entry(0, 0x000FFFFF, GDT_DATA_PL3);
        flush_gdt(&gp);
}