#pragma once

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

enum Ring { Ring0 = 0x0, Ring1 = 0x1, Ring2 = 0x2, Ring3 = 0x3 };

struct __attribute__((packed)) Access {
  bool accessed : 1;      // if 0, is set by processor when accessed
  bool readwrite : 1;     // code seg: read toggle; data seg: write toggle
  bool direction : 1;     // code seg: conforming bit; data seg: direction bit
  bool executable : 1;    // executable bit
  bool segment : 1;       // true for code/data; false for gates/tss
  enum Ring privilege : 2; // descriptor privilege level
  bool present : 1;       // true for every active segment
};
_Static_assert(sizeof(struct Access) == 1, "access byte size");

static const struct Access null_access = {false, false, false, false, false, false, false};
static const struct Access ktext_access = {.readwrite = true, .executable = true, .segment = true, .present = true};
static const struct Access kdata_access = {.readwrite = true, .segment = true, .present = true};

// Segment Descriptor
// A memory structure (part of a table) that tells the CPU the attributes of a given segment
// |31|  |  |  |  |  |  |24|23|22|21|20|19|  |  |16|15|  |  |  |  |  |  | 8| 7|  |  |  |  |  |  | 0|
// |   base_31_24          | G|DB|  | A| lim_19_16 |   access              |   base_23_16          |
// |   base_15_0                                   |   limit_15_0                                  |
// |31|  |  |  |  |  |  |  |  |  |  |  |  |  |  |16|15|  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0|
// limit    size of segment - 1, either in bytes or in 4KiB chunks (check flags)
// base     address of segment
// access
// flags    defines the segment chunks and 16/32 bit
struct __attribute__((packed)) SegmentDescriptor_t {
  uint16_t limit_15_0;      // low bits of segment limit
  uint16_t base_15_0;       // low bits of segment base address
  uint8_t base_23_16;       // middle bits of segment base address
  uint8_t access;           // access byte
  uint8_t limit_19_16 : 4;  // high bits of segment limit
  // flags
  bool a : 1;               // unused, available for software use
  bool rsv : 1;             // reserved
  bool db : 1;              // false => 16-bit seg; true => 32-bit seg
  bool granularity : 1;     // limit scaled by 4k when set
  uint8_t base_31_24;       // high bits of segment address
};
_Static_assert(sizeof(struct SegmentDescriptor_t) == 8, "segment descriptor size");

void SegmentDescriptor(struct SegmentDescriptor_t *self, unsigned base, unsigned limit, uint8_t access);

void gdt_install();

enum SegmentIndex {
  ktextDescriptor = 2 * sizeof(struct SegmentDescriptor_t),
  kdataDescriptor = 3 * sizeof(struct SegmentDescriptor_t),
};