alias GDTEntry = LibGDT::GDTEntry
alias GDTPointer = LibGDT::GDTPointer
alias TSSEntry = LibTSS::TSSEntry

private lib LibGDT
  @[Packed]
  struct GDTEntry
    limit_low : UInt16
    base_low : UInt16
    base_middle : UInt8
    access : UInt8
    granularity : UInt8
    base_high : UInt8
  end

  @[Packed]
  struct GDTPointer
    limit : UInt16
    base : UInt32
  end

  fun gdt_flush(gdt : UInt32)
end

private lib LibTSS
  @[Packed]
  struct TSSEntry
    prev_tss : UInt32
    esp0 : UInt32
    ss0 : UInt32
    esp1 : UInt32
    ss1 : UInt32
    esp2 : UInt32
    ss2 : UInt32
    cr3 : UInt32
    eip : UInt32
    eflags : UInt32
    eax : UInt32
    ecx : UInt32
    edx : UInt32
    ebx : UInt32
    esp : UInt32
    ebp : UInt32
    esi : UInt32
    edi : UInt32
    es : UInt32
    cs : UInt32
    ss : UInt32
    ds : UInt32
    fs : UInt32
    gs : UInt32
    ldt : UInt32
    trap : UInt32
    iomap_base : UInt32
  end

  fun tss_flush
end

struct GDT
  @@instance = Singleton(GDT).new
  @ptr = uninitialized GDTPointer
  @gdt = uninitialized GDTEntry[6]
  @tss = uninitialized TSSEntry

  def self.init
    instance = @@instance.get
    instance.@ptr.limit = sizeof(GDTEntry) * 6;
    instance.@ptr.base = pointerof(instance.@gdt).address.to_u32

    instance.set_gate 0u32, 0u32, 0u32, 0u8, 0u8
    instance.set_gate 1u32, 0u32, 0xFFFFFFFFu32, 0x9Au8, 0xCFu8
    instance.set_gate 2u32, 0u32, 0xFFFFFFFFu32, 0x92u8, 0xCFu8
    instance.set_gate 3u32, 0u32, 0xFFFFFFFFu32, 0xFAu8, 0xCFu8
    instance.set_gate 4u32, 0u32, 0xFFFFFFFFu32, 0xF2u8, 0xCFu8

    instance.write_tss 5u32, 0x10u16, 0x00u32

    print "before_flush"

    LibGDT.gdt_flush pointerof(instance.@ptr).address.to_u32
    print "after_flush"
    LibTSS.tss_flush
  end

  def set_gate(num : UInt32, base : UInt32, limit : UInt32, access : UInt8, gran : UInt8)
    @gdt[num].base_low = base.to_u16
    @gdt[num].base_middle = (base >> 16).to_u8
    @gdt[num].base_high = (base >> 24).to_u8
    @gdt[num].limit_low = limit.to_u16
    @gdt[num].granularity = ((limit >> 16) & 0x0F).to_u8
    @gdt[num].granularity |= (gran & 0xF0).to_u8
    @gdt[num].access = access
  end

  def write_tss(num : UInt32, ss0 : UInt16, esp0 : UInt32)
    base = pointerof(@tss).address.to_u32
    limit = base + sizeof(TSSEntry)

    set_gate num, base, limit, 0xE9u8, 0x00u8
    memset pointerof(@tss).as Void*, 0u8, sizeof(TSSEntry).to_u32

    @tss.ss0 = ss0
    @tss.esp0 = esp0
    @tss.cs = 0x0B
    @tss.ss = @tss.ds = @tss.es = @tss.fs = @tss.gs = 0x13;
  end
end