use {
    spin::Lazy,
    x86_64::{
        structures::{
            gdt::{Descriptor, GlobalDescriptorTable, SegmentSelector},
            tss::TaskStateSegment,
        },
        VirtAddr,
    },
};

pub const DOUBLE_FAULT_IX: u16 = 0;

pub unsafe fn init() {
    use x86_64::instructions::{
        segmentation::{Segment, CS, SS},
        tables::load_tss,
    };

    log::trace!("Initialising GDT");
    GDT.0.load();
    CS::set_reg(GDT.1.kcode);
    SS::set_reg(GDT.1.kdata);
    load_tss(GDT.1.tss);
}

struct Selectors {
    kcode: SegmentSelector,
    kdata: SegmentSelector,
    tss:   SegmentSelector,
}

static TSS: Lazy<TaskStateSegment> = Lazy::new(|| {
    let mut tss = TaskStateSegment::new();
    tss.interrupt_stack_table[usize::from(DOUBLE_FAULT_IX)] = {
        const SIZE: usize = 5 * 1024;
        let stack = unsafe {
            alloc::alloc::alloc_zeroed(
                alloc::alloc::Layout::from_size_align(SIZE, 1).expect("stack pointer"),
            )
        };
        VirtAddr::from_ptr(stack) + SIZE
    };
    tss
});

static GDT: Lazy<(GlobalDescriptorTable, Selectors)> = Lazy::new(|| {
    let mut gdt = GlobalDescriptorTable::new();
    let sels = Selectors {
        kcode: gdt.add_entry(Descriptor::kernel_code_segment()),
        kdata: gdt.add_entry(Descriptor::kernel_data_segment()),
        tss:   gdt.add_entry(Descriptor::tss_segment(&TSS)),
    };
    (gdt, sels)
});