use core::mem;
use core::sync::atomic::{AtomicBool, ATOMIC_BOOL_INIT, Ordering};

/// This must be used by the kernel to ensure that context switches are done atomically
/// Compare and exchange this to true when beginning a context switch on any CPU
/// The Context::switch_to function will set it back to false, allowing other CPU's to switch
/// This must be done, as no locks can be held on the stack during switch
pub static CONTEXT_SWITCH_LOCK: AtomicBool = ATOMIC_BOOL_INIT;

#[derive(Debug)]
pub struct Context {
    /// RFLAGS register
    rflags: usize,
    /// RBX register
    rbx: usize,
    /// R12 register
    r12: usize,
    /// R13 register
    r13: usize,
    /// R14 register
    r14: usize,
    /// R15 register
    r15: usize,
    /// Base pointer
    rbp: usize,
    /// Stack pointer
    rsp: usize,
    /// Page table pointer
    cr3: usize
}

impl Context {
    pub fn new() -> Context {
        Context {
            rflags: 0,
            rbx: 0,
            r12: 0,
            r13: 0,
            r14: 0,
            r15: 0,
            rbp: 0,
            rsp: 0,
            cr3: 0
        }
    }

    pub fn set_stack(&mut self, address: usize) {
        self.rsp = address;
    }

    /// Switch to the next context by restoring its stack and registers
    #[inline(never)]
    #[naked]
    pub unsafe fn switch_to(&mut self, next: &mut Context) {
        asm!("xchg bx, bx" : : : "memory" : "intel", "volatile");

/*
        asm!("fxsave [$0]" : : "r"(self.fx) : "memory" : "intel", "volatile");
        self.loadable = true;
        if next.loadable {
            asm!("fxrstor [$0]" : : "r"(next.fx) : "memory" : "intel", "volatile");
        }else{
            asm!("fninit" : : : "memory" : "intel", "volatile");
        }
*/

        asm!("pushfq ; pop $0" : "=r"(self.rflags) : : "memory" : "intel", "volatile");
        asm!("push $0 ; popfq" : : "r"(next.rflags) : "memory" : "intel", "volatile");

        asm!("mov $0, rbx" : "=r"(self.rbx) : : "memory" : "intel", "volatile");
        asm!("mov rbx, $0" : : "r"(next.rbx) : "memory" : "intel", "volatile");

        asm!("mov $0, r12" : "=r"(self.r12) : : "memory" : "intel", "volatile");
        asm!("mov r12, $0" : : "r"(next.r12) : "memory" : "intel", "volatile");

        asm!("mov $0, r13" : "=r"(self.r13) : : "memory" : "intel", "volatile");
        asm!("mov r13, $0" : : "r"(next.r13) : "memory" : "intel", "volatile");

        asm!("mov $0, r14" : "=r"(self.r14) : : "memory" : "intel", "volatile");
        asm!("mov r14, $0" : : "r"(next.r14) : "memory" : "intel", "volatile");

        asm!("mov $0, r15" : "=r"(self.r15) : : "memory" : "intel", "volatile");
        asm!("mov r15, $0" : : "r"(next.r15) : "memory" : "intel", "volatile");

        asm!("mov $0, rsp" : "=r"(self.rsp) : : "memory" : "intel", "volatile");
        asm!("mov rsp, $0" : : "r"(next.rsp) : "memory" : "intel", "volatile");

        asm!("mov $0, rbp" : "=r"(self.rbp) : : "memory" : "intel", "volatile");
        asm!("mov rbp, $0" : : "r"(next.rbp) : "memory" : "intel", "volatile");

        /* TODO
        asm!("mov $0, cr3" : "=r"(self.cr3) : : "memory" : "intel", "volatile");
        asm!("mov cr3, $0" : : "r"(self.cr3) : "memory" : "intel", "volatile");
        */

        //CONTEXT_SWITCH_LOCK.store(false, Ordering::SeqCst);
    }
}