use alloc::{boxed::Box, string::String, sync::Arc};
use core::ops::Deref;
use core::sync::atomic::{AtomicBool, AtomicI32, AtomicU64, AtomicU8, Ordering};
use core::{alloc::Layout, cell::UnsafeCell, fmt, ptr::NonNull};

#[cfg(feature = "preempt")]
use core::sync::atomic::AtomicUsize;

#[cfg(feature = "tls")]
use axhal::tls::TlsArea;

use axhal::arch::TaskContext;
use memory_addr::{align_up_4k, VirtAddr};

use crate::task_ext::AxTaskExt;
use crate::{AxRunQueue, AxTask, AxTaskRef, WaitQueue};

/// A unique identifier for a thread.
#[derive(Debug, Clone, Copy, Eq, PartialEq)]
pub struct TaskId(u64);

/// The possible states of a task.
#[repr(u8)]
#[derive(Debug, Clone, Copy, Eq, PartialEq)]
pub(crate) enum TaskState {
    Running = 1,
    Ready = 2,
    Blocked = 3,
    Exited = 4,
}

/// The inner task structure.
pub struct TaskInner {
    id: TaskId,
    name: String,
    is_idle: bool,
    is_init: bool,

    entry: Option<*mut dyn FnOnce()>,
    state: AtomicU8,

    in_wait_queue: AtomicBool,
    #[cfg(feature = "irq")]
    in_timer_list: AtomicBool,

    #[cfg(feature = "preempt")]
    need_resched: AtomicBool,
    #[cfg(feature = "preempt")]
    preempt_disable_count: AtomicUsize,

    exit_code: AtomicI32,
    wait_for_exit: WaitQueue,

    kstack: Option<TaskStack>,
    ctx: UnsafeCell<TaskContext>,
    task_ext: AxTaskExt,

    #[cfg(feature = "tls")]
    tls: TlsArea,
}

impl TaskId {
    fn new() -> Self {
        static ID_COUNTER: AtomicU64 = AtomicU64::new(1);
        Self(ID_COUNTER.fetch_add(1, Ordering::Relaxed))
    }

    /// Convert the task ID to a `u64`.
    pub const fn as_u64(&self) -> u64 {
        self.0
    }
}

impl From<u8> for TaskState {
    #[inline]
    fn from(state: u8) -> Self {
        match state {
            1 => Self::Running,
            2 => Self::Ready,
            3 => Self::Blocked,
            4 => Self::Exited,
            _ => unreachable!(),
        }
    }
}

unsafe impl Send for TaskInner {}
unsafe impl Sync for TaskInner {}

impl TaskInner {
    /// Create a new task with the given entry function and stack size.
    pub fn new<F>(entry: F, name: String, stack_size: usize) -> Self
    where
        F: FnOnce() + Send + 'static,
    {
        let mut t = Self::new_common(TaskId::new(), name);
        debug!("new task: {}", t.id_name());
        let kstack = TaskStack::alloc(align_up_4k(stack_size));

        #[cfg(feature = "tls")]
        let tls = VirtAddr::from(t.tls.tls_ptr() as usize);
        #[cfg(not(feature = "tls"))]
        let tls = VirtAddr::from(0);

        t.entry = Some(Box::into_raw(Box::new(entry)));
        t.ctx_mut().init(task_entry as usize, kstack.top(), tls);
        t.kstack = Some(kstack);
        if t.name == "idle" {
            t.is_idle = true;
        }
        t
    }

    /// Gets the ID of the task.
    pub const fn id(&self) -> TaskId {
        self.id
    }

    /// Gets the name of the task.
    pub fn name(&self) -> &str {
        self.name.as_str()
    }

    /// Get a combined string of the task ID and name.
    pub fn id_name(&self) -> alloc::string::String {
        alloc::format!("Task({}, {:?})", self.id.as_u64(), self.name)
    }

    /// Wait for the task to exit, and return the exit code.
    ///
    /// It will return immediately if the task has already exited (but not dropped).
    pub fn join(&self) -> Option<i32> {
        self.wait_for_exit
            .wait_until(|| self.state() == TaskState::Exited);
        Some(self.exit_code.load(Ordering::Acquire))
    }

    /// Returns the pointer to the user-defined task extended data.
    ///
    /// # Safety
    ///
    /// The caller should not access the pointer directly, use [`TaskExtRef::task_ext`]
    /// or [`TaskExtMut::task_ext_mut`] instead.
    ///
    /// [`TaskExtRef::task_ext`]: crate::task_ext::TaskExtRef::task_ext
    /// [`TaskExtMut::task_ext_mut`]: crate::task_ext::TaskExtMut::task_ext_mut
    pub unsafe fn task_ext_ptr(&self) -> *mut u8 {
        self.task_ext.as_ptr()
    }

    /// Initialize the user-defined task extended data.
    ///
    /// Returns a reference to the task extended data if it has not been
    /// initialized yet (empty), otherwise returns [`None`].
    pub fn init_task_ext<T: Sized>(&mut self, data: T) -> Option<&T> {
        if self.task_ext.is_empty() {
            self.task_ext.write(data).map(|data| &*data)
        } else {
            None
        }
    }
}

// private methods
impl TaskInner {
    fn new_common(id: TaskId, name: String) -> Self {
        Self {
            id,
            name,
            is_idle: false,
            is_init: false,
            entry: None,
            state: AtomicU8::new(TaskState::Ready as u8),
            in_wait_queue: AtomicBool::new(false),
            #[cfg(feature = "irq")]
            in_timer_list: AtomicBool::new(false),
            #[cfg(feature = "preempt")]
            need_resched: AtomicBool::new(false),
            #[cfg(feature = "preempt")]
            preempt_disable_count: AtomicUsize::new(0),
            exit_code: AtomicI32::new(0),
            wait_for_exit: WaitQueue::new(),
            kstack: None,
            ctx: UnsafeCell::new(TaskContext::new()),
            task_ext: AxTaskExt::empty(),
            #[cfg(feature = "tls")]
            tls: TlsArea::alloc(),
        }
    }

    /// Creates an "init task" using the current CPU states, to use as the
    /// current task.
    ///
    /// As it is the current task, no other task can switch to it until it
    /// switches out.
    ///
    /// And there is no need to set the `entry`, `kstack` or `tls` fields, as
    /// they will be filled automatically when the task is switches out.
    pub(crate) fn new_init(name: String) -> Self {
        let mut t = Self::new_common(TaskId::new(), name);
        t.is_init = true;
        if t.name == "idle" {
            t.is_idle = true;
        }
        t
    }

    pub(crate) fn into_arc(self) -> AxTaskRef {
        Arc::new(AxTask::new(self))
    }

    #[inline]
    pub(crate) fn state(&self) -> TaskState {
        self.state.load(Ordering::Acquire).into()
    }

    #[inline]
    pub(crate) fn set_state(&self, state: TaskState) {
        self.state.store(state as u8, Ordering::Release)
    }

    #[inline]
    pub(crate) fn is_running(&self) -> bool {
        matches!(self.state(), TaskState::Running)
    }

    #[inline]
    pub(crate) fn is_ready(&self) -> bool {
        matches!(self.state(), TaskState::Ready)
    }

    #[inline]
    pub(crate) fn is_blocked(&self) -> bool {
        matches!(self.state(), TaskState::Blocked)
    }

    #[inline]
    pub(crate) const fn is_init(&self) -> bool {
        self.is_init
    }

    #[inline]
    pub(crate) const fn is_idle(&self) -> bool {
        self.is_idle
    }

    #[inline]
    pub(crate) fn in_wait_queue(&self) -> bool {
        self.in_wait_queue.load(Ordering::Acquire)
    }

    #[inline]
    pub(crate) fn set_in_wait_queue(&self, in_wait_queue: bool) {
        self.in_wait_queue.store(in_wait_queue, Ordering::Release);
    }

    #[inline]
    #[cfg(feature = "irq")]
    pub(crate) fn in_timer_list(&self) -> bool {
        self.in_timer_list.load(Ordering::Acquire)
    }

    #[inline]
    #[cfg(feature = "irq")]
    pub(crate) fn set_in_timer_list(&self, in_timer_list: bool) {
        self.in_timer_list.store(in_timer_list, Ordering::Release);
    }

    #[inline]
    #[cfg(feature = "preempt")]
    pub(crate) fn set_preempt_pending(&self, pending: bool) {
        self.need_resched.store(pending, Ordering::Release)
    }

    #[inline]
    #[cfg(feature = "preempt")]
    pub(crate) fn can_preempt(&self, current_disable_count: usize) -> bool {
        self.preempt_disable_count.load(Ordering::Acquire) == current_disable_count
    }

    #[inline]
    #[cfg(feature = "preempt")]
    pub(crate) fn disable_preempt(&self) {
        self.preempt_disable_count.fetch_add(1, Ordering::Relaxed);
    }

    #[inline]
    #[cfg(feature = "preempt")]
    pub(crate) fn enable_preempt(&self, resched: bool) {
        if self.preempt_disable_count.fetch_sub(1, Ordering::Relaxed) == 1 && resched {
            // If current task is pending to be preempted, do rescheduling.
            Self::current_check_preempt_pending();
        }
    }

    #[cfg(feature = "preempt")]
    fn current_check_preempt_pending() {
        let curr = crate::current();
        if curr.need_resched.load(Ordering::Acquire) && curr.can_preempt(0) {
            let mut rq = crate::RUN_QUEUE.lock();
            if curr.need_resched.load(Ordering::Acquire) {
                rq.preempt_resched();
            }
        }
    }

    pub(crate) fn notify_exit(&self, exit_code: i32, rq: &mut AxRunQueue) {
        self.exit_code.store(exit_code, Ordering::Release);
        self.wait_for_exit.notify_all_locked(false, rq);
    }

    #[inline]
    pub(crate) const unsafe fn ctx_mut_ptr(&self) -> *mut TaskContext {
        self.ctx.get()
    }

    /// Returns a mutable reference to the task context.
    #[inline]
    pub const fn ctx_mut(&mut self) -> &mut TaskContext {
        self.ctx.get_mut()
    }

    /// Returns the top address of the kernel stack.
    #[inline]
    pub const fn kernel_stack_top(&self) -> Option<VirtAddr> {
        match &self.kstack {
            Some(s) => Some(s.top()),
            None => None,
        }
    }
}

impl fmt::Debug for TaskInner {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("TaskInner")
            .field("id", &self.id)
            .field("name", &self.name)
            .field("state", &self.state())
            .finish()
    }
}

impl Drop for TaskInner {
    fn drop(&mut self) {
        debug!("task drop: {}", self.id_name());
    }
}

struct TaskStack {
    ptr: NonNull<u8>,
    layout: Layout,
}

impl TaskStack {
    pub fn alloc(size: usize) -> Self {
        let layout = Layout::from_size_align(size, 16).unwrap();
        Self {
            ptr: NonNull::new(unsafe { alloc::alloc::alloc(layout) }).unwrap(),
            layout,
        }
    }

    pub const fn top(&self) -> VirtAddr {
        unsafe { core::mem::transmute(self.ptr.as_ptr().add(self.layout.size())) }
    }
}

impl Drop for TaskStack {
    fn drop(&mut self) {
        unsafe { alloc::alloc::dealloc(self.ptr.as_ptr(), self.layout) }
    }
}

use core::mem::ManuallyDrop;

/// A wrapper of [`AxTaskRef`] as the current task.
///
/// It won't change the reference count of the task when created or dropped.
pub struct CurrentTask(ManuallyDrop<AxTaskRef>);

impl CurrentTask {
    pub(crate) fn try_get() -> Option<Self> {
        let ptr: *const super::AxTask = axhal::cpu::current_task_ptr();
        if !ptr.is_null() {
            Some(Self(unsafe { ManuallyDrop::new(AxTaskRef::from_raw(ptr)) }))
        } else {
            None
        }
    }

    pub(crate) fn get() -> Self {
        Self::try_get().expect("current task is uninitialized")
    }

    /// Converts [`CurrentTask`] to [`AxTaskRef`].
    pub fn as_task_ref(&self) -> &AxTaskRef {
        &self.0
    }

    pub(crate) fn clone(&self) -> AxTaskRef {
        self.0.deref().clone()
    }

    pub(crate) fn ptr_eq(&self, other: &AxTaskRef) -> bool {
        Arc::ptr_eq(&self.0, other)
    }

    pub(crate) unsafe fn init_current(init_task: AxTaskRef) {
        assert!(init_task.is_init());
        #[cfg(feature = "tls")]
        axhal::arch::write_thread_pointer(init_task.tls.tls_ptr() as usize);
        let ptr = Arc::into_raw(init_task);
        axhal::cpu::set_current_task_ptr(ptr);
    }

    pub(crate) unsafe fn set_current(prev: Self, next: AxTaskRef) {
        let Self(arc) = prev;
        ManuallyDrop::into_inner(arc); // `call Arc::drop()` to decrease prev task reference count.
        let ptr = Arc::into_raw(next);
        axhal::cpu::set_current_task_ptr(ptr);
    }
}

impl Deref for CurrentTask {
    type Target = TaskInner;
    fn deref(&self) -> &Self::Target {
        self.0.deref()
    }
}

extern "C" fn task_entry() -> ! {
    // release the lock that was implicitly held across the reschedule
    unsafe { crate::RUN_QUEUE.force_unlock() };
    #[cfg(feature = "irq")]
    axhal::arch::enable_irqs();
    let task = crate::current();
    if let Some(entry) = task.entry {
        unsafe { Box::from_raw(entry)() };
    }
    crate::exit(0);
}