//! The interface to a collector

use std::fmt::{self, Debug, Formatter};
use std::sync::Arc;
use std::ptr::NonNull;
use std::marker::PhantomData;

use slog::{Logger, o};

use zerogc::{Gc, GcSafe, GcSystem, Trace, GcSimpleAlloc, NullTrace, TraceImmutable, GcVisitor};

use crate::{CollectorContext};
use crate::state::{CollectionManager, RawContext};

/// A specific implementation of a collector
pub unsafe trait RawCollectorImpl: 'static + Sized {
    /// A dynamic pointer to a GC object
    ///
    /// The simple collector implements this as
    /// a trait object pointer.
    type GcDynPointer: Copy + Debug + 'static;

    /// A pointer to this collector
    ///
    /// Must be a ZST if the collector is a singleton.
    type Ptr: CollectorPtr<Self>;

    /// The type that manages this collector's state
    type Manager: CollectionManager<Self, Context=Self::RawContext>;

    /// The context
    type RawContext: RawContext<Self>;

    /// True if this collector is a singleton
    ///
    /// If the collector allows multiple instances,
    /// this *must* be false
    const SINGLETON: bool;

    /// True if this collector is thread-safe.
    const SYNC: bool;

    /// Convert the specified value into a dyn pointer
    unsafe fn create_dyn_pointer<T: Trace>(t: *mut T) -> Self::GcDynPointer;

    /// Initialize an instance of the collector
    ///
    /// Must panic if the collector is not a singleton
    fn init(logger: Logger) -> NonNull<Self>;

    /// The id of this collector
    #[inline]
    fn id(&self) -> CollectorId<Self> {
        CollectorId { ptr: unsafe { Self::Ptr::from_raw(self as *const _ as *mut _) } }
    }
    unsafe fn gc_write_barrier<'gc, T, V>(
        owner: &Gc<'gc, T, CollectorId<Self>>,
        value: &Gc<'gc, V, CollectorId<Self>>,
        field_offset: usize
    ) where T: GcSafe + ?Sized + 'gc, V: GcSafe + ?Sized + 'gc;
    /// The logger associated with this collector
    fn logger(&self) -> &Logger;

    fn manager(&self) -> &Self::Manager;

    fn should_collect(&self) -> bool;

    fn allocated_size(&self) -> crate::utils::MemorySize;

    unsafe fn perform_raw_collection(&self, contexts: &[*mut Self::RawContext]);
}

/// A thread safe collector
pub unsafe trait SyncCollector: RawCollectorImpl + Sync {

}

/// A collector implemented as a singleton
///
/// This only has one instance
pub unsafe trait SingletonCollector: RawCollectorImpl<Ptr=PhantomData<&'static Self>> {
    /// When the collector is a singleton,
    /// return the global implementation
    fn global_ptr() -> *const Self;

    /// Initialize the global singleton
    ///
    /// Panics if already initialized
    fn init_global(logger: Logger);
}

impl<C: RawCollectorImpl> PartialEq for CollectorId<C> {
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        self.ptr == other.ptr
    }
}
impl<C: RawCollectorImpl> Eq for CollectorId<C> {}
impl<C: RawCollectorImpl> Clone for CollectorId<C> {
    #[inline]
    fn clone(&self) -> Self {
        *self
    }
}
impl<C: RawCollectorImpl> Copy for CollectorId<C> {}
impl<C: RawCollectorImpl> Debug for CollectorId<C> {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        let mut debug = f.debug_struct("CollectorId");
        if !C::SINGLETON {
            debug.field("ptr", &format_args!("{:p}", self.ptr.as_ptr()));
        }
        debug.finish()
    }
}

/// An unchecked pointer to a collector
pub unsafe trait CollectorPtr<C: RawCollectorImpl<Ptr=Self>>: Copy + Eq
    + self::sealed::Sealed + 'static {
    /// A weak reference to the pointer
    type Weak: Clone + 'static;

    unsafe fn from_raw(ptr: *mut C) -> Self;
    unsafe fn clone_owned(&self) -> Self;
    fn as_ptr(&self) -> *mut C;
    unsafe fn drop(self);
    fn upgrade_weak_raw(weak: &Self::Weak) -> Option<Self>;
    #[inline]
    fn upgrade_weak(weak: &Self::Weak) -> Option<CollectorRef<C>> {
        match Self::upgrade_weak_raw(weak) {
            Some(ptr) => Some(CollectorRef { ptr }),
            None => None
        }
    }
    unsafe fn assume_weak_valid(weak: &Self::Weak) -> Self;
    unsafe fn create_weak(&self) -> Self::Weak;
}
/// This is implemented as a
/// raw pointer via [Arc::into_raw]
unsafe impl<C: RawCollectorImpl<Ptr=Self>> CollectorPtr<C> for NonNull<C> {
    type Weak = std::sync::Weak<C>;

    #[inline]
    unsafe fn from_raw(ptr: *mut C) -> Self {
        assert!(!C::SINGLETON, "Collector is a singleton!");
        debug_assert!(!ptr.is_null());
        NonNull::new_unchecked(ptr)
    }

    #[inline]
    unsafe fn clone_owned(&self) -> Self {
        let original = Arc::from_raw(self.as_ptr());
        let cloned = Arc::clone(&original);
        std::mem::forget(original);
        NonNull::new_unchecked(Arc::into_raw(cloned) as *mut _)
    }

    #[inline]
    fn as_ptr(&self) -> *mut C {
        NonNull::as_ptr(*self)
    }

    #[inline]
    unsafe fn drop(self) {
        drop(Arc::from_raw(self.as_ptr() as *const _))
    }

    #[inline]
    fn upgrade_weak_raw(weak: &Self::Weak) -> Option<Self> {
        match weak.upgrade() {
            Some(arc) => {
                Some(unsafe {
                    Self::from_raw(Arc::into_raw(arc) as *mut _)
                })
            },
            None => None
        }
    }

    #[inline]
    unsafe fn assume_weak_valid(weak: &Self::Weak) -> Self {
        debug_assert!(
            weak.upgrade().is_some(),
            "Dead collector"
        );
        NonNull::new_unchecked(weak.as_ptr() as *mut _)
    }

    #[inline]
    unsafe fn create_weak(&self) -> Self::Weak {
        let arc = Arc::from_raw(self.as_ptr());
        let weak = Arc::downgrade(&arc);
        std::mem::forget(arc);
        weak
    }
}
/// Dummy implementation
impl<C: RawCollectorImpl> self::sealed::Sealed for NonNull<C> {}
unsafe impl<C: SingletonCollector<Ptr=Self>> CollectorPtr<C> for PhantomData<&'static C> {
    type Weak = PhantomData<&'static C>;

    #[inline]
    unsafe fn from_raw(ptr: *mut C) -> Self {
        assert!(C::SINGLETON, "Expected a singleton");
        debug_assert_eq!(ptr, C::global_ptr() as *mut _);
        PhantomData
    }

    #[inline]
    unsafe fn clone_owned(&self) -> Self {
        *self
    }

    #[inline]
    fn as_ptr(&self) -> *mut C {
        assert!(C::SINGLETON, "Expected a singleton");
        C::global_ptr() as *mut C
    }

    #[inline]
    unsafe fn drop(self) {}

    #[inline]
    fn upgrade_weak_raw(weak: &Self::Weak) -> Option<Self> {
        assert!(C::SINGLETON);
        Some(*weak) // gloal is always valid
    }

    #[inline]
    unsafe fn assume_weak_valid(weak: &Self::Weak) -> Self {
        assert!(C::SINGLETON); // global is always valid
        *weak
    }

    #[inline]
    unsafe fn create_weak(&self) -> Self::Weak {
        *self
    }
}
/// Dummy implementation
impl<C: SingletonCollector> self::sealed::Sealed for PhantomData<&'static C> {}

/// Uniquely identifies the collector in case there are
/// multiple collectors.
///
/// If there are multiple collectors `cfg!(feature="multiple-collectors")`,
/// we need to use a pointer to tell them apart.
/// Otherwise, this is a zero-sized structure.
///
/// As long as our memory is valid,
/// it implies this pointer is too.
#[repr(C)]
pub struct CollectorId<C: RawCollectorImpl> {
    /// This is in essence a borrowed reference to
    /// the collector.
    ///
    /// Depending on whether or not the collector is a singleton,
    ///
    /// We don't know whether the underlying memory will be valid.
    ptr: C::Ptr,
}
impl<C: RawCollectorImpl> CollectorId<C> {
    #[inline]
    pub const unsafe fn from_raw(ptr: C::Ptr) -> CollectorId<C> {
        CollectorId { ptr }
    }
    #[inline]
    pub unsafe fn as_ref(&self) -> &C {
        &*self.ptr.as_ptr()
    }
    #[inline]
    pub unsafe fn weak_ref(&self) -> WeakCollectorRef<C> {
        WeakCollectorRef { weak: self.ptr.create_weak() }
    }
}
unsafe impl<C: RawCollectorImpl> ::zerogc::CollectorId for CollectorId<C> {
    type System = CollectorRef<C>;

    #[inline(always)]
    unsafe fn gc_write_barrier<'gc, T, V>(
        owner: &Gc<'gc, T, Self>,
        value: &Gc<'gc, V, Self>,
        field_offset: usize
    ) where T: GcSafe + ?Sized + 'gc, V: GcSafe + ?Sized + 'gc {
        C::gc_write_barrier(owner, value, field_offset)
    }

    #[inline]
    unsafe fn assume_valid_system(&self) -> &Self::System {
        // TODO: Make the API nicer? (avoid borrowing and indirection)
        assert_eq!(
            std::mem::size_of::<Self>(),
            std::mem::size_of::<CollectorRef<C>>()
        );
        &*(self as *const CollectorId<C> as *const CollectorRef<C>)
    }
}
unsafe impl<C: RawCollectorImpl> Trace for CollectorId<C> {
    const NEEDS_TRACE: bool = false;
    #[inline(always)]
    fn visit<V: GcVisitor>(&mut self, _visitor: &mut V) -> Result<(), V::Err> {
        Ok(())
    }
}
unsafe impl<C: RawCollectorImpl> TraceImmutable for CollectorId<C> {
    #[inline(always)]
    fn visit_immutable<V: GcVisitor>(&self, _visitor: &mut V) -> Result<(), <V as GcVisitor>::Err> {
        Ok(())
    }
}
unsafe impl<C: RawCollectorImpl> NullTrace for CollectorId<C> {}

pub struct WeakCollectorRef<C: RawCollectorImpl> {
    weak: <C::Ptr as CollectorPtr<C>>::Weak,
}
impl<C: RawCollectorImpl> WeakCollectorRef<C> {
    #[inline]
    pub unsafe fn assume_valid(&self) -> CollectorId<C> {
        CollectorId { ptr: C::Ptr::assume_weak_valid(&self.weak) }
    }
    pub fn ensure_valid<R>(&self, func: impl FnOnce(CollectorId<C>) -> R) -> R {
        self.try_ensure_valid(|id| match id{
            Some(id) => func(id),
            None => panic!("Dead collector")
        })
    }
    #[inline]
    pub fn try_ensure_valid<R>(&self, func: impl FnOnce(Option<CollectorId<C>>) -> R) -> R{
        func(C::Ptr::upgrade_weak(&self.weak).map(|r| r.id()))
    }
}

pub unsafe trait RawSimpleAlloc: RawCollectorImpl {
    fn alloc<'gc, T: GcSafe + 'gc>(context: &'gc CollectorContext<Self>, value: T) -> Gc<'gc, T, CollectorId<Self>>;
}
unsafe impl<'gc, T, C> GcSimpleAlloc<'gc, T> for CollectorContext<C>
    where T: GcSafe + 'gc, C: RawSimpleAlloc {
    #[inline]
    fn alloc(&'gc self, value: T) -> Gc<'gc, T, Self::Id> {
        C::alloc(self, value)
    }
}

/// A reference to the collector.
///
/// TODO: Devise better name
#[repr(C)]
pub struct CollectorRef<C: RawCollectorImpl> {
    /// When using singleton collectors, this is a ZST.
    ///
    /// When using multiple collectors, this is just an [Arc].
    ///
    /// It is implemented as a raw pointer around [Arc::into_raw]
    ptr: C::Ptr
}
/// We actually are thread safe ;)
unsafe impl<C: SyncCollector> Send for CollectorRef<C> {}
#[cfg(feature = "sync")]
unsafe impl<C: SyncCollector> Sync for CollectorRef<C> {}

/// Internal trait for initializing a collector
#[doc(hidden)]
pub trait CollectorInit<C: RawCollectorImpl<Ptr=Self>>: CollectorPtr<C> {
    fn create() -> CollectorRef<C> {
        Self::with_logger(Logger::root(
            slog::Discard,
            o!()
        ))
    }
    fn with_logger(logger: Logger) -> CollectorRef<C>;
}

impl<C: RawCollectorImpl<Ptr=NonNull<C>>> CollectorInit<C> for NonNull<C> {
    fn with_logger(logger: Logger) -> CollectorRef<C> {
        assert!(!C::SINGLETON);
        let raw_ptr = C::init(logger);
        CollectorRef { ptr: raw_ptr }
    }
}
impl<C> CollectorInit<C> for PhantomData<&'static C>
    where C: SingletonCollector {
    fn with_logger(logger: Logger) -> CollectorRef<C> {
        assert!(C::SINGLETON);
        C::init_global(logger); // TODO: Is this safe?
        // NOTE: The raw pointer is implicit (now that we're leaked)
        CollectorRef { ptr: PhantomData }
    }
}


impl<C: RawCollectorImpl> CollectorRef<C> {
    #[inline]
    pub fn create() -> Self where C::Ptr: CollectorInit<C> {
        <C::Ptr as CollectorInit<C>>::create()
    }

    #[inline]
    pub fn with_logger(logger: Logger) -> Self where C::Ptr: CollectorInit<C> {
        <C::Ptr as CollectorInit<C>>::with_logger(logger)
    }

    #[inline]
    pub(crate) fn clone_internal(&self) -> CollectorRef<C> {
        CollectorRef { ptr: unsafe { self.ptr.clone_owned() } }
    }

    #[inline]
    pub fn as_raw(&self) -> &C {
        unsafe { &*self.ptr.as_ptr() }
    }

    /// The id of this collector
    #[inline]
    pub fn id(&self) -> CollectorId<C> {
        CollectorId { ptr: self.ptr  }
    }

    /// Convert this collector into a unique context
    ///
    /// The single-threaded implementation only allows a single context,
    /// so this method is nessicary to support it.
    pub fn into_context(self) -> CollectorContext<C> {
        unsafe { CollectorContext::register_root(&self) }
    }
}
impl<C: SyncCollector> CollectorRef<C> {

    /// Create a new context bound to this collector
    ///
    /// Warning: Only one collector should be created per thread.
    /// Doing otherwise can cause deadlocks/panics.
    pub fn create_context(&self) -> CollectorContext<C> {
        unsafe { CollectorContext::register_root(&self) }
    }
}
impl<C: RawCollectorImpl> Drop for CollectorRef<C> {
    #[inline]
    fn drop(&mut self) {
        unsafe { self.ptr.drop(); }
    }
}

unsafe impl<C: RawCollectorImpl> GcSystem for CollectorRef<C> {
    type Id = CollectorId<C>;
    type Context = CollectorContext<C>;
}

mod sealed {
    pub trait Sealed {}
}