//   Copyright 2015 Colin Sherratt
//
//   Licensed under the Apache License, Version 2.0 (the "License");
//   you may not use this file except in compliance with the License.
//   You may obtain a copy of the License at
//
//       http://www.apache.org/licenses/LICENSE-2.0
//
//   Unless required by applicable law or agreed to in writing, software
//   distributed under the License is distributed on an "AS IS" BASIS,
//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//   See the License for the specific language governing permissions and
//   limitations under the License.


use std::sync::atomic::AtomicPtr;
use std::sync::Arc;
use std::mem;
use std::ptr;
use std::ops::Deref;
use std::fmt::{self, Debug, Formatter};
use std::marker::PhantomData;
use std::sync::atomic::Ordering;

/// An Atom wraps an AtomicPtr, it allows for safe mutation of an atomic
/// into common Rust Types.
pub struct Atom<P> where P: IntoRawPtr + FromRawPtr {
    inner: AtomicPtr<()>,
    data: PhantomData<P>
}

impl<P> Debug for Atom<P> where P: IntoRawPtr + FromRawPtr {
    fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
        write!(f, "atom({:?})", self.inner.load(Ordering::Relaxed))
    }
}

impl<P> Atom<P> where P: IntoRawPtr + FromRawPtr {
    /// Create a empty Atom
    pub fn empty() -> Atom<P> {
        Atom {
            inner: AtomicPtr::new(ptr::null_mut()),
            data: PhantomData
        }
    }

    /// Create a new Atomic from Pointer P
    pub fn new(value: P) -> Atom<P> {
        Atom {
            inner: AtomicPtr::new(unsafe { value.into_raw() }),
            data: PhantomData
        }
    }

    /// Swap a new value into the Atom, This will try multiple
    /// times until it succeeds. The old value will be returned.
    pub fn swap(&self, v: P) -> Option<P> {
        let new = unsafe { v.into_raw() };
        let old = self.inner.swap(new, Ordering::AcqRel);
        if !old.is_null() {
            Some(unsafe { FromRawPtr::from_raw(old) })
        } else {
            None
        }
    }

    /// Take the value of the Atom replacing it with null pointer
    /// Returning the contents. If the contents was a `null` pointer the
    /// result will be `None`.
    pub fn take(&self) -> Option<P> {
        let old = self.inner.swap(ptr::null_mut(), Ordering::Acquire);
        if !old.is_null() {
            Some(unsafe { FromRawPtr::from_raw(old) })
        } else {
            None
        }
    }

    /// This will do a `CAS` setting the value only if it is NULL
    /// this will return `None` if the value was written,
    /// otherwise a `Some(v)` will be returned, where the value was
    /// the same value that you passed into this function
    pub fn set_if_none(&self, v: P) -> Option<P> {
        let new = unsafe { v.into_raw() };
        let old = self.inner.compare_and_swap(ptr::null_mut(), new, Ordering::Release);
        if !old.is_null() {
            Some(unsafe { FromRawPtr::from_raw(new) })
        } else {
            None
        }
    }

    /// Take the current content, write it into P then do a CAS to extent this
    /// Atom with the previous contents. This can be used to create a LIFO
    ///
    /// Returns true if this set this migrated the Atom from null.
    pub fn replace_and_set_next(&self, mut value: P) -> bool
        where P: GetNextMut<NextPtr=Option<P>> {
        unsafe {
            let next = value.get_next() as *mut Option<P>;
            let raw = value.into_raw();
            // Iff next was set to Some(P) we want to 
            // assert that it was droppeds
            drop(ptr::read(next));
            loop {
                let pcurrent = self.inner.load(Ordering::Relaxed);
                let current = if pcurrent.is_null() {
                    None
                } else {
                    Some(FromRawPtr::from_raw(pcurrent))
                };
                ptr::write(next, current);
                let last = self.inner.compare_and_swap(pcurrent, raw, Ordering::AcqRel);
                if last == pcurrent {
                    return last.is_null();
                }
            }
        }
    }

    /// Check to see if an atom is None
    ///
    /// This only means that the contents was None when it was measured
    pub fn is_none(&self) -> bool {
        self.inner.load(Ordering::Relaxed).is_null()
    }
}

impl<P> Drop for Atom<P> where P: IntoRawPtr + FromRawPtr  {
    fn drop(&mut self) {
        unsafe {
            let ptr = self.inner.load(Ordering::Relaxed);
            if !ptr.is_null() {
                let _: P = FromRawPtr::from_raw(ptr);
            }
        }
    }
}

unsafe impl<P> Send for Atom<P> where P: IntoRawPtr + FromRawPtr {}
unsafe impl<P> Sync for Atom<P> where P: IntoRawPtr + FromRawPtr {}

/// Convert from into a raw pointer
pub trait IntoRawPtr {
    unsafe fn into_raw(self) -> *mut ();
}

/// Convert from a raw ptr into a pointer
pub trait FromRawPtr {
    unsafe fn from_raw(ptr: *mut ()) -> Self;
}

impl<T> IntoRawPtr for Box<T> {
    unsafe fn into_raw(self) -> *mut () {
        mem::transmute(self)
    }
}

impl<T> FromRawPtr for Box<T> {
    unsafe fn from_raw(ptr: *mut ()) -> Box<T> {
        mem::transmute(ptr)
    }
}

impl<T> IntoRawPtr for Arc<T> {
    unsafe fn into_raw(self) -> *mut () {
        mem::transmute(self)
    }
}

impl<T> FromRawPtr for Arc<T> {
    unsafe fn from_raw(ptr: *mut ()) -> Arc<T> {
        mem::transmute(ptr)
    }
}

/// Transforms lifetime of the second pointer to match the first.
#[inline]
unsafe fn copy_lifetime<'a, S: ?Sized, T: ?Sized + 'a>(_ptr: &'a S, 
                                                       ptr: &T) -> &'a T {
    mem::transmute(ptr)
}


/// Transforms lifetime of the second pointer to match the first.
#[inline]
unsafe fn copy_mut_lifetime<'a, S: ?Sized, T: ?Sized + 'a>(_ptr: &'a S, 
                                                    ptr: &mut T) -> &'a mut T {
    mem::transmute(ptr)
}


/// This is a restricted version of the Atom. It allows for only
/// `set_if_none` to be called.
///
/// `swap` and `take` can be used only with a mutable reference. Meaning
/// that AtomSetOnce is not usable as a 
#[derive(Debug)]
pub struct AtomSetOnce<P> where P: IntoRawPtr + FromRawPtr {
    inner: Atom<P>
}

impl<P> AtomSetOnce<P>
    where P: IntoRawPtr + FromRawPtr {

    /// Create a empty AtomSetOnce
    pub fn empty() -> AtomSetOnce<P> {
        AtomSetOnce { inner: Atom::empty() }
    }

    /// Create a new AtomSetOnce from Pointer P
    pub fn new(value: P) -> AtomSetOnce<P> {
        AtomSetOnce { inner: Atom::new(value) }
    }

    /// This will do a `CAS` setting the value only if it is NULL
    /// this will return `OK(())` if the value was written,
    /// otherwise a `Err(P)` will be returned, where the value was
    /// the same value that you passed into this function
    pub fn set_if_none(&self, v: P) -> Option<P> {
        self.inner.set_if_none(v)
    }

    /// Convert an AtomSetOnce into an Atom
    pub fn into_atom(self) -> Atom<P> { self.inner }

    /// Allow access to the atom if exclusive access is granted
    pub fn atom(&mut self) -> &mut Atom<P> { &mut self.inner }

    /// Check to see if an atom is None
    ///
    /// This only means that the contents was None when it was measured
    pub fn is_none(&self) -> bool { self.inner.is_none() }
}

impl<T, P> AtomSetOnce<P>
    where P: IntoRawPtr + FromRawPtr + Deref<Target=T> {

    /// If the Atom is set, get the value
    pub fn get<'a>(&'a self) -> Option<&'a T> {
        let ptr = self.inner.inner.load(Ordering::Acquire);
        if ptr.is_null() {
            None
        } else {
            unsafe {
                // This is safe since ptr cannot be changed once it is set
                // which means that this is now a Arc or a Box.
                let v: P = FromRawPtr::from_raw(ptr);
                let out = copy_lifetime(self, &*v);
                mem::forget(v);
                Some(out)
            }
        }
    }
}

impl<T> AtomSetOnce<Box<T>> {
    /// If the Atom is set, get the value
    pub fn get_mut<'a>(&'a mut self) -> Option<&'a mut T> {
        let ptr = self.inner.inner.load(Ordering::Acquire);
        if ptr.is_null() {
            None
        } else {
            unsafe {
                // This is safe since ptr cannot be changed once it is set
                // which means that this is now a Arc or a Box.
                let mut v: Box<T> = FromRawPtr::from_raw(ptr);
                let out = copy_mut_lifetime(self, &mut *v);
                mem::forget(v);
                Some(out)
            }
        }
    }
}

impl<T> AtomSetOnce<T> where T: Clone+IntoRawPtr+FromRawPtr {
    /// Duplicate the inner pointer if it is set
    pub fn dup<'a>(&self) -> Option<T> {
        let ptr = self.inner.inner.load(Ordering::Acquire);
        if ptr.is_null() {
            None
        } else {
            unsafe {
                // This is safe since ptr cannot be changed once it is set
                // which means that this is now a Arc or a Box.
                let v: T = FromRawPtr::from_raw(ptr);
                let out = v.clone();
                mem::forget(v);
                Some(out)
            }
        }
    }
}

/// This is a utility Trait that fetches the next ptr from
/// an object.
pub trait GetNextMut {
    type NextPtr;
    fn get_next(&mut self) -> &mut Self::NextPtr;
}