//! The global allocator.
//!
//! This contains primitives for the cross-thread allocator.

use prelude::*;

use core::{mem, ops};

use {brk, sync};
use bookkeeper::{self, Bookkeeper, Allocator};

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

/// Alias for the wrapper type of the thread-local variable holding the local allocator.
#[cfg(feature = "tls")]
type ThreadLocalAllocator = MoveCell<Option<LazyInit<fn() -> LocalAllocator, LocalAllocator>>>;

/// The global default allocator.
// TODO: Remove these filthy function pointers.
static GLOBAL_ALLOCATOR: sync::Mutex<LazyInit<fn() -> GlobalAllocator, GlobalAllocator>> =
    sync::Mutex::new(LazyInit::new(GlobalAllocator::init));
#[cfg(feature = "tls")]
tls! {
    /// The thread-local allocator.
    static THREAD_ALLOCATOR: ThreadLocalAllocator = MoveCell::new(Some(LazyInit::new(LocalAllocator::init)));
}

/// Temporarily get the allocator.
///
/// This is simply to avoid repeating ourself, so we let this take care of the hairy stuff:
///
/// 1. Initialize the allocator if needed.
/// 2. If the allocator is not yet initialized, fallback to the global allocator.
/// 3. Unlock/move temporarily out of reference.
///
/// This is a macro due to the lack of generic closure, which makes it impossible to have one
/// closure for both cases (global and local).
// TODO: Instead of falling back to the global allocator, the thread dtor should be set such that
// it run after the TLS keys that might be declared.
macro_rules! get_allocator {
    (|$v:ident| $b:expr) => {{
        // Get the thread allocator, if TLS is enabled
        #[cfg(feature = "tls")]
        {
            THREAD_ALLOCATOR.with(|thread_alloc| {
                if let Some(mut thread_alloc_original) = thread_alloc.replace(None) {
                    let res = {
                        // Call the closure involved.
                        let $v = thread_alloc_original.get();
                        $b
                    };

                    // Put back the original allocator.
                    thread_alloc.replace(Some(thread_alloc_original));

                    res
                } else {
                    // The local allocator seems to have been deinitialized, for this reason we fallback to
                    // the global allocator.

                    // Lock the global allocator.
                    let mut guard = GLOBAL_ALLOCATOR.lock();

                    // Call the block in question.
                    let $v = guard.get();
                    $b
                }
            })
        }

        // TLS is disabled, just use the global allocator.
        #[cfg(not(feature = "tls"))]
        {
            // Lock the global allocator.
            let mut guard = GLOBAL_ALLOCATOR.lock();

            // Call the block in question.
            let $v = guard.get();
            $b
        }
    }}
}

/// Derives `Deref` and `DerefMut` to the `inner` field.
///
/// This requires importing `core::ops`.
macro_rules! derive_deref {
    ($imp:ty, $target:ty) => {
        impl ops::Deref for $imp {
            type Target = $target;

            fn deref(&self) -> &$target {
                &self.inner
            }
        }

        impl ops::DerefMut for $imp {
            fn deref_mut(&mut self) -> &mut $target {
                &mut self.inner
            }
        }
    };
}

/// Global SBRK-based allocator.
///
/// This will extend the data segment whenever new memory is needed. Since this includes leaving
/// userspace, this shouldn't be used when other allocators are available (i.e. the bookkeeper is
/// local).
struct GlobalAllocator {
    // The inner bookkeeper.
    inner: Bookkeeper,
}

impl GlobalAllocator {
    /// Initialize the global allocator.
    fn init() -> GlobalAllocator {
        // The initial acquired segment.
        let (aligner, initial_segment, excessive) =
            brk::get(4 * bookkeeper::EXTRA_ELEMENTS * mem::size_of::<Block>(), mem::align_of::<Block>());

        // Initialize the new allocator.
        let mut res = GlobalAllocator {
            inner: Bookkeeper::new(unsafe {
                Vec::from_raw_parts(initial_segment, 0)
            }),
        };

        // Free the secondary space.
        res.push(aligner);
        res.push(excessive);

        res
    }
}

derive_deref!(GlobalAllocator, Bookkeeper);

impl Allocator for GlobalAllocator {
    #[inline]
    fn alloc_fresh(&mut self, size: usize, align: usize) -> Block {
        // Obtain what you need.
        let (alignment_block, res, excessive) = brk::get(size, align);

        // Add it to the list. This will not change the order, since the pointer is higher than all
        // the previous blocks (BRK extends the data segment). Although, it is worth noting that
        // the stack is higher than the program break.
        self.push(alignment_block);
        self.push(excessive);

        res
    }
}

/// A local allocator.
///
/// This acquires memory from the upstream (global) allocator, which is protected by a `Mutex`.
#[cfg(feature = "tls")]
pub struct LocalAllocator {
    // The inner bookkeeper.
    inner: Bookkeeper,
}

impl LocalAllocator {
    /// Initialize the local allocator.
    #[cfg(feature = "tls")]
    fn init() -> LocalAllocator {
        /// The destructor of the local allocator.
        ///
        /// This will simply free everything to the global allocator.
        extern fn dtor(alloc: &ThreadLocalAllocator) {
            // This is important! The thread destructors guarantee no other, and thus one could use the
            // allocator _after_ this destructor have been finished. In fact, this is a real problem,
            // and happens when using `Arc` and terminating the main thread, for this reason we place
            // `None` as a permanent marker indicating that the allocator is deinitialized. After such
            // a state is in place, all allocation calls will be redirected to the global allocator,
            // which is of course still usable at this moment.
            let alloc = alloc.replace(None).expect("Thread-local allocator is already freed.");

            // Lock the global allocator.
            let mut global_alloc = GLOBAL_ALLOCATOR.lock();
            let global_alloc = global_alloc.get();

            // TODO: we know this is sorted, so we could abuse that fact to faster insertion in the
            // global allocator.

            alloc.into_inner().inner.for_each(move |block| global_alloc.free(block));
        }

        // The initial acquired segment.
        let initial_segment = GLOBAL_ALLOCATOR
            .lock()
            .get()
            .alloc(4 * bookkeeper::EXTRA_ELEMENTS * mem::size_of::<Block>(), mem::align_of::<Block>());

        unsafe {
            // Register the thread destructor on the current thread.
            THREAD_ALLOCATOR.register_thread_destructor(dtor)
                .expect("Unable to register a thread destructor.");

            LocalAllocator {
                inner: Bookkeeper::new(Vec::from_raw_parts(initial_segment, 0)),
            }
        }
    }

    /// Shuld we memtrim this allocator?
    ///
    /// The idea is to free memory to the global allocator to unify small stubs and avoid
    /// fragmentation and thread accumulation.
    fn should_memtrim(&self) -> bool {
        // TODO: Tweak this.

        /// The fragmentation scale constant.
        ///
        /// This is used for determining the minimum avarage block size before memtrimming.
        const FRAGMENTATION_SCALE: usize = 10;
        /// The local memtrim limit.
        ///
        /// Whenever an allocator has more free bytes than this value, it will be memtrimmed.
        const LOCAL_MEMTRIM_LIMIT: usize = 16384;

        self.total_bytes() < FRAGMENTATION_SCALE * self.len() || self.total_bytes() > LOCAL_MEMTRIM_LIMIT
    }
}

#[cfg(feature = "tls")]
derive_deref!(LocalAllocator, Bookkeeper);

#[cfg(feature = "tls")]
impl Allocator for LocalAllocator {
    #[inline]
    fn alloc_fresh(&mut self, size: usize, align: usize) -> Block {
        // Get the block from the global allocator. Please note that we cannot canonicalize `size`,
        // due to freeing excessive blocks would change the order.
        GLOBAL_ALLOCATOR.lock().get().alloc(size, align)
    }

    #[inline]
    fn on_new_memory(&mut self) {
        if self.should_memtrim() {
            // Lock the global allocator.
            let mut global_alloc = GLOBAL_ALLOCATOR.lock();
            let global_alloc = global_alloc.get();

            while let Some(block) = self.pop() {
                // Pop'n'free.
                global_alloc.free(block);

                // Memtrim 'till we can't memtrim anymore.
                if !self.should_memtrim() { break; }
            }
        }
    }
}

/// Allocate a block of memory.
///
/// # Errors
///
/// The OOM handler handles out-of-memory conditions.
#[inline]
pub fn alloc(size: usize, align: usize) -> *mut u8 {
    get_allocator!(|alloc| *Pointer::from(alloc.alloc(size, align)))
}

/// Free a buffer.
///
/// Note that this do not have to be a buffer allocated through ralloc. The only requirement is
/// that it is not used after the free.
///
/// # Important!
///
/// You should only allocate buffers allocated through `ralloc`. Anything else is considered
/// invalid.
///
/// # Errors
///
/// The OOM handler handles out-of-memory conditions.
///
/// # Safety
///
/// Rust assume that the allocation symbols returns correct values. For this reason, freeing
/// invalid pointers might introduce memory unsafety.
///
/// Secondly, freeing an used buffer can introduce use-after-free.
#[inline]
pub unsafe fn free(ptr: *mut u8, size: usize) {
    get_allocator!(|alloc| alloc.free(Block::from_raw_parts(Pointer::new(ptr), size)))
}

/// Reallocate memory.
///
/// Reallocate the buffer starting at `ptr` with size `old_size`, to a buffer starting at the
/// returned pointer with size `size`.
///
/// # Important!
///
/// You should only reallocate buffers allocated through `ralloc`. Anything else is considered
/// invalid.
///
/// # Errors
///
/// The OOM handler handles out-of-memory conditions.
///
/// # Safety
///
/// Due to being able to potentially memcpy an arbitrary buffer, as well as shrinking a buffer,
/// this is marked unsafe.
#[inline]
pub unsafe fn realloc(ptr: *mut u8, old_size: usize, size: usize, align: usize) -> *mut u8 {
    get_allocator!(|alloc| {
        *Pointer::from(alloc.realloc(
            Block::from_raw_parts(Pointer::new(ptr), old_size),
            size,
            align
        ))
    })
}

/// Try to reallocate the buffer _inplace_.
///
/// In case of success, return the new buffer's size. On failure, return the old size.
///
/// This can be used to shrink (truncate) a buffer as well.
///
/// # Safety
///
/// Due to being able to shrink (and thus free) the buffer, this is marked unsafe.
#[inline]
pub unsafe fn realloc_inplace(ptr: *mut u8, old_size: usize, size: usize) -> Result<(), ()> {
    get_allocator!(|alloc| {
        if alloc.realloc_inplace(
            Block::from_raw_parts(Pointer::new(ptr), old_size),
            size
        ).is_ok() {
            Ok(())
        } else {
            Err(())
        }
    })
}