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    // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use core::ptr::Unique;
    use core::mem;
    use core::slice;
    use heap;
    use super::oom;
    use super::boxed::Box;
    use core::ops::Drop;
    use core::cmp;

    /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
    /// a buffer of memory on the heap without having to worry about all the corner cases
    /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
    /// In particular:
    ///
    /// * Produces heap::EMPTY on zero-sized types
    /// * Produces heap::EMPTY on zero-length allocations
    /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics)
    /// * Guards against 32-bit systems allocating more than isize::MAX bytes
    /// * Guards against overflowing your length
    /// * Aborts on OOM
    /// * Avoids freeing heap::EMPTY
    /// * Contains a ptr::Unique and thus endows the user with all related benefits
    ///
    /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
    /// free its memory, but it *won't* try to Drop its contents. It is up to the user of RawVec
    /// to handle the actual things *stored* inside of a RawVec.
    ///
    /// Note that a RawVec always forces its capacity to be usize::MAX for zero-sized types.
    /// This enables you to use capacity growing logic catch the overflows in your length
    /// that might occur with zero-sized types.
    ///
    /// However this means that you need to be careful when roundtripping this type
    /// with a `Box<[T]>`: `cap()` won't yield the len. However `with_capacity`,
    /// `shrink_to_fit`, and `from_box` will actually set RawVec's private capacity
    /// field. This allows zero-sized types to not be special-cased by consumers of
    /// this type.
    pub struct RawVec<T> {
        ptr: Unique<T>,
        cap: usize,
    }

    impl<T> RawVec<T> {
        /// Creates the biggest possible RawVec without allocating. If T has positive
        /// size, then this makes a RawVec with capacity 0. If T has 0 size, then it
        /// it makes a RawVec with capacity `usize::MAX`. Useful for implementing
        /// delayed allocation.
        pub fn new() -> Self {
            unsafe {
                // !0 is usize::MAX. This branch should be stripped at compile time.
                let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };

                // heap::EMPTY doubles as "unallocated" and "zero-sized allocation"
                RawVec {
                    ptr: Unique::new(heap::EMPTY as *mut T),
                    cap: cap,
                }
            }
        }

        /// Creates a RawVec with exactly the capacity and alignment requirements
        /// for a `[T; cap]`. This is equivalent to calling RawVec::new when `cap` is 0
        /// or T is zero-sized. Note that if `T` is zero-sized this means you will *not*
        /// get a RawVec with the requested capacity!
        ///
        /// # Panics
        ///
        /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM
        pub fn with_capacity(cap: usize) -> Self {
            unsafe {
                let elem_size = mem::size_of::<T>();

                let alloc_size = cap.checked_mul(elem_size).expect("capacity overflow");
                alloc_guard(alloc_size);

                // handles ZSTs and `cap = 0` alike
                let ptr = if alloc_size == 0 {
                    heap::EMPTY as *mut u8
                } else {
                    let align = mem::align_of::<T>();
                    let ptr = heap::allocate(alloc_size, align);
                    if ptr.is_null() {
                        oom()
                    }
                    ptr
                };

                RawVec {
                    ptr: Unique::new(ptr as *mut _),
                    cap: cap,
                }
            }
        }

        /// Reconstitutes a RawVec from a pointer and capacity.
        ///
        /// # Undefined Behavior
        ///
        /// The ptr must be allocated, and with the given capacity. The
        /// capacity cannot exceed `isize::MAX` (only a concern on 32-bit systems).
        /// If the ptr and capacity come from a RawVec, then this is guaranteed.
        pub unsafe fn from_raw_parts(ptr: *mut T, cap: usize) -> Self {
            RawVec {
                ptr: Unique::new(ptr),
                cap: cap,
            }
        }

        /// Converts a `Box<[T]>` into a `RawVec<T>`.
        pub fn from_box(mut slice: Box<[T]>) -> Self {
            unsafe {
                let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
                mem::forget(slice);
                result
            }
        }
    }

    impl<T> RawVec<T> {
        /// Gets a raw pointer to the start of the allocation. Note that this is
        /// heap::EMPTY if `cap = 0` or T is zero-sized. In the former case, you must
        /// be careful.
        pub fn ptr(&self) -> *mut T {
            *self.ptr
        }

        /// Gets the capacity of the allocation.
        ///
        /// This will always be `usize::MAX` if `T` is zero-sized.
        #[inline(always)]
        pub fn cap(&self) -> usize {
            if mem::size_of::<T>() == 0 {
                !0
            } else {
                self.cap
            }
        }

        /// Doubles the size of the type's backing allocation. This is common enough
        /// to want to do that it's easiest to just have a dedicated method. Slightly
        /// more efficient logic can be provided for this than the general case.
        ///
        /// This function is ideal for when pushing elements one-at-a-time because
        /// you don't need to incur the costs of the more general computations
        /// reserve needs to do to guard against overflow. You do however need to
        /// manually check if your `len == cap`.
        ///
        /// # Panics
        ///
        /// * Panics if T is zero-sized on the assumption that you managed to exhaust
        ///   all `usize::MAX` slots in your imaginary buffer.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM
        ///
        /// # Examples
        ///
        /// ```ignore
        /// struct MyVec<T> {
        ///     buf: RawVec<T>,
        ///     len: usize,
        /// }
        ///
        /// impl<T> MyVec<T> {
        ///     pub fn push(&mut self, elem: T) {
        ///         if self.len == self.buf.cap() { self.buf.double(); }
        ///         // double would have aborted or panicked if the len exceeded
        ///         // `isize::MAX` so this is safe to do unchecked now.
        ///         unsafe {
        ///             ptr::write(self.buf.ptr().offset(self.len as isize), elem);
        ///         }
        ///         self.len += 1;
        ///     }
        /// }
        /// ```
        #[inline(never)]
        #[cold]
        pub fn double(&mut self) {
            unsafe {
                let elem_size = mem::size_of::<T>();

                // since we set the capacity to usize::MAX when elem_size is
                // 0, getting to here necessarily means the RawVec is overfull.
                assert!(elem_size != 0, "capacity overflow");

                let align = mem::align_of::<T>();

                let (new_cap, ptr) = if self.cap == 0 {
                    // skip to 4 because tiny Vec's are dumb; but not if that would cause overflow
                    let new_cap = if elem_size > (!0) / 8 { 1 } else { 4 };
                    let ptr = heap::allocate(new_cap * elem_size, align);
                    (new_cap, ptr)
                } else {
                    // Since we guarantee that we never allocate more than isize::MAX bytes,
                    // `elem_size * self.cap <= isize::MAX` as a precondition, so this can't overflow
                    let new_cap = 2 * self.cap;
                    let new_alloc_size = new_cap * elem_size;
                    alloc_guard(new_alloc_size);
                    let ptr = heap::reallocate(self.ptr() as *mut _,
                                               self.cap * elem_size,
                                               new_alloc_size,
                                               align);
                    (new_cap, ptr)
                };

                // If allocate or reallocate fail, we'll get `null` back
                if ptr.is_null() {
                    oom()
                }

                self.ptr = Unique::new(ptr as *mut _);
                self.cap = new_cap;
            }
        }

        /// Attempts to double the size of the type's backing allocation in place. This is common
        /// enough to want to do that it's easiest to just have a dedicated method. Slightly
        /// more efficient logic can be provided for this than the general case.
        ///
        /// Returns true if the reallocation attempt has succeeded, or false otherwise.
        ///
        /// # Panics
        ///
        /// * Panics if T is zero-sized on the assumption that you managed to exhaust
        ///   all `usize::MAX` slots in your imaginary buffer.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        #[inline(never)]
        #[cold]
        pub fn double_in_place(&mut self) -> bool {
            unsafe {
                let elem_size = mem::size_of::<T>();
                let align = mem::align_of::<T>();

                // since we set the capacity to usize::MAX when elem_size is
                // 0, getting to here necessarily means the RawVec is overfull.
                assert!(elem_size != 0, "capacity overflow");

                // Since we guarantee that we never allocate more than isize::MAX bytes,
                // `elem_size * self.cap <= isize::MAX` as a precondition, so this can't overflow
                let new_cap = 2 * self.cap;
                let new_alloc_size = new_cap * elem_size;

                alloc_guard(new_alloc_size);
                let size = heap::reallocate_inplace(self.ptr() as *mut _,
                                                    self.cap * elem_size,
                                                    new_alloc_size,
                                                    align);
                if size >= new_alloc_size {
                    // We can't directly divide `size`.
                    self.cap = new_cap;
                }
                size >= new_alloc_size
            }
        }

        /// Ensures that the buffer contains at least enough space to hold
        /// `used_cap + needed_extra_cap` elements. If it doesn't already,
        /// will reallocate the minimum possible amount of memory necessary.
        /// Generally this will be exactly the amount of memory necessary,
        /// but in principle the allocator is free to give back more than
        /// we asked for.
        ///
        /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
        /// the requested space. This is not really unsafe, but the unsafe
        /// code *you* write that relies on the behavior of this function may break.
        ///
        /// # Panics
        ///
        /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM
        pub fn reserve_exact(&mut self, used_cap: usize, needed_extra_cap: usize) {
            unsafe {
                let elem_size = mem::size_of::<T>();
                let align = mem::align_of::<T>();

                // NOTE: we don't early branch on ZSTs here because we want this
                // to actually catch "asking for more than usize::MAX" in that case.
                // If we make it past the first branch then we are guaranteed to
                // panic.

                // Don't actually need any more capacity.
                // Wrapping in case they gave a bad `used_cap`.
                if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
                    return;
                }

                // Nothing we can really do about these checks :(
                let new_cap = used_cap.checked_add(needed_extra_cap).expect("capacity overflow");
                let new_alloc_size = new_cap.checked_mul(elem_size).expect("capacity overflow");
                alloc_guard(new_alloc_size);

                let ptr = if self.cap == 0 {
                    heap::allocate(new_alloc_size, align)
                } else {
                    heap::reallocate(self.ptr() as *mut _,
                                     self.cap * elem_size,
                                     new_alloc_size,
                                     align)
                };

                // If allocate or reallocate fail, we'll get `null` back
                if ptr.is_null() {
                    oom()
                }

                self.ptr = Unique::new(ptr as *mut _);
                self.cap = new_cap;
            }
        }

        /// Calculates the buffer's new size given that it'll hold `used_cap +
        /// needed_extra_cap` elements. This logic is used in amortized reserve methods.
        /// Returns `(new_capacity, new_alloc_size)`.
        fn amortized_new_size(&self, used_cap: usize, needed_extra_cap: usize) -> (usize, usize) {
            let elem_size = mem::size_of::<T>();
            // Nothing we can really do about these checks :(
            let required_cap = used_cap.checked_add(needed_extra_cap)
                .expect("capacity overflow");
            // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
            let double_cap = self.cap * 2;
            // `double_cap` guarantees exponential growth.
            let new_cap = cmp::max(double_cap, required_cap);
            let new_alloc_size = new_cap.checked_mul(elem_size).expect("capacity overflow");
            (new_cap, new_alloc_size)
        }

        /// Ensures that the buffer contains at least enough space to hold
        /// `used_cap + needed_extra_cap` elements. If it doesn't already have
        /// enough capacity, will reallocate enough space plus comfortable slack
        /// space to get amortized `O(1)` behavior. Will limit this behavior
        /// if it would needlessly cause itself to panic.
        ///
        /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
        /// the requested space. This is not really unsafe, but the unsafe
        /// code *you* write that relies on the behavior of this function may break.
        ///
        /// This is ideal for implementing a bulk-push operation like `extend`.
        ///
        /// # Panics
        ///
        /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM
        ///
        /// # Examples
        ///
        /// ```ignore
        /// struct MyVec<T> {
        ///     buf: RawVec<T>,
        ///     len: usize,
        /// }
        ///
        /// impl<T> MyVec<T> {
        ///     pub fn push_all(&mut self, elems: &[T]) {
        ///         self.buf.reserve(self.len, elems.len());
        ///         // reserve would have aborted or panicked if the len exceeded
        ///         // `isize::MAX` so this is safe to do unchecked now.
        ///         for x in elems {
        ///             unsafe {
        ///                 ptr::write(self.buf.ptr().offset(self.len as isize), x.clone());
        ///             }
        ///             self.len += 1;
        ///         }
        ///     }
        /// }
        /// ```
        pub fn reserve(&mut self, used_cap: usize, needed_extra_cap: usize) {
            unsafe {
                let elem_size = mem::size_of::<T>();
                let align = mem::align_of::<T>();

                // NOTE: we don't early branch on ZSTs here because we want this
                // to actually catch "asking for more than usize::MAX" in that case.
                // If we make it past the first branch then we are guaranteed to
                // panic.

                // Don't actually need any more capacity.
                // Wrapping in case they give a bad `used_cap`
                if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
                    return;
                }

                let (new_cap, new_alloc_size) = self.amortized_new_size(used_cap, needed_extra_cap);
                // FIXME: may crash and burn on over-reserve
                alloc_guard(new_alloc_size);

                let ptr = if self.cap == 0 {
                    heap::allocate(new_alloc_size, align)
                } else {
                    heap::reallocate(self.ptr() as *mut _,
                                     self.cap * elem_size,
                                     new_alloc_size,
                                     align)
                };

                // If allocate or reallocate fail, we'll get `null` back
                if ptr.is_null() {
                    oom()
                }

                self.ptr = Unique::new(ptr as *mut _);
                self.cap = new_cap;
            }
        }

        /// Attempts to ensure that the buffer contains at least enough space to hold
        /// `used_cap + needed_extra_cap` elements. If it doesn't already have
        /// enough capacity, will reallocate in place enough space plus comfortable slack
        /// space to get amortized `O(1)` behaviour. Will limit this behaviour
        /// if it would needlessly cause itself to panic.
        ///
        /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
        /// the requested space. This is not really unsafe, but the unsafe
        /// code *you* write that relies on the behaviour of this function may break.
        ///
        /// Returns true if the reallocation attempt has succeeded, or false otherwise.
        ///
        /// # Panics
        ///
        /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        pub fn reserve_in_place(&mut self, used_cap: usize, needed_extra_cap: usize) -> bool {
            unsafe {
                let elem_size = mem::size_of::<T>();
                let align = mem::align_of::<T>();

                // NOTE: we don't early branch on ZSTs here because we want this
                // to actually catch "asking for more than usize::MAX" in that case.
                // If we make it past the first branch then we are guaranteed to
                // panic.

                // Don't actually need any more capacity. If the current `cap` is 0, we can't
                // reallocate in place.
                // Wrapping in case they give a bad `used_cap`
                if self.cap().wrapping_sub(used_cap) >= needed_extra_cap || self.cap == 0 {
                    return false;
                }

                let (_, new_alloc_size) = self.amortized_new_size(used_cap, needed_extra_cap);
                // FIXME: may crash and burn on over-reserve
                alloc_guard(new_alloc_size);

                let size = heap::reallocate_inplace(self.ptr() as *mut _,
                                                    self.cap * elem_size,
                                                    new_alloc_size,
                                                    align);
                if size >= new_alloc_size {
                    self.cap = new_alloc_size / elem_size;
                }
                size >= new_alloc_size
            }
        }

        /// Shrinks the allocation down to the specified amount. If the given amount
        /// is 0, actually completely deallocates.
        ///
        /// # Panics
        ///
        /// Panics if the given amount is *larger* than the current capacity.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM.
        pub fn shrink_to_fit(&mut self, amount: usize) {
            let elem_size = mem::size_of::<T>();
            let align = mem::align_of::<T>();

            // Set the `cap` because they might be about to promote to a `Box<[T]>`
            if elem_size == 0 {
                self.cap = amount;
                return;
            }

            // This check is my waterloo; it's the only thing Vec wouldn't have to do.
            assert!(self.cap >= amount, "Tried to shrink to a larger capacity");

            if amount == 0 {
                mem::replace(self, RawVec::new());
            } else if self.cap != amount {
                unsafe {
                    // Overflow check is unnecessary as the vector is already at
                    // least this large.
                    let ptr = heap::reallocate(self.ptr() as *mut _,
                                               self.cap * elem_size,
                                               amount * elem_size,
                                               align);
                    if ptr.is_null() {
                        oom()
                    }
                    self.ptr = Unique::new(ptr as *mut _);
                }
                self.cap = amount;
            }
        }

        /// Converts the entire buffer into `Box<[T]>`.
        ///
        /// While it is not *strictly* Undefined Behavior to call
        /// this procedure while some of the RawVec is uninitialized,
        /// it certainly makes it trivial to trigger it.
        ///
        /// Note that this will correctly reconstitute any `cap` changes
        /// that may have been performed. (see description of type for details)
        pub unsafe fn into_box(self) -> Box<[T]> {
            // NOTE: not calling `cap()` here, actually using the real `cap` field!
            let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
            let output: Box<[T]> = Box::from_raw(slice);
            mem::forget(self);
            output
        }
    }

    unsafe impl<#[may_dangle] T> Drop for RawVec<T> {
        /// Frees the memory owned by the RawVec *without* trying to Drop its contents.
        fn drop(&mut self) {
            let elem_size = mem::size_of::<T>();
            if elem_size != 0 && self.cap != 0 {
                let align = mem::align_of::<T>();

                let num_bytes = elem_size * self.cap;
                unsafe {
                    heap::deallocate(*self.ptr as *mut _, num_bytes, align);
                }
            }
        }
    }

    // We need to guarantee the following:
    // * We don't ever allocate `> isize::MAX` byte-size objects
    // * We don't overflow `usize::MAX` and actually allocate too little
    //
    // On 64-bit we just need to check for overflow since trying to allocate
    // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
    // an extra guard for this in case we're running on a platform which can use
    // all 4GB in user-space. e.g. PAE or x32

    #[inline]
    fn alloc_guard(alloc_size: usize) {
        if mem::size_of::<usize>() < 8 {
            assert!(alloc_size <= ::core::isize::MAX as usize,
                    "capacity overflow");
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn reserve_does_not_overallocate() {
            {
                let mut v: RawVec<u32> = RawVec::new();
                // First `reserve` allocates like `reserve_exact`
                v.reserve(0, 9);
                assert_eq!(9, v.cap());
            }

            {
                let mut v: RawVec<u32> = RawVec::new();
                v.reserve(0, 7);
                assert_eq!(7, v.cap());
                // 97 if more than double of 7, so `reserve` should work
                // like `reserve_exact`.
                v.reserve(7, 90);
                assert_eq!(97, v.cap());
            }

            {
                let mut v: RawVec<u32> = RawVec::new();
                v.reserve(0, 12);
                assert_eq!(12, v.cap());
                v.reserve(12, 3);
                // 3 is less than half of 12, so `reserve` must grow
                // exponentially. At the time of writing this test grow
                // factor is 2, so new capacity is 24, however, grow factor
                // of 1.5 is OK too. Hence `>= 18` in assert.
                assert!(v.cap() >= 12 + 12 / 2);
            }
        }

    }

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