table.rs.html -- source
Description: Source to the Rust file `libstd/collections/hash/table.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2014-2015 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 alloc::{Global, Alloc, Layout, LayoutErr, handle_alloc_error};
    use collections::CollectionAllocErr;
    use hash::{BuildHasher, Hash, Hasher};
    use marker;
    use mem::{size_of, needs_drop};
    use mem;
    use ops::{Deref, DerefMut};
    use ptr::{self, Unique, NonNull};
    use hint;

    use self::BucketState::*;

    /// Integer type used for stored hash values.
    ///
    /// No more than bit_width(usize) bits are needed to select a bucket.
    ///
    /// The most significant bit is ours to use for tagging `SafeHash`.
    ///
    /// (Even if we could have usize::MAX bytes allocated for buckets,
    /// each bucket stores at least a `HashUint`, so there can be no more than
    /// usize::MAX / size_of(usize) buckets.)
    type HashUint = usize;

    const EMPTY_BUCKET: HashUint = 0;
    const EMPTY: usize = 1;

    /// Special `Unique<HashUint>` that uses the lower bit of the pointer
    /// to expose a boolean tag.
    /// Note: when the pointer is initialized to EMPTY `.ptr()` will return
    /// null and the tag functions shouldn't be used.
    struct TaggedHashUintPtr(Unique<HashUint>);

    impl TaggedHashUintPtr {
        #[inline]
        unsafe fn new(ptr: *mut HashUint) -> Self {
            debug_assert!(ptr as usize & 1 == 0 || ptr as usize == EMPTY as usize);
            TaggedHashUintPtr(Unique::new_unchecked(ptr))
        }

        #[inline]
        fn set_tag(&mut self, value: bool) {
            let mut usize_ptr = self.0.as_ptr() as usize;
            unsafe {
                if value {
                    usize_ptr |= 1;
                } else {
                    usize_ptr &= !1;
                }
                self.0 = Unique::new_unchecked(usize_ptr as *mut HashUint)
            }
        }

        #[inline]
        fn tag(&self) -> bool {
            (self.0.as_ptr() as usize) & 1 == 1
        }

        #[inline]
        fn ptr(&self) -> *mut HashUint {
            (self.0.as_ptr() as usize & !1) as *mut HashUint
        }
    }

    /// The raw hashtable, providing safe-ish access to the unzipped and highly
    /// optimized arrays of hashes, and key-value pairs.
    ///
    /// This design is a lot faster than the naive
    /// `Vec<Option<(u64, K, V)>>`, because we don't pay for the overhead of an
    /// option on every element, and we get a generally more cache-aware design.
    ///
    /// Essential invariants of this structure:
    ///
    ///   - if `t.hashes[i] == EMPTY_BUCKET`, then `Bucket::at_index(&t, i).raw`
    ///     points to 'undefined' contents. Don't read from it. This invariant is
    ///     enforced outside this module with the `EmptyBucket`, `FullBucket`,
    ///     and `SafeHash` types.
    ///
    ///   - An `EmptyBucket` is only constructed at an index with
    ///     a hash of EMPTY_BUCKET.
    ///
    ///   - A `FullBucket` is only constructed at an index with a
    ///     non-EMPTY_BUCKET hash.
    ///
    ///   - A `SafeHash` is only constructed for non-`EMPTY_BUCKET` hash. We get
    ///     around hashes of zero by changing them to 0x8000_0000_0000_0000,
    ///     which will likely map to the same bucket, while not being confused
    ///     with "empty".
    ///
    ///   - Both "arrays represented by pointers" are the same length:
    ///     `capacity`. This is set at creation and never changes. The arrays
    ///     are unzipped and are more cache aware (scanning through 8 hashes
    ///     brings in at most 2 cache lines, since they're all right beside each
    ///     other). This layout may waste space in padding such as in a map from
    ///     u64 to u8, but is a more cache conscious layout as the key-value pairs
    ///     are only very shortly probed and the desired value will be in the same
    ///     or next cache line.
    ///
    /// You can kind of think of this module/data structure as a safe wrapper
    /// around just the "table" part of the hashtable. It enforces some
    /// invariants at the type level and employs some performance trickery,
    /// but in general is just a tricked out `Vec<Option<(u64, K, V)>>`.
    ///
    /// The hashtable also exposes a special boolean tag. The tag defaults to false
    /// when the RawTable is created and is accessible with the `tag` and `set_tag`
    /// functions.
    pub struct RawTable<K, V> {
        capacity_mask: usize,
        size: usize,
        hashes: TaggedHashUintPtr,

        // Because K/V do not appear directly in any of the types in the struct,
        // inform rustc that in fact instances of K and V are reachable from here.
        marker: marker::PhantomData<(K, V)>,
    }

    // An unsafe view of a RawTable bucket
    // Valid indexes are within [0..table_capacity)
    pub struct RawBucket<K, V> {
        hash_start: *mut HashUint,
        // We use *const to ensure covariance with respect to K and V
        pair_start: *const (K, V),
        idx: usize,
        _marker: marker::PhantomData<(K, V)>,
    }

    impl<K, V> Copy for RawBucket<K, V> {}
    impl<K, V> Clone for RawBucket<K, V> {
        fn clone(&self) -> RawBucket<K, V> {
            *self
        }
    }

    pub struct Bucket<K, V, M> {
        raw: RawBucket<K, V>,
        table: M,
    }

    impl<K, V, M: Copy> Copy for Bucket<K, V, M> {}
    impl<K, V, M: Copy> Clone for Bucket<K, V, M> {
        fn clone(&self) -> Bucket<K, V, M> {
            *self
        }
    }

    pub struct EmptyBucket<K, V, M> {
        raw: RawBucket<K, V>,
        table: M,
    }

    pub struct FullBucket<K, V, M> {
        raw: RawBucket<K, V>,
        table: M,
    }

    pub type FullBucketMut<'table, K, V> = FullBucket<K, V, &'table mut RawTable<K, V>>;

    pub enum BucketState<K, V, M> {
        Empty(EmptyBucket<K, V, M>),
        Full(FullBucket<K, V, M>),
    }

    // A GapThenFull encapsulates the state of two consecutive buckets at once.
    // The first bucket, called the gap, is known to be empty.
    // The second bucket is full.
    pub struct GapThenFull<K, V, M> {
        gap: EmptyBucket<K, V, ()>,
        full: FullBucket<K, V, M>,
    }

    /// A hash that is not zero, since we use a hash of zero to represent empty
    /// buckets.
    #[derive(PartialEq, Copy, Clone)]
    pub struct SafeHash {
        hash: HashUint,
    }

    impl SafeHash {
        /// Peek at the hash value, which is guaranteed to be non-zero.
        #[inline(always)]
        pub fn inspect(&self) -> HashUint {
            self.hash
        }

        #[inline(always)]
        pub fn new(hash: u64) -> Self {
            // We need to avoid 0 in order to prevent collisions with
            // EMPTY_HASH. We can maintain our precious uniform distribution
            // of initial indexes by unconditionally setting the MSB,
            // effectively reducing the hashes by one bit.
            //
            // Truncate hash to fit in `HashUint`.
            let hash_bits = size_of::<HashUint>() * 8;
            SafeHash { hash: (1 << (hash_bits - 1)) | (hash as HashUint) }
        }
    }

    /// We need to remove hashes of 0. That's reserved for empty buckets.
    /// This function wraps up `hash_keyed` to be the only way outside this
    /// module to generate a SafeHash.
    pub fn make_hash<T: ?Sized, S>(hash_state: &S, t: &T) -> SafeHash
        where T: Hash,
              S: BuildHasher
    {
        let mut state = hash_state.build_hasher();
        t.hash(&mut state);
        SafeHash::new(state.finish())
    }

    // `replace` casts a `*HashUint` to a `*SafeHash`. Since we statically
    // ensure that a `FullBucket` points to an index with a non-zero hash,
    // and a `SafeHash` is just a `HashUint` with a different name, this is
    // safe.
    //
    // This test ensures that a `SafeHash` really IS the same size as a
    // `HashUint`. If you need to change the size of `SafeHash` (and
    // consequently made this test fail), `replace` needs to be
    // modified to no longer assume this.
    #[test]
    fn can_alias_safehash_as_hash() {
        assert_eq!(size_of::<SafeHash>(), size_of::<HashUint>())
    }

    // RawBucket methods are unsafe as it's possible to
    // make a RawBucket point to invalid memory using safe code.
    impl<K, V> RawBucket<K, V> {
        unsafe fn hash(&self) -> *mut HashUint {
            self.hash_start.add(self.idx)
        }
        unsafe fn pair(&self) -> *mut (K, V) {
            self.pair_start.add(self.idx) as *mut (K, V)
        }
        unsafe fn hash_pair(&self) -> (*mut HashUint, *mut (K, V)) {
            (self.hash(), self.pair())
        }
    }

    // Buckets hold references to the table.
    impl<K, V, M> FullBucket<K, V, M> {
        /// Borrow a reference to the table.
        pub fn table(&self) -> &M {
            &self.table
        }
        /// Borrow a mutable reference to the table.
        pub fn table_mut(&mut self) -> &mut M {
            &mut self.table
        }
        /// Move out the reference to the table.
        pub fn into_table(self) -> M {
            self.table
        }
        /// Get the raw index.
        pub fn index(&self) -> usize {
            self.raw.idx
        }
        /// Get the raw bucket.
        pub fn raw(&self) -> RawBucket<K, V> {
            self.raw
        }
    }

    impl<K, V, M> EmptyBucket<K, V, M> {
        /// Borrow a reference to the table.
        pub fn table(&self) -> &M {
            &self.table
        }
        /// Borrow a mutable reference to the table.
        pub fn table_mut(&mut self) -> &mut M {
            &mut self.table
        }
    }

    impl<K, V, M> Bucket<K, V, M> {
        /// Get the raw index.
        pub fn index(&self) -> usize {
            self.raw.idx
        }
        /// get the table.
        pub fn into_table(self) -> M {
            self.table
        }
    }

    impl<K, V, M> Deref for FullBucket<K, V, M>
        where M: Deref<Target = RawTable<K, V>>
    {
        type Target = RawTable<K, V>;
        fn deref(&self) -> &RawTable<K, V> {
            &self.table
        }
    }

    /// `Put` is implemented for types which provide access to a table and cannot be invalidated
    ///  by filling a bucket. A similar implementation for `Take` is possible.
    pub trait Put<K, V> {
        unsafe fn borrow_table_mut(&mut self) -> &mut RawTable<K, V>;
    }

    impl<'t, K, V> Put<K, V> for &'t mut RawTable<K, V> {
        unsafe fn borrow_table_mut(&mut self) -> &mut RawTable<K, V> {
            *self
        }
    }

    impl<K, V, M> Put<K, V> for Bucket<K, V, M>
        where M: Put<K, V>
    {
        unsafe fn borrow_table_mut(&mut self) -> &mut RawTable<K, V> {
            self.table.borrow_table_mut()
        }
    }

    impl<K, V, M> Put<K, V> for FullBucket<K, V, M>
        where M: Put<K, V>
    {
        unsafe fn borrow_table_mut(&mut self) -> &mut RawTable<K, V> {
            self.table.borrow_table_mut()
        }
    }

    impl<K, V, M: Deref<Target = RawTable<K, V>>> Bucket<K, V, M> {
        pub fn new(table: M, hash: SafeHash) -> Bucket<K, V, M> {
            Bucket::at_index(table, hash.inspect() as usize)
        }

        pub fn new_from(r: RawBucket<K, V>, t: M)
            -> Bucket<K, V, M>
        {
            Bucket {
                raw: r,
                table: t,
            }
        }

        pub fn at_index(table: M, ib_index: usize) -> Bucket<K, V, M> {
            // if capacity is 0, then the RawBucket will be populated with bogus pointers.
            // This is an uncommon case though, so avoid it in release builds.
            debug_assert!(table.capacity() > 0,
                          "Table should have capacity at this point");
            let ib_index = ib_index & table.capacity_mask;
            Bucket {
                raw: table.raw_bucket_at(ib_index),
                table,
            }
        }

        pub fn first(table: M) -> Bucket<K, V, M> {
            Bucket {
                raw: table.raw_bucket_at(0),
                table,
            }
        }

        // "So a few of the first shall be last: for many be called,
        // but few chosen."
        //
        // We'll most likely encounter a few buckets at the beginning that
        // have their initial buckets near the end of the table. They were
        // placed at the beginning as the probe wrapped around the table
        // during insertion. We must skip forward to a bucket that won't
        // get reinserted too early and won't unfairly steal others spot.
        // This eliminates the need for robin hood.
        pub fn head_bucket(table: M) -> Bucket<K, V, M> {
            let mut bucket = Bucket::first(table);

            loop {
                bucket = match bucket.peek() {
                    Full(full) => {
                        if full.displacement() == 0 {
                            // This bucket occupies its ideal spot.
                            // It indicates the start of another "cluster".
                            bucket = full.into_bucket();
                            break;
                        }
                        // Leaving this bucket in the last cluster for later.
                        full.into_bucket()
                    }
                    Empty(b) => {
                        // Encountered a hole between clusters.
                        b.into_bucket()
                    }
                };
                bucket.next();
            }
            bucket
        }

        /// Reads a bucket at a given index, returning an enum indicating whether
        /// it's initialized or not. You need to match on this enum to get
        /// the appropriate types to call most of the other functions in
        /// this module.
        pub fn peek(self) -> BucketState<K, V, M> {
            match unsafe { *self.raw.hash() } {
                EMPTY_BUCKET => {
                    Empty(EmptyBucket {
                        raw: self.raw,
                        table: self.table,
                    })
                }
                _ => {
                    Full(FullBucket {
                        raw: self.raw,
                        table: self.table,
                    })
                }
            }
        }

        /// Modifies the bucket in place to make it point to the next slot.
        pub fn next(&mut self) {
            self.raw.idx = self.raw.idx.wrapping_add(1) & self.table.capacity_mask;
        }

        /// Modifies the bucket in place to make it point to the previous slot.
        pub fn prev(&mut self) {
            self.raw.idx = self.raw.idx.wrapping_sub(1) & self.table.capacity_mask;
        }
    }

    impl<K, V, M: Deref<Target = RawTable<K, V>>> EmptyBucket<K, V, M> {
        #[inline]
        pub fn next(self) -> Bucket<K, V, M> {
            let mut bucket = self.into_bucket();
            bucket.next();
            bucket
        }

        #[inline]
        pub fn into_bucket(self) -> Bucket<K, V, M> {
            Bucket {
                raw: self.raw,
                table: self.table,
            }
        }

        pub fn gap_peek(self) -> Result<GapThenFull<K, V, M>, Bucket<K, V, M>> {
            let gap = EmptyBucket {
                raw: self.raw,
                table: (),
            };

            match self.next().peek() {
                Full(bucket) => {
                    Ok(GapThenFull {
                        gap,
                        full: bucket,
                    })
                }
                Empty(e) => Err(e.into_bucket()),
            }
        }
    }

    impl<K, V, M> EmptyBucket<K, V, M>
        where M: Put<K, V>
    {
        /// Puts given key and value pair, along with the key's hash,
        /// into this bucket in the hashtable. Note how `self` is 'moved' into
        /// this function, because this slot will no longer be empty when
        /// we return! A `FullBucket` is returned for later use, pointing to
        /// the newly-filled slot in the hashtable.
        ///
        /// Use `make_hash` to construct a `SafeHash` to pass to this function.
        pub fn put(mut self, hash: SafeHash, key: K, value: V) -> FullBucket<K, V, M> {
            unsafe {
                *self.raw.hash() = hash.inspect();
                ptr::write(self.raw.pair(), (key, value));

                self.table.borrow_table_mut().size += 1;
            }

            FullBucket {
                raw: self.raw,
                table: self.table,
            }
        }
    }

    impl<K, V, M: Deref<Target = RawTable<K, V>>> FullBucket<K, V, M> {
        #[inline]
        pub fn next(self) -> Bucket<K, V, M> {
            let mut bucket = self.into_bucket();
            bucket.next();
            bucket
        }

        #[inline]
        pub fn into_bucket(self) -> Bucket<K, V, M> {
            Bucket {
                raw: self.raw,
                table: self.table,
            }
        }

        /// Duplicates the current position. This can be useful for operations
        /// on two or more buckets.
        pub fn stash(self) -> FullBucket<K, V, Self> {
            FullBucket {
                raw: self.raw,
                table: self,
            }
        }

        /// Get the distance between this bucket and the 'ideal' location
        /// as determined by the key's hash stored in it.
        ///
        /// In the cited blog posts above, this is called the "distance to
        /// initial bucket", or DIB. Also known as "probe count".
        pub fn displacement(&self) -> usize {
            // Calculates the distance one has to travel when going from
            // `hash mod capacity` onwards to `idx mod capacity`, wrapping around
            // if the destination is not reached before the end of the table.
            (self.raw.idx.wrapping_sub(self.hash().inspect() as usize)) & self.table.capacity_mask
        }

        #[inline]
        pub fn hash(&self) -> SafeHash {
            unsafe { SafeHash { hash: *self.raw.hash() } }
        }

        /// Gets references to the key and value at a given index.
        pub fn read(&self) -> (&K, &V) {
            unsafe {
                let pair_ptr = self.raw.pair();
                (&(*pair_ptr).0, &(*pair_ptr).1)
            }
        }
    }

    // We take a mutable reference to the table instead of accepting anything that
    // implements `DerefMut` to prevent fn `take` from being called on `stash`ed
    // buckets.
    impl<'t, K, V> FullBucket<K, V, &'t mut RawTable<K, V>> {
        /// Removes this bucket's key and value from the hashtable.
        ///
        /// This works similarly to `put`, building an `EmptyBucket` out of the
        /// taken bucket.
        pub fn take(self) -> (EmptyBucket<K, V, &'t mut RawTable<K, V>>, K, V) {
            self.table.size -= 1;

            unsafe {
                *self.raw.hash() = EMPTY_BUCKET;
                let (k, v) = ptr::read(self.raw.pair());
                (EmptyBucket {
                     raw: self.raw,
                     table: self.table,
                 },
                k,
                v)
            }
        }
    }

    // This use of `Put` is misleading and restrictive, but safe and sufficient for our use cases
    // where `M` is a full bucket or table reference type with mutable access to the table.
    impl<K, V, M> FullBucket<K, V, M>
        where M: Put<K, V>
    {
        pub fn replace(&mut self, h: SafeHash, k: K, v: V) -> (SafeHash, K, V) {
            unsafe {
                let old_hash = ptr::replace(self.raw.hash() as *mut SafeHash, h);
                let (old_key, old_val) = ptr::replace(self.raw.pair(), (k, v));

                (old_hash, old_key, old_val)
            }
        }
    }

    impl<K, V, M> FullBucket<K, V, M>
        where M: Deref<Target = RawTable<K, V>> + DerefMut
    {
        /// Gets mutable references to the key and value at a given index.
        pub fn read_mut(&mut self) -> (&mut K, &mut V) {
            unsafe {
                let pair_ptr = self.raw.pair();
                (&mut (*pair_ptr).0, &mut (*pair_ptr).1)
            }
        }
    }

    impl<'t, K, V, M> FullBucket<K, V, M>
        where M: Deref<Target = RawTable<K, V>> + 't
    {
        /// Exchange a bucket state for immutable references into the table.
        /// Because the underlying reference to the table is also consumed,
        /// no further changes to the structure of the table are possible;
        /// in exchange for this, the returned references have a longer lifetime
        /// than the references returned by `read()`.
        pub fn into_refs(self) -> (&'t K, &'t V) {
            unsafe {
                let pair_ptr = self.raw.pair();
                (&(*pair_ptr).0, &(*pair_ptr).1)
            }
        }
    }

    impl<'t, K, V, M> FullBucket<K, V, M>
        where M: Deref<Target = RawTable<K, V>> + DerefMut + 't
    {
        /// This works similarly to `into_refs`, exchanging a bucket state
        /// for mutable references into the table.
        pub fn into_mut_refs(self) -> (&'t mut K, &'t mut V) {
            unsafe {
                let pair_ptr = self.raw.pair();
                (&mut (*pair_ptr).0, &mut (*pair_ptr).1)
            }
        }
    }

    impl<K, V, M> GapThenFull<K, V, M>
        where M: Deref<Target = RawTable<K, V>>
    {
        #[inline]
        pub fn full(&self) -> &FullBucket<K, V, M> {
            &self.full
        }

        pub fn into_table(self) -> M {
            self.full.into_table()
        }

        pub fn shift(mut self) -> Result<GapThenFull<K, V, M>, Bucket<K, V, M>> {
            unsafe {
                let (gap_hash, gap_pair) = self.gap.raw.hash_pair();
                let (full_hash, full_pair) = self.full.raw.hash_pair();
                *gap_hash = mem::replace(&mut *full_hash, EMPTY_BUCKET);
                ptr::copy_nonoverlapping(full_pair, gap_pair, 1);
            }

            let FullBucket { raw: prev_raw, .. } = self.full;

            match self.full.next().peek() {
                Full(bucket) => {
                    self.gap.raw = prev_raw;

                    self.full = bucket;

                    Ok(self)
                }
                Empty(b) => Err(b.into_bucket()),
            }
        }
    }

    // Returns a Layout which describes the allocation required for a hash table,
    // and the offset of the array of (key, value) pairs in the allocation.
    fn calculate_layout<K, V>(capacity: usize) -> Result<(Layout, usize), LayoutErr> {
        let hashes = Layout::array::<HashUint>(capacity)?;
        let pairs = Layout::array::<(K, V)>(capacity)?;
        hashes.extend(pairs).map(|(layout, _)| {
            // LLVM seems to have trouble properly const-propagating pairs.align(),
            // possibly due to the use of NonZeroUsize. This little hack allows it
            // to generate optimal code.
            //
            // See https://github.com/rust-lang/rust/issues/51346 for more details.
            (
                layout,
                hashes.size() + hashes.padding_needed_for(mem::align_of::<(K, V)>()),
            )
        })
    }

    pub(crate) enum Fallibility {
        Fallible,
        Infallible,
    }

    use self::Fallibility::*;

    impl<K, V> RawTable<K, V> {
        /// Does not initialize the buckets. The caller should ensure they,
        /// at the very least, set every hash to EMPTY_BUCKET.
        /// Returns an error if it cannot allocate or capacity overflows.
        unsafe fn new_uninitialized_internal(
            capacity: usize,
            fallibility: Fallibility,
        ) -> Result<RawTable<K, V>, CollectionAllocErr> {
            if capacity == 0 {
                return Ok(RawTable {
                    size: 0,
                    capacity_mask: capacity.wrapping_sub(1),
                    hashes: TaggedHashUintPtr::new(EMPTY as *mut HashUint),
                    marker: marker::PhantomData,
                });
            }

            // Allocating hashmaps is a little tricky. We need to allocate two
            // arrays, but since we know their sizes and alignments up front,
            // we just allocate a single array, and then have the subarrays
            // point into it.
            let (layout, _) = calculate_layout::<K, V>(capacity)?;
            let buffer = Global.alloc(layout).map_err(|e| match fallibility {
                Infallible => handle_alloc_error(layout),
                Fallible => e,
            })?;

            Ok(RawTable {
                capacity_mask: capacity.wrapping_sub(1),
                size: 0,
                hashes: TaggedHashUintPtr::new(buffer.cast().as_ptr()),
                marker: marker::PhantomData,
            })
        }

        /// Does not initialize the buckets. The caller should ensure they,
        /// at the very least, set every hash to EMPTY_BUCKET.
        unsafe fn new_uninitialized(capacity: usize) -> RawTable<K, V> {
            match Self::new_uninitialized_internal(capacity, Infallible) {
                Err(CollectionAllocErr::CapacityOverflow) => panic!("capacity overflow"),
                Err(CollectionAllocErr::AllocErr) => unreachable!(),
                Ok(table) => { table }
            }
        }

        fn raw_bucket_at(&self, index: usize) -> RawBucket<K, V> {
            let (_, pairs_offset) = calculate_layout::<K, V>(self.capacity())
                .unwrap_or_else(|_| unsafe { hint::unreachable_unchecked() });
            let buffer = self.hashes.ptr() as *mut u8;
            unsafe {
                RawBucket {
                    hash_start: buffer as *mut HashUint,
                    pair_start: buffer.add(pairs_offset) as *const (K, V),
                    idx: index,
                    _marker: marker::PhantomData,
                }
            }
        }

        fn new_internal(
            capacity: usize,
            fallibility: Fallibility,
        ) -> Result<RawTable<K, V>, CollectionAllocErr> {
            unsafe {
                let ret = RawTable::new_uninitialized_internal(capacity, fallibility)?;
                if capacity > 0 {
                    ptr::write_bytes(ret.hashes.ptr(), 0, capacity);
                }
                Ok(ret)
            }
        }

        /// Tries to create a new raw table from a given capacity. If it cannot allocate,
        /// it returns with AllocErr.
        pub fn try_new(capacity: usize) -> Result<RawTable<K, V>, CollectionAllocErr> {
            Self::new_internal(capacity, Fallible)
        }

        /// Creates a new raw table from a given capacity. All buckets are
        /// initially empty.
        pub fn new(capacity: usize) -> RawTable<K, V> {
            match Self::new_internal(capacity, Infallible) {
                Err(CollectionAllocErr::CapacityOverflow) => panic!("capacity overflow"),
                Err(CollectionAllocErr::AllocErr) => unreachable!(),
                Ok(table) => { table }
            }
        }

        /// The hashtable's capacity, similar to a vector's.
        pub fn capacity(&self) -> usize {
            self.capacity_mask.wrapping_add(1)
        }

        /// The number of elements ever `put` in the hashtable, minus the number
        /// of elements ever `take`n.
        pub fn size(&self) -> usize {
            self.size
        }

        fn raw_buckets(&self) -> RawBuckets<K, V> {
            RawBuckets {
                raw: self.raw_bucket_at(0),
                elems_left: self.size,
                marker: marker::PhantomData,
            }
        }

        pub fn iter(&self) -> Iter<K, V> {
            Iter {
                iter: self.raw_buckets(),
            }
        }

        pub fn iter_mut(&mut self) -> IterMut<K, V> {
            IterMut {
                iter: self.raw_buckets(),
                _marker: marker::PhantomData,
            }
        }

        pub fn into_iter(self) -> IntoIter<K, V> {
            let RawBuckets { raw, elems_left, .. } = self.raw_buckets();
            // Replace the marker regardless of lifetime bounds on parameters.
            IntoIter {
                iter: RawBuckets {
                    raw,
                    elems_left,
                    marker: marker::PhantomData,
                },
                table: self,
            }
        }

        pub fn drain(&mut self) -> Drain<K, V> {
            let RawBuckets { raw, elems_left, .. } = self.raw_buckets();
            // Replace the marker regardless of lifetime bounds on parameters.
            Drain {
                iter: RawBuckets {
                    raw,
                    elems_left,
                    marker: marker::PhantomData,
                },
                table: NonNull::from(self),
                marker: marker::PhantomData,
            }
        }

        /// Drops buckets in reverse order. It leaves the table in an inconsistent
        /// state and should only be used for dropping the table's remaining
        /// entries. It's used in the implementation of Drop.
        unsafe fn rev_drop_buckets(&mut self) {
            // initialize the raw bucket past the end of the table
            let mut raw = self.raw_bucket_at(self.capacity());
            let mut elems_left = self.size;

            while elems_left != 0 {
                raw.idx -= 1;

                if *raw.hash() != EMPTY_BUCKET {
                    elems_left -= 1;
                    ptr::drop_in_place(raw.pair());
                }
            }
        }

        /// Set the table tag
        pub fn set_tag(&mut self, value: bool) {
            self.hashes.set_tag(value)
        }

        /// Get the table tag
        pub fn tag(&self) -> bool {
            self.hashes.tag()
        }
    }

    /// A raw iterator. The basis for some other iterators in this module. Although
    /// this interface is safe, it's not used outside this module.
    struct RawBuckets<'a, K, V> {
        raw: RawBucket<K, V>,
        elems_left: usize,

        // Strictly speaking, this should be &'a (K,V), but that would
        // require that K:'a, and we often use RawBuckets<'static...> for
        // move iterations, so that messes up a lot of other things. So
        // just use `&'a (K,V)` as this is not a publicly exposed type
        // anyway.
        marker: marker::PhantomData<&'a ()>,
    }

    // FIXME(#26925) Remove in favor of `#[derive(Clone)]`
    impl<'a, K, V> Clone for RawBuckets<'a, K, V> {
        fn clone(&self) -> RawBuckets<'a, K, V> {
            RawBuckets {
                raw: self.raw,
                elems_left: self.elems_left,
                marker: marker::PhantomData,
            }
        }
    }

    impl<'a, K, V> Iterator for RawBuckets<'a, K, V> {
        type Item = RawBucket<K, V>;

        fn next(&mut self) -> Option<RawBucket<K, V>> {
            if self.elems_left == 0 {
                return None;
            }

            loop {
                unsafe {
                    let item = self.raw;
                    self.raw.idx += 1;
                    if *item.hash() != EMPTY_BUCKET {
                        self.elems_left -= 1;
                        return Some(item);
                    }
                }
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.elems_left, Some(self.elems_left))
        }
    }

    impl<'a, K, V> ExactSizeIterator for RawBuckets<'a, K, V> {
        fn len(&self) -> usize {
            self.elems_left
        }
    }

    /// Iterator over shared references to entries in a table.
    pub struct Iter<'a, K: 'a, V: 'a> {
        iter: RawBuckets<'a, K, V>,
    }

    unsafe impl<'a, K: Sync, V: Sync> Sync for Iter<'a, K, V> {}
    unsafe impl<'a, K: Sync, V: Sync> Send for Iter<'a, K, V> {}

    // FIXME(#26925) Remove in favor of `#[derive(Clone)]`
    impl<'a, K, V> Clone for Iter<'a, K, V> {
        fn clone(&self) -> Iter<'a, K, V> {
            Iter {
                iter: self.iter.clone(),
            }
        }
    }

    /// Iterator over mutable references to entries in a table.
    pub struct IterMut<'a, K: 'a, V: 'a> {
        iter: RawBuckets<'a, K, V>,
        // To ensure invariance with respect to V
        _marker: marker::PhantomData<&'a mut V>,
    }

    unsafe impl<'a, K: Sync, V: Sync> Sync for IterMut<'a, K, V> {}
    // Both K: Sync and K: Send are correct for IterMut's Send impl,
    // but Send is the more useful bound
    unsafe impl<'a, K: Send, V: Send> Send for IterMut<'a, K, V> {}

    impl<'a, K: 'a, V: 'a> IterMut<'a, K, V> {
        pub fn iter(&self) -> Iter<K, V> {
            Iter {
                iter: self.iter.clone(),
            }
        }
    }

    /// Iterator over the entries in a table, consuming the table.
    pub struct IntoIter<K, V> {
        table: RawTable<K, V>,
        iter: RawBuckets<'static, K, V>,
    }

    unsafe impl<K: Sync, V: Sync> Sync for IntoIter<K, V> {}
    unsafe impl<K: Send, V: Send> Send for IntoIter<K, V> {}

    impl<K, V> IntoIter<K, V> {
        pub fn iter(&self) -> Iter<K, V> {
            Iter {
                iter: self.iter.clone(),
            }
        }
    }

    /// Iterator over the entries in a table, clearing the table.
    pub struct Drain<'a, K: 'a, V: 'a> {
        table: NonNull<RawTable<K, V>>,
        iter: RawBuckets<'static, K, V>,
        marker: marker::PhantomData<&'a RawTable<K, V>>,
    }

    unsafe impl<'a, K: Sync, V: Sync> Sync for Drain<'a, K, V> {}
    unsafe impl<'a, K: Send, V: Send> Send for Drain<'a, K, V> {}

    impl<'a, K, V> Drain<'a, K, V> {
        pub fn iter(&self) -> Iter<K, V> {
            Iter {
                iter: self.iter.clone(),
            }
        }
    }

    impl<'a, K, V> Iterator for Iter<'a, K, V> {
        type Item = (&'a K, &'a V);

        fn next(&mut self) -> Option<(&'a K, &'a V)> {
            self.iter.next().map(|raw| unsafe {
                let pair_ptr = raw.pair();
                (&(*pair_ptr).0, &(*pair_ptr).1)
            })
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    impl<'a, K, V> ExactSizeIterator for Iter<'a, K, V> {
        fn len(&self) -> usize {
            self.iter.len()
        }
    }

    impl<'a, K, V> Iterator for IterMut<'a, K, V> {
        type Item = (&'a K, &'a mut V);

        fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
            self.iter.next().map(|raw| unsafe {
                let pair_ptr = raw.pair();
                (&(*pair_ptr).0, &mut (*pair_ptr).1)
            })
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    impl<'a, K, V> ExactSizeIterator for IterMut<'a, K, V> {
        fn len(&self) -> usize {
            self.iter.len()
        }
    }

    impl<K, V> Iterator for IntoIter<K, V> {
        type Item = (SafeHash, K, V);

        fn next(&mut self) -> Option<(SafeHash, K, V)> {
            self.iter.next().map(|raw| {
                self.table.size -= 1;
                unsafe {
                    let (k, v) = ptr::read(raw.pair());
                    (SafeHash { hash: *raw.hash() }, k, v)
                }
            })
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    impl<K, V> ExactSizeIterator for IntoIter<K, V> {
        fn len(&self) -> usize {
            self.iter().len()
        }
    }

    impl<'a, K, V> Iterator for Drain<'a, K, V> {
        type Item = (SafeHash, K, V);

        #[inline]
        fn next(&mut self) -> Option<(SafeHash, K, V)> {
            self.iter.next().map(|raw| {
                unsafe {
                    self.table.as_mut().size -= 1;
                    let (k, v) = ptr::read(raw.pair());
                    (SafeHash { hash: ptr::replace(&mut *raw.hash(), EMPTY_BUCKET) }, k, v)
                }
            })
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    impl<'a, K, V> ExactSizeIterator for Drain<'a, K, V> {
        fn len(&self) -> usize {
            self.iter.len()
        }
    }

    impl<'a, K: 'a, V: 'a> Drop for Drain<'a, K, V> {
        fn drop(&mut self) {
            self.for_each(drop);
        }
    }

    impl<K: Clone, V: Clone> Clone for RawTable<K, V> {
        fn clone(&self) -> RawTable<K, V> {
            unsafe {
                let cap = self.capacity();
                let mut new_ht = RawTable::new_uninitialized(cap);

                let mut new_buckets = new_ht.raw_bucket_at(0);
                let mut buckets = self.raw_bucket_at(0);
                while buckets.idx < cap {
                    *new_buckets.hash() = *buckets.hash();
                    if *new_buckets.hash() != EMPTY_BUCKET {
                        let pair_ptr = buckets.pair();
                        let kv = ((*pair_ptr).0.clone(), (*pair_ptr).1.clone());
                        ptr::write(new_buckets.pair(), kv);
                    }
                    buckets.idx += 1;
                    new_buckets.idx += 1;
                }

                new_ht.size = self.size();
                new_ht.set_tag(self.tag());

                new_ht
            }
        }
    }

    unsafe impl<#[may_dangle] K, #[may_dangle] V> Drop for RawTable<K, V> {
        fn drop(&mut self) {
            if self.capacity() == 0 {
                return;
            }

            // This is done in reverse because we've likely partially taken
            // some elements out with `.into_iter()` from the front.
            // Check if the size is 0, so we don't do a useless scan when
            // dropping empty tables such as on resize.
            // Also avoid double drop of elements that have been already moved out.
            unsafe {
                if needs_drop::<(K, V)>() {
                    // avoid linear runtime for types that don't need drop
                    self.rev_drop_buckets();
                }
            }

            let (layout, _) = calculate_layout::<K, V>(self.capacity())
                .unwrap_or_else(|_| unsafe { hint::unreachable_unchecked() });
            unsafe {
                Global.dealloc(NonNull::new_unchecked(self.hashes.ptr()).cast(), layout);
                // Remember how everything was allocated out of one buffer
                // during initialization? We only need one call to free here.
            }
        }
    }

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