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哈希集合

基于 Entry[]? _entries 实现。

// \runtime\src\libraries\System.Private.CoreLib\src\System\Collections\Generic\HashSet.cs
namespace System.Collections.Generic
{
    // 和 Dictionary<TKey, TValue> 一样基于array实现
    public class HashSet<T> : ICollection<T>, ISet<T>, IReadOnlyCollection<T>, IReadOnlySet<T>, ISerializable, IDeserializationCallback
    {
        // 栈分配int数据的阈值
        private const int StackAllocThreshold = 100;

        // 缩容阈值条件，未使用到1/3
        private const int ShrinkThreshold = 3;
        private const int StartOfFreeList = -3;

        // 桶，存放数据所在的index
        private int[]? _buckets;
        // 实体，具体数据：hashcode next value
        private Entry[]? _entries;
#if TARGET_64BIT
        private ulong _fastModMultiplier;
#endif
        // 数据实体的数量
        private int _count;
        private int _freeList;
        private int _freeCount;
        // 版本号，用于标识迭代时确保没有修改集合
        private int _version;
        private IEqualityComparer<T>? _comparer;

        #region Constructors

        public HashSet() : this((IEqualityComparer<T>?)null) { }

        public HashSet(IEqualityComparer<T>? comparer)
        {
            // For reference types, we always want to store a comparer instance, either
            // the one provided, or if one wasn't provided, the default (accessing
            // EqualityComparer<TKey>.Default with shared generics on every dictionary
            // access can add measurable overhead).  For value types, if no comparer is
            // provided, or if the default is provided, we'd prefer to use
            // EqualityComparer<TKey>.Default.Equals on every use, enabling the JIT to
            // devirtualize and possibly inline the operation.
            if (!typeof(T).IsValueType)
            {
                _comparer = comparer ?? EqualityComparer<T>.Default;

                // Special-case EqualityComparer<string>.Default, StringComparer.Ordinal, and StringComparer.OrdinalIgnoreCase.
                // We use a non-randomized comparer for improved perf, falling back to a randomized comparer if the
                // hash buckets become unbalanced.
                if (typeof(T) == typeof(string) &&
                    NonRandomizedStringEqualityComparer.GetStringComparer(_comparer!) is IEqualityComparer<string> stringComparer)
                {
                    _comparer = (IEqualityComparer<T>)stringComparer;
                }
            }
            else if (comparer is not null && // first check for null to avoid forcing default comparer instantiation unnecessarily
                     comparer != EqualityComparer<T>.Default)
            {
                _comparer = comparer;
            }
        }

        public HashSet(int capacity) : this(capacity, null) { }

        public HashSet(IEnumerable<T> collection) : this(collection, null) { }

        public HashSet(IEnumerable<T> collection, IEqualityComparer<T>? comparer) : this(comparer)
        {
            if (collection == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
            }

            if (collection is HashSet<T> otherAsHashSet && EqualityComparersAreEqual(this, otherAsHashSet))
            {
                ConstructFrom(otherAsHashSet);
            }
            else
            {
                // To avoid excess resizes, first set size based on collection's count. The collection may
                // contain duplicates, so call TrimExcess if resulting HashSet is larger than the threshold.
                if (collection is ICollection<T> coll)
                {
                    int count = coll.Count;
                    if (count > 0)
                    {
                        Initialize(count);
                    }
                }

                UnionWith(collection);

                if (_count > 0 && _entries!.Length / _count > ShrinkThreshold)
                {
                    TrimExcess();
                }
            }
        }

        public HashSet(int capacity, IEqualityComparer<T>? comparer) : this(comparer)
        {
            if (capacity < 0)
            {
                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.capacity);
            }

            if (capacity > 0)
            {
                Initialize(capacity);
            }
        }

        /// <summary>Initializes the HashSet from another HashSet with the same element type and equality comparer.</summary>
        private void ConstructFrom(HashSet<T> source)
        {
            if (source.Count == 0)
            {
                // As well as short-circuiting on the rest of the work done,
                // this avoids errors from trying to access source._buckets
                // or source._entries when they aren't initialized.
                return;
            }

            int capacity = source._buckets!.Length;
            int threshold = HashHelpers.ExpandPrime(source.Count + 1);

            if (threshold >= capacity)
            {
                _buckets = (int[])source._buckets.Clone();
                _entries = (Entry[])source._entries!.Clone();
                _freeList = source._freeList;
                _freeCount = source._freeCount;
                _count = source._count;
#if TARGET_64BIT
                _fastModMultiplier = source._fastModMultiplier;
#endif
            }
            else
            {
                Initialize(source.Count);

                Entry[]? entries = source._entries;
                for (int i = 0; i < source._count; i++)
                {
                    ref Entry entry = ref entries![i];
                    if (entry.Next >= -1)
                    {
                        AddIfNotPresent(entry.Value, out _);
                    }
                }
            }

            Debug.Assert(Count == source.Count);
        }

        #endregion

        #region ICollection<T> methods

        void ICollection<T>.Add(T item) => AddIfNotPresent(item, out _);

        /// <summary>Removes all elements from the <see cref="HashSet{T}"/> object.</summary>
        public void Clear()
        {
            int count = _count;
            if (count > 0)
            {
                Debug.Assert(_buckets != null, "_buckets should be non-null");
                Debug.Assert(_entries != null, "_entries should be non-null");

                Array.Clear(_buckets);
                _count = 0;
                _freeList = -1;
                _freeCount = 0;
                Array.Clear(_entries, 0, count);
            }
        }

        /// <summary>Determines whether the <see cref="HashSet{T}"/> contains the specified element.</summary>
        /// <param name="item">The element to locate in the <see cref="HashSet{T}"/> object.</param>
        /// <returns>true if the <see cref="HashSet{T}"/> object contains the specified element; otherwise, false.</returns>
        public bool Contains(T item) => FindItemIndex(item) >= 0;

        /// <summary>Gets the index of the item in <see cref="_entries"/>, or -1 if it's not in the set.</summary>
        private int FindItemIndex(T item)
        {
            int[]? buckets = _buckets;
            if (buckets != null)
            {
                Entry[]? entries = _entries;
                Debug.Assert(entries != null, "Expected _entries to be initialized");

                uint collisionCount = 0;
                IEqualityComparer<T>? comparer = _comparer;

                if (typeof(T).IsValueType && // comparer can only be null for value types; enable JIT to eliminate entire if block for ref types
                    comparer == null)
                {
                    // ValueType: Devirtualize with EqualityComparer<TValue>.Default intrinsic
                    int hashCode = item!.GetHashCode();
                    int i = GetBucketRef(hashCode) - 1; // Value in _buckets is 1-based
                    while (i >= 0)
                    {
                        ref Entry entry = ref entries[i];
                        if (entry.HashCode == hashCode && EqualityComparer<T>.Default.Equals(entry.Value, item))
                        {
                            return i;
                        }
                        i = entry.Next;

                        collisionCount++;
                        if (collisionCount > (uint)entries.Length)
                        {
                            // The chain of entries forms a loop, which means a concurrent update has happened.
                            ThrowHelper.ThrowInvalidOperationException_ConcurrentOperationsNotSupported();
                        }
                    }
                }
                else
                {
                    Debug.Assert(comparer is not null);
                    int hashCode = item != null ? comparer.GetHashCode(item) : 0;
                    int i = GetBucketRef(hashCode) - 1; // Value in _buckets is 1-based
                    while (i >= 0)
                    {
                        ref Entry entry = ref entries[i];
                        if (entry.HashCode == hashCode && comparer.Equals(entry.Value, item))
                        {
                            return i;
                        }
                        i = entry.Next;

                        collisionCount++;
                        if (collisionCount > (uint)entries.Length)
                        {
                            // The chain of entries forms a loop, which means a concurrent update has happened.
                            ThrowHelper.ThrowInvalidOperationException_ConcurrentOperationsNotSupported();
                        }
                    }
                }
            }

            return -1;
        }
        
        // 生成hashcode对应的bucket值，64bit平台仅加速%运算
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private ref int GetBucketRef(int hashCode)
        {
            int[] buckets = _buckets!;
#if TARGET_64BIT
            return ref buckets[HashHelpers.FastMod((uint)hashCode, (uint)buckets.Length, _fastModMultiplier)];
#else
            return ref buckets[(uint)hashCode % (uint)buckets.Length];
#endif
        }

        // 移除元素
        public bool Remove(T item)
        {
            if (_buckets != null)
            {
                Entry[]? entries = _entries;
                Debug.Assert(entries != null, "entries should be non-null");

                uint collisionCount = 0;
                int last = -1;

                IEqualityComparer<T>? comparer = _comparer;
                Debug.Assert(typeof(T).IsValueType || comparer is not null);
                int hashCode =
                    typeof(T).IsValueType && comparer == null ? item!.GetHashCode() :
                    item is not null ? comparer!.GetHashCode(item) :
                    0;

                ref int bucket = ref GetBucketRef(hashCode);
                int i = bucket - 1; // Value in buckets is 1-based

                while (i >= 0)
                {
                    ref Entry entry = ref entries[i];

                    if (entry.HashCode == hashCode && (comparer?.Equals(entry.Value, item) ?? EqualityComparer<T>.Default.Equals(entry.Value, item)))
                    {
                        if (last < 0)
                        {
                            bucket = entry.Next + 1; // Value in buckets is 1-based
                        }
                        else
                        {
                            entries[last].Next = entry.Next;
                        }

                        Debug.Assert((StartOfFreeList - _freeList) < 0, "shouldn't underflow because max hashtable length is MaxPrimeArrayLength = 0x7FEFFFFD(2146435069) _freelist underflow threshold 2147483646");
                        entry.Next = StartOfFreeList - _freeList;

                        if (RuntimeHelpers.IsReferenceOrContainsReferences<T>())
                        {
                            entry.Value = default!;
                        }

                        _freeList = i;
                        _freeCount++;
                        return true;
                    }

                    last = i;
                    i = entry.Next;

                    collisionCount++;
                    if (collisionCount > (uint)entries.Length)
                    {
                        // The chain of entries forms a loop; which means a concurrent update has happened.
                        // Break out of the loop and throw, rather than looping forever.
                        ThrowHelper.ThrowInvalidOperationException_ConcurrentOperationsNotSupported();
                    }
                }
            }

            return false;
        }
        
        // 元素数量
        public int Count => _count - _freeCount;

        bool ICollection<T>.IsReadOnly => false;

        #endregion

        #region IEnumerable methods

        public Enumerator GetEnumerator() => new Enumerator(this);

        IEnumerator<T> IEnumerable<T>.GetEnumerator() =>
            Count == 0 ? SZGenericArrayEnumerator<T>.Empty :
            GetEnumerator();

        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<T>)this).GetEnumerator();

        #endregion

        #region HashSet methods

        // 添加元素，返回true表示添加成功，false表示已存在
        public bool Add(T item) => AddIfNotPresent(item, out _);
        
        // 搜索指定值是否存在
        public bool TryGetValue(T equalValue, [MaybeNullWhen(false)] out T actualValue)
        {
            if (_buckets != null)
            {
                int index = FindItemIndex(equalValue);
                if (index >= 0)
                {
                    actualValue = _entries![index].Value;
                    return true;
                }
            }

            actualValue = default;
            return false;
        }
        
        // 并集
        public void UnionWith(IEnumerable<T> other)
        {
            if (other == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
            }

            foreach (T item in other)
            {
                AddIfNotPresent(item, out _);
            }
        }

        // 交集
        public void IntersectWith(IEnumerable<T> other)
        {
            if (other == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
            }

            // Intersection of anything with empty set is empty set, so return if count is 0.
            // Same if the set intersecting with itself is the same set.
            if (Count == 0 || other == this)
            {
                return;
            }

            // If other is known to be empty, intersection is empty set; remove all elements, and we're done.
            if (other is ICollection<T> otherAsCollection)
            {
                if (otherAsCollection.Count == 0)
                {
                    Clear();
                    return;
                }

                // Faster if other is a hashset using same equality comparer; so check
                // that other is a hashset using the same equality comparer.
                if (other is HashSet<T> otherAsSet && EqualityComparersAreEqual(this, otherAsSet))
                {
                    IntersectWithHashSetWithSameComparer(otherAsSet);
                    return;
                }
            }

            IntersectWithEnumerable(other);
        }

        // 差集：this-other
        public void ExceptWith(IEnumerable<T> other)
        {
            if (other == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
            }

            // This is already the empty set; return.
            if (Count == 0)
            {
                return;
            }

            // Special case if other is this; a set minus itself is the empty set.
            if (other == this)
            {
                Clear();
                return;
            }

            // Remove every element in other from this.
            foreach (T element in other)
            {
                Remove(element);
            }
        }

        /// <summary>Modifies the current <see cref="HashSet{T}"/> object to contain only elements that are present either in that object or in the specified collection, but not both.</summary>
        /// <param name="other">The collection to compare to the current <see cref="HashSet{T}"/> object.</param>
        public void SymmetricExceptWith(IEnumerable<T> other)
        {
            if (other == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
            }

            // If set is empty, then symmetric difference is other.
            if (Count == 0)
            {
                UnionWith(other);
                return;
            }

            // Special-case this; the symmetric difference of a set with itself is the empty set.
            if (other == this)
            {
                Clear();
                return;
            }

            // If other is a HashSet, it has unique elements according to its equality comparer,
            // but if they're using different equality comparers, then assumption of uniqueness
            // will fail. So first check if other is a hashset using the same equality comparer;
            // symmetric except is a lot faster and avoids bit array allocations if we can assume
            // uniqueness.
            if (other is HashSet<T> otherAsSet && EqualityComparersAreEqual(this, otherAsSet))
            {
                SymmetricExceptWithUniqueHashSet(otherAsSet);
            }
            else
            {
                SymmetricExceptWithEnumerable(other);
            }
        }

        // 是否是子集
        public bool IsSubsetOf(IEnumerable<T> other)
        {
            if (other == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
            }
            
            if (Count == 0 || other == this)
            {
                return true;
            }

            // Faster if other has unique elements according to this equality comparer; so check
            // that other is a hashset using the same equality comparer.
            if (other is HashSet<T> otherAsSet && EqualityComparersAreEqual(this, otherAsSet))
            {
                // if this has more elements then it can't be a subset
                if (Count > otherAsSet.Count)
                {
                    return false;
                }

                // already checked that we're using same equality comparer. simply check that
                // each element in this is contained in other.
                return IsSubsetOfHashSetWithSameComparer(otherAsSet);
            }

            (int uniqueCount, int unfoundCount) = CheckUniqueAndUnfoundElements(other, returnIfUnfound: false);
            return uniqueCount == Count && unfoundCount >= 0;
        }

        /// <summary>Determines whether a <see cref="HashSet{T}"/> object is a proper subset of the specified collection.</summary>
        /// <param name="other">The collection to compare to the current <see cref="HashSet{T}"/> object.</param>
        /// <returns>true if the <see cref="HashSet{T}"/> object is a proper subset of <paramref name="other"/>; otherwise, false.</returns>
        public bool IsProperSubsetOf(IEnumerable<T> other)
        {
            if (other == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
            }

            // No set is a proper subset of itself.
            if (other == this)
            {
                return false;
            }

            if (other is ICollection<T> otherAsCollection)
            {
                // No set is a proper subset of an empty set.
                if (otherAsCollection.Count == 0)
                {
                    return false;
                }

                // The empty set is a proper subset of anything but the empty set.
                if (Count == 0)
                {
                    return otherAsCollection.Count > 0;
                }

                // Faster if other is a hashset (and we're using same equality comparer).
                if (other is HashSet<T> otherAsSet && EqualityComparersAreEqual(this, otherAsSet))
                {
                    if (Count >= otherAsSet.Count)
                    {
                        return false;
                    }

                    // This has strictly less than number of items in other, so the following
                    // check suffices for proper subset.
                    return IsSubsetOfHashSetWithSameComparer(otherAsSet);
                }
            }

            (int uniqueCount, int unfoundCount) = CheckUniqueAndUnfoundElements(other, returnIfUnfound: false);
            return uniqueCount == Count && unfoundCount > 0;
        }

        /// <summary>Determines whether a <see cref="HashSet{T}"/> object is a proper superset of the specified collection.</summary>
        /// <param name="other">The collection to compare to the current <see cref="HashSet{T}"/> object.</param>
        /// <returns>true if the <see cref="HashSet{T}"/> object is a superset of <paramref name="other"/>; otherwise, false.</returns>
        public bool IsSupersetOf(IEnumerable<T> other)
        {
            if (other == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
            }

            // A set is always a superset of itself.
            if (other == this)
            {
                return true;
            }

            // Try to fall out early based on counts.
            if (other is ICollection<T> otherAsCollection)
            {
                // If other is the empty set then this is a superset.
                if (otherAsCollection.Count == 0)
                {
                    return true;
                }

                // Try to compare based on counts alone if other is a hashset with same equality comparer.
                if (other is HashSet<T> otherAsSet &&
                    EqualityComparersAreEqual(this, otherAsSet) &&
                    otherAsSet.Count > Count)
                {
                    return false;
                }
            }

            foreach (T element in other)
            {
                if (!Contains(element))
                {
                    return false;
                }
            }

            return true;
        }

        /// <summary>Determines whether a <see cref="HashSet{T}"/> object is a proper superset of the specified collection.</summary>
        /// <param name="other">The collection to compare to the current <see cref="HashSet{T}"/> object.</param>
        /// <returns>true if the <see cref="HashSet{T}"/> object is a proper superset of <paramref name="other"/>; otherwise, false.</returns>
        public bool IsProperSupersetOf(IEnumerable<T> other)
        {
            if (other == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
            }

            // The empty set isn't a proper superset of any set, and a set is never a strict superset of itself.
            if (Count == 0 || other == this)
            {
                return false;
            }

            if (other is ICollection<T> otherAsCollection)
            {
                // If other is the empty set then this is a superset.
                if (otherAsCollection.Count == 0)
                {
                    // Note that this has at least one element, based on above check.
                    return true;
                }

                // Faster if other is a hashset with the same equality comparer
                if (other is HashSet<T> otherAsSet && EqualityComparersAreEqual(this, otherAsSet))
                {
                    if (otherAsSet.Count >= Count)
                    {
                        return false;
                    }

                    // Now perform element check.
                    return otherAsSet.IsSubsetOfHashSetWithSameComparer(this);
                }
            }

            // Couldn't fall out in the above cases; do it the long way
            (int uniqueCount, int unfoundCount) = CheckUniqueAndUnfoundElements(other, returnIfUnfound: true);
            return uniqueCount < Count && unfoundCount == 0;
        }

        /// <summary>Determines whether the current <see cref="HashSet{T}"/> object and a specified collection share common elements.</summary>
        /// <param name="other">The collection to compare to the current <see cref="HashSet{T}"/> object.</param>
        /// <returns>true if the <see cref="HashSet{T}"/> object and <paramref name="other"/> share at least one common element; otherwise, false.</returns>
        public bool Overlaps(IEnumerable<T> other)
        {
            if (other == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
            }

            if (Count == 0)
            {
                return false;
            }

            // Set overlaps itself
            if (other == this)
            {
                return true;
            }

            foreach (T element in other)
            {
                if (Contains(element))
                {
                    return true;
                }
            }

            return false;
        }

        /// <summary>Determines whether a <see cref="HashSet{T}"/> object and the specified collection contain the same elements.</summary>
        /// <param name="other">The collection to compare to the current <see cref="HashSet{T}"/> object.</param>
        /// <returns>true if the <see cref="HashSet{T}"/> object is equal to <paramref name="other"/>; otherwise, false.</returns>
        public bool SetEquals(IEnumerable<T> other)
        {
            if (other == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.other);
            }

            // A set is equal to itself.
            if (other == this)
            {
                return true;
            }

            // Faster if other is a hashset and we're using same equality comparer.
            if (other is HashSet<T> otherAsSet && EqualityComparersAreEqual(this, otherAsSet))
            {
                // Attempt to return early: since both contain unique elements, if they have
                // different counts, then they can't be equal.
                if (Count != otherAsSet.Count)
                {
                    return false;
                }

                // Already confirmed that the sets have the same number of distinct elements, so if
                // one is a subset of the other then they must be equal.
                return IsSubsetOfHashSetWithSameComparer(otherAsSet);
            }
            else
            {
                // If this count is 0 but other contains at least one element, they can't be equal.
                if (Count == 0 &&
                    other is ICollection<T> otherAsCollection &&
                    otherAsCollection.Count > 0)
                {
                    return false;
                }

                (int uniqueCount, int unfoundCount) = CheckUniqueAndUnfoundElements(other, returnIfUnfound: true);
                return uniqueCount == Count && unfoundCount == 0;
            }
        }

        public void CopyTo(T[] array) => CopyTo(array, 0, Count);

        /// <summary>Copies the elements of a <see cref="HashSet{T}"/> object to an array, starting at the specified array index.</summary>
        /// <param name="array">The destination array.</param>
        /// <param name="arrayIndex">The zero-based index in array at which copying begins.</param>
        public void CopyTo(T[] array, int arrayIndex) => CopyTo(array, arrayIndex, Count);

        public void CopyTo(T[] array, int arrayIndex, int count)
        {
            if (array == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
            }

            ArgumentOutOfRangeException.ThrowIfNegative(arrayIndex);
            ArgumentOutOfRangeException.ThrowIfNegative(count);

            // Will the array, starting at arrayIndex, be able to hold elements? Note: not
            // checking arrayIndex >= array.Length (consistency with list of allowing
            // count of 0; subsequent check takes care of the rest)
            if (arrayIndex > array.Length || count > array.Length - arrayIndex)
            {
                ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
            }

            Entry[]? entries = _entries;
            for (int i = 0; i < _count && count != 0; i++)
            {
                ref Entry entry = ref entries![i];
                if (entry.Next >= -1)
                {
                    array[arrayIndex++] = entry.Value;
                    count--;
                }
            }
        }

        /// <summary>Removes all elements that match the conditions defined by the specified predicate from a <see cref="HashSet{T}"/> collection.</summary>
        public int RemoveWhere(Predicate<T> match)
        {
            if (match == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
            }

            Entry[]? entries = _entries;
            int numRemoved = 0;
            for (int i = 0; i < _count; i++)
            {
                ref Entry entry = ref entries![i];
                if (entry.Next >= -1)
                {
                    // Cache value in case delegate removes it
                    T value = entry.Value;
                    if (match(value))
                    {
                        // Check again that remove actually removed it.
                        if (Remove(value))
                        {
                            numRemoved++;
                        }
                    }
                }
            }

            return numRemoved;
        }

        /// <summary>Gets the <see cref="IEqualityComparer"/> object that is used to determine equality for the values in the set.</summary>
        public IEqualityComparer<T> Comparer
        {
            get
            {
                if (typeof(T) == typeof(string))
                {
                    Debug.Assert(_comparer is not null, "The comparer should never be null for a reference type.");
                    return (IEqualityComparer<T>)IInternalStringEqualityComparer.GetUnderlyingEqualityComparer((IEqualityComparer<string?>)_comparer);
                }
                else
                {
                    return _comparer ?? EqualityComparer<T>.Default;
                }
            }
        }

        /// <summary>Ensures that this hash set can hold the specified number of elements without growing.</summary>
        public int EnsureCapacity(int capacity)
        {
            if (capacity < 0)
            {
                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.capacity);
            }

            int currentCapacity = _entries == null ? 0 : _entries.Length;
            if (currentCapacity >= capacity)
            {
                return currentCapacity;
            }

            if (_buckets == null)
            {
                return Initialize(capacity);
            }

            int newSize = HashHelpers.GetPrime(capacity);
            Resize(newSize, forceNewHashCodes: false);
            return newSize;
        }

        private void Resize() => Resize(HashHelpers.ExpandPrime(_count), forceNewHashCodes: false);

        private void Resize(int newSize, bool forceNewHashCodes)
        {
            // Value types never rehash
            Debug.Assert(!forceNewHashCodes || !typeof(T).IsValueType);
            Debug.Assert(_entries != null, "_entries should be non-null");
            Debug.Assert(newSize >= _entries.Length);

            var entries = new Entry[newSize];

            int count = _count;
            Array.Copy(_entries, entries, count);

            if (!typeof(T).IsValueType && forceNewHashCodes)
            {
                Debug.Assert(_comparer is NonRandomizedStringEqualityComparer);
                IEqualityComparer<T> comparer = _comparer = (IEqualityComparer<T>)((NonRandomizedStringEqualityComparer)_comparer).GetRandomizedEqualityComparer();

                for (int i = 0; i < count; i++)
                {
                    ref Entry entry = ref entries[i];
                    if (entry.Next >= -1)
                    {
                        entry.HashCode = entry.Value != null ? comparer.GetHashCode(entry.Value) : 0;
                    }
                }
            }

            // Assign member variables after both arrays allocated to guard against corruption from OOM if second fails
            _buckets = new int[newSize];
#if TARGET_64BIT
            _fastModMultiplier = HashHelpers.GetFastModMultiplier((uint)newSize);
#endif
            for (int i = 0; i < count; i++)
            {
                ref Entry entry = ref entries[i];
                if (entry.Next >= -1)
                {
                    ref int bucket = ref GetBucketRef(entry.HashCode);
                    entry.Next = bucket - 1; // Value in _buckets is 1-based
                    bucket = i + 1;
                }
            }

            _entries = entries;
        }

        /// <summary>
        /// Sets the capacity of a <see cref="HashSet{T}"/> object to the actual number of elements it contains,
        /// rounded up to a nearby, implementation-specific value.
        /// </summary>
        public void TrimExcess()
        {
            int capacity = Count;

            int newSize = HashHelpers.GetPrime(capacity);
            Entry[]? oldEntries = _entries;
            int currentCapacity = oldEntries == null ? 0 : oldEntries.Length;
            if (newSize >= currentCapacity)
            {
                return;
            }

            int oldCount = _count;
            _version++;
            Initialize(newSize);
            Entry[]? entries = _entries;
            int count = 0;
            for (int i = 0; i < oldCount; i++)
            {
                int hashCode = oldEntries![i].HashCode; // At this point, we know we have entries.
                if (oldEntries[i].Next >= -1)
                {
                    ref Entry entry = ref entries![count];
                    entry = oldEntries[i];
                    ref int bucket = ref GetBucketRef(hashCode);
                    entry.Next = bucket - 1; // Value in _buckets is 1-based
                    bucket = count + 1;
                    count++;
                }
            }

            _count = capacity;
            _freeCount = 0;
        }

        #endregion

        #region Helper methods

        /// <summary>Returns an <see cref="IEqualityComparer"/> object that can be used for equality testing of a <see cref="HashSet{T}"/> object.</summary>
        public static IEqualityComparer<HashSet<T>> CreateSetComparer() => new HashSetEqualityComparer<T>();

        // 初始化指定容量，返回实际容量（向上取质数）
        private int Initialize(int capacity)
        {
            int size = HashHelpers.GetPrime(capacity);
            var buckets = new int[size];
            var entries = new Entry[size];

            _freeList = -1;
            _buckets = buckets;
            _entries = entries;
#if TARGET_64BIT
            _fastModMultiplier = HashHelpers.GetFastModMultiplier((uint)size);
#endif
            return size;
        }
        
        // set不包含元素则添加，返回是否添加成功
        private bool AddIfNotPresent(T value, out int location)
        {
            if (_buckets == null)
            {
                Initialize(0);
            }
            Debug.Assert(_buckets != null);

            Entry[]? entries = _entries;
            Debug.Assert(entries != null, "expected entries to be non-null");

            IEqualityComparer<T>? comparer = _comparer;
            int hashCode;

            uint collisionCount = 0;
            ref int bucket = ref Unsafe.NullRef<int>();

            if (typeof(T).IsValueType && // comparer can only be null for value types; enable JIT to eliminate entire if block for ref types
                comparer == null)
            {
                hashCode = value!.GetHashCode();
                bucket = ref GetBucketRef(hashCode);
                int i = bucket - 1; // Value in _buckets is 1-based

                // ValueType: Devirtualize with EqualityComparer<TValue>.Default intrinsic
                while (i >= 0)
                {
                    ref Entry entry = ref entries[i];
                    if (entry.HashCode == hashCode && EqualityComparer<T>.Default.Equals(entry.Value, value))
                    {
                        location = i;
                        return false;
                    }
                    i = entry.Next;

                    collisionCount++;
                    if (collisionCount > (uint)entries.Length)
                    {
                        // The chain of entries forms a loop, which means a concurrent update has happened.
                        ThrowHelper.ThrowInvalidOperationException_ConcurrentOperationsNotSupported();
                    }
                }
            }
            else
            {
                Debug.Assert(comparer is not null);
                hashCode = value != null ? comparer.GetHashCode(value) : 0;
                bucket = ref GetBucketRef(hashCode);
                int i = bucket - 1; // Value in _buckets is 1-based
                while (i >= 0)
                {
                    ref Entry entry = ref entries[i];
                    if (entry.HashCode == hashCode && comparer.Equals(entry.Value, value))
                    {
                        location = i;
                        return false;
                    }
                    i = entry.Next;

                    collisionCount++;
                    if (collisionCount > (uint)entries.Length)
                    {
                        // The chain of entries forms a loop, which means a concurrent update has happened.
                        ThrowHelper.ThrowInvalidOperationException_ConcurrentOperationsNotSupported();
                    }
                }
            }

            int index;
            if (_freeCount > 0)
            {
                index = _freeList;
                _freeCount--;
                Debug.Assert((StartOfFreeList - entries![_freeList].Next) >= -1, "shouldn't overflow because `next` cannot underflow");
                _freeList = StartOfFreeList - entries[_freeList].Next;
            }
            else
            {
                int count = _count;
                if (count == entries.Length)
                {
                    Resize();
                    bucket = ref GetBucketRef(hashCode);
                }
                index = count;
                _count = count + 1;
                entries = _entries;
            }

            {
                ref Entry entry = ref entries![index];
                entry.HashCode = hashCode;
                entry.Next = bucket - 1; // Value in _buckets is 1-based
                entry.Value = value;
                bucket = index + 1;
                _version++;
                location = index;
            }

            // Value types never rehash
            if (!typeof(T).IsValueType && collisionCount > HashHelpers.HashCollisionThreshold && comparer is NonRandomizedStringEqualityComparer)
            {
                // If we hit the collision threshold we'll need to switch to the comparer which is using randomized string hashing
                // i.e. EqualityComparer<string>.Default.
                Resize(entries.Length, forceNewHashCodes: true);
                location = FindItemIndex(value);
                Debug.Assert(location >= 0);
            }

            return true;
        }

        /// <summary>
        /// Implementation Notes:
        /// If other is a hashset and is using same equality comparer, then checking subset is
        /// faster. Simply check that each element in this is in other.
        ///
        /// Note: if other doesn't use same equality comparer, then Contains check is invalid,
        /// which is why callers must take are of this.
        ///
        /// If callers are concerned about whether this is a proper subset, they take care of that.
        /// </summary>
        internal bool IsSubsetOfHashSetWithSameComparer(HashSet<T> other)
        {
            foreach (T item in this)
            {
                if (!other.Contains(item))
                {
                    return false;
                }
            }

            return true;
        }

        /// <summary>
        /// If other is a hashset that uses same equality comparer, intersect is much faster
        /// because we can use other's Contains
        /// </summary>
        private void IntersectWithHashSetWithSameComparer(HashSet<T> other)
        {
            Entry[]? entries = _entries;
            for (int i = 0; i < _count; i++)
            {
                ref Entry entry = ref entries![i];
                if (entry.Next >= -1)
                {
                    T item = entry.Value;
                    if (!other.Contains(item))
                    {
                        Remove(item);
                    }
                }
            }
        }

        /// <summary>
        /// Iterate over other. If contained in this, mark an element in bit array corresponding to
        /// its position in _slots. If anything is unmarked (in bit array), remove it.
        ///
        /// This attempts to allocate on the stack, if below StackAllocThreshold.
        /// </summary>
        private unsafe void IntersectWithEnumerable(IEnumerable<T> other)
        {
            Debug.Assert(_buckets != null, "_buckets shouldn't be null; callers should check first");

            // Keep track of current last index; don't want to move past the end of our bit array
            // (could happen if another thread is modifying the collection).
            int originalCount = _count;
            int intArrayLength = BitHelper.ToIntArrayLength(originalCount);

            Span<int> span = stackalloc int[StackAllocThreshold];
            BitHelper bitHelper = intArrayLength <= StackAllocThreshold ?
                new BitHelper(span.Slice(0, intArrayLength), clear: true) :
                new BitHelper(new int[intArrayLength], clear: false);

            // Mark if contains: find index of in slots array and mark corresponding element in bit array.
            foreach (T item in other)
            {
                int index = FindItemIndex(item);
                if (index >= 0)
                {
                    bitHelper.MarkBit(index);
                }
            }

            // If anything unmarked, remove it. Perf can be optimized here if BitHelper had a
            // FindFirstUnmarked method.
            for (int i = 0; i < originalCount; i++)
            {
                ref Entry entry = ref _entries![i];
                if (entry.Next >= -1 && !bitHelper.IsMarked(i))
                {
                    Remove(entry.Value);
                }
            }
        }

        /// <summary>
        /// if other is a set, we can assume it doesn't have duplicate elements, so use this
        /// technique: if can't remove, then it wasn't present in this set, so add.
        ///
        /// As with other methods, callers take care of ensuring that other is a hashset using the
        /// same equality comparer.
        /// </summary>
        /// <param name="other"></param>
        private void SymmetricExceptWithUniqueHashSet(HashSet<T> other)
        {
            foreach (T item in other)
            {
                if (!Remove(item))
                {
                    AddIfNotPresent(item, out _);
                }
            }
        }

        /// <summary>
        /// Implementation notes:
        ///
        /// Used for symmetric except when other isn't a HashSet. This is more tedious because
        /// other may contain duplicates. HashSet technique could fail in these situations:
        /// 1. Other has a duplicate that's not in this: HashSet technique would add then
        /// remove it.
        /// 2. Other has a duplicate that's in this: HashSet technique would remove then add it
        /// back.
        /// In general, its presence would be toggled each time it appears in other.
        ///
        /// This technique uses bit marking to indicate whether to add/remove the item. If already
        /// present in collection, it will get marked for deletion. If added from other, it will
        /// get marked as something not to remove.
        ///
        /// </summary>
        /// <param name="other"></param>
        private unsafe void SymmetricExceptWithEnumerable(IEnumerable<T> other)
        {
            int originalCount = _count;
            int intArrayLength = BitHelper.ToIntArrayLength(originalCount);

            Span<int> itemsToRemoveSpan = stackalloc int[StackAllocThreshold / 2];
            BitHelper itemsToRemove = intArrayLength <= StackAllocThreshold / 2 ?
                new BitHelper(itemsToRemoveSpan.Slice(0, intArrayLength), clear: true) :
                new BitHelper(new int[intArrayLength], clear: false);

            Span<int> itemsAddedFromOtherSpan = stackalloc int[StackAllocThreshold / 2];
            BitHelper itemsAddedFromOther = intArrayLength <= StackAllocThreshold / 2 ?
                new BitHelper(itemsAddedFromOtherSpan.Slice(0, intArrayLength), clear: true) :
                new BitHelper(new int[intArrayLength], clear: false);

            foreach (T item in other)
            {
                int location;
                if (AddIfNotPresent(item, out location))
                {
                    // wasn't already present in collection; flag it as something not to remove
                    // *NOTE* if location is out of range, we should ignore. BitHelper will
                    // detect that it's out of bounds and not try to mark it. But it's
                    // expected that location could be out of bounds because adding the item
                    // will increase _lastIndex as soon as all the free spots are filled.
                    itemsAddedFromOther.MarkBit(location);
                }
                else
                {
                    // already there...if not added from other, mark for remove.
                    // *NOTE* Even though BitHelper will check that location is in range, we want
                    // to check here. There's no point in checking items beyond originalCount
                    // because they could not have been in the original collection
                    if (location < originalCount && !itemsAddedFromOther.IsMarked(location))
                    {
                        itemsToRemove.MarkBit(location);
                    }
                }
            }

            // if anything marked, remove it
            for (int i = 0; i < originalCount; i++)
            {
                if (itemsToRemove.IsMarked(i))
                {
                    Remove(_entries![i].Value);
                }
            }
        }

        /// <summary>
        /// Determines counts that can be used to determine equality, subset, and superset. This
        /// is only used when other is an IEnumerable and not a HashSet. If other is a HashSet
        /// these properties can be checked faster without use of marking because we can assume
        /// other has no duplicates.
        ///
        /// The following count checks are performed by callers:
        /// 1. Equals: checks if unfoundCount = 0 and uniqueFoundCount = _count; i.e. everything
        /// in other is in this and everything in this is in other
        /// 2. Subset: checks if unfoundCount >= 0 and uniqueFoundCount = _count; i.e. other may
        /// have elements not in this and everything in this is in other
        /// 3. Proper subset: checks if unfoundCount > 0 and uniqueFoundCount = _count; i.e
        /// other must have at least one element not in this and everything in this is in other
        /// 4. Proper superset: checks if unfound count = 0 and uniqueFoundCount strictly less
        /// than _count; i.e. everything in other was in this and this had at least one element
        /// not contained in other.
        ///
        /// An earlier implementation used delegates to perform these checks rather than returning
        /// an ElementCount struct; however this was changed due to the perf overhead of delegates.
        /// </summary>
        /// <param name="other"></param>
        /// <param name="returnIfUnfound">Allows us to finish faster for equals and proper superset
        /// because unfoundCount must be 0.</param>
        private (int UniqueCount, int UnfoundCount) CheckUniqueAndUnfoundElements(IEnumerable<T> other, bool returnIfUnfound)
        {
            // Need special case in case this has no elements.
            if (_count == 0)
            {
                int numElementsInOther = 0;
                foreach (T item in other)
                {
                    numElementsInOther++;
                    break; // break right away, all we want to know is whether other has 0 or 1 elements
                }

                return (UniqueCount: 0, UnfoundCount: numElementsInOther);
            }

            Debug.Assert((_buckets != null) && (_count > 0), "_buckets was null but count greater than 0");

            int originalCount = _count;
            int intArrayLength = BitHelper.ToIntArrayLength(originalCount);

            Span<int> span = stackalloc int[StackAllocThreshold];
            BitHelper bitHelper = intArrayLength <= StackAllocThreshold ?
                new BitHelper(span.Slice(0, intArrayLength), clear: true) :
                new BitHelper(new int[intArrayLength], clear: false);

            int unfoundCount = 0; // count of items in other not found in this
            int uniqueFoundCount = 0; // count of unique items in other found in this

            foreach (T item in other)
            {
                int index = FindItemIndex(item);
                if (index >= 0)
                {
                    if (!bitHelper.IsMarked(index))
                    {
                        // Item hasn't been seen yet.
                        bitHelper.MarkBit(index);
                        uniqueFoundCount++;
                    }
                }
                else
                {
                    unfoundCount++;
                    if (returnIfUnfound)
                    {
                        break;
                    }
                }
            }

            return (uniqueFoundCount, unfoundCount);
        }

        /// <summary>
        /// Checks if equality comparers are equal. This is used for algorithms that can
        /// speed up if it knows the other item has unique elements. I.e. if they're using
        /// different equality comparers, then uniqueness assumption between sets break.
        /// </summary>
        internal static bool EqualityComparersAreEqual(HashSet<T> set1, HashSet<T> set2) => set1.Comparer.Equals(set2.Comparer);

#endregion

        private struct Entry
        {
            public int HashCode;
            /// <summary>
            /// 0-based index of next entry in chain: -1 means end of chain
            /// also encodes whether this entry _itself_ is part of the free list by changing sign and subtracting 3,
            /// so -2 means end of free list, -3 means index 0 but on free list, -4 means index 1 but on free list, etc.
            /// </summary>
            public int Next;
            public T Value;
        }        
    }
}

枚举器

public struct Enumerator : IEnumerator<T>
{
    private readonly HashSet<T> _hashSet;
    private readonly int _version;
    private int _index;
    private T _current;

    internal Enumerator(HashSet<T> hashSet)
    {
        _hashSet = hashSet;
        _version = hashSet._version;
        _index = 0;
        _current = default!;
    }

    public bool MoveNext()
    {
        if (_version != _hashSet._version)
        {
            ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion();
        }

        // Use unsigned comparison since we set index to dictionary.count+1 when the enumeration ends.
        // dictionary.count+1 could be negative if dictionary.count is int.MaxValue
        while ((uint)_index < (uint)_hashSet._count)
        {
            ref Entry entry = ref _hashSet._entries![_index++];
            if (entry.Next >= -1)
            {
                _current = entry.Value;
                return true;
            }
        }

        _index = _hashSet._count + 1;
        _current = default!;
        return false;
    }

    public T Current => _current;

    public void Dispose() { }

    object? IEnumerator.Current
    {
        get
        {
            if (_index == 0 || (_index == _hashSet._count + 1))
            {
                ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumOpCantHappen();
            }

            return _current;
        }
    }

    void IEnumerator.Reset()
    {
        if (_version != _hashSet._version)
        {
            ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion();
        }

        _index = 0;
        _current = default!;
    }
}