using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.Contracts;
#if !SILVERLIGHT
using System.Runtime.Serialization;
#endif
using System.Security.Permissions;
using System.Text;
using System.Diagnostics.CodeAnalysis;
using System.Security;
#if SILVERLIGHT
using System.Core; // for System.Core.SR
#endif

namespace System.Collections.Generic {

    /// <summary>
    /// Implementation notes:
    /// This uses an array-based implementation similar to Dictionary<T>, using a buckets array
    /// to map hash values to the Slots array. Items in the Slots array that hash to the same value
    /// are chained together through the "next" indices. 
    /// 
    /// The capacity is always prime; so during resizing, the capacity is chosen as the next prime
    /// greater than double the last capacity. 
    /// 
    /// The underlying data structures are lazily initialized. Because of the observation that, 
    /// in practice, hashtables tend to contain only a few elements, the initial capacity is
    /// set very small (3 elements) unless the ctor with a collection is used.
    /// 
    /// The +/- 1 modifications in methods that add, check for containment, etc allow us to 
    /// distinguish a hash code of 0 from an uninitialized bucket. This saves us from having to 
    /// reset each bucket to -1 when resizing. See Contains, for example.
    /// 
    /// Set methods such as UnionWith, IntersectWith, ExceptWith, and SymmetricExceptWith modify
    /// this set.
    /// 
    /// Some operations can perform faster if we can assume "other" contains unique elements
    /// according to this equality comparer. The only times this is efficient to check is if
    /// other is a hashset. Note that checking that it's a hashset alone doesn't suffice; we
    /// also have to check that the hashset is using the same equality comparer. If other 
    /// has a different equality comparer, it will have unique elements according to its own
    /// equality comparer, but not necessarily according to ours. Therefore, to go these 
    /// optimized routes we check that other is a hashset using the same equality comparer.
    /// 
    /// A HashSet with no elements has the properties of the empty set. (See IsSubset, etc. for 
    /// special empty set checks.)
    /// 
    /// A couple of methods have a special case if other is this (e.g. SymmetricExceptWith). 
    /// If we didn't have these checks, we could be iterating over the set and modifying at
    /// the same time. 
    /// </summary>
    /// <typeparam name="T"></typeparam>
    [DebuggerTypeProxy(typeof(System.Collections.Generic.HashSetDebugView<>))]
    [DebuggerDisplay("Count = {Count}")]
    [SuppressMessage("Microsoft.Naming","CA1710:IdentifiersShouldHaveCorrectSuffix", Justification="By design")]
#if SILVERLIGHT
    public class HashSet<T> : ICollection<T>, ISet<T>, IReadOnlyCollection<T>
#else
    [Serializable()]
    [System.Security.Permissions.HostProtection(MayLeakOnAbort = true)]
    public class HashSet<T> : ICollection<T>, ISerializable, IDeserializationCallback, ISet<T>, IReadOnlyCollection<T>
#endif
    {

        // store lower 31 bits of hash code
        private const int Lower31BitMask = 0x7FFFFFFF;
        // cutoff point, above which we won't do stackallocs. This corresponds to 100 integers.
        private const int StackAllocThreshold = 100;
        // when constructing a hashset from an existing collection, it may contain duplicates, 
        // so this is used as the max acceptable excess ratio of capacity to count. Note that
        // this is only used on the ctor and not to automatically shrink if the hashset has, e.g,
        // a lot of adds followed by removes. Users must explicitly shrink by calling TrimExcess.
        // This is set to 3 because capacity is acceptable as 2x rounded up to nearest prime.
        private const int ShrinkThreshold = 3;

#if !SILVERLIGHT
        // constants for serialization
        private const String CapacityName = "Capacity";
        private const String ElementsName = "Elements";
        private const String ComparerName = "Comparer";
        private const String VersionName = "Version";
#endif

        private int[] m_buckets;
        private Slot[] m_slots;
        private int m_count;
        private int m_lastIndex;
        private int m_freeList;
        private IEqualityComparer<T> m_comparer;
        private int m_version;

#if !SILVERLIGHT
        // temporary variable needed during deserialization
        private SerializationInfo m_siInfo;
#endif

        #region Constructors

        public HashSet()
            : this(EqualityComparer<T>.Default) { }

        public HashSet(IEqualityComparer<T> comparer) {
            if (comparer == null) {
                comparer = EqualityComparer<T>.Default;
            }

            this.m_comparer = comparer;
            m_lastIndex = 0;
            m_count = 0;
            m_freeList = -1;
            m_version = 0;
        }

        public HashSet(IEnumerable<T> collection)
            : this(collection, EqualityComparer<T>.Default) { }

        /// <summary>
        /// Implementation Notes:
        /// Since resizes are relatively expensive (require rehashing), this attempts to minimize 
        /// the need to resize by setting the initial capacity based on size of collection. 
        /// </summary>
        /// <param name="collection"></param>
        /// <param name="comparer"></param>
        public HashSet(IEnumerable<T> collection, IEqualityComparer<T> comparer)
            : this(comparer) {
            if (collection == null) {
                throw new ArgumentNullException("collection");
            }
            Contract.EndContractBlock();

            // to avoid excess resizes, first set size based on collection's count. Collection
            // may contain duplicates, so call TrimExcess if resulting hashset is larger than
            // threshold
            int suggestedCapacity = 0;
            ICollection<T> coll = collection as ICollection<T>;
            if (coll != null) {
                suggestedCapacity = coll.Count;
            }
            Initialize(suggestedCapacity);

            this.UnionWith(collection);
            if ((m_count == 0 && m_slots.Length > HashHelpers.GetMinPrime()) ||
                (m_count > 0 && m_slots.Length / m_count > ShrinkThreshold)) {
                TrimExcess();
            }
        }

#if !SILVERLIGHT
        protected HashSet(SerializationInfo info, StreamingContext context) {
            // We can't do anything with the keys and values until the entire graph has been 
            // deserialized and we have a reasonable estimate that GetHashCode is not going to 
            // fail.  For the time being, we'll just cache this.  The graph is not valid until 
            // OnDeserialization has been called.
            m_siInfo = info;
        }
#endif

        #endregion

        #region ICollection<T> methods

        /// <summary>
        /// Add item to this hashset. This is the explicit implementation of the ICollection<T>
        /// interface. The other Add method returns bool indicating whether item was added.
        /// </summary>
        /// <param name="item">item to add</param>
        void ICollection<T>.Add(T item) {
            AddIfNotPresent(item);
        }

        /// <summary>
        /// Remove all items from this set. This clears the elements but not the underlying 
        /// buckets and slots array. Follow this call by TrimExcess to release these.
        /// </summary>
        public void Clear() {
            if (m_lastIndex > 0) {
                Debug.Assert(m_buckets != null, "m_buckets was null but m_lastIndex > 0");

                // clear the elements so that the gc can reclaim the references.
                // clear only up to m_lastIndex for m_slots 
                Array.Clear(m_slots, 0, m_lastIndex);
                Array.Clear(m_buckets, 0, m_buckets.Length);
                m_lastIndex = 0;
                m_count = 0;
                m_freeList = -1;
            }
            m_version++;
        }

        /// <summary>
        /// Checks if this hashset contains the item
        /// </summary>
        /// <param name="item">item to check for containment</param>
        /// <returns>true if item contained; false if not</returns>
        public bool Contains(T item) {
            if (m_buckets != null) {
                int hashCode = InternalGetHashCode(item);
                // see note at "HashSet" level describing why "- 1" appears in for loop
                for (int i = m_buckets[hashCode % m_buckets.Length] - 1; i >= 0; i = m_slots[i].next) {
                    if (m_slots[i].hashCode == hashCode && m_comparer.Equals(m_slots[i].value, item)) {
                        return true;
                    }
                }
            }
            // either m_buckets is null or wasn't found
            return false;
        }

        /// <summary>
        /// Copy items in this hashset to array, starting at arrayIndex
        /// </summary>
        /// <param name="array">array to add items to</param>
        /// <param name="arrayIndex">index to start at</param>
        public void CopyTo(T[] array, int arrayIndex) {
            CopyTo(array, arrayIndex, m_count);
        }

        /// <summary>
        /// Remove item from this hashset
        /// </summary>
        /// <param name="item">item to remove</param>
        /// <returns>true if removed; false if not (i.e. if the item wasn't in the HashSet)</returns>
        public bool Remove(T item) {
            if (m_buckets != null) {
                int hashCode = InternalGetHashCode(item);
                int bucket = hashCode % m_buckets.Length;
                int last = -1;
                for (int i = m_buckets[bucket] - 1; i >= 0; last = i, i = m_slots[i].next) {
                    if (m_slots[i].hashCode == hashCode && m_comparer.Equals(m_slots[i].value, item)) {
                        if (last < 0) {
                            // first iteration; update buckets
                            m_buckets[bucket] = m_slots[i].next + 1;
                        }
                        else {
                            // subsequent iterations; update 'next' pointers
                            m_slots[last].next = m_slots[i].next;
                        }
                        m_slots[i].hashCode = -1;
                        m_slots[i].value = default(T);
                        m_slots[i].next = m_freeList;

                        m_count--;
                        m_version++;
                        if (m_count == 0) {
                            m_lastIndex = 0;
                            m_freeList = -1;
                        }
                        else {
                            m_freeList = i;
                        }
                        return true;
                    }
                }
            }
            // either m_buckets is null or wasn't found
            return false;
        }

        /// <summary>
        /// Number of elements in this hashset
        /// </summary>
        public int Count {
            get { return m_count; }
        }

        /// <summary>
        /// Whether this is readonly
        /// </summary>
        bool ICollection<T>.IsReadOnly {
            get { return false; }
        }

        #endregion

        #region IEnumerable methods

        public Enumerator GetEnumerator() {
            return new Enumerator(this);
        }

        IEnumerator<T> IEnumerable<T>.GetEnumerator() {
            return new Enumerator(this);
        }

        IEnumerator IEnumerable.GetEnumerator() {
            return new Enumerator(this);
        }

        #endregion

        #region ISerializable methods

#if !SILVERLIGHT
        [SecurityPermissionAttribute(SecurityAction.LinkDemand, Flags = SecurityPermissionFlag.SerializationFormatter)]
        [SecurityCritical]
        public virtual void GetObjectData(SerializationInfo info, StreamingContext context) {
            if (info == null) {
                throw new ArgumentNullException("info");
            }

            // need to serialize version to avoid problems with serializing while enumerating
            info.AddValue(VersionName, m_version);

#if FEATURE_RANDOMIZED_STRING_HASHING && !FEATURE_NETCORE
            info.AddValue(ComparerName, HashHelpers.GetEqualityComparerForSerialization(m_comparer), typeof(IEqualityComparer<T>));
#else
            info.AddValue(ComparerName, m_comparer, typeof(IEqualityComparer<T>));
#endif

            info.AddValue(CapacityName, m_buckets == null ? 0 : m_buckets.Length);
            if (m_buckets != null) {
                T[] array = new T[m_count];
                CopyTo(array);
                info.AddValue(ElementsName, array, typeof(T[]));
            }
        }
#endif
        #endregion

        #region IDeserializationCallback methods

#if !SILVERLIGHT
        public virtual void OnDeserialization(Object sender) {

            if (m_siInfo == null) {
                // It might be necessary to call OnDeserialization from a container if the 
                // container object also implements OnDeserialization. However, remoting will 
                // call OnDeserialization again. We can return immediately if this function is 
                // called twice. Note we set m_siInfo to null at the end of this method.
                return;
            }

            int capacity = m_siInfo.GetInt32(CapacityName);
            m_comparer = (IEqualityComparer<T>)m_siInfo.GetValue(ComparerName, typeof(IEqualityComparer<T>));
            m_freeList = -1;

            if (capacity != 0) {
                m_buckets = new int[capacity];
                m_slots = new Slot[capacity];

                T[] array = (T[])m_siInfo.GetValue(ElementsName, typeof(T[]));

                if (array == null) {
                    throw new SerializationException(SR.GetString(SR.Serialization_MissingKeys));
                }

                // there are no resizes here because we already set capacity above
                for (int i = 0; i < array.Length; i++) {
                    AddIfNotPresent(array[i]);
                }
            }
            else {
                m_buckets = null;
            }

            m_version = m_siInfo.GetInt32(VersionName);
            m_siInfo = null;
        }
#endif

        #endregion

        #region HashSet methods

        /// <summary>
        /// Add item to this HashSet. Returns bool indicating whether item was added (won't be 
        /// added if already present)
        /// </summary>
        /// <param name="item"></param>
        /// <returns>true if added, false if already present</returns>
        public bool Add(T item) {
            return AddIfNotPresent(item);
        }

        /// <summary>
        /// Take the union of this HashSet with other. Modifies this set.
        /// 
        /// Implementation note: GetSuggestedCapacity (to increase capacity in advance avoiding 
        /// multiple resizes ended up not being useful in practice; quickly gets to the 
        /// point where it's a wasteful check.
        /// </summary>
        /// <param name="other">enumerable with items to add</param>
        public void UnionWith(IEnumerable<T> other) {
            if (other == null) {
                throw new ArgumentNullException("other");
            }
            Contract.EndContractBlock();

            foreach (T item in other) {
                AddIfNotPresent(item);
            }
        }

        /// <summary>
        /// Takes the intersection of this set with other. Modifies this set.
        /// 
        /// Implementation Notes: 
        /// We get better perf if other is a hashset using same equality comparer, because we 
        /// get constant contains check in other. Resulting cost is O(n1) to iterate over this.
        /// 
        /// If we can't go above route, iterate over the other and mark intersection by checking
        /// contains in this. Then loop over and delete any unmarked elements. Total cost is n2+n1. 
        /// 
        /// Attempts to return early based on counts alone, using the property that the 
        /// intersection of anything with the empty set is the empty set.
        /// </summary>
        /// <param name="other">enumerable with items to add </param>
        public void IntersectWith(IEnumerable<T> other) {
            if (other == null) {
                throw new ArgumentNullException("other");
            }
            Contract.EndContractBlock();

            // intersection of anything with empty set is empty set, so return if count is 0
            if (m_count == 0) {
                return;
            }

            // if other is empty, intersection is empty set; remove all elements and we're done
            // can only figure this out if implements ICollection<T>. (IEnumerable<T> has no count)
            ICollection<T> otherAsCollection = other as ICollection<T>;
            if (otherAsCollection != null) {
                if (otherAsCollection.Count == 0) {
                    Clear();
                    return;
                }

                HashSet<T> otherAsSet = other as HashSet<T>;
                // faster if other is a hashset using same equality comparer; so check 
                // that other is a hashset using the same equality comparer.
                if (otherAsSet != null && AreEqualityComparersEqual(this, otherAsSet)) {
                    IntersectWithHashSetWithSameEC(otherAsSet);
                    return;
                }
            }

            IntersectWithEnumerable(other);
        }

        /// <summary>
        /// Remove items in other from this set. Modifies this set.
        /// </summary>
        /// <param name="other">enumerable with items to remove</param>
        public void ExceptWith(IEnumerable<T> other) {
            if (other == null) {
                throw new ArgumentNullException("other");
            }
            Contract.EndContractBlock();

            // this is already the enpty set; return
            if (m_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>
        /// Takes symmetric difference (XOR) with other and this set. Modifies this set.
        /// </summary>
        /// <param name="other">enumerable with items to XOR</param>
        public void SymmetricExceptWith(IEnumerable<T> other) {
            if (other == null) {
                throw new ArgumentNullException("other");
            }
            Contract.EndContractBlock();

            // if set is empty, then symmetric difference is other
            if (m_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;
            }

            HashSet<T> otherAsSet = other as HashSet<T>;
            // 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 (otherAsSet != null && AreEqualityComparersEqual(this, otherAsSet)) {
                SymmetricExceptWithUniqueHashSet(otherAsSet);
            }
            else {
                SymmetricExceptWithEnumerable(other);
            }
        }

        /// <summary>
        /// Checks if this is a subset of other.
        /// 
        /// Implementation Notes:
        /// The following properties are used up-front to avoid element-wise checks:
        /// 1. If this is the empty set, then it's a subset of anything, including the empty set
        /// 2. If other has unique elements according to this equality comparer, and this has more
        /// elements than other, then it can't be a subset.
        /// 
        /// Furthermore, if other is a hashset using the same equality comparer, we can use a 
        /// faster element-wise check.
        /// </summary>
        /// <param name="other"></param>
        /// <returns>true if this is a subset of other; false if not</returns>
        public bool IsSubsetOf(IEnumerable<T> other) {
            if (other == null) {
                throw new ArgumentNullException("other");
            }
            Contract.EndContractBlock();

            // The empty set is a subset of any set
            if (m_count == 0) {
                return true;
            }

            HashSet<T> otherAsSet = other as HashSet<T>;
            // faster if other has unique elements according to this equality comparer; so check 
            // that other is a hashset using the same equality comparer.
            if (otherAsSet != null && AreEqualityComparersEqual(this, otherAsSet)) {
                // if this has more elements then it can't be a subset
                if (m_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 IsSubsetOfHashSetWithSameEC(otherAsSet);
            }
            else {
                ElementCount result = CheckUniqueAndUnfoundElements(other, false);
                return (result.uniqueCount == m_count && result.unfoundCount >= 0);
            }
        }

        /// <summary>
        /// Checks if this is a proper subset of other (i.e. strictly contained in)
        /// 
        /// Implementation Notes:
        /// The following properties are used up-front to avoid element-wise checks:
        /// 1. If this is the empty set, then it's a proper subset of a set that contains at least
        /// one element, but it's not a proper subset of the empty set.
        /// 2. If other has unique elements according to this equality comparer, and this has >=
        /// the number of elements in other, then this can't be a proper subset.
        /// 
        /// Furthermore, if other is a hashset using the same equality comparer, we can use a 
        /// faster element-wise check.
        /// </summary>
        /// <param name="other"></param>
        /// <returns>true if this is a proper subset of other; false if not</returns>
        public bool IsProperSubsetOf(IEnumerable<T> other) {
            if (other == null) {
                throw new ArgumentNullException("other");
            }
            Contract.EndContractBlock();

            ICollection<T> otherAsCollection = other as ICollection<T>;
            if (otherAsCollection != null) {
                // the empty set is a proper subset of anything but the empty set
                if (m_count == 0) {
                    return otherAsCollection.Count > 0;
                }
                HashSet<T> otherAsSet = other as HashSet<T>;
                // faster if other is a hashset (and we're using same equality comparer)
                if (otherAsSet != null && AreEqualityComparersEqual(this, otherAsSet)) {
                    if (m_count >= otherAsSet.Count) {
                        return false;
                    }
                    // this has strictly less than number of items in other, so the following
                    // check suffices for proper subset.
                    return IsSubsetOfHashSetWithSameEC(otherAsSet);
                }
            }

            ElementCount result = CheckUniqueAndUnfoundElements(other, false);
            return (result.uniqueCount == m_count && result.unfoundCount > 0);

        }

        /// <summary>
        /// Checks if this is a superset of other
        /// 
        /// Implementation Notes:
        /// The following properties are used up-front to avoid element-wise checks:
        /// 1. If other has no elements (it's the empty set), then this is a superset, even if this
        /// is also the empty set.
        /// 2. If other has unique elements according to this equality comparer, and this has less 
        /// than the number of elements in other, then this can't be a superset
        /// 
        /// </summary>
        /// <param name="other"></param>
        /// <returns>true if this is a superset of other; false if not</returns>
        public bool IsSupersetOf(IEnumerable<T> other) {
            if (other == null) {
                throw new ArgumentNullException("other");
            }
            Contract.EndContractBlock();

            // try to fall out early based on counts
            ICollection<T> otherAsCollection = other as ICollection<T>;
            if (otherAsCollection != null) {
                // if other is the empty set then this is a superset
                if (otherAsCollection.Count == 0) {
                    return true;
                }
                HashSet<T> otherAsSet = other as HashSet<T>;
                // try to compare based on counts alone if other is a hashset with
                // same equality comparer
                if (otherAsSet != null && AreEqualityComparersEqual(this, otherAsSet)) {
                    if (otherAsSet.Count > m_count) {
                        return false;
                    }
                }
            }

            return ContainsAllElements(other);
        }

        /// <summary>
        /// Checks if this is a proper superset of other (i.e. other strictly contained in this)
        /// 
        /// Implementation Notes: 
        /// This is slightly more complicated than above because we have to keep track if there
        /// was at least one element not contained in other.
        /// 
        /// The following properties are used up-front to avoid element-wise checks:
        /// 1. If this is the empty set, then it can't be a proper superset of any set, even if 
        /// other is the empty set.
        /// 2. If other is an empty set and this contains at least 1 element, then this is a proper
        /// superset.
        /// 3. If other has unique elements according to this equality comparer, and other's count
        /// is greater than or equal to this count, then this can't be a proper superset
        /// 
        /// Furthermore, if other has unique elements according to this equality comparer, we can
        /// use a faster element-wise check.
        /// </summary>
        /// <param name="other"></param>
        /// <returns>true if this is a proper superset of other; false if not</returns>
        public bool IsProperSupersetOf(IEnumerable<T> other) {
            if (other == null) {
                throw new ArgumentNullException("other");
            }
            Contract.EndContractBlock();

            // the empty set isn't a proper superset of any set.
            if (m_count == 0) {
                return false;
            }

            ICollection<T> otherAsCollection = other as ICollection<T>;
            if (otherAsCollection != null) {
                // 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;
                }
                HashSet<T> otherAsSet = other as HashSet<T>;
                // faster if other is a hashset with the same equality comparer
                if (otherAsSet != null && AreEqualityComparersEqual(this, otherAsSet)) {
                    if (otherAsSet.Count >= m_count) {
                        return false;
                    }
                    // now perform element check
                    return ContainsAllElements(otherAsSet);
                }
            }
            // couldn't fall out in the above cases; do it the long way
            ElementCount result = CheckUniqueAndUnfoundElements(other, true);
            return (result.uniqueCount < m_count && result.unfoundCount == 0);

        }

        /// <summary>
        /// Checks if this set overlaps other (i.e. they share at least one item)
        /// </summary>
        /// <param name="other"></param>
        /// <returns>true if these have at least one common element; false if disjoint</returns>
        public bool Overlaps(IEnumerable<T> other) {
            if (other == null) {
                throw new ArgumentNullException("other");
            }
            Contract.EndContractBlock();

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

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

        /// <summary>
        /// Checks if this and other contain the same elements. This is set equality: 
        /// duplicates and order are ignored
        /// </summary>
        /// <param name="other"></param>
        /// <returns></returns>
        public bool SetEquals(IEnumerable<T> other) {
            if (other == null) {
                throw new ArgumentNullException("other");
            }
            Contract.EndContractBlock();

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

                // already confirmed that the sets have the same number of distinct elements, so if
                // one is a superset of the other then they must be equal
                return ContainsAllElements(otherAsSet);
            }
            else {
                ICollection<T> otherAsCollection = other as ICollection<T>;
                if (otherAsCollection != null) {
                    // if this count is 0 but other contains at least one element, they can't be equal
                    if (m_count == 0 && otherAsCollection.Count > 0) {
                        return false;
                    }
                }
                ElementCount result = CheckUniqueAndUnfoundElements(other, true);
                return (result.uniqueCount == m_count && result.unfoundCount == 0);
            }
        }

        public void CopyTo(T[] array) { CopyTo(array, 0, m_count); }

        public void CopyTo(T[] array, int arrayIndex, int count) {
            if (array == null) {
                throw new ArgumentNullException("array");
            }
            Contract.EndContractBlock();

            // check array index valid index into array
            if (arrayIndex < 0) {
                throw new ArgumentOutOfRangeException("arrayIndex", SR.GetString(SR.ArgumentOutOfRange_NeedNonNegNum));
            }

            // also throw if count less than 0
            if (count < 0) {
                throw new ArgumentOutOfRangeException("count", SR.GetString(SR.ArgumentOutOfRange_NeedNonNegNum));
            }

            // will 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) {
                throw new ArgumentException(SR.GetString(SR.Arg_ArrayPlusOffTooSmall));
            }

            int numCopied = 0;
            for (int i = 0; i < m_lastIndex && numCopied < count; i++) {
                if (m_slots[i].hashCode >= 0) {
                    array[arrayIndex + numCopied] = m_slots[i].value;
                    numCopied++;
                }
            }
        }

        /// <summary>
        /// Remove elements that match specified predicate. Returns the number of elements removed
        /// </summary>
        /// <param name="match"></param>
        /// <returns></returns>
        public int RemoveWhere(Predicate<T> match) {
            if (match == null) {
                throw new ArgumentNullException("match");
            }
            Contract.EndContractBlock();

            int numRemoved = 0;
            for (int i = 0; i < m_lastIndex; i++) {
                if (m_slots[i].hashCode >= 0) {
                    // cache value in case delegate removes it
                    T value = m_slots[i].value;
                    if (match(value)) {
                        // check again that remove actually removed it
                        if (Remove(value)) {
                            numRemoved++;
                        }
                    }
                }
            }
            return numRemoved;
        }

        /// <summary>
        /// Gets the IEqualityComparer that is used to determine equality of keys for 
        /// the HashSet.
        /// </summary>
        public IEqualityComparer<T> Comparer {
            get {
                return m_comparer;
            }
        }

        /// <summary>
        /// Sets the capacity of this list to the size of the list (rounded up to nearest prime),
        /// unless count is 0, in which case we release references.
        /// 
        /// This method can be used to minimize a list's memory overhead once it is known that no
        /// new elements will be added to the list. To completely clear a list and release all 
        /// memory referenced by the list, execute the following statements:
        /// 
        /// list.Clear();
        /// list.TrimExcess(); 
        /// </summary>
        public void TrimExcess() {
            Debug.Assert(m_count >= 0, "m_count is negative");

            if (m_count == 0) {
                // if count is zero, clear references
                m_buckets = null;
                m_slots = null;
                m_version++;
            }
            else {
                Debug.Assert(m_buckets != null, "m_buckets was null but m_count > 0");

                // similar to IncreaseCapacity but moves down elements in case add/remove/etc
                // caused fragmentation
                int newSize = HashHelpers.GetPrime(m_count);
                Slot[] newSlots = new Slot[newSize];
                int[] newBuckets = new int[newSize];

                // move down slots and rehash at the same time. newIndex keeps track of current 
                // position in newSlots array
                int newIndex = 0;
                for (int i = 0; i < m_lastIndex; i++) {
                    if (m_slots[i].hashCode >= 0) {
                        newSlots[newIndex] = m_slots[i];

                        // rehash
                        int bucket = newSlots[newIndex].hashCode % newSize;
                        newSlots[newIndex].next = newBuckets[bucket] - 1;
                        newBuckets[bucket] = newIndex + 1;

                        newIndex++;
                    }
                }

                Debug.Assert(newSlots.Length <= m_slots.Length, "capacity increased after TrimExcess");

                m_lastIndex = newIndex;
                m_slots = newSlots;
                m_buckets = newBuckets;
                m_freeList = -1;
            }
        }

#if !SILVERLIGHT || FEATURE_NETCORE
        /// <summary>
        /// Used for deep equality of HashSet testing
        /// </summary>
        /// <returns></returns>
        public static IEqualityComparer<HashSet<T>> CreateSetComparer() {
            return new HashSetEqualityComparer<T>();
        }
#endif

        #endregion

        #region Helper methods

        /// <summary>
        /// Initializes buckets and slots arrays. Uses suggested capacity by finding next prime
        /// greater than or equal to capacity.
        /// </summary>
        /// <param name="capacity"></param>
        private void Initialize(int capacity) {
            Debug.Assert(m_buckets == null, "Initialize was called but m_buckets was non-null");

            int size = HashHelpers.GetPrime(capacity);

            m_buckets = new int[size];
            m_slots = new Slot[size];
        }

        /// <summary>
        /// Expand to new capacity. New capacity is next prime greater than or equal to suggested 
        /// size. This is called when the underlying array is filled. This performs no 
        /// defragmentation, allowing faster execution; note that this is reasonable since 
        /// AddIfNotPresent attempts to insert new elements in re-opened spots.
        /// </summary>
        /// <param name="sizeSuggestion"></param>
        private void IncreaseCapacity() {
            Debug.Assert(m_buckets != null, "IncreaseCapacity called on a set with no elements");

            int newSize = HashHelpers.ExpandPrime(m_count);
            if (newSize <= m_count) {
                throw new ArgumentException(SR.GetString(SR.Arg_HSCapacityOverflow));
            }

            // Able to increase capacity; copy elements to larger array and rehash
            SetCapacity(newSize, false);
        }

        /// <summary>
        /// Set the underlying buckets array to size newSize and rehash.  Note that newSize
        /// *must* be a prime.  It is very likely that you want to call IncreaseCapacity()
        /// instead of this method.
        /// </summary>
        private void SetCapacity(int newSize, bool forceNewHashCodes) { 
            Contract.Assert(HashHelpers.IsPrime(newSize), "New size is not prime!");

            Contract.Assert(m_buckets != null, "SetCapacity called on a set with no elements");

            Slot[] newSlots = new Slot[newSize];
            if (m_slots != null) {
                Array.Copy(m_slots, 0, newSlots, 0, m_lastIndex);
            }

            if(forceNewHashCodes) {
                for(int i = 0; i < m_lastIndex; i++) {
                    if(newSlots[i].hashCode != -1) {
                        newSlots[i].hashCode = InternalGetHashCode(newSlots[i].value);
                    }
                }
            }

            int[] newBuckets = new int[newSize];
            for (int i = 0; i < m_lastIndex; i++) {
                int bucket = newSlots[i].hashCode % newSize;
                newSlots[i].next = newBuckets[bucket] - 1;
                newBuckets[bucket] = i + 1;
            }
            m_slots = newSlots;
            m_buckets = newBuckets;
        }

        /// <summary>
        /// Adds value to HashSet if not contained already
        /// Returns true if added and false if already present
        /// </summary>
        /// <param name="value">value to find</param>
        /// <returns></returns>
        private bool AddIfNotPresent(T value) {
            if (m_buckets == null) {
                Initialize(0);
            }

            int hashCode = InternalGetHashCode(value);
            int bucket = hashCode % m_buckets.Length;
#if FEATURE_RANDOMIZED_STRING_HASHING && !FEATURE_NETCORE
            int collisionCount = 0;
#endif
            for (int i = m_buckets[hashCode % m_buckets.Length] - 1; i >= 0; i = m_slots[i].next) {
                if (m_slots[i].hashCode == hashCode && m_comparer.Equals(m_slots[i].value, value)) {
                    return false;
                }
#if FEATURE_RANDOMIZED_STRING_HASHING && !FEATURE_NETCORE
                collisionCount++;
#endif
            }

            int index;
            if (m_freeList >= 0) {
                index = m_freeList;
                m_freeList = m_slots[index].next;
            }
            else {
                if (m_lastIndex == m_slots.Length) {
                    IncreaseCapacity();
                    // this will change during resize
                    bucket = hashCode % m_buckets.Length;
                }
                index = m_lastIndex;
                m_lastIndex++;
            }
            m_slots[index].hashCode = hashCode;
            m_slots[index].value = value;
            m_slots[index].next = m_buckets[bucket] - 1;
            m_buckets[bucket] = index + 1;
            m_count++;
            m_version++;

#if FEATURE_RANDOMIZED_STRING_HASHING && !FEATURE_NETCORE
            if(collisionCount > HashHelpers.HashCollisionThreshold && HashHelpers.IsWellKnownEqualityComparer(m_comparer)) {
                m_comparer = (IEqualityComparer<T>) HashHelpers.GetRandomizedEqualityComparer(m_comparer);
                SetCapacity(m_buckets.Length, true);
            }
#endif // FEATURE_RANDOMIZED_STRING_HASHING

            return true;
        }

        /// <summary>
        /// Checks if this contains of other's elements. Iterates over other's elements and 
        /// returns false as soon as it finds an element in other that's not in this.
        /// Used by SupersetOf, ProperSupersetOf, and SetEquals.
        /// </summary>
        /// <param name="other"></param>
        /// <returns></returns>
        private bool ContainsAllElements(IEnumerable<T> other) {
            foreach (T element in other) {
                if (!Contains(element)) {
                    return false;
                }
            }
            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>
        /// <param name="other"></param>
        /// <returns></returns>
        private bool IsSubsetOfHashSetWithSameEC(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>
        /// <param name="other"></param>
        private void IntersectWithHashSetWithSameEC(HashSet<T> other) {
            for (int i = 0; i < m_lastIndex; i++) {
                if (m_slots[i].hashCode >= 0) {
                    T item = m_slots[i].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 m_slots. If anything is unmarked (in bit array), remove it.
        /// 
        /// This attempts to allocate on the stack, if below StackAllocThreshold.
        /// </summary>
        /// <param name="other"></param>
        [System.Security.SecuritySafeCritical]
        private unsafe void IntersectWithEnumerable(IEnumerable<T> other) {
            Debug.Assert(m_buckets != null, "m_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 originalLastIndex = m_lastIndex;
            int intArrayLength = BitHelper.ToIntArrayLength(originalLastIndex);

            BitHelper bitHelper;
            if (intArrayLength <= StackAllocThreshold) {
                int* bitArrayPtr = stackalloc int[intArrayLength];
                bitHelper = new BitHelper(bitArrayPtr, intArrayLength);
            }
            else {
                int[] bitArray = new int[intArrayLength];
                bitHelper = new BitHelper(bitArray, intArrayLength);
            }

            // mark if contains: find index of in slots array and mark corresponding element in bit array
            foreach (T item in other) {
                int index = InternalIndexOf(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 < originalLastIndex; i++) {
                if (m_slots[i].hashCode >= 0 && !bitHelper.IsMarked(i)) {
                    Remove(m_slots[i].value);
                }
            }
        }

        /// <summary>
        /// Used internally by set operations which have to rely on bit array marking. This is like
        /// Contains but returns index in slots array. 
        /// </summary>
        /// <param name="item"></param>
        /// <returns></returns>
        private int InternalIndexOf(T item) {
            Debug.Assert(m_buckets != null, "m_buckets was null; callers should check first");

            int hashCode = InternalGetHashCode(item);
            for (int i = m_buckets[hashCode % m_buckets.Length] - 1; i >= 0; i = m_slots[i].next) {
                if ((m_slots[i].hashCode) == hashCode && m_comparer.Equals(m_slots[i].value, item)) {
                    return i;
                }
            }
            // wasn't found
            return -1;
        }

        /// <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);
                }
            }
        }

        /// <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>
        [System.Security.SecuritySafeCritical]
        private unsafe void SymmetricExceptWithEnumerable(IEnumerable<T> other) {
            int originalLastIndex = m_lastIndex;
            int intArrayLength = BitHelper.ToIntArrayLength(originalLastIndex);

            BitHelper itemsToRemove;
            BitHelper itemsAddedFromOther;
            if (intArrayLength <= StackAllocThreshold / 2) {
                int* itemsToRemovePtr = stackalloc int[intArrayLength];
                itemsToRemove = new BitHelper(itemsToRemovePtr, intArrayLength);

                int* itemsAddedFromOtherPtr = stackalloc int[intArrayLength];
                itemsAddedFromOther = new BitHelper(itemsAddedFromOtherPtr, intArrayLength);
            }
            else {
                int[] itemsToRemoveArray = new int[intArrayLength];
                itemsToRemove = new BitHelper(itemsToRemoveArray, intArrayLength);

                int[] itemsAddedFromOtherArray = new int[intArrayLength];
                itemsAddedFromOther = new BitHelper(itemsAddedFromOtherArray, intArrayLength);
            }

            foreach (T item in other) {
                int location = 0;
                bool added = AddOrGetLocation(item, out location);
                if (added) {
                    // 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 m_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 originalLastIndex
                    // because they could not have been in the original collection
                    if (location < originalLastIndex && !itemsAddedFromOther.IsMarked(location)) {
                        itemsToRemove.MarkBit(location);
                    }
                }
            }

            // if anything marked, remove it
            for (int i = 0; i < originalLastIndex; i++) {
                if (itemsToRemove.IsMarked(i)) {
                    Remove(m_slots[i].value);
                }
            }
        }

        /// <summary>
        /// Add if not already in hashset. Returns an out param indicating index where added. This 
        /// is used by SymmetricExcept because it needs to know the following things:
        /// - whether the item was already present in the collection or added from other
        /// - where it's located (if already present, it will get marked for removal, otherwise
        /// marked for keeping)
        /// </summary>
        /// <param name="value"></param>
        /// <param name="location"></param>
        /// <returns></returns>
        private bool AddOrGetLocation(T value, out int location) {
            Debug.Assert(m_buckets != null, "m_buckets is null, callers should have checked");

            int hashCode = InternalGetHashCode(value);
            int bucket = hashCode % m_buckets.Length;
            for (int i = m_buckets[hashCode % m_buckets.Length] - 1; i >= 0; i = m_slots[i].next) {
                if (m_slots[i].hashCode == hashCode && m_comparer.Equals(m_slots[i].value, value)) {
                    location = i;
                    return false; //already present
                }
            }
            int index;
            if (m_freeList >= 0) {
                index = m_freeList;
                m_freeList = m_slots[index].next;
            }
            else {
                if (m_lastIndex == m_slots.Length) {
                    IncreaseCapacity();
                    // this will change during resize
                    bucket = hashCode % m_buckets.Length;
                }
                index = m_lastIndex;
                m_lastIndex++;
            }
            m_slots[index].hashCode = hashCode;
            m_slots[index].value = value;
            m_slots[index].next = m_buckets[bucket] - 1;
            m_buckets[bucket] = index + 1;
            m_count++;
            m_version++;
            location = index;
            return true;
        }

        /// <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 = m_count; i.e. everything 
        /// in other is in this and everything in this is in other
        /// 2. Subset: checks if unfoundCount >= 0 and uniqueFoundCount = m_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 = m_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 m_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>
        /// <returns></returns>
        [System.Security.SecuritySafeCritical]
        private unsafe ElementCount CheckUniqueAndUnfoundElements(IEnumerable<T> other, bool returnIfUnfound) {
            ElementCount result;

            // need special case in case this has no elements. 
            if (m_count == 0) {
                int numElementsInOther = 0;
                foreach (T item in other) {
                    numElementsInOther++;
                    // break right away, all we want to know is whether other has 0 or 1 elements
                    break;
                }
                result.uniqueCount = 0;
                result.unfoundCount = numElementsInOther;
                return result;
            }


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

            int originalLastIndex = m_lastIndex;
            int intArrayLength = BitHelper.ToIntArrayLength(originalLastIndex);

            BitHelper bitHelper;
            if (intArrayLength <= StackAllocThreshold) {
                int* bitArrayPtr = stackalloc int[intArrayLength];
                bitHelper = new BitHelper(bitArrayPtr, intArrayLength);
            }
            else {
                int[] bitArray = new int[intArrayLength];
                bitHelper = new BitHelper(bitArray, intArrayLength);
            }

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

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

            result.uniqueCount = uniqueFoundCount;
            result.unfoundCount = unfoundCount;
            return result;
        }

        /// <summary>
        /// Copies this to an array. Used for DebugView
        /// </summary>
        /// <returns></returns>
        internal T[] ToArray() {
            T[] newArray = new T[Count];
            CopyTo(newArray);
            return newArray;
        }

        /// <summary>
        /// Internal method used for HashSetEqualityComparer. Compares set1 and set2 according 
        /// to specified comparer.
        /// 
        /// Because items are hashed according to a specific equality comparer, we have to resort
        /// to n^2 search if they're using different equality comparers.
        /// </summary>
        /// <param name="set1"></param>
        /// <param name="set2"></param>
        /// <param name="comparer"></param>
        /// <returns></returns>
        internal static bool HashSetEquals(HashSet<T> set1, HashSet<T> set2, IEqualityComparer<T> comparer) {
            // handle null cases first
            if (set1 == null) {
                return (set2 == null);
            }
            else if (set2 == null) {
                // set1 != null
                return false;
            }

            // all comparers are the same; this is faster
            if (AreEqualityComparersEqual(set1, set2)) {
                if (set1.Count != set2.Count) {
                    return false;
                }
                // suffices to check subset
                foreach (T item in set2) {
                    if (!set1.Contains(item)) {
                        return false;
                    }
                }
                return true;
            }
            else {  // n^2 search because items are hashed according to their respective ECs
                foreach (T set2Item in set2) {
                    bool found = false;
                    foreach (T set1Item in set1) {
                        if (comparer.Equals(set2Item, set1Item)) {
                            found = true;
                            break;
                        }
                    }
                    if (!found) {
                        return false;
                    }
                }
                return true;
            }
        }

        /// <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>
        /// <param name="set1"></param>
        /// <param name="set2"></param>
        /// <returns></returns>
        private static bool AreEqualityComparersEqual(HashSet<T> set1, HashSet<T> set2) {
            return set1.Comparer.Equals(set2.Comparer);
        }

        /// <summary>
        /// Workaround Comparers that throw ArgumentNullException for GetHashCode(null).
        /// </summary>
        /// <param name="item"></param>
        /// <returns>hash code</returns>
        private int InternalGetHashCode(T item) {
            if (item == null) {
                return 0;
            } 
            return m_comparer.GetHashCode(item) & Lower31BitMask;
        }

        #endregion

        // used for set checking operations (using enumerables) that rely on counting
        internal struct ElementCount {
            internal int uniqueCount;
            internal int unfoundCount;
        }

        internal struct Slot {
            internal int hashCode;      // Lower 31 bits of hash code, -1 if unused
            internal T value;
            internal int next;          // Index of next entry, -1 if last
        }

#if !SILVERLIGHT
        [Serializable()]
        [System.Security.Permissions.HostProtection(MayLeakOnAbort = true)]
#endif
        public struct Enumerator : IEnumerator<T>, System.Collections.IEnumerator {
            private HashSet<T> set;
            private int index;
            private int version;
            private T current;

            internal Enumerator(HashSet<T> set) {
                this.set = set;
                index = 0;
                version = set.m_version;
                current = default(T);
            }

            public void Dispose() {
            }

            public bool MoveNext() {
                if (version != set.m_version) {
                    throw new InvalidOperationException(SR.GetString(SR.InvalidOperation_EnumFailedVersion));
                }

                while (index < set.m_lastIndex) {
                    if (set.m_slots[index].hashCode >= 0) {
                        current = set.m_slots[index].value;
                        index++;
                        return true;
                    }
                    index++;
                }
                index = set.m_lastIndex + 1;
                current = default(T);
                return false;
            }

            public T Current {
                get {
                    return current;
                }
            }

            Object System.Collections.IEnumerator.Current {
                get {
                    if (index == 0 || index == set.m_lastIndex + 1) {
                        throw new InvalidOperationException(SR.GetString(SR.InvalidOperation_EnumOpCantHappen));
                    }
                    return Current;
                }
            }

            void System.Collections.IEnumerator.Reset() {
                if (version != set.m_version) {
                    throw new InvalidOperationException(SR.GetString(SR.InvalidOperation_EnumFailedVersion));
                }

                index = 0;
                current = default(T);
            }
        }
    }

}