Added a bi-directional dictionary

src/Microsoft.Experimental.Collections/Microsoft/Collections/Extensions/BidirectionalDictionary.KeyCollection.cs

@@ -0,0 +1,157 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Diagnostics;
+
+namespace Microsoft.Collections.Extensions
+{
+    public partial class BidirectionalDictionary<TFirst, TSecond>
+    {
+        /// <summary>
+        /// Represents the collection of keys in a <see cref="BidirectionalDictionary{TFirst, TSecond}" />. This class cannot be inherited.
+        /// </summary>
+        [DebuggerTypeProxy(typeof(DictionaryKeyCollectionDebugView<,>))]
+        [DebuggerDisplay("Count = {Count}")]
+        public sealed class KeyCollection : ICollection<TFirst>, IReadOnlyCollection<TFirst>
+        {
+            private readonly BidirectionalDictionary<TFirst, TSecond> _bidirectionalDictionary;
+
+            /// <summary>
+            /// Gets the number of elements contained in the <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection" />.
+            /// </summary>
+            /// <returns>The number of elements contained in the <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection" />.</returns>
+            public int Count => _bidirectionalDictionary.Count;
+
+            bool ICollection<TFirst>.IsReadOnly => true;
+
+            internal KeyCollection(BidirectionalDictionary<TFirst, TSecond> bidirectionalDictionary)
+            {
+                _bidirectionalDictionary = bidirectionalDictionary;
+            }
+
+            /// <summary>
+            /// Determines whether the <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection" /> contains a specific value.
+            /// </summary>
+            /// <param name="firstKey">The object to locate in the <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection" />.</param>
+            /// <returns>true if <paramref name="firstKey" /> is found in the <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection" />; otherwise, false.</returns>
+            public bool Contains(TFirst firstKey) => _bidirectionalDictionary.ContainsKey(firstKey);
+
+            /// <summary>
+            /// Returns an enumerator that iterates through the <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection" />.
+            /// </summary>
+            /// <returns>A <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection.Enumerator" /> for the <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection" />.</returns>
+            public Enumerator GetEnumerator() => new Enumerator(_bidirectionalDictionary);
+
+            void ICollection<TFirst>.Add(TFirst item) => throw new NotSupportedException(Strings.NotSupported_KeyCollectionSet);
+
+            void ICollection<TFirst>.Clear() => throw new NotSupportedException(Strings.NotSupported_KeyCollectionSet);
+
+            void ICollection<TFirst>.CopyTo(TFirst[] array, int arrayIndex)
+            {
+                if (array == null)
+                {
+                    throw new ArgumentNullException(nameof(array));
+                }
+                if ((uint)arrayIndex > (uint)array.Length)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(arrayIndex), Strings.ArgumentOutOfRange_NeedNonNegNum);
+                }
+                if (array.Length - arrayIndex < Count)
+                {
+                    throw new ArgumentException(Strings.Arg_ArrayPlusOffTooSmall);
+                }
+
+                int count = _bidirectionalDictionary._shared.Count;
+                Entry[] entries = _bidirectionalDictionary._entries;
+                int currentArrayIndex = arrayIndex;
+                for (int i = 0; i < count; ++i)
+                {
+                    Entry entry = entries[i];
+                    if (entry.Next >= -1)
+                    {
+                        array[currentArrayIndex++] = entry.Key;
+                    }
+                }
+            }
+
+            bool ICollection<TFirst>.Remove(TFirst item) => throw new NotSupportedException(Strings.NotSupported_KeyCollectionSet);
+
+            IEnumerator<TFirst> IEnumerable<TFirst>.GetEnumerator() => GetEnumerator();
+
+            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
+            /// <summary>
+            /// Enumerates the elements of a <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection" />.
+            /// </summary>
+            public struct Enumerator : IEnumerator<TFirst>
+            {
+                private readonly BidirectionalDictionary<TFirst, TSecond> _bidirectionalDictionary;
+                private readonly int _version;
+                private int _index;
+                private TFirst _current;
+
+                /// <summary>
+                /// Gets the element at the current position of the enumerator.
+                /// </summary>
+                /// <returns>The element in the <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection" /> at the current position of the enumerator.</returns>
+                public TFirst Current => _current;
+
+                object IEnumerator.Current => _current;
+
+                internal Enumerator(BidirectionalDictionary<TFirst, TSecond> bidirectionalDictionary)
+                {
+                    _bidirectionalDictionary = bidirectionalDictionary;
+                    _version = bidirectionalDictionary._shared.Version;
+                    _index = 0;
+                    _current = default;
+                }
+
+                /// <summary>
+                /// Releases all resources used by the <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection.Enumerator" />.
+                /// </summary>
+                public void Dispose()
+                {
+                }
+
+                /// <summary>
+                /// Advances the enumerator to the next element of the <see cref="BidirectionalDictionary{TFirst, TSecond}.KeyCollection" />.
+                /// </summary>
+                /// <returns>true if the enumerator was successfully advanced to the next element; false if the enumerator has passed the end of the collection.</returns>
+                /// <exception cref="InvalidOperationException">The collection was modified after the enumerator was created.</exception>
+                public bool MoveNext()
+                {
+                    BidirectionalDictionaryShared shared = _bidirectionalDictionary._shared;
+                    if (_version != shared.Version)
+                    {
+                        throw new InvalidOperationException(Strings.InvalidOperation_EnumFailedVersion);
+                    }
+
+                    Entry[] entries = _bidirectionalDictionary._entries;
+                    while ((uint)_index < (uint)shared.Count)
+                    {
+                        Entry entry = entries[_index++];
+
+                        if (entry.Next >= -1)
+                        {
+                            _current = entry.Key;
+                            return true;
+                        }
+                    }
+                    _current = default;
+                    return false;
+                }
+
+                void IEnumerator.Reset()
+                {
+                    if (_version != _bidirectionalDictionary._shared.Version)
+                    {
+                        throw new InvalidOperationException(Strings.InvalidOperation_EnumFailedVersion);
+                    }
+
+                    _index = 0;
+                    _current = default;
+                }
+            }
+        }
+    }
+}