using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Threading;

namespace Umbraco.Core.Collections
{
    /// <summary>
    /// A thread-safe representation of a <see cref="HashSet{T}"/>.
    /// Enumerating this collection is thread-safe and will only operate on a clone that is generated before returning the enumerator.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    [Serializable]
    public class ConcurrentHashSet<T> : ICollection<T>
    {
        private readonly HashSet<T> _innerSet = new HashSet<T>();
        private readonly ReaderWriterLockSlim _instanceLocker = new ReaderWriterLockSlim(LockRecursionPolicy.NoRecursion);

        /// <summary>
        /// Returns an enumerator that iterates through the collection.
        /// </summary>
        /// <returns>
        /// A <see cref="T:System.Collections.Generic.IEnumerator`1"/> that can be used to iterate through the collection.
        /// </returns>
        /// <filterpriority>1</filterpriority>
        public IEnumerator<T> GetEnumerator()
        {
            return GetThreadSafeClone().GetEnumerator();
        }

        /// <summary>
        /// Returns an enumerator that iterates through a collection.
        /// </summary>
        /// <returns>
        /// An <see cref="T:System.Collections.IEnumerator"/> object that can be used to iterate through the collection.
        /// </returns>
        /// <filterpriority>2</filterpriority>
        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        /// <summary>
        /// Removes the first occurrence of a specific object from the <see cref="T:System.Collections.Generic.ICollection`1"/>.
        /// </summary>
        /// <returns>
        /// true if <paramref name="item"/> was successfully removed from the <see cref="T:System.Collections.Generic.ICollection`1"/>; otherwise, false. This method also returns false if <paramref name="item"/> is not found in the original <see cref="T:System.Collections.Generic.ICollection`1"/>.
        /// </returns>
        /// <param name="item">The object to remove from the <see cref="T:System.Collections.Generic.ICollection`1"/>.</param><exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only.</exception>
        public bool Remove(T item)
        {
            try
            {
                _instanceLocker.EnterWriteLock();
                return _innerSet.Remove(item);
            }
            finally
            {
                if (_instanceLocker.IsWriteLockHeld)
                    _instanceLocker.ExitWriteLock();
            }
        }


        /// <summary>
        /// Gets the number of elements contained in the <see cref="T:System.Collections.ICollection"/>.
        /// </summary>
        /// <returns>
        /// The number of elements contained in the <see cref="T:System.Collections.ICollection"/>.
        /// </returns>
        /// <filterpriority>2</filterpriority>
        public int Count
        {
            get
            {
                try
                {
                    _instanceLocker.EnterReadLock();
                    return _innerSet.Count;
                }
                finally
                {
                    if (_instanceLocker.IsReadLockHeld)
                        _instanceLocker.ExitReadLock();
                }

            }
        }

        /// <summary>
        /// Gets a value indicating whether the <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only.
        /// </summary>
        /// <returns>
        /// true if the <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only; otherwise, false.
        /// </returns>
        public bool IsReadOnly => false;

        /// <summary>
        /// Adds an item to the <see cref="T:System.Collections.Generic.ICollection`1"/>.
        /// </summary>
        /// <param name="item">The object to add to the <see cref="T:System.Collections.Generic.ICollection`1"/>.</param><exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only.</exception>
        public void Add(T item)
        {
            try
            {
                _instanceLocker.EnterWriteLock();
                _innerSet.Add(item);
            }
            finally
            {
                if (_instanceLocker.IsWriteLockHeld)
                    _instanceLocker.ExitWriteLock();
            }
        }

        /// <summary>
        /// Attempts to add an item to the collection
        /// </summary>
        /// <param name="item"></param>
        /// <returns></returns>
        public bool TryAdd(T item)
        {
            if (Contains(item)) return false;
            try
            {
                _instanceLocker.EnterWriteLock();

                //double check
                if (_innerSet.Contains(item)) return false;
                _innerSet.Add(item);
                return true;
            }
            finally
            {
                if (_instanceLocker.IsWriteLockHeld)
                    _instanceLocker.ExitWriteLock();
            }
        }

        /// <summary>
        /// Removes all items from the <see cref="T:System.Collections.Generic.ICollection`1"/>.
        /// </summary>
        /// <exception cref="T:System.NotSupportedException">The <see cref="T:System.Collections.Generic.ICollection`1"/> is read-only. </exception>
        public void Clear()
        {
            try
            {
                _instanceLocker.EnterWriteLock();
                _innerSet.Clear();
            }
            finally
            {
                if (_instanceLocker.IsWriteLockHeld)
                    _instanceLocker.ExitWriteLock();
            }
        }

        /// <summary>
        /// Determines whether the <see cref="T:System.Collections.Generic.ICollection`1"/> contains a specific value.
        /// </summary>
        /// <returns>
        /// true if <paramref name="item"/> is found in the <see cref="T:System.Collections.Generic.ICollection`1"/>; otherwise, false.
        /// </returns>
        /// <param name="item">The object to locate in the <see cref="T:System.Collections.Generic.ICollection`1"/>.</param>
        public bool Contains(T item)
        {
            try
            {
                _instanceLocker.EnterReadLock();
                return _innerSet.Contains(item);
            }
            finally
            {
                if (_instanceLocker.IsReadLockHeld)
                    _instanceLocker.ExitReadLock();
            }
        }

        /// <summary>
        /// Copies the elements of the <see cref="T:System.Collections.Concurrent.IProducerConsumerCollection`1"/> to an <see cref="T:System.Array"/>, starting at a specified index.
        /// </summary>
        /// <param name="array">The one-dimensional <see cref="T:System.Array"/> that is the destination of the elements copied from the <see cref="T:System.Collections.Concurrent.IProducerConsumerCollection`1"/>. The array must have zero-based indexing.</param><param name="index">The zero-based index in <paramref name="array"/> at which copying begins.</param><exception cref="T:System.ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in Visual Basic).</exception><exception cref="T:System.ArgumentOutOfRangeException"><paramref name="index"/> is less than zero.</exception><exception cref="T:System.ArgumentException"><paramref name="index"/> is equal to or greater than the length of the <paramref name="array"/> -or- The number of elements in the source <see cref="T:System.Collections.Concurrent.ConcurrentQueue`1"/> is greater than the available space from <paramref name="index"/> to the end of the destination <paramref name="array"/>.</exception>
        public void CopyTo(T[] array, int index)
        {
            var clone = GetThreadSafeClone();
            clone.CopyTo(array, index);
        }

        private HashSet<T> GetThreadSafeClone()
        {
            HashSet<T> clone = null;
            try
            {
                _instanceLocker.EnterReadLock();
                clone = new HashSet<T>(_innerSet, _innerSet.Comparer);
            }
            finally
            {
                if (_instanceLocker.IsReadLockHeld)
                    _instanceLocker.ExitReadLock();
            }
            return clone;
        }

        /// <summary>
        /// Copies the elements of the <see cref="T:System.Collections.ICollection"/> to an <see cref="T:System.Array"/>, starting at a particular <see cref="T:System.Array"/> index.
        /// </summary>
        /// <param name="array">The one-dimensional <see cref="T:System.Array"/> that is the destination of the elements copied from <see cref="T:System.Collections.ICollection"/>. The <see cref="T:System.Array"/> must have zero-based indexing. </param><param name="index">The zero-based index in <paramref name="array"/> at which copying begins. </param><exception cref="T:System.ArgumentNullException"><paramref name="array"/> is null. </exception><exception cref="T:System.ArgumentOutOfRangeException"><paramref name="index"/> is less than zero. </exception><exception cref="T:System.ArgumentException"><paramref name="array"/> is multidimensional.-or- The number of elements in the source <see cref="T:System.Collections.ICollection"/> is greater than the available space from <paramref name="index"/> to the end of the destination <paramref name="array"/>. </exception><exception cref="T:System.ArgumentException">The type of the source <see cref="T:System.Collections.ICollection"/> cannot be cast automatically to the type of the destination <paramref name="array"/>. </exception><filterpriority>2</filterpriority>
        public void CopyTo(Array array, int index)
        {
            var clone = GetThreadSafeClone();
            Array.Copy(clone.ToArray(), 0, array, index, clone.Count);
        }
    }
}