// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Collections;
using System.Collections.Generic;
using Microsoft.AspNetCore.Internal;
using Microsoft.Extensions.Primitives;

namespace Microsoft.AspNetCore.Http
{
    /// <summary>
    /// The HttpRequest query string collection
    /// </summary>
    internal class QueryCollectionInternal : IQueryCollection
    {
        private AdaptiveCapacityDictionary<string, StringValues> Store { get; }

        /// <summary>
        /// Initializes a new instance of <see cref="QueryCollection"/>.
        /// </summary>
        /// <param name="store">The backing store.</param>
        internal QueryCollectionInternal(AdaptiveCapacityDictionary<string, StringValues> store)
        {
            Store = store;
        }

        /// <summary>
        /// Gets the associated set of values from the collection.
        /// </summary>
        /// <param name="key">The key name.</param>
        /// <returns>the associated value from the collection as a StringValues or StringValues.Empty if the key is not present.</returns>
        public StringValues this[string key] => TryGetValue(key, out var value) ? value : StringValues.Empty;

        /// <summary>
        /// Gets the number of elements contained in the <see cref="QueryCollection" />;.
        /// </summary>
        /// <returns>The number of elements contained in the <see cref="QueryCollection" />.</returns>
        public int Count => Store.Count;

        /// <summary>
        /// Gets the collection of query names in this instance.
        /// </summary>
        public ICollection<string> Keys => Store.Keys;

        /// <summary>
        /// Determines whether the <see cref="QueryCollection" /> contains a specific key.
        /// </summary>
        /// <param name="key">The key.</param>
        /// <returns>true if the <see cref="QueryCollection" /> contains a specific key; otherwise, false.</returns>
        public bool ContainsKey(string key) => Store.ContainsKey(key);

        /// <summary>
        /// Retrieves a value from the collection.
        /// </summary>
        /// <param name="key">The key.</param>
        /// <param name="value">The value.</param>
        /// <returns>true if the <see cref="QueryCollection" /> contains the key; otherwise, false.</returns>
        public bool TryGetValue(string key, out StringValues value) => Store.TryGetValue(key, out value);

        /// <summary>
        /// Returns an enumerator that iterates through a collection.
        /// </summary>
        /// <returns>An <see cref="Enumerator" /> object that can be used to iterate through the collection.</returns>
        public Enumerator GetEnumerator() => new Enumerator(Store.GetEnumerator());

        /// <summary>
        /// Returns an enumerator that iterates through a collection.
        /// </summary>
        /// <returns>An <see cref="IEnumerator{T}" /> object that can be used to iterate through the collection.</returns>
        IEnumerator<KeyValuePair<string, StringValues>> IEnumerable<KeyValuePair<string, StringValues>>.GetEnumerator()
            => Store.GetEnumerator();

        /// <summary>
        /// Returns an enumerator that iterates through a collection.
        /// </summary>
        /// <returns>An <see cref="IEnumerator" /> object that can be used to iterate through the collection.</returns>
        IEnumerator IEnumerable.GetEnumerator() => Store.GetEnumerator();

        /// <summary>
        /// Enumerates a <see cref="QueryCollection"/>.
        /// </summary>
        public struct Enumerator : IEnumerator<KeyValuePair<string, StringValues>>
        {
            // Do NOT make this readonly, or MoveNext will not work
            private AdaptiveCapacityDictionary<string, StringValues>.Enumerator _dictionaryEnumerator;
            private readonly bool _notEmpty;

            internal Enumerator(AdaptiveCapacityDictionary<string, StringValues>.Enumerator dictionaryEnumerator)
            {
                _dictionaryEnumerator = dictionaryEnumerator;
                _notEmpty = true;
            }

            /// <summary>
            /// Advances the enumerator to the next element of the <see cref="HeaderDictionary"/>.
            /// </summary>
            /// <returns><see langword="true"/> if the enumerator was successfully advanced to the next element;
            /// <see langword="false"/> if the enumerator has passed the end of the collection.</returns>
            public bool MoveNext()
            {
                if (_notEmpty)
                {
                    return _dictionaryEnumerator.MoveNext();
                }
                return false;
            }

            /// <summary>
            /// Gets the element at the current position of the enumerator.
            /// </summary>
            public KeyValuePair<string, StringValues> Current => _notEmpty ? _dictionaryEnumerator.Current : default;

            /// <inheritdoc />
            public void Dispose()
            {
            }

            object IEnumerator.Current => Current;

            void IEnumerator.Reset()
            {
                if (_notEmpty)
                {
                    ((IEnumerator)_dictionaryEnumerator).Reset();
                }
            }
        }
    }
}
