// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

using System.Collections.Generic;

namespace System.Linq
{
    public static partial class EnumerableEx
    {
        /// <summary>
        /// Returns elements with a distinct key value by using the default equality comparer to compare key values.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <typeparam name="TKey">Key type.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="keySelector">Key selector.</param>
        /// <returns>Sequence that contains the elements from the source sequence with distinct key values.</returns>
        public static IEnumerable<TSource> Distinct<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));

            return DistinctCore(source, keySelector, EqualityComparer<TKey>.Default);
        }

        /// <summary>
        /// Returns elements with a distinct key value by using the specified equality comparer to compare key values.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <typeparam name="TKey">Key type.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="keySelector">Key selector.</param>
        /// <param name="comparer">Comparer used to compare key values.</param>
        /// <returns>Sequence that contains the elements from the source sequence with distinct key values.</returns>
        public static IEnumerable<TSource> Distinct<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));
            if (comparer == null)
                throw new ArgumentNullException(nameof(comparer));

            return DistinctCore(source, keySelector, comparer);
        }

        private static IEnumerable<TSource> DistinctCore<TSource, TKey>(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
        {
            var set = new HashSet<TKey>(comparer);

            foreach (var item in source)
            {
                var key = keySelector(item);

                if (set.Add(key))
                {
                    yield return item;
                }
            }
        }
    }
}