#region License and Terms
// MoreLINQ - Extensions to LINQ to Objects
// Copyright (c) 2008 Jonathan Skeet. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#endregion

namespace MoreLinq
{
    using System;
    using System.Collections.Generic;

    static partial class MoreEnumerable
    {
        /// <summary>
        /// Returns all distinct elements of the given source, where "distinctness"
        /// is determined via a projection and the default equality comparer for the projected type.
        /// </summary>
        /// <remarks>
        /// This operator uses deferred execution and streams the results, although
        /// a set of already-seen keys is retained. If a key is seen multiple times,
        /// only the first element with that key is returned.
        /// </remarks>
        /// <typeparam name="TSource">Type of the source sequence</typeparam>
        /// <typeparam name="TKey">Type of the projected element</typeparam>
        /// <param name="source">Source sequence</param>
        /// <param name="keySelector">Projection for determining "distinctness"</param>
        /// <returns>A sequence consisting of distinct elements from the source sequence,
        /// comparing them by the specified key projection.</returns>

        public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> keySelector)
        {
            return source.DistinctBy(keySelector, null);
        }

        /// <summary>
        /// Returns all distinct elements of the given source, where "distinctness"
        /// is determined via a projection and the specified comparer for the projected type.
        /// </summary>
        /// <remarks>
        /// This operator uses deferred execution and streams the results, although
        /// a set of already-seen keys is retained. If a key is seen multiple times,
        /// only the first element with that key is returned.
        /// </remarks>
        /// <typeparam name="TSource">Type of the source sequence</typeparam>
        /// <typeparam name="TKey">Type of the projected element</typeparam>
        /// <param name="source">Source sequence</param>
        /// <param name="keySelector">Projection for determining "distinctness"</param>
        /// <param name="comparer">The equality comparer to use to determine whether or not keys are equal.
        /// If null, the default equality comparer for <c>TSource</c> is used.</param>
        /// <returns>A sequence consisting of distinct elements from the source sequence,
        /// comparing them by the specified key projection.</returns>

        public static IEnumerable<TSource> DistinctBy<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));

            return _(); IEnumerable<TSource> _()
            {
                var knownKeys = new HashSet<TKey>(comparer);
                foreach (var element in source)
                {
                    if (knownKeys.Add(keySelector(element)))
                        yield return element;
                }
            }
        }
    }
}
