// I don't want all of MoreLINQ (https://github.com/morelinq/MoreLINQ) so we'll borrow just this

#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

using System;
using System.Collections.Generic;

namespace Artemis.Core
{
    /// <summary>
    ///     A static class providing <see cref="IEnumerable{T}" /> extensions
    /// </summary>
    // ReSharper disable once InconsistentNaming
    public static class IEnumerableExtensions
    {
        /// <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> _()
            {
                HashSet<TKey> knownKeys = new(comparer);
                foreach (TSource element in source)
                {
                    if (knownKeys.Add(keySelector(element)))
                        yield return element;
                }
            }
        }
    }
}