#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 the minimal elements of the given sequence, based on
        /// the given projection.
        /// </summary>
        /// <remarks>
        /// This overload uses the default comparer for the projected type.
        /// This operator uses deferred execution. The results are evaluated
        /// and cached on first use to returned sequence.
        /// </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="selector">Selector to use to pick the results to compare</param>
        /// <returns>The minimal element, according to the projection.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="selector"/> is null</exception>
        /// <exception cref="InvalidOperationException"><paramref name="source"/> is empty</exception>

        public static IExtremaEnumerable<TSource> MinBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> selector)
        {
            return source.MinBy(selector, null);
        }

        /// <summary>
        /// Returns the minimal elements of the given sequence, based on
        /// the given projection and the specified comparer for projected values.
        /// </summary>
        /// <remarks>
        /// This operator uses deferred execution. The results are evaluated
        /// and cached on first use to returned sequence.
        /// </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="selector">Selector to use to pick the results to compare</param>
        /// <param name="comparer">Comparer to use to compare projected values</param>
        /// <returns>The minimal element, according to the projection.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/>, <paramref name="selector"/>
        /// or <paramref name="comparer"/> is null</exception>
        /// <exception cref="InvalidOperationException"><paramref name="source"/> is empty</exception>

        public static IExtremaEnumerable<TSource> MinBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> selector, IComparer<TKey> comparer)
        {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (selector == null) throw new ArgumentNullException(nameof(selector));

            comparer = comparer ?? Comparer<TKey>.Default;
            return new ExtremaEnumerable<TSource, TKey>(source, selector, (x, y) => -Math.Sign(comparer.Compare(x, y)));
        }
    }
}
