DistinctBy.cs

#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


#if NO_HASHSET
using System.Linq;
#endif

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("source");
            if (keySelector == null) throw new ArgumentNullException("keySelector");
            return DistinctByImpl(source, keySelector, comparer);
        }

        private static IEnumerable<TSource> DistinctByImpl<TSource, TKey>(IEnumerable<TSource> source,
            Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
        {
#if !NO_HASHSET
            var knownKeys = new HashSet<TKey>(comparer);
            foreach (var element in source)
            {
                if (knownKeys.Add(keySelector(element)))
                {
                    yield return element;
                }
            }
#else
            //
            // On platforms where LINQ is available but no HashSet<T>
            // (like on Silverlight), implement this operator using 
            // existing LINQ operators. Using GroupBy is slightly less
            // efficient since it has do all the grouping work before
            // it can start to yield any one element from the source.
            //

            return source.GroupBy(keySelector, comparer).Select(g => g.First());
#endif
        }
    }
}

MaxBy.cs

#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 maximal element of the given sequence, based on
        /// the given projection.
        /// </summary>
        /// <remarks>
        /// If more than one element has the maximal projected value, the first
        /// one encountered will be returned. This overload uses the default comparer
        /// for the projected type. This operator uses immediate execution, but
        /// only buffers a single result (the current maximal element).
        /// </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 maximal 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 TSource MaxBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> selector)
        {
            return source.MaxBy(selector, Comparer<TKey>.Default);
        }

        /// <summary>
        /// Returns the maximal element of the given sequence, based on
        /// the given projection and the specified comparer for projected values. 
        /// </summary>
        /// <remarks>
        /// If more than one element has the maximal projected value, the first
        /// one encountered will be returned. This overload uses the default comparer
        /// for the projected type. This operator uses immediate execution, but
        /// only buffers a single result (the current maximal element).
        /// </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 maximal 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 TSource MaxBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> selector, IComparer<TKey> comparer)
        {
            if (source == null) throw new ArgumentNullException("source");
            if (selector == null) throw new ArgumentNullException("selector");
            if (comparer == null) throw new ArgumentNullException("comparer");
            using (var sourceIterator = source.GetEnumerator())
            {
                if (!sourceIterator.MoveNext())
                {
                    throw new InvalidOperationException("Sequence contains no elements");
                }
                var max = sourceIterator.Current;
                var maxKey = selector(max);
                while (sourceIterator.MoveNext())
                {
                    var candidate = sourceIterator.Current;
                    var candidateProjected = selector(candidate);
                    if (comparer.Compare(candidateProjected, maxKey) > 0)
                    {
                        max = candidate;
                        maxKey = candidateProjected;
                    }
                }
                return max;
            }
        }
    }
}

MinBy.cs

#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 element of the given sequence, based on
        /// the given projection.
        /// </summary>
        /// <remarks>
        /// If more than one element has the minimal projected value, the first
        /// one encountered will be returned. This overload uses the default comparer
        /// for the projected type. This operator uses immediate execution, but
        /// only buffers a single result (the current minimal element).
        /// </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 TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> selector)
        {
            return source.MinBy(selector, Comparer<TKey>.Default);
        }

        /// <summary>
        /// Returns the minimal element of the given sequence, based on
        /// the given projection and the specified comparer for projected values.
        /// </summary>
        /// <remarks>
        /// If more than one element has the minimal projected value, the first
        /// one encountered will be returned. This overload uses the default comparer
        /// for the projected type. This operator uses immediate execution, but
        /// only buffers a single result (the current minimal element).
        /// </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 TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> selector, IComparer<TKey> comparer)
        {
            if (source == null) throw new ArgumentNullException("source");
            if (selector == null) throw new ArgumentNullException("selector");
            if (comparer == null) throw new ArgumentNullException("comparer");
            using (var sourceIterator = source.GetEnumerator())
            {
                if (!sourceIterator.MoveNext())
                {
                    throw new InvalidOperationException("Sequence contains no elements");
                }
                var min = sourceIterator.Current;
                var minKey = selector(min);
                while (sourceIterator.MoveNext())
                {
                    var candidate = sourceIterator.Current;
                    var candidateProjected = selector(candidate);
                    if (comparer.Compare(candidateProjected, minKey) < 0)
                    {
                        min = candidate;
                        minKey = candidateProjected;
                    }
                }
                return min;
            }
        }
    }
}