TakeFromGrouped

EnumerableExtensions.cs

using System;
using System.Collections.Generic;

namespace TakeFromGroup
{
    public static class EnumerableExtensions
    {
        public static IEnumerable<T> TakeFromGrouped<T, TGroup>(this IEnumerable<T> enumerable,
                                                                Func<T, TGroup> groupSelector,
                                                                Func<T, T, T> takeFunc)
        {
            var taken = new Dictionary<TGroup, T>();

            foreach (T value in enumerable)
            {
                T currentTaken;
                TGroup groupKey = groupSelector(value);
                if (taken.TryGetValue(groupKey, out currentTaken))
                {
                    currentTaken = takeFunc(value, currentTaken);
                }
                else
                {
                    currentTaken = value;
                }
                taken[groupKey] = currentTaken;
            }
            return taken.Values;
        }

        public static IEnumerable<T> TakeMinFromGrouped<T, TGroup, TKey>(this IEnumerable<T> enumerable,
                                                                         Func<T, TGroup> groupSelector,
                                                                         Func<T, TKey> keySelector,
                                                                         IComparer<TKey> comparer = null)
        {
            SetDefaultIfNull(ref comparer);
            return enumerable.TakeFromGrouped(groupSelector,
                (x, y) => comparer.Compare(keySelector(x), keySelector(y)) < 0 ? x : y);
        }

        public static IEnumerable<T> TakeMaxFromGrouped<T, TGroup, TKey>(this IEnumerable<T> enumerable,
                                                                         Func<T, TGroup> groupSelector,
                                                                         Func<T, TKey> keySelector,
                                                                         IComparer<TKey> comparer = null)
        {
            SetDefaultIfNull(ref comparer);
            return enumerable
                .TakeFromGrouped(groupSelector,
                    (x, y) => comparer.Compare(keySelector(x), keySelector(y)) > 0 ? x : y);
        }

        private static void SetDefaultIfNull<T>(ref IComparer<T> comparer)
        {
            if (comparer == null)
            {
                comparer = Comparer<T>.Default;
            }
        }
    }
}