using System;

using System.Collections.Generic;

using System.Linq;

​

static class Program

{

    static void Main()

    {

        var source = Enumerable.Range(1, 1_000_000);

        {

            var mem0 = GC.GetTotalAllocatedBytes(true);

            source.GroupBy(x => x % 1000).Select(g => g.MaxBy(x => x % 3333)).Count();

            var mem1 = GC.GetTotalAllocatedBytes(true);

            Console.WriteLine($"Allocated: {mem1 - mem0:#,0} bytes");

        }

        {

            var mem0 = GC.GetTotalAllocatedBytes(true);

            source.GroupMaxBy(x => x % 1000, x => x % 3333).Count();

            var mem1 = GC.GetTotalAllocatedBytes(true);

            Console.WriteLine($"Allocated: {mem1 - mem0:#,0} bytes");

        }

    }

​

    /// <summary>

    /// Groups the elements of a sequence according to a specified key selector

    /// function, and then returns the maximum element in each group according to

    /// a specified value selector function.

    /// </summary>

    public static IEnumerable<TSource> GroupMaxBy<TSource, TKey, TValue>(

        this IEnumerable<TSource> source,

        Func<TSource, TKey> keySelector,

        Func<TSource, TValue> valueSelector,

        IEqualityComparer<TKey> keyComparer = default,

        IComparer<TValue> valueComparer = default)

    {

        // Arguments validation omitted

        valueComparer ??= Comparer<TValue>.Default;

        var dictionary = new Dictionary<TKey, (TSource Item, TValue Value)>(keyComparer);

        foreach (var item in source)

        {

            var key = keySelector(item);

            var value = valueSelector(item);

            if (dictionary.TryGetValue(key, out var existing) &&

                valueComparer.Compare(existing.Value, value) >= 0) continue;

            dictionary[key] = (item, value);

        }

        foreach (var entry in dictionary.Values)

            yield return entry.Item;

    }

}