linq_ext.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Extensions.Linq
{
    public static class XEnumerable
    {
        public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> source) where T : class =>
            source.Where(x => x is {})!;

        public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> source) where T : struct =>
            source.Where(x => x is {}).Select(x => x.GetValueOrDefault());

        public static T? FirstOrNull<T>(this IEnumerable<T> source) where T : class =>
            source.FirstOrDefault();

        public static T? FirstOrNull<T>(this IEnumerable<T?> source) where T : struct =>
            source.FirstOrDefault();

        public static T? FirstOrNull<T>(this IEnumerable<T> source, object? _ = null) where T : struct =>
            source.Cast<T?>().FirstOrDefault();

        public static T? SingleOrNull<T>(this IEnumerable<T> source) where T : class =>
            source.SingleOrDefault();

        public static T? SingleOrNull<T>(this IEnumerable<T> source, object? _ = null) where T : struct =>
            source.Cast<T?>().SingleOrDefault();

        public static IEnumerable<TResult> SelectWithState<T, TResult, TState>(
            this IEnumerable<T> source,
            Func<T, TState, (TResult Result, TState NewState)> mapFunc,
            TState initialState)
        {
            var state = initialState;
            return source.Select(x =>
            {
                var (result, newState) = mapFunc(x, state);
                state = newState;
                return result;
            });
        }
        
        public static TSource? MaxByOrNull<TSource, TKey>(
            this IEnumerable<TSource> source,
            Func<TSource, TKey> selector,
            IComparer<TKey>? comparer = null) where TSource : class
        {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (selector == null) throw new ArgumentNullException(nameof(selector));
            comparer ??= Comparer<TKey>.Default;

            using (var sourceIterator = source.GetEnumerator())
            {
                if (!sourceIterator.MoveNext())
                    return default;

                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;
            }
        }
        
        public static TSource? MaxByOrNull<TSource, TKey>(
            this IEnumerable<TSource> source,
            Func<TSource, TKey> selector,
            IComparer<TKey>? comparer = null,
            object? _ = null) where TSource : struct
        {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (selector == null) throw new ArgumentNullException(nameof(selector));
            comparer ??= Comparer<TKey>.Default;

            using (var sourceIterator = source.GetEnumerator())
            {
                if (!sourceIterator.MoveNext())
                    return default;

                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;
            }
        }

        public static TSource? MinByOrNull<TSource, TKey>(
            this IEnumerable<TSource> source,
            Func<TSource, TKey> selector,
            IComparer<TKey>? comparer = null) where TSource : class
        {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (selector == null) throw new ArgumentNullException(nameof(selector));
            comparer ??= Comparer<TKey>.Default;

            using var sourceIterator = source.GetEnumerator();
            if (!sourceIterator.MoveNext())
                return default;

            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;
        }
        
        public static TSource? MinByOrNull<TSource, TKey>(
            this IEnumerable<TSource> source,
            Func<TSource, TKey> selector,
            IComparer<TKey>? comparer = null,
            object? _ = null) where TSource : struct
        {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (selector == null) throw new ArgumentNullException(nameof(selector));
            comparer ??= Comparer<TKey>.Default;

            using var sourceIterator = source.GetEnumerator();
            if (!sourceIterator.MoveNext())
                return default;

            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;
        }

        public static IEnumerable<TSource> Except<TSource>(this IEnumerable<TSource> source, TSource element,
            IEqualityComparer<TSource>? comparer = null)
        {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (element is null) throw new ArgumentNullException(nameof(element));
            comparer ??= EqualityComparer<TSource>.Default;

            using (var sourceIterator = source.GetEnumerator())
            {
                while (sourceIterator.MoveNext())
                {
                    var candidate = sourceIterator.Current;
                    if (comparer.Equals(candidate, element))
                        continue;

                    yield return candidate;
                }
            }
        }

        public static IEnumerable<T[]> SliceOn<T>(this IEnumerable<T> source, int batchCapacity)
        {
            if (source is null)
                throw new ArgumentNullException(nameof(source));

            return source.SliceOnInternal(batchCapacity);
        }

        private static IEnumerable<T[]> SliceOnInternal<T>(this IEnumerable<T> source, int batchCapacity)
        {
            var buffer = new List<T>(batchCapacity);
            foreach (var item in source)
            {
                buffer.Add(item);

                if (buffer.Count == batchCapacity)
                {
                    yield return buffer.ToArray();
                    buffer.Clear();
                }
            }

            if (buffer.Count > 0)
                yield return buffer.ToArray();
        }
    }
}