using System;
using System.Collections.Generic;
using System.Linq;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Engines;
using BenchmarkDotNet.Running;

public static class EnumerableExtensions
{
    static readonly Func<object, bool> notNullTest = x => x is not null;

    public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> source)
        where T : class =>
            source.Where((Func<T?, bool>)notNullTest)!;

    public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> source)
        where T : struct =>
            source.Where(x => x.HasValue).Select(x => x!.Value);
}

public class BenchmarkClass
{
    const int dataLength = 100_000;
    int?[] intData = new int?[dataLength];
    string?[] stringData = new string?[dataLength];

    public BenchmarkClass()
    {
        var random = new Random();

        for (int idx = 0; idx < dataLength; ++idx)
        {
            var value = random.Next(100);
            if (value != 0)
            {
                intData[idx] = value;
                stringData[idx] = value.ToString();
            }
        }
    }

    [Benchmark]
    public void Integer_OfType() => intData.OfType<int>().Consume(new Consumer());

    [Benchmark]
    public void Integer_WhereNotNull() => intData.WhereNotNull().Consume(new Consumer());

    [Benchmark]
    public void String_OfType() => stringData.OfType<string>().Consume(new Consumer());

    [Benchmark]
    public void String_WhereNotNull() => stringData.WhereNotNull().Consume(new Consumer());
}

public static class Program
{
    public static void Main() =>
        BenchmarkRunner.Run<BenchmarkClass>();
}