iuh

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;

public class BigFileMergeSort
{
    private static void SplitAndSort(string inputFile, string outputFile)
    {
        var lines = new List<string>(100_000);
        var part = 0;
        var size = 0;

        Task blocker = Task.CompletedTask;

        using (var reader = new StreamReader(inputFile))
        {
            while (!reader.EndOfStream)
            {
                string line = reader.ReadLine()!;
                lines.Add(line);

                size += line.Length;

                if (size > 128_000_000)
                {
                    size = 0;

                    var fname = $"{outputFile}part{part++}.txt";
                    var flines = lines.ToList();

                    Task.WaitAll(new[] { blocker });
                    blocker = Task.Run(() => Jazda(flines, fname));

                    lines.Clear();
                }
            }

            Task.WaitAll(new[] { blocker });

            if (lines.Count > 0)
            {
                Jazda(lines, $"{outputFile}part{part++}.txt");
            }
        }

        static void Jazda(List<string> flines, string fname)
        {
            flines.Sort();
            File.WriteAllLines(fname, flines);
        }
    }

    public class FilePart
    {
        public required StreamReader S { get; set; }
        public required string? L { get; set; }
        public void Go() => L = S.ReadLine();
        public bool GameOver => L is null;
    }

    private static void Merge(string outputFile)
    {
        var parts = new List<FilePart>();
        var flush = new List<string>();
        var size = 0;

        foreach (var file in Directory.GetFiles("C:\\temp\\sort", $"*part*.txt"))
        {
            parts.Add(new FilePart() { S = new StreamReader(file), L = null });
        }

        parts.ForEach(x => x.Go());

        Task blocker = Task.CompletedTask;

        while (parts.Count > 0)
        {
            var kd = parts.OrderBy(x => x.L).First();

            flush.Add(kd.L!);
            size += kd.L!.Length;

            if (size > 128_000_000)
            {
                Task.WaitAll(new[] { blocker });

                size = 0;
                var tmp = flush;

                blocker = Task.Run(() =>
                {
                    File.AppendAllLines(outputFile, tmp);
                    tmp.Clear();
                });

                flush = new List<string>();
            }

            kd.Go();

            if (kd.GameOver)
            {
                parts.Remove(kd);
            }
        }

        Task.WaitAll(new[] { blocker });

        if (flush.Count > 0)
        {
            File.AppendAllLines(outputFile, flush);
        }
    }

    public static void Main()
    {
        string inputFile = "C:\\temp\\sort\\testfile.txt";
        string outputFile = "C:\\temp\\sort\\output";

        var sw = Stopwatch.StartNew();
        SplitAndSort(inputFile, outputFile);
        Merge(outputFile);
        Console.WriteLine(sw.Elapsed.TotalSeconds);
    }
}