Very simple code for sorting big text file in csharp

big_file_sort_in_csharp.cs

/*
2_000_000 => 16 sec
5_ => 32
10_ => 1:10
20_ => 2:58
40_ => 13:44 file size = 2.5 Gb
*/

using System.IO;
using System.Diagnostics;
using static System.Console;
int count = args.Length > 0 && int.TryParse(args[0], out var val) ? val : 2_000_000;

var sw = Stopwatch.StartNew();
var alf = get_alf();

if (Directory.Exists(Constants.TMP))
    Directory.Delete(Constants.TMP, true);
Directory.CreateDirectory(Constants.TMP);

var comp = new SimpleComparer();

do_it(count);
sw.Stop();
WriteLine(sw.Elapsed);

char[] get_alf()
{
    var list = new List<char>();
    for (char x = 'A'; x <= 'Z'; x++)
        list.Add(x);
    for (char x = 'a'; x <= 'z'; x++)
        list.Add(x);
    list.Add(' ');

    return list.ToArray();
}

string gen_str()
{
    var range = Enumerable.Range(0, Random.Shared.Next(20, 100));
    var chars = range.Select(x => alf[Random.Shared.Next(0, alf.Length)]).ToArray();
    return new string(chars);
}

int gen_num() => Random.Shared.Next(0, 10_000);

void do_it(int count)
{
    var source = $"L{count}.txt";
    File.WriteAllLines(source, Enumerable.Range(0, count).Select(x => $"{gen_num()}.{gen_str()}"));
    int num = 0;
    foreach (var chunk in File.ReadAllLines(source).Chunk(200_000))
        File.WriteAllLines(Path.Combine($".\\{Constants.TMP}\\", $"{(++num):D5}.txt"), chunk.OrderBy(x => x, comp));
    SortFiles($"S{count}.txt");
}

void SortFiles(string outputFile)
{
    var readers = Directory.GetFiles(Path.Combine($".\\{Constants.TMP}\\")).Select(x => new StreamReader(x));
    try
    {
        var lines = readers.Select(x => new TxtReader
        {
            Line = x.ReadLine(),
            Reader = x
        }).OrderBy(x => x.Line, comparer: comp).ToList();

        using var writer = new StreamWriter(outputFile);
        Reorder();
        void Reorder()
        {
            if (lines.Count == 1)
                return;

            int i = 0;
            while (comp.Compare(lines[i].Line, lines[i + 1].Line) > 0)
            {
                var t = lines[i];
                lines[i] = lines[i + 1];
                lines[i + 1] = t;
                i++;
                if (i + 1 == lines.Count)
                    return;
            }
        }

        while (lines.Count > 0)
        {
            var current = lines[0];
            writer.WriteLine(current.Line);

            if (current.Reader.EndOfStream)
            {
                lines.Remove(current);
                WriteLine($"{DateTime.Now:s} : {lines.Count}");
                continue;
            }
            current.Line = current.Reader.ReadLine();
            Reorder();
        }
    }
    finally
    {
        foreach (var r in readers)
            r.Dispose();
    }
}

public class TxtReader
{
    public string Line;
    public StreamReader Reader;
}
class SimpleComparer : IComparer<string>
{
    public int Compare(string s1, string s2)
    {
        var p1 = s1.IndexOf('.');
        var v1 = s1.AsSpan(p1 + 2);

        var p2 = s2.IndexOf('.');
        var v2 = s2.AsSpan(p2 + 2);

        var vc =
            v1.CompareTo(v2, StringComparison.Ordinal);

        if (vc != 0)
            return vc;
        else
        {
            var n1 = Int32.Parse(s1.AsSpan(0, p1));
            var n2 = Int32.Parse(s2.AsSpan(0, p2));
            return n1.CompareTo(n2);
        }
    }
}

public static class Constants
{
    public const string TMP = "tmp";
}