test.txt

sdfsdfdsf

WhitespaceFilter.cs

public class WhitespaceFilter : Stream
{
    public WhitespaceFilter(Stream sink) => _sink = sink;

    private Stream _sink;
    private static readonly Regex reg = new Regex(@"(?<=[^])\t{2,}|(?<=[>])\s{2,}(?=[<])|(?<=[>])\s{2,11}(?=[<])|(?=[\n])\s{2,}");

    public override bool CanRead => true;

    public override bool CanSeek => true;

    public override bool CanWrite => true;

    public override void Flush() => _sink.Flush();

    public override long Length => 0;

    private long _position;

    public override long Position
    {
        get { return _position; }
        set { _position = value; }
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        return _sink.Read(buffer, offset, count);
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        return _sink.Seek(offset, origin);
    }

    public override void SetLength(long value)
    {
        _sink.SetLength(value);
    }

    public override void Close()
    {
        _sink.Close();
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        var data = new byte[count];
        Buffer.BlockCopy(buffer, offset, data, 0, count);
        var html = Encoding.Default.GetString(buffer);

        html = reg.Replace(html, string.Empty);

        var outdata = Encoding.Default.GetBytes(html);
        _sink.Write(outdata, 0, outdata.GetLength(0));
    }
}