diff.cs

public class DiffBinaryWriter : BinaryWriter
{
    long prev;
    public DiffBinaryWriter(Stream s, long start) : base(s)
    {
        prev = start;
        WriteValue(prev);
    }

    public void Diff(long cur)
    {
        var diff = cur - prev;
        prev = cur;
        if (diff <= 0)
            throw new ArgumentOutOfRangeException($"{cur} is not greater than {prev}, values aren't sorted");

        WriteValue(diff);
    }

    private void WriteValue(long diff)
    {
        if (diff > int.MaxValue)
        {
            // probably rare
            Write7BitEncodedInt(0); // indicate that we need to read a full long here
            Write(diff);
            return;
        }

        Write7BitEncodedInt((int)diff);
    }
}