2.cs

public static long ComputeSum(string fileName, 
    Func<ReadOnlyMemory<byte>, long, int, (long, int)> sumComputer)
{
    using var fs = new FileStream(fileName, FileMode.Open);
    using var lease = MemoryPool<byte>.Shared.Rent(MinBufferSize);
    var buffer = lease.Memory;

    long sum = 0;
    int n = 0;
    while (true) {
        var count = fs.Read(buffer.Span);
        if (count == 0)
            return sum + n;
        (sum, n) = sumComputer(buffer.Slice(0, count), sum, n);
    }
}

private static (long, int) ComputeSum(ReadOnlyMemory<byte> buffer, long sum, int n)
{
    var span = buffer.Span;
    foreach (var b in span) {
        if (b < 128)
            n = (n << 7) + b;
        else {
            sum += (n << 7) + b - 128;
            n = 0;
        }
    }
    return (sum, n);
}