这是一个公认的校验和算法吗?

I have to calculate a check digit for a 20-digit barcode. I have a spec which includes some pseudo code but it looks nasty. Particularly the part with division & quotient. There's plenty of scope for this to fail with loss of precision or rounding etc. I want to know if this is a recognised checksum so that I can use a tested implementation, ideally in c# or java.I have to calculate a check digit for a 20-digi