some mathematic problems

[davidchen2014]

Hi, I'm newcomer here and interested at some basic source code. But I met some trouble when reading some source code phrases at https://github.com/bcgit/bc-java/blob/master/core/src/main/java/org/bouncycastle/math/ec/custom/sec/SecP128R1Field.java
Especially at following part:

I can understand these phrase Nat128.mul(x, y, tt); .
But I don't understand next phrase reduce(tt, z);
especially at following part:

I can understand that the reduce process is rearranging array X to array Z., but don't understand the structure behind these and why do this.

And next question is: I also don't understand isZero method in the same class

Why always shifting left 1 or shifting right 1 among these code?

I'm very appreciated if receiving your returning message, guidance , related samples and mathematics-related materials!

Thanks in advance.

[davidchen2014]

I have solved these problems by myself, and understand knowledge behind these.
These codes is related to secp128r1 fast-reduce procedure. Furthermore I have learned secp192r1 and secp256r1 fast-reduce procedures.