any reference documentation of NURBS?

I'm reading OCCT source code, and meet problem.

I've read "The NURBS Book 2nd edition". In that book, the knots is stored in sequence format. In OCCT, knots is stored in 2 sequences: knots and mults.

In that book, the algorithm to calculate N_i_p is based on expanded sequence of knots. In OCCT, in src/BSplClib/BSplClib.cxx in function "BSplCLib::Eval" seems like equation 2.5 in the book, but some different.

in the book

N_i_p_u=(U-knots[i])/(knots[i+p]-knots[i])*N_i_(p-1)_u+(knots[i+p+1]-U)/(knots[i+p+1]-knots[i+1])*N_(i+1)_(p-1)_u

the coefficient of first item at right side seems like X in "BSplCLib::Eval", and coefficient of the second item at right side seems like Y. Look at the equation, the 2 coefficients summary is not 1

also the denominator of the 2 coefficient could be 0. In OCCT, knots stores in 2 sequences: knots and mults, but in the book the knots are expanded into 1 sequence. So if "BSplCLib::Eval" is based on that equation, the knots should be in 1 sequence, if the mults is bigger than 1, (knots[Dpi] - knots[Sti]) is possible to be 0. I tried, if the mults is bigger than 1 at begin or end, it's ok, if the mults is bigger than 1 at internal position, divide 0 error would happen.

in the code there is no comment or detailed description of the input parameter of "BSplCLib::Eval", so I'm not sure the input format, whatever mults is not input parameter of "BSplCLib::Eval", so I guess the input knots is expanded sequence.

also, there are some other differences between the book and the source code. In the book, periodic B-spline is not introduced, but in the source code, there is periodic B-spline.

is there any reference documentation of the NURBS algorithm in OCCT? I want to read and understand OCCT better.