Shortened codes generator matrix computation

[David Lucas]

For now, as explained in Huffman and Pless' Fundamentals of Error Correcting Codes, I compute the generator matrix of a shortened code as follows:

• I puncture a Parity Check matrix of its original code on the positions chosen for shortening.

• This new matrix is a PC for the shortened code

• Then I compute its generator matrix using the PC matrix.

It might be possible to do something faster.