说明：  Flexible construction of girth-six QC-LDPC Codes
(The program constructs girth-six QC-LDPC codes given row-weight,k, column-weight,j, and sub-matrix size,m. The obtained parity-check matrix is stored in H. The construction has flexibility in that j,k and m can be varied.The program achieves girth-six by applying the row-column constraint. There are m rows in j submatrices. No two rows should share the same column more than once. This constraint is applied to avoid 4-cycles. The construction has flexibility in that j,k and m can be varied.)