enc.c

来自「I have aatched C code for enc.」· C语言 代码 · 共 142 行

/* ENC.C - Encoding procedures. */

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#include "rand.h"
#include "alloc.h"
#include "mod2sparse.h"
#include "mod2dense.h"
#include "mod2convert.h"
#include "rcode.h"
#include "enc.h"


/* The procedures in this module obtain the generator matrix to use for
   encoding from the global variables declared in rcode.h */


/* ENCODE A BLOCK USING A SPARSE REPRESENTATION OF THE GENERATOR MATRIX. */

void sparse_encode
( char *sblk,
  char *cblk
)
{
  int i, j;

  mod2entry *e;
  char *x, *y;

  x = chk_alloc (M, sizeof *x);
  y = chk_alloc (M, sizeof *y);

  /* Multiply the vector of source bits by the systematic columns of the 
     parity check matrix, giving x.  Also copy these bits to the coded block. */

  for (i = 0; i<M; i++) x[i] = 0;

  for (j = M; j<N; j++)
  { 
    cblk[cols[j]] = sblk[j-M];

    if (sblk[j-M]==1)
    { for (e = mod2sparse_first_in_col(H,cols[j]);
           !mod2sparse_at_end(e);
           e = mod2sparse_next_in_col(e))
      { x[mod2sparse_row(e)] ^= 1;
      }
    }
  }
 
  /* Solve Ly=x for y by forward substitution, then U(cblk)=y by backward
     substitution. */

  if (!mod2sparse_forward_sub(L,rows,x,y)
   || !mod2sparse_backward_sub(U,cols,y,cblk))
  { 
    abort(); /* Shouldn't occur, even if the parity check matrix has 
                redundant rows */
  }

  free(x);
  free(y);
}


/* ENCODE A BLOCK USING DENSE REPRESENTATION OF GENERATOR MATRIX. */

void dense_encode
( char *sblk,
  char *cblk,
  mod2dense *u,
  mod2dense *v
)
{
  int j;

  /* Copy source bits to the systematic part of the coded block. */

  for (j = M; j<N; j++) 
  { cblk[cols[j]] = sblk[j-M];
  }

  /* Multiply by Inv(A) X B to produce check bits. */

  for (j = M; j<N; j++)
  { mod2dense_set(u,j-M,0,sblk[j-M]); 
  }
  
  mod2dense_multiply(G,u,v);

  /* Copy check bits to the right places in the coded block. */

  for (j = 0; j<M; j++)
  { cblk[cols[j]] = mod2dense_get(v,j,0);
  }
}


/* ENCODE A BLOCK USING MIXED REPRESENTATION OF GENERATOR MATRIX. */

void mixed_encode
( char *sblk,
  char *cblk,
  mod2dense *u,
  mod2dense *v
)
{
  mod2entry *e;
  int j;

  /* Multiply the vector of source bits by the message bit columns of the 
     parity check matrix.  Also copy these bits to the coded block.  Take
     account of how columns have been reordered. */

  mod2dense_clear(u);

  for (j = M; j<N; j++)
  { 
    cblk[cols[j]] = sblk[j-M];

    if (sblk[j-M]==1)
    { for (e = mod2sparse_first_in_col(H,cols[j]);
           !mod2sparse_at_end(e);
           e = mod2sparse_next_in_col(e))
      { (void) mod2dense_flip(u,mod2sparse_row(e),0);
      }
    }
  }

  /* Multiply by Inv(A) to produce check bits. */

  mod2dense_multiply(G,u,v);

  /* Copy check bits to the right places in the coded block. */

  for (j = 0; j<M; j++)
  { cblk[cols[j]] = mod2dense_get(v,j,0);
  }
}