/* DEC.C - Decoding procedures. *//* Copyright (c) 2000, 2001 by Radford M. Neal  * * Permission is granted for anyone to copy, use, or modify this program  * for purposes of research or education, provided this copyright notice  * is retained, and note is made of any changes that have been made.  * * This program is distributed without any warranty, express or implied. * As this program was written for research purposes only, it has not been * tested to the degree that would be advisable in any important application. * All use of this program is entirely at the user's own risk. *//* NOTE:  See decoding.html for general documentation on the decoding methods */#include <stdio.h>#include <stdlib.h>#include <math.h>#include "alloc.h"#include "mod2sparse.h"#include "mod2dense.h"#include "mod2convert.h"#include "rand.h"#include "rcode.h"#include "check.h"#include "dec.h"#include "enc.h"/* GLOBAL VARIABLES.  Declared in dec.h. */decoding_method dec_method;	/* Decoding method to use */int table;	/* Trace option, 2 for a table of decoding details */int block_no;	/* Number of current block, from zero */int max_iter;	/* Maximum number of iteratons of decoding to do */char *gen_file;	/* Generator file for Enum_block and Enum_bit *//* DECODE BY EXHAUSTIVE ENUMERATION.  Decodes by trying all possible source   messages (and hence all possible codewords, unless the parity check matrix   was redundant).  If the last argument is 1, it sets dblk to the most likely   entire block; if this argument is 0, each bit of dblk is set to the most   likely value for that bit.  The marginal probabilities of each bit being 1   are returned in bitpr.   The function value returned is the total number of codewords tried (which    will be the same for all blocks).  The return valued is "unsigned" because   it might conceivably be as big as 2^31.   The parity check matrix and other data are taken from the global variables   declared in rcode.h.   The number of message bits should not be greater than 31 for this procedure.   The setup procedure immediately below checks this, reads the generator file,   and outputs headers for the detailed trace file, if required. */void enum_decode_setup(void){  read_gen(gen_file,0,0);  if (N-M>31)  { fprintf(stderr,"Trying to decode messages with %d bits by exhaustive enumeration is absurd!\n",      N-M);    exit(1);    }  if (table==2)  { printf("  block   decoding  likelihood\n");  }}unsigned enum_decode( double *lratio,	/* Likelihood ratios for bits */  char *dblk, 		/* Place to stored decoded message */  double *bitpr,	/* Place to store marginal bit probabilities */  int max_block		/* Maximize probability of whole block being correct? */){  mod2dense *u, *v;  double lk, maxlk, tpr;  double *bpr, *lk0, *lk1;  char sblk[31];  char *cblk;  unsigned d;  int i, j;  if (N-M>31) abort();  /* Allocate needed space. */  bpr = bitpr;  if (bpr==0 && max_block==0)  { bpr = chk_alloc (N, sizeof *bpr);  }  cblk = chk_alloc (N, sizeof *cblk);  if (type=='d')  { u = mod2dense_allocate(N-M,1);    v = mod2dense_allocate(M,1);  }  if (type=='m')  { u = mod2dense_allocate(M,1);    v = mod2dense_allocate(M,1);  }  lk0 = chk_alloc (N, sizeof *lk0);  lk1 = chk_alloc (N, sizeof *lk1);  /* Pre-compute likelihoods for bits. */  for (j = 0; j<N; j++)  { 
	lk0[j] = 1/(1+lratio[j]);    lk1[j] = 1 - lk0[j];  }  /* Initialize marginal bit probabilities. */  if (bpr)  { for (j = 0; j<N; j++) bpr[j] = 0.0;  }  /* Exhaustively try all possible decoded messages. */  tpr = 0.0;  for (d = 0; d<=(1<<(N-M))-1; d++)  {    /* Unpack message into source block. */    for (i = N-M-1; i>=0; i--)    { sblk[i] = (d>>i)&1;    }    /* Find full codeword for this message. */    switch (type)    { case 's':      { sparse_encode (sblk, cblk);        break;      }      case 'd':      { dense_encode (sblk, cblk, u, v);        break;      }      case 'm':      { mixed_encode (sblk, cblk, u, v);        break;      }    }    /* Compute likelihood for this decoding. */    lk = 1;    for (j = 0; j<N; j++)    { lk *= cblk[j]==0 ? lk0[j] : lk1[j];    }    /* Update maximum likelihood decoding. */    if (max_block)    { if (d==0 || lk>maxlk)      { for (j = 0; j<N; j++)        { dblk[j] = cblk[j];        }        maxlk = lk;      }    }    /* Update bit probabilities. */     if (bpr)    { for (j = 0; j<N; j++)       { if (cblk[j]==1)         { bpr[j] += lk;        }      }      tpr += lk;    }    /* Output data to trace file. */    if (table==2)    { printf("%7d %10x  %10.4e\n",block_no,d,lk);    }  }  /* Normalize bit probabilities. */  if (bpr)  { for (j = 0; j<N; j++) bpr[j] /= tpr;  }  /* Decoding to maximize bit-by-bit success, if that's what's wanted.      In case of a tie, decode to a 1. */  if (!max_block)  { for (j = 0; j<N; j++)     { dblk[j] = bpr[j]>=0.5;    }  }  /* Free space. */  if (bpr!=0 && bpr!=bitpr) free(bpr);  free(cblk);  free(lk0);  free(lk1);  return 1<<(N-M);}/* DECODE USING PROBABILITY PROPAGATION.  Tries to find the most probable    values for the bits of the codeword, given a parity check matrix (H), and   likelihood ratios (lratio) for each bit.  If max_iter is positive, up to    that many iterations of probability propagation are done, stopping before    then if the tentative decoding is a valid codeword.  If max_iter is    negative, abs(max_iter) iterations are done, regardless of whether a    codeword was found earlier.   Returns the number of iterations done (as an "unsigned" for consistency   with enum_decode).  Regardless of whether or not a valid codeword was    reached, the bit vector from thresholding the bit-by-bit probabilities is    stored in dblk, and the resulting parity checks are stored in pchk (all    will be zero if the codeword is valid).  The final probabilities for each    bit being a 1 are stored in bprb.   The setup procedure immediately below outputs headers for the detailed trace   file, if required.*/void prprp_decode_setup (void){  if (table==2)  { printf(     "  block  iter  changed  perrs    loglik   Eperrs   Eloglik  entropy\n");  }}unsigned prprp_decode( mod2sparse *H,	/* Parity check matrix */  double *lratio,	/* Likelihood ratios for bits */  char *dblk,		/* Place to store decoding */  char *pchk,		/* Place to store parity checks */  double *bprb		/* Place to store bit probabilities */){   int N, n, c;  N = mod2sparse_cols(H);  /* Initialize probability and likelihood ratios, and find initial guess. */  initprp(H,lratio,dblk,bprb);  /* Do up to abs(max_iter) iterations of probability propagation, stopping     early if a codeword is found, unless max_iter is negative. */
  //如果加了負(fù)號，則必須達(dá)到次數(shù)，沒有加的話是碼子就可以了！  for (n = 0; ; n++)  {     c = check(H,dblk,pchk);    if (table==2)    { printf("%7d %5d %8.1f %6d %+9.2f %8.1f %+9.2f  %7.1f\n", block_no, n, changed(lratio,dblk,N), c, loglikelihood(lratio,dblk,N),        expected_parity_errors(H,bprb), expected_loglikelihood(lratio,bprb,N), entropy(bprb,N));    }       if (n==max_iter || n==-max_iter || (max_iter>0 && c==0))    {
	  break;     }    iterprp(H,lratio,dblk,bprb);  }  return n;}/* INITIALIZE PROBABILITY PROPAGATION.  Stores initial ratios, probabilities,   and guess at decoding. */void initprp( mod2sparse *H,	/* Parity check matrix */  double *lratio,	/* Likelihood ratios for bits */  char *dblk,		/* Place to store decoding */  double *bprb		/* Place to store bit probabilities, 0 if not wanted */){   mod2entry *e;
  /*typedef struct mod2entry /* Structure representing a non-zero entry, or
			      //the header for a row or column               
 {
  int row, col;		  /* Row and column indexes of this entry, starting
                             //at 0, and with -1 for a row or column header  

  struct mod2entry *left, *right,  /* Pointers to entries adjacent in row  
                   *up, *down;     /*   and column, or to headers.  Free   
                                   /*   entries are linked by 'left'.

  double pr, lr;	  /* Probability and likelihood ratios - not used by the mod2sparse module itself             
} mod2entry;*/  int N;  int j;  N = mod2sparse_cols(H);  for (j = 0; j<N; j++)  { 
	for (e = mod2sparse_first_in_col(H,j); !mod2sparse_at_end(e);  e = mod2sparse_next_in_col(e))    { 
	  e->pr = lratio[j];      e->lr = 1;    }    if (bprb) bprb[j] = 1 - 1/(1+lratio[j]);//如何解讀該行語法??    dblk[j] = (lratio[j]>=1);  }}/* DO ONE ITERATION OF PROBABILITY PROPAGATION. *///一次概率傳播過程。void iterprp( mod2sparse *H,	/* Parity check matrix */  double *lratio,	/* Likelihood ratios for bits */  char *dblk,		/* Place to store decoding */  double *bprb		/* Place to store bit probabilities, 0 if not wanted */){  double pr, dl, t;  mod2entry *e;  int N, M;  int i, j;  M = mod2sparse_rows(H);  N = mod2sparse_cols(H);  /* Recompute likelihood ratios. */  for (i = 0; i<M; i++)  { 
	dl = 1;    for (e = mod2sparse_first_in_row(H,i); !mod2sparse_at_end(e);  e = mod2sparse_next_in_row(e))    { 
	  e->lr = dl;      dl *= 2/(1+e->pr) - 1;    }    dl = 1;    for (e = mod2sparse_last_in_row(H,i); !mod2sparse_at_end(e); e = mod2sparse_prev_in_row(e))    { 
	  t = e->lr * dl;      e->lr = (1-t)/(1+t);      dl *= 2/(1+e->pr) - 1;    }  }  /* Recompute probability ratios.  Also find the next guess based on the     individually most likely values. */  for (j = 0; j<N; j++)  { 
	pr = lratio[j];    for (e = mod2sparse_first_in_col(H,j); !mod2sparse_at_end(e);e = mod2sparse_next_in_col(e))    { 
	  e->pr = pr;      pr *= e->lr;    }    /*
	if (isnan(pr))    { 
		pr = 1;    }
    */    if (bprb) bprb[j] = 1 - 1/(1+pr);    dblk[j] = pr>=1;    pr = 1;    for (e = mod2sparse_last_in_col(H,j); !mod2sparse_at_end(e); e = mod2sparse_prev_in_col(e))    { 
	  e->pr *= pr;      /* 
	  if (isnan(e->pr))       { 
		  e->pr = 1;      }
	  */      pr *= e->lr;    }  }}