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/* $Id: filt.c,v 1.1 2002/10/20 18:19:17 tommy Exp $ */
/*
* Copyright (c) 2002 Tom Marshall <tommy@tig-grr.com>
*
* This program is free software. It may be distributed under the terms
* in the file LICENSE, found in the top level of the distribution.
*
* filt.c: The Bayes filter implementation.
* See http://www.paulgraham.com/spam.html for discussion.
*/
#include "config.h"
#include "dbg.h"
#include "str.h"
#include "lex.h"
#include "vec.h"
#include "dbh.h"
#include "filt.h"
#define DEVIATION(n) fabs((n)-0.5f)
/* Dump the contents of a statistics structure */
void statdump( stats_t* pstat, int fd )
{
char iobuf[IOBUFSIZE];
char* p;
discrim_t* pp;
p = iobuf;
p += sprintf( iobuf, "# Spamicity: %f\n", pstat->spamicity );
for (pp = pstat->extrema; pp < pstat->extrema + pstat->keepers; pp++)
{
if (pp->key.len)
{
strcpy( p, "# '" ); p += 3;
strncpylwr( p, pp->key.p, pp->key.len ); p += pp->key.len;
p += snprintf( p, 28, "' -> %f\n", pp->prob );
if( p+MAXWORDLEN+32 > (iobuf+1) )
{
write( fd, iobuf, p-iobuf );
p = iobuf;
}
}
}
if( p != iobuf )
{
write( fd, iobuf, p-iobuf );
}
}
void bayesfilt( dbt_t* pglist, dbt_t* pblist, vec_t* pmlist, stats_t* pstats )
{
veciter_t iter;
str_t* pword;
double prob, product, invproduct, dev;
double slotdev, hitdev;
#ifdef NON_EQUIPROBABLE
/* There is an argument that we should (go?) by number of *words* here. */
double msg_prob = ((double)pblist->nitems / (double)pglist->nitems);
#endif
discrim_t* pp;
discrim_t* hit;
for (pp = pstats->extrema; pp < pstats->extrema+pstats->keepers; pp++)
{
pp->key.p = NULL;
pp->key.len = 0;
pp->prob = 0.5f;
}
vec_first( pmlist, &iter );
while( (pword = veciter_get( &iter )) != NULL )
{
double goodness = pglist->getcount( pglist, pword );
double spamness = pblist->getcount( pblist, pword );
uint goodtotal = pglist->getmsgcount( pglist );
uint spamtotal = pblist->getmsgcount( pblist );
if( goodness + spamness < MINIMUM_FREQ )
{
#ifdef NON_EQUIPROBABLE
/*
* In the absence of evidence, the probability that a new word will
* be spam is the historical ratio of spam words to nonspam words.
*/
prob = msg_prob;
#else
prob = UNKNOWN_WORD;
#endif
}
else
{
double goodprob = goodtotal ? min( 1.0, (goodness / goodtotal) ) : 0.0;
double spamprob = spamtotal ? min( 1.0, (spamness / spamtotal) ) : 0.0;
assert( goodtotal > 0 || spamtotal > 0 );
#ifdef NON_EQUIPROBABLE
prob = (spamprob * msg_prob) / ((goodprob * (1 - msg_prob)) + (spamprob * msg_prob));
#else
prob = spamprob / (goodprob + spamprob);
#endif
prob = minmax( prob, 0.01, 0.99 );
}
/* update the list of tokens with maximum deviation */
dev = DEVIATION(prob);
hit = NULL;
hitdev = 0;
for (pp = pstats->extrema; pp < pstats->extrema+pstats->keepers; pp++)
{
/* don't allow duplicate tokens in the stats.extrema */
if( pp->key.len > 0 && str_casecmp( pword, &pp->key ) == 0 )
{
hit = NULL;
break;
}
slotdev = DEVIATION(pp->prob);
if (dev>slotdev && dev>hitdev)
{
hit = pp;
hitdev = slotdev;
}
}
if (hit)
{
hit->prob = prob;
hit->key = *pword;
}
veciter_next( &iter );
}
veciter_destroy( &iter );
/*
* Bayes' theorem.
* For discussion, see <http://www.mathpages.com/home/kmath267.htm>.
*/
product = invproduct = 1.0f;
for (pp = pstats->extrema; pp < pstats->extrema+pstats->keepers; pp++)
{
if( pp->prob == 0 )
{
break;
}
else
{
product *= pp->prob;
invproduct *= (1 - pp->prob);
}
}
pstats->spamicity = product / (product + invproduct);
}
bool_t bvec_loadmsg( vec_t* pthis, lex_t* plex, tok_t* ptok )
{
str_t w;
lex_nexttoken( plex, ptok );
while( ptok->tt != eof && ptok->tt != from )
{
w.p = ptok->p;
w.len = ptok->len;
vec_addtail( pthis, &w );
lex_nexttoken( plex, ptok );
}
return true;
}
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