svn commit: r586111 - /spamassassin/branches/bug-5293-pluginized-bayes/lib/Mail/SpamAssassin/Plugin/OSBF.pm

[jm...@apache.org]

Author: jm
Date: Thu Oct 18 13:09:13 2007
New Revision: 586111

URL: http://svn.apache.org/viewvc?rev=586111&view=rev
Log:
implement the Bayes chain rule algorithm

Modified:
    spamassassin/branches/bug-5293-pluginized-bayes/lib/Mail/SpamAssassin/Plugin/OSBF.pm

Modified: spamassassin/branches/bug-5293-pluginized-bayes/lib/Mail/SpamAssassin/Plugin/OSBF.pm
URL: http://svn.apache.org/viewvc/spamassassin/branches/bug-5293-pluginized-bayes/lib/Mail/SpamAssassin/Plugin/OSBF.pm?rev=586111&r1=586110&r2=586111&view=diff
==============================================================================
--- spamassassin/branches/bug-5293-pluginized-bayes/lib/Mail/SpamAssassin/Plugin/OSBF.pm (original)
+++ spamassassin/branches/bug-5293-pluginized-bayes/lib/Mail/SpamAssassin/Plugin/OSBF.pm Thu Oct 18 13:09:13 2007
@@ -710,7 +710,8 @@
     goto skip;
   }
 
-  $score = Mail::SpamAssassin::Bayes::Combine::combine($ns, $nn, \@sorted);
+  $score = bcr_combine($ns, $nn, \@sorted);
+  # $score = Mail::SpamAssassin::Bayes::Combine::combine($ns, $nn, \@sorted);
 
   # Couldn't come up with a probability?
   goto skip unless defined $score;
@@ -769,6 +770,47 @@
   return $score;
 }
 
+# combine using the Bayes chain rule algorithm described in the
+# EDDC paper
+
+sub bcr_combine {
+  my ($ns, $nn, $probary) = @_;
+
+  my $PCs = $ns / ($ns + $nn);        # P(Ci) where i = "spam"
+  my $PCh = $nn / ($ns + $nn);        # P(Ci) where i = "ham"
+  if ($ns + $nn == 0) {
+    return;         # cannot combine without both ham *and* spam
+  }
+
+  my $Ps = 0.5;                       # accumulators
+  my $Ph = 0.5;
+  foreach my $p (@$probary) {
+    # note: the denominator is
+    #
+    #   Sum (i=1 .. m) ( P (Fj | Ci) . P (Ci) )
+    #
+    # we only have two classes, s and h, with P(f | s) = $p, P(f | h) = 1-$p
+    #
+    #   => ($p) * $PCs + (1-$p) * $PCh
+
+    my $invp = 1-$p;
+    my $denom = $p * $PCs + $invp * $PCh;
+    $Ps *=    $p / $denom;
+    $Ph *= $invp / $denom;
+
+    # avoid underflow; if they've become ~0, reset to a non-0 value
+    # so that further multiplications still work
+    if ($Ps == 0) { $Ps = 1e-300; }
+    if ($Ph == 0) { $Ph = 1e-300; }
+  }
+
+  # pick the lowest value (a tip from
+  # http://www.bgl.nu/bogofilter/BcrFisher.html )
+  if ($Ph < $Ps) { $Ps = 1-$Ph; } else { $Ph = 1-$Ps; }
+
+  return $Ps;
+}
+
 ###########################################################################
 
 # Plugin hook.
@@ -1250,7 +1292,7 @@
   # 0.99 instead of 0.125, which makes more sense for us
   my $K1 = 0.25;
   my $K2 = 10;
-  my $K3 = 4;
+  my $K3 = 1;
 
   my $Sumf = $s + $n; die "assert: Sumf == 0" unless $Sumf;
   my $WdotSumf = $weight * $Sumf;