[Fix] Fix probabilities overflow

diff --git a/src/libstat/classifiers/bayes.c b/src/libstat/classifiers/bayes.c
index 405168f910abf9fc80dc4f6ddc0e5eb752a6ade3..66d84a14d33c37554f8d1a19a11cda72e9a5b154 100644 (file)
--- a/src/libstat/classifiers/bayes.c
+++ b/src/libstat/classifiers/bayes.c
@@ -53,10 +53,26 @@ static double
 inv_chi_square(struct rspamd_task *task, double value, int freedom_deg)
 {
        double prob, sum, m;
+       double log_prob, log_m;
        int i;
 
        errno = 0;
        m = -value;
+
+       /* Handle extreme negative values that would cause exp() underflow */
+       if (value < -700) {
+               /* Very strong confidence, return 0 */
+               msg_debug_bayes("extreme negative value: %f, returning 0", value);
+               return 0.0;
+       }
+
+       /* Handle extreme positive values that would cause overflow */
+       if (value > 700) {
+               /* No confidence, return 1 */
+               msg_debug_bayes("extreme positive value: %f, returning 1", value);
+               return 1.0;
+       }
+
        prob = exp(value);
 
        if (errno == ERANGE) {
@@ -75,6 +91,8 @@ inv_chi_square(struct rspamd_task *task, double value, int freedom_deg)
        }
 
        sum = prob;
+       log_prob = value;     /* log of current prob term */
+       log_m = log(fabs(m)); /* log of |m| for numerical stability */
 
        msg_debug_bayes("m: %f, probability: %g", m, prob);
 
@@ -83,11 +101,33 @@ inv_chi_square(struct rspamd_task *task, double value, int freedom_deg)
         * prob is e ^ x (small value since x is normally less than zero
         * So we integrate over degrees of freedom and produce the total result
         * from 1.0 (no confidence) to 0.0 (full confidence)
+        * Use logarithmic arithmetic to prevent overflow
         */
        for (i = 1; i < freedom_deg; i++) {
-               prob *= m / (double) i;
+               /* Calculate next term using logarithms to prevent overflow */
+               log_prob += log_m - log((double) i);
+
+               /* Check if the log probability is too negative (term becomes negligible) */
+               if (log_prob < -700) {
+                       msg_debug_bayes("term %d became negligible, stopping series", i);
+                       break;
+               }
+
+               /* Check if the log probability is too positive (would cause overflow) */
+               if (log_prob > 700) {
+                       msg_debug_bayes("series diverging at term %d, returning 1.0", i);
+                       return 1.0;
+               }
+
+               prob = exp(log_prob);
                sum += prob;
-               msg_debug_bayes("i=%d, probability: %g, sum: %g", i, prob, sum);
+               msg_debug_bayes("i=%d, log_prob: %g, probability: %g, sum: %g", i, log_prob, prob, sum);
+
+               /* Early termination if sum is getting too large */
+               if (sum > 1e10) {
+                       msg_debug_bayes("sum too large (%g), returning 1.0", sum);
+                       return 1.0;
+               }
        }
 
        return MIN(1.0, sum);