star-phor

commit 17725ded63f3bb869670747b45ed00b3c0ec08f3
parent 5bde959937eaf8935c9a775544a26db3282b4272
Author: Eduardo Fontana Lazzari <edufonlaz@gmail.com>
Date:   Mon,  6 Apr 2026 13:15:41 +0200

Clamp variance to avoid negative values

In zero (or near-zero) variance cases, floating-point rounding errors
can produce small negative variance values. Clamp the result to 0 to
prevent propagating invalid values to the standard error computation,
specially considering it depends on the square root of the variance.

Diffstat:
M
src/sphor_accum.c
 | 
3
+++

1 file changed, 3 insertions(+), 0 deletions(-)
diff --git a/src/sphor_accum.c b/src/sphor_accum.c
@@ -147,6 +147,9 @@ accum_compute_estim
   /* Compute the sample variance. This formula is equivalent to
    * var = 1 / (n-1)  sum(x_i - <x>)^2 */
   var = (sum2 - sum * sum / samples) / (samples - 1);
+  /* Clamp variance to 0 to guard against small negative values
+   * caused by floating-point rounding errors */
+  var = var < 0 ? 0 : var;
   std_err = sqrt (var / samples);
 
   estim->mean = mean;