diff options
Diffstat (limited to 'src/lib/ndpi_analyze.c')
| -rw-r--r-- | src/lib/ndpi_analyze.c | 16 |
1 files changed, 9 insertions, 7 deletions
diff --git a/src/lib/ndpi_analyze.c b/src/lib/ndpi_analyze.c index 26f2ae041..7b53e20ea 100644 --- a/src/lib/ndpi_analyze.c +++ b/src/lib/ndpi_analyze.c @@ -87,8 +87,6 @@ void ndpi_reset_data_analysis(struct ndpi_analyze_struct *d) { Add a new point to analyze */ void ndpi_data_add_value(struct ndpi_analyze_struct *s, const u_int32_t value) { - float tmp_mu; - if(s->sum_total == 0) s->min_val = s->max_val = value; else { @@ -105,10 +103,14 @@ void ndpi_data_add_value(struct ndpi_analyze_struct *s, const u_int32_t value) { s->next_value_insert_index = 0; } - /* Update stddev */ - tmp_mu = s->stddev.mu; - s->stddev.mu = ((s->stddev.mu * (s->num_data_entries - 1)) + value) / s->num_data_entries; - s->stddev.q = s->stddev.q + (value - tmp_mu)*(value - s->stddev.mu); + /* + Optimized stddev calculation + + https://www.khanacademy.org/math/probability/data-distributions-a1/summarizing-spread-distributions/a/calculating-standard-deviation-step-by-step + https://math.stackexchange.com/questions/683297/how-to-calculate-standard-deviation-without-detailed-historical-data + http://mathcentral.uregina.ca/QQ/database/QQ.09.02/carlos1.html + */ + s->stddev.sum_square_total += value * value; } /* ********************************************************************************* */ @@ -138,7 +140,7 @@ u_int32_t ndpi_data_max(struct ndpi_analyze_struct *s) { return(s->max_val); } /* Compute the variance on all values */ float ndpi_data_variance(struct ndpi_analyze_struct *s) { - return(s->num_data_entries ? (s->stddev.q / s->num_data_entries) : 0); + return(s->num_data_entries ? ((float)s->stddev.sum_square_total - (float)((s->sum_total * s->sum_total) / (float)(s->num_data_entries))) / (float)s->num_data_entries : 0); } /* ********************************************************************************* */ |