Commit 1b8997b3 authored by Damien Lespiau's avatar Damien Lespiau

stats: Add support for quartiles (and thus median)

More stuff, quite useful characteristics of a dataset.
Signed-off-by: default avatarDamien Lespiau <damien.lespiau@intel.com>
parent 3839bacd
......@@ -23,6 +23,7 @@
*/
#include <math.h>
#include <stdlib.h>
#include <string.h>
#include "igt_core.h"
......@@ -94,6 +95,7 @@ void igt_stats_init(igt_stats_t *stats, unsigned int capacity)
void igt_stats_fini(igt_stats_t *stats)
{
free(stats->values);
free(stats->sorted);
}
......@@ -158,6 +160,7 @@ void igt_stats_push(igt_stats_t *stats, uint64_t value)
stats->values[stats->n_values++] = value;
stats->mean_variance_valid = false;
stats->sorted_array_valid = false;
if (value < stats->min)
stats->min = value;
......@@ -220,6 +223,134 @@ uint64_t igt_stats_get_range(igt_stats_t *stats)
return igt_stats_get_max(stats) - igt_stats_get_min(stats);
}
static int cmp_u64(const void *pa, const void *pb)
{
const uint64_t *a = pa, *b = pb;
if (*a < *b)
return -1;
if (*a > *b)
return 1;
return 0;
}
static void igt_stats_ensure_sorted_values(igt_stats_t *stats)
{
if (stats->sorted_array_valid)
return;
if (!stats->sorted) {
stats->sorted = calloc(stats->capacity, sizeof(*stats->values));
igt_assert(stats->sorted);
}
memcpy(stats->sorted, stats->values,
sizeof(*stats->values) * stats->n_values);
qsort(stats->sorted, stats->n_values, sizeof(*stats->values), cmp_u64);
stats->sorted_array_valid = true;
}
/*
* We use Tukey's hinge for our quartiles determination.
* ends (end, lower_end) are exclusive.
*/
static double
igt_stats_get_median_internal(igt_stats_t *stats,
unsigned int start, unsigned int end,
unsigned int *lower_end /* out */,
unsigned int *upper_start /* out */)
{
unsigned int mid, n_values = end - start;
double median;
igt_stats_ensure_sorted_values(stats);
/* odd number of data points */
if (n_values % 2 == 1) {
/* median is the value in the middle (actual datum) */
mid = start + n_values / 2;
median = stats->sorted[mid];
/* the two halves contain the median value */
if (lower_end)
*lower_end = mid + 1;
if (upper_start)
*upper_start = mid;
/* even number of data points */
} else {
/*
* The middle is in between two indexes, 'mid' points at the
* lower one. The median is then the average between those two
* values.
*/
mid = start + n_values / 2 - 1;
median = (stats->sorted[mid] + stats->sorted[mid + 1]) / 2.;
if (lower_end)
*lower_end = mid + 1;
if (upper_start)
*upper_start = mid + 1;
}
return median;
}
/**
* igt_stats_get_quartiles:
* @stats: An #igt_stats_t instance
* @q1: (out): lower or 25th quartile
* @q2: (out): median or 50th quartile
* @q3: (out): upper or 75th quartile
*
* Retrieves the [quartiles](https://en.wikipedia.org/wiki/Quartile) of the
* @stats dataset.
*/
void igt_stats_get_quartiles(igt_stats_t *stats,
double *q1, double *q2, double *q3)
{
unsigned int lower_end, upper_start;
double ret;
if (stats->n_values < 3) {
if (q1)
*q1 = 0.;
if (q2)
*q2 = 0.;
if (q3)
*q3 = 0.;
return;
}
ret = igt_stats_get_median_internal(stats, 0, stats->n_values,
&lower_end, &upper_start);
if (q2)
*q2 = ret;
ret = igt_stats_get_median_internal(stats, 0, lower_end, NULL, NULL);
if (q1)
*q1 = ret;
ret = igt_stats_get_median_internal(stats, upper_start, stats->n_values,
NULL, NULL);
if (q3)
*q3 = ret;
}
/**
* igt_stats_get_median:
* @stats: An #igt_stats_t instance
*
* Retrieves the median of the @stats dataset.
*/
double igt_stats_get_median(igt_stats_t *stats)
{
return igt_stats_get_median_internal(stats, 0, stats->n_values,
NULL, NULL);
}
/*
* Algorithm popularised by Knuth in:
*
......
......@@ -41,8 +41,10 @@ typedef struct {
unsigned int capacity;
unsigned int is_population : 1;
unsigned int mean_variance_valid : 1;
unsigned int sorted_array_valid : 1;
uint64_t min, max;
double mean, variance;
uint64_t *sorted;
} igt_stats_t;
void igt_stats_init(igt_stats_t *stats, unsigned int capacity);
......@@ -55,7 +57,10 @@ void igt_stats_push_array(igt_stats_t *stats,
uint64_t igt_stats_get_min(igt_stats_t *stats);
uint64_t igt_stats_get_max(igt_stats_t *stats);
uint64_t igt_stats_get_range(igt_stats_t *stats);
void igt_stats_get_quartiles(igt_stats_t *stats,
double *q1, double *q2, double *q3);
double igt_stats_get_mean(igt_stats_t *stats);
double igt_stats_get_median(igt_stats_t *stats);
double igt_stats_get_variance(igt_stats_t *stats);
double igt_stats_get_std_deviation(igt_stats_t *stats);
......
......@@ -25,6 +25,8 @@
#include "igt_core.h"
#include "igt_stats.h"
#define ARRAY_SIZE(arr) (sizeof(arr)/sizeof(arr[0]))
static void push_fixture_1(igt_stats_t *stats)
{
igt_stats_push(stats, 2);
......@@ -78,6 +80,44 @@ static void test_range(void)
igt_assert(igt_stats_get_range(&stats) == 8);
}
/*
* Examples taken from: https://en.wikipedia.org/wiki/Quartile
* The values are shifted a bit to test we do indeed start by sorting data
* set.
*/
static void test_quartiles(void)
{
static const uint64_t s1[] =
{ 47, 49, 6, 7, 15, 36, 39, 40, 41, 42, 43 };
static const uint64_t s2[] = { 40, 41, 7, 15, 36, 39 };
igt_stats_t stats;
double q1, q2, q3;
/* s1, odd number of data points */
igt_stats_init(&stats, ARRAY_SIZE(s1));
igt_stats_push_array(&stats, s1, ARRAY_SIZE(s1));
igt_stats_get_quartiles(&stats, &q1, &q2, &q3);
igt_assert_eq_double(q1, 25.5);
igt_assert_eq_double(q2, 40);
igt_assert_eq_double(q3, 42.5);
igt_assert_eq_double(igt_stats_get_median(&stats), 40);
igt_stats_fini(&stats);
/* s1, even number of data points */
igt_stats_init(&stats, ARRAY_SIZE(s2));
igt_stats_push_array(&stats, s2, ARRAY_SIZE(s2));
igt_stats_get_quartiles(&stats, &q1, &q2, &q3);
igt_assert_eq_double(q1, 15);
igt_assert_eq_double(q2, 37.5);
igt_assert_eq_double(q3, 40);
igt_assert_eq_double(igt_stats_get_median(&stats), 37.5);
igt_stats_fini(&stats);
}
static void test_mean(void)
{
igt_stats_t stats;
......@@ -150,6 +190,7 @@ igt_simple_main
test_init();
test_min_max();
test_range();
test_quartiles();
test_mean();
test_invalidate_mean();
test_std_deviation();
......
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