lib/igt_stats.c

/*
 * Copyright © 2015 Intel Corporation
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 *
 */

#include <math.h>
#include <string.h>

#include "igt_core.h"
#include "igt_stats.h"

void igt_stats_init(igt_stats_t *stats, unsigned int capacity)
{
	memset(stats, 0, sizeof(*stats));

	stats->values = calloc(capacity, sizeof(*stats->values));
	igt_assert(stats->values);
	stats->capacity = capacity;
}

void igt_stats_fini(igt_stats_t *stats)
{
	free(stats->values);
}

void igt_stats_push(igt_stats_t *stats, uint64_t value)
{
	igt_assert(stats->n_values < stats->capacity);
	stats->values[stats->n_values++] = value;
	stats->mean_variance_valid = false;
}

/*
 * Algorithm popularised by Knuth in:
 *
 * The Art of Computer Programming, volume 2: Seminumerical Algorithms,
 * 3rd edn., p. 232. Boston: Addison-Wesley
 *
 * Source: https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance
 */
static void igt_stats_knuth_mean_variance(igt_stats_t *stats)
{
	double mean = 0., m2 = 0.;
	unsigned int i;

	if (stats->mean_variance_valid)
		return;

	for (i = 0; i < stats->n_values; i++) {
		double delta = stats->values[i] - mean;

		mean += delta / (i + 1);
		m2 += delta * (stats->values[i] - mean);
	}

	stats->mean = mean;
	stats->variance = m2 / stats->n_values;
	stats->mean_variance_valid = true;
}

double igt_stats_get_mean(igt_stats_t *stats)
{
	igt_stats_knuth_mean_variance(stats);

	return stats->mean;
}

double igt_stats_get_variance(igt_stats_t *stats)
{
	igt_stats_knuth_mean_variance(stats);

	return stats->variance;
}

double igt_stats_get_std_deviation(igt_stats_t *stats)
{
	igt_stats_knuth_mean_variance(stats);

	return sqrt(stats->variance);
}