/**
 * \file
 *
 * calculate the available entropy. This is taken from timed_entropyd.
 *
 * \author  Georg Hopp <georg@steffers.org
 *
 * \copyright
 * Copyright © 2012  Georg Hopp
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <math.h>
#include <sys/types.h>

#define min(x, y)    ((x)<(y)?(x):(y))

/**
 * This is taken from timer_entropyd and modified so
 * that the constant 1/log(2.0) is not calculated but
 * set directly.
 *
 * As far as i can say this correlates to the shannon
 * entropy algorithm with equal probabilities
 * for entropy where the entropy units are bits.
 *
 * But actually i am no mathemacian and my analysis capabilities
 * are limited. Additionally i have not analysed the linux random
 * character device code, so i trusted the code in timer_entropyd.
 */
int
get_entropy_bits(const unsigned char * data, size_t ndata)
{
	size_t        byte_count[256];
	size_t        iterator;
	static double log2inv = 1.442695; //!< 1 / log(2.0): the entropy unit size
	double        entropy = 0.0;

	memset(byte_count, 0, sizeof(byte_count));

	/**
	 * first get the amount each byte occurs in the array
	 */
	for (iterator = 0; iterator < ndata; iterator++) {
		byte_count[data[iterator]]++;
	}

	/**
	 * calculate the entropy value
	 */
	for (iterator = 0; iterator < 256; iterator++) {
		double probability = (double)byte_count[iterator] / (double)ndata;

		if (0.0 < probability) {
			entropy += probability * log2inv * (log(1.0 / probability));
		}
	}

	/**
	 * prepare for use with linux kernel ioctl RNDADDENTROPY
	 */
	entropy *= (double)ndata;
	entropy = (entropy < 0.0)? 0.0 : entropy;
	entropy = min((double)(ndata * 8), entropy);

	return entropy;
}

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