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ffmpeg / libavutil / random.c @ 80a289b9

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/*
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 * Mersenne Twister Random Algorithm
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 * Copyright (c) 2006 Ryan Martell.
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 * Based on A C-program for MT19937, with initialization improved 2002/1/26. Coded by
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 * Takuji Nishimura and Makoto Matsumoto.
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 *
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 * This file is part of FFmpeg.
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 *
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 * FFmpeg is free software; you can redistribute it and/or
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 * modify it under the terms of the GNU Lesser General Public
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 * License as published by the Free Software Foundation; either
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 * version 2.1 of the License, or (at your option) any later version.
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 *
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 * FFmpeg is distributed in the hope that it will be useful,
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 * but WITHOUT ANY WARRANTY; without even the implied warranty of
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 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
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 * Lesser General Public License for more details.
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 *
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 * You should have received a copy of the GNU Lesser General Public
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 * License along with FFmpeg; if not, write to the Free Software
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 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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 */
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/**
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see http://en.wikipedia.org/wiki/Mersenne_twister for an explanation of this algorithm.
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*/
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#include <stdio.h>
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#include "random.h"
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//#define DEBUG
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#ifdef DEBUG
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#include "common.h"
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#include "log.h"
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#endif
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/* Period parameters */
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#define M 397
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#define A 0x9908b0df /* constant vector a */
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#define UPPER_MASK 0x80000000 /* most significant w-r bits */
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#define LOWER_MASK 0x7fffffff /* least significant r bits */
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/** initializes mt[AV_RANDOM_N] with a seed */
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void av_init_random(unsigned int seed, AVRandomState *state)
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{
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    int index;
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    /*
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     This differs from the wikipedia article.  Source is from the Makoto
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     Makoto Matsumoto and Takuji Nishimura code, with the following comment:
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     */
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     /* See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier. */
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     /* In the previous versions, MSBs of the seed affect   */
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     /* only MSBs of the array mt[].                        */
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    state->mt[0] = seed & 0xffffffff;
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    for (index = 1; index < AV_RANDOM_N; index++) {
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        unsigned int prev= state->mt[index - 1];
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        state->mt[index] = (1812433253UL * (prev ^ (prev>>30)) + index) & 0xffffffff;
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    }
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    state->index= index; // will cause it to generate untempered numbers the first iteration
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}
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/** generate AV_RANDOM_N words at one time (which will then be tempered later) (av_random calls this; you shouldn't) */
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void av_random_generate_untempered_numbers(AVRandomState *state)
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{
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    int kk;
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    unsigned int y;
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    for (kk = 0; kk < AV_RANDOM_N - M; kk++) {
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        y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
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        state->mt[kk] = state->mt[kk + M] ^ (y >> 1) ^ ((y&1)*A);
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    }
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    for (; kk < AV_RANDOM_N - 1; kk++) {
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        y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
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        state->mt[kk] = state->mt[kk + (M - AV_RANDOM_N)] ^ (y >> 1) ^ ((y&1)*A);
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    }
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    y = (state->mt[AV_RANDOM_N - 1] & UPPER_MASK) | (state->mt[0] & LOWER_MASK);
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    state->mt[AV_RANDOM_N - 1] = state->mt[M - 1] ^ (y >> 1) ^ ((y&1)*A);
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    state->index = 0;
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}
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#ifdef DEBUG
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void av_benchmark_random(void)
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{
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    int x=0;
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    int i, j;
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    AVRandomState state;
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    av_init_random(0xdeadbeef, &state);
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    for (j = 0; j < 100; j++) {
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        START_TIMER;
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        x+= av_random(&state);
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        STOP_TIMER("first call to av_random");
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        for (i = 1; i < AV_RANDOM_N; i++) {
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            START_TIMER;
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            x+= av_random(&state);
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            STOP_TIMER("AV_RANDOM_N calls of av_random");
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        }
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    }
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    av_log(NULL, AV_LOG_ERROR, "final value:%X\n", x);
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}
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#endif