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libavutil/lfg.c

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00001 /*
00002  * Lagged Fibonacci PRNG
00003  * Copyright (c) 2008 Michael Niedermayer
00004  *
00005  * This file is part of FFmpeg.
00006  *
00007  * FFmpeg is free software; you can redistribute it and/or
00008  * modify it under the terms of the GNU Lesser General Public
00009  * License as published by the Free Software Foundation; either
00010  * version 2.1 of the License, or (at your option) any later version.
00011  *
00012  * FFmpeg is distributed in the hope that it will be useful,
00013  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00014  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
00015  * Lesser General Public License for more details.
00016  *
00017  * You should have received a copy of the GNU Lesser General Public
00018  * License along with FFmpeg; if not, write to the Free Software
00019  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
00020  */
00021 
00022 #include <inttypes.h>
00023 #include "lfg.h"
00024 #include "md5.h"
00025 #include "intreadwrite.h"
00026 #include "attributes.h"
00027 
00028 void av_cold av_lfg_init(AVLFG *c, unsigned int seed){
00029     uint8_t tmp[16]={0};
00030     int i;
00031 
00032     for(i=8; i<64; i+=4){
00033         AV_WL32(tmp, seed); tmp[4]=i;
00034         av_md5_sum(tmp, tmp,  16);
00035         c->state[i  ]= AV_RL32(tmp);
00036         c->state[i+1]= AV_RL32(tmp+4);
00037         c->state[i+2]= AV_RL32(tmp+8);
00038         c->state[i+3]= AV_RL32(tmp+12);
00039     }
00040     c->index=0;
00041 }
00042 
00043 void av_bmg_get(AVLFG *lfg, double out[2])
00044 {
00045     double x1, x2, w;
00046 
00047     do {
00048         x1 = 2.0/UINT_MAX*av_lfg_get(lfg) - 1.0;
00049         x2 = 2.0/UINT_MAX*av_lfg_get(lfg) - 1.0;
00050         w = x1*x1 + x2*x2;
00051     } while (w >= 1.0);
00052 
00053     w = sqrt((-2.0 * log(w)) / w);
00054     out[0] = x1 * w;
00055     out[1] = x2 * w;
00056 }
00057 
00058 #ifdef TEST
00059 #include "log.h"
00060 #include "timer.h"
00061 
00062 int main(void)
00063 {
00064     int x=0;
00065     int i, j;
00066     AVLFG state;
00067 
00068     av_lfg_init(&state, 0xdeadbeef);
00069     for (j = 0; j < 10000; j++) {
00070         START_TIMER
00071         for (i = 0; i < 624; i++) {
00072 //            av_log(NULL,AV_LOG_ERROR, "%X\n", av_lfg_get(&state));
00073             x+=av_lfg_get(&state);
00074         }
00075         STOP_TIMER("624 calls of av_lfg_get");
00076     }
00077     av_log(NULL, AV_LOG_ERROR, "final value:%X\n", x);
00078 
00079     /* BMG usage example */
00080     {
00081         double mean   = 1000;
00082         double stddev = 53;
00083 
00084         av_lfg_init(&state, 42);
00085 
00086         for (i = 0; i < 1000; i += 2) {
00087             double bmg_out[2];
00088             av_bmg_get(&state, bmg_out);
00089             av_log(NULL, AV_LOG_INFO,
00090                    "%f\n%f\n",
00091                    bmg_out[0] * stddev + mean,
00092                    bmg_out[1] * stddev + mean);
00093         }
00094     }
00095 
00096     return 0;
00097 }
00098 #endif

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