av_random() by Ryan Martell rdm4 name-server-seperator martellventures d0t com

Originally committed as revision 7885 to svn://svn.ffmpeg.org/ffmpeg/trunk

1 files changed, 104 insertions, 0 deletions

diff --git a/libavutil/random.c b/libavutil/random.c
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+++ b/libavutil/random.c
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+/*
+ * Mersenne Twister Random Algorithm
+ * Copyright (c) 2006 Ryan Martell.
+ * Based on A C-program for MT19937, with initialization improved 2002/1/26. Coded by
+ * Takuji Nishimura and Makoto Matsumoto.
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+
+/**
+see http://en.wikipedia.org/wiki/Mersenne_twister for an explanation of this algorithm.
+*/
+#include <stdio.h>
+#include "random.h"
+
+//#define DEBUG
+
+#ifdef DEBUG
+#include "common.h"
+#include "log.h"
+#endif
+
+
+/* Period parameters */
+#define M 397
+#define A 0x9908b0df /* constant vector a */
+#define UPPER_MASK 0x80000000 /* most significant w-r bits */
+#define LOWER_MASK 0x7fffffff /* least significant r bits */
+
+/** initializes mt[AV_RANDOM_N] with a seed */
+void av_init_random(unsigned int seed, AVRandomState *state)
+{
+    int index;
+
+    /*
+     This differs from the wikipedia article.  Source is from the Makoto
+     Makoto Matsumoto and Takuji Nishimura code, with the following comment:
+     */
+     /* See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier. */
+     /* In the previous versions, MSBs of the seed affect   */
+     /* only MSBs of the array mt[].                        */
+    state->mt[0] = seed & 0xffffffff;
+    for (index = 1; index < AV_RANDOM_N; index++) {
+        unsigned int prev= state->mt[index - 1];
+        state->mt[index] = (1812433253UL * (prev ^ (prev>>30)) + index) & 0xffffffff;
+    }
+    state->index= index; // will cause it to generate untempered numbers the first iteration
+}
+
+/** generate AV_RANDOM_N words at one time (which will then be tempered later) (av_random calls this; you shouldn't) */
+void av_random_generate_untempered_numbers(AVRandomState *state)
+{
+    int kk;
+    unsigned int y;
+
+    for (kk = 0; kk < AV_RANDOM_N - M; kk++) {
+        y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
+        state->mt[kk] = state->mt[kk + M] ^ (y >> 1) ^ ((y&1)*A);
+    }
+    for (; kk < AV_RANDOM_N - 1; kk++) {
+        y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
+        state->mt[kk] = state->mt[kk + (M - AV_RANDOM_N)] ^ (y >> 1) ^ ((y&1)*A);
+    }
+    y = (state->mt[AV_RANDOM_N - 1] & UPPER_MASK) | (state->mt[0] & LOWER_MASK);
+    state->mt[AV_RANDOM_N - 1] = state->mt[M - 1] ^ (y >> 1) ^ ((y&1)*A);
+    state->index = 0;
+}
+
+#ifdef DEBUG
+void av_benchmark_random(void)
+{
+    int x=0;
+    int i, j;
+    AVRandomState state;
+
+    av_init_random(0xdeadbeef, &state);
+    for (j = 0; j < 100; j++) {
+        START_TIMER;
+        x+= av_random(&state);
+        STOP_TIMER("first call to av_random");
+        for (i = 1; i < AV_RANDOM_N; i++) {
+            START_TIMER;
+            x+= av_random(&state);
+            STOP_TIMER("AV_RANDOM_N calls of av_random");
+        }
+    }
+    av_log(NULL, AV_LOG_ERROR, "final value:%X\n", x);
+}
+#endif