random_seed: rewrite generic code.

The new code is faster and reuses the previous state in case of
multiple calls. For testing the iterations are reduced and several
entropy sources are disabled.
Its based on SHA-1
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/random_seed.c

@@ -22,8 +22,15 @@
 #include <fcntl.h>
 #include <math.h>
 #include <time.h>
+#include <string.h>
 #include "timer.h"
 #include "random_seed.h"
+#include "sha.h"
+#include "intreadwrite.h"
+
+#ifndef TEST
+#define TEST 0
+#endif
 
 static int read_random(uint32_t *dst, const char *file)
 {
@@ -40,34 +47,44 @@ static int read_random(uint32_t *dst, const char *file)
 
 static uint32_t get_generic_seed(void)
 {
+    uint8_t tmp[av_sha_size];
+    struct AVSHA *sha = (void*)tmp;
     clock_t last_t  = 0;
-    int bits        = 0;
-    uint64_t random = 0;
-    unsigned i;
-    float s = 0.000000000001;
+    static uint64_t i = 0;
+    static uint32_t buffer[512] = {0};
+    unsigned char digest[32];
+    uint64_t last_i = i;
 
-    for (i = 0; bits < 64; i++) {
+    if(TEST){
+        memset(buffer, 0, sizeof(buffer));
+        last_i = i = 0;
+    }else{
+#ifdef AV_READ_TIME
+        buffer[13] ^= AV_READ_TIME();
+        buffer[41] ^= AV_READ_TIME()>>32;
+#endif
+    }
+
+    for (;;) {
         clock_t t = clock();
-        if (last_t && fabs(t - last_t) > s || t == (clock_t) -1) {
-            if (i < 10000 && s < (1 << 24)) {
-                s += s;
-                i = t = 0;
-            } else {
-                random = 2 * random + (i & 1);
-                bits++;
-            }
+
+        if(last_t == t){
+            buffer[i&511]++;
+        }else{
+            buffer[++i&511]+= (t-last_t) % 3294638521U;
+            if(last_i && i-last_i > 4 || i-last_i > 64 || TEST && i-last_i > 8)
+                break;
         }
         last_t = t;
     }
-#ifdef AV_READ_TIME
-    random ^= AV_READ_TIME();
-#else
-    random ^= clock();
-#endif
 
-    random += random >> 32;
+    if(TEST)
+        buffer[0] = buffer[1] = 0;
 
-    return random;
+    av_sha_init(sha, 160);
+    av_sha_update(sha, (uint8_t*)buffer, sizeof(buffer));
+    av_sha_final(sha, digest);
+    return AV_RB32(digest) + AV_RB32(digest+32);
 }
 
 uint32_t av_get_random_seed(void)
@@ -81,7 +98,7 @@ uint32_t av_get_random_seed(void)
     return get_generic_seed();
 }
 
-#ifdef TEST
+#if TEST
 #undef printf
 #define N 256
 #include <stdio.h>