commit 171fd8999fe8fdf04b5e836ff4eb0106ac6d7501
author Alex Converse <aconverse@google.com> Wed Oct 14 11:03:14 2015 -0700
committer Alex Converse <aconverse@google.com> Sat Oct 24 13:38:23 2015 -0700
tree 700ba6f690e476975f4e63d77c1079703965fba1
parent d162934bdca89b156194732226746993356344ef

palette: Replace rand() call with custom LCG.

The custom LCG is based on the POSIX recommend constants for a 16-bit
rand(). This implementation uses less computation than typical standard
library procedures which have been extended for 32-bit support, is
guaranteed to be reentrant, and identical everywhere.

Change-Id: I3140bbd566f44ab820d131c584a5d4ec6134c5a0
Ref: http://pubs.opengroup.org/onlinepubs/9699919799/functions/rand.html

vp10/encoder/palette.c

diff --git a/vp10/encoder/palette.c b/vp10/encoder/palette.c
index b6fd653..fd5f402 100644
--- a/vp10/encoder/palette.c
+++ b/vp10/encoder/palette.c
@@ -39,12 +39,20 @@
   }
 }
 
+// Generate a random number in the range [0, 32768).
+static unsigned int lcg_rand16(unsigned int *state) {
+  *state = *state * 1103515245 + 12345;
+  return *state / 65536 % 32768;
+}
+
 static void calc_centroids(const double *data, double *centroids,
                            const uint8_t *indices, int n, int k, int dim) {
   int i, j, index;
   int count[PALETTE_MAX_SIZE];
+  unsigned int rand_state = data[0];
 
-  srand((unsigned int) data[0]);
+  assert(n <= 32768);
+
   memset(count, 0, sizeof(count[0]) * k);
   memset(centroids, 0, sizeof(centroids[0]) * k * dim);
 
@@ -59,8 +67,7 @@
 
   for (i = 0; i < k; ++i) {
     if (count[i] == 0) {
-      // TODO(huisu): replace rand() with something else.
-      memcpy(centroids + i * dim, data + (rand() % n) * dim,
+      memcpy(centroids + i * dim, data + (lcg_rand16(&rand_state) % n) * dim,
                  sizeof(centroids[0]) * dim);
     } else {
       const double norm = 1.0 / count[i];