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palette: Replace rand() call with custom LCG.

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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
This commit is contained in:
Alex Converse 2015-10-14 11:03:14 -07:00
parent d162934bdc
commit 171fd8999f
1 changed files with 10 additions and 3 deletions
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13
vp10/encoder/palette.c
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@ -39,12 +39,20 @@ void vp10_calc_indices(const double *data, const double *centroids,
}
}
// 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];
assert(n <= 32768);
srand((unsigned int) data[0]);
memset(count, 0, sizeof(count[0]) * k);
memset(centroids, 0, sizeof(centroids[0]) * k * dim);
@ -59,8 +67,7 @@ static void calc_centroids(const double *data, double *centroids,
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];