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Fix the random generator in HistogramCombineStochastic.

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It was a bad implementation of a Lehmer random number generator
(the saturation was done wrong and mostly & was used instead of % .....).

That lead to "for" loop stuck with the same values given a specific seed,
hence wasted "for" loops (e.g. seed getting at 374988608 and modulo of 64
later leads to 0 even when updating the seed with the old formula).

As the "for" loops now always return a proper pair of histograms, their
number can greatly be reduced, hence a speedup.

Change-Id: I9f5b44d66cc96fd4824189d92276c3756c8ead5b
This commit is contained in:
Vincent Rabaud 2017-02-17 20:31:24 +01:00
parent 027151ca6c
commit fa63a96603
1 changed files with 18 additions and 23 deletions
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41
src/enc/histogram_enc.c
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@ -596,11 +596,11 @@ static VP8LHistogram* HistogramCombineEntropyBin(
return cur_combo;
}
// Implement a Lehmer random number generator with a multiplicative constant of
// 48271 and a modulo constant of 2^31 − 1.
static uint32_t MyRand(uint32_t* const seed) {
*seed = (*seed * 16807ull) & 0xffffffffu;
if (*seed == 0) {
*seed = 1;
}
*seed = (uint32_t)(((uint64_t)(*seed) * 48271ul) % 2147483647ul);
assert(*seed > 0);
return *seed;
}
@ -782,15 +782,13 @@ static int HistogramCombineGreedy(VP8LHistogramSet* const image_histo) {
// afterwards, 0 otherwise.
static void HistogramCombineStochastic(VP8LHistogramSet* const image_histo,
VP8LHistogram* tmp_histo,
VP8LHistogram* best_combo, int quality,
VP8LHistogram* best_combo,
int min_cluster_size, int* do_greedy) {
int iter;
uint32_t seed = 0;
uint32_t seed = 1;
int tries_with_no_success = 0;
int image_histo_size = image_histo->size;
const int iter_mult = (quality < 25) ? 2 : 2 + (quality - 25) / 8;
const int outer_iters = image_histo_size * iter_mult;
const int num_pairs = image_histo_size / 2;
const int outer_iters = image_histo_size;
const int num_tries_no_success = outer_iters / 2;
VP8LHistogram** const histograms = image_histo->histograms;
@ -803,8 +801,11 @@ static void HistogramCombineStochastic(VP8LHistogramSet* const image_histo,
double best_cost_diff = 0.;
int best_idx1 = -1, best_idx2 = 1;
int j;
int num_tries =
(num_pairs < image_histo_size) ? num_pairs : image_histo_size;
const uint32_t rand_range = (image_histo_size - 1) * image_histo_size;
// 6/10 was chosen empirically.
// TODO(vrabaud): use less magic constants in that code.
const int num_tries = (6 * image_histo_size) / 10;
// If the stochastic method has not worked for a while (10 iterations) and
// if it requires less iterations to finish off with a greedy approach, go
// for it.
@ -818,19 +819,13 @@ static void HistogramCombineStochastic(VP8LHistogramSet* const image_histo,
num_tries * (outer_iters - iter));
if (*do_greedy) break;
seed += iter;
for (j = 0; j < num_tries; ++j) {
double curr_cost_diff;
// Choose two histograms at random and try to combine them.
uint32_t idx1, idx2;
const uint32_t tmp = (j & 7) + 1;
const uint32_t diff =
(tmp < 3) ? tmp : MyRand(&seed) % (image_histo_size - 1);
idx1 = MyRand(&seed) % image_histo_size;
idx2 = (idx1 + diff + 1) % image_histo_size;
if (idx1 == idx2) {
continue;
}
// Choose two different histograms at random and try to combine them.
const uint32_t tmp = MyRand(&seed) % rand_range;
const uint32_t idx1 = tmp / (image_histo_size - 1);
uint32_t idx2 = tmp % (image_histo_size - 1);
if (idx2 >= idx1) ++idx2;
// Calculate cost reduction on combining.
curr_cost_diff = HistogramAddEval(histograms[idx1], histograms[idx2],
@ -967,7 +962,7 @@ int VP8LGetHistoImageSymbols(int xsize, int ysize,
const int threshold_size = (int)(1 + (x * x * x) * (MAX_HISTO_GREEDY - 1));
int do_greedy;
HistogramCombineStochastic(image_histo, tmp_histos->histograms[0],
cur_combo, quality, threshold_size, &do_greedy);
cur_combo, threshold_size, &do_greedy);
if (do_greedy && !HistogramCombineGreedy(image_histo)) {
goto Error;
}