From 5183326ba864b444e211ff89f576fd440977eb84 Mon Sep 17 00:00:00 2001 From: Vincent Rabaud Date: Wed, 22 Feb 2017 14:57:29 +0100 Subject: [PATCH] Refactor the greedy histogram merge. This is preparatory work for an upcoming commit. No impact on speed or compression. Change-Id: I62488ae6d2dbb5398f1604068c212ab8e26e82bc --- src/enc/histogram_enc.c | 107 +++++++++++++++++++++------------------- 1 file changed, 56 insertions(+), 51 deletions(-) diff --git a/src/enc/histogram_enc.c b/src/enc/histogram_enc.c index e0e7fcd7..37f95f12 100644 --- a/src/enc/histogram_enc.c +++ b/src/enc/histogram_enc.c @@ -643,55 +643,71 @@ static void HistoQueueClear(HistoQueue* const histo_queue) { WebPSafeFree(histo_queue->queue); } -static void SwapHistogramPairs(HistogramPair *p1, - HistogramPair *p2) { - const HistogramPair tmp = *p1; - *p1 = *p2; - *p2 = tmp; +// Pop a specific pair in the queue by replacing it with the last one +// and shrinking the queue. +static void HistoQueuePopPair(HistoQueue* const histo_queue, + HistogramPair* const pair) { + assert(pair >= histo_queue->queue && + pair < (histo_queue->queue + histo_queue->size)); + assert(histo_queue->size > 0); + *pair = histo_queue->queue[histo_queue->size - 1]; + --histo_queue->size; } -// Given a valid priority queue in range [0, queue_size) this function checks -// whether histo_queue[queue_size] should be accepted and swaps it with the -// front if it is smaller. Otherwise, it leaves it as is. -static void UpdateQueueFront(HistoQueue* const histo_queue) { - if (histo_queue->queue[histo_queue->size].cost_diff >= 0) return; - - if (histo_queue->queue[histo_queue->size].cost_diff < - histo_queue->queue[0].cost_diff) { - SwapHistogramPairs(histo_queue->queue, - histo_queue->queue + histo_queue->size); +// Check whether a pair in the queue should be updated as head or not. +static void HistoQueueUpdateHead(HistoQueue* const histo_queue, + HistogramPair* const pair) { + assert(pair->cost_diff < 0.); + assert(pair >= histo_queue->queue && + pair < (histo_queue->queue + histo_queue->size)); + assert(histo_queue->size > 0); + if (pair->cost_diff < histo_queue->queue[0].cost_diff) { + // Replace the best pair. + const HistogramPair tmp = histo_queue->queue[0]; + histo_queue->queue[0] = *pair; + *pair = tmp; } - ++histo_queue->size; - - // We cannot add more elements than the capacity. - // The allocation adds an extra element to the official capacity so that - // histo_queue->queue[histo_queue->max_size] is read/written within bound. - assert(histo_queue->size <= histo_queue->max_size); } -// ----------------------------------------------------------------------------- - -static void PreparePair(VP8LHistogram** histograms, int idx1, int idx2, - HistogramPair* const pair) { - VP8LHistogram* h1; - VP8LHistogram* h2; +// Create a pair from indices "idx1" and "idx2" provided its cost +// is inferior to "threshold", a negative entropy. +// It returns the cost of the pair, or 0. if it superior to threshold. +static double HistoQueuePush(HistoQueue* const histo_queue, + VP8LHistogram** const histograms, int idx1, + int idx2, double threshold) { + const VP8LHistogram* h1; + const VP8LHistogram* h2; + HistogramPair pair; double sum_cost; + assert(threshold <= 0.); if (idx1 > idx2) { const int tmp = idx2; idx2 = idx1; idx1 = tmp; } - pair->idx1 = idx1; - pair->idx2 = idx2; + pair.idx1 = idx1; + pair.idx2 = idx2; h1 = histograms[idx1]; h2 = histograms[idx2]; sum_cost = h1->bit_cost_ + h2->bit_cost_; - pair->cost_combo = 0.; - GetCombinedHistogramEntropy(h1, h2, sum_cost, &pair->cost_combo); - pair->cost_diff = pair->cost_combo - sum_cost; + pair.cost_combo = 0.; + GetCombinedHistogramEntropy(h1, h2, sum_cost + threshold, &pair.cost_combo); + pair.cost_diff = pair.cost_combo - sum_cost; + + // Do not even consider the pair if it does not improve the entropy. + if (pair.cost_diff >= threshold) return 0.; + + // We cannot add more elements than the capacity. + assert(histo_queue->size < histo_queue->max_size); + histo_queue->queue[histo_queue->size++] = pair; + HistoQueueUpdateHead(histo_queue, &histo_queue->queue[histo_queue->size - 1]); + + return pair.cost_diff; } +// ----------------------------------------------------------------------------- + // Combines histograms by continuously choosing the one with the highest cost // reduction. static int HistogramCombineGreedy(VP8LHistogramSet* const image_histo) { @@ -714,13 +730,11 @@ static int HistogramCombineGreedy(VP8LHistogramSet* const image_histo) { clusters[i] = i; for (j = i + 1; j < image_histo_size; ++j) { // Initialize positions array. - PreparePair(histograms, i, j, &histo_queue.queue[histo_queue.size]); - UpdateQueueFront(&histo_queue); + HistoQueuePush(&histo_queue, histograms, i, j, 0.); } } while (image_histo_size > 1 && histo_queue.size > 0) { - HistogramPair* copy_to; const int idx1 = histo_queue.queue[0].idx1; const int idx2 = histo_queue.queue[0].idx2; HistogramAdd(histograms[idx2], histograms[idx1], histograms[idx1]); @@ -733,31 +747,22 @@ static int HistogramCombineGreedy(VP8LHistogramSet* const image_histo) { } --image_histo_size; - // Remove pairs intersecting the just combined best pair. This will - // therefore pop the head of the queue. - copy_to = histo_queue.queue; - for (i = 0; i < histo_queue.size; ++i) { + // Remove pairs intersecting the just combined best pair. + for (i = 0; i < histo_queue.size;) { HistogramPair* const p = histo_queue.queue + i; if (p->idx1 == idx1 || p->idx2 == idx1 || p->idx1 == idx2 || p->idx2 == idx2) { - // Do not copy the invalid pair. - continue; + HistoQueuePopPair(&histo_queue, p); + } else { + HistoQueueUpdateHead(&histo_queue, p); + ++i; } - if (p->cost_diff < histo_queue.queue[0].cost_diff) { - // Replace the top of the queue if we found better. - SwapHistogramPairs(histo_queue.queue, p); - } - SwapHistogramPairs(copy_to, p); - ++copy_to; } - histo_queue.size = (int)(copy_to - histo_queue.queue); // Push new pairs formed with combined histogram to the queue. for (i = 0; i < image_histo_size; ++i) { if (clusters[i] != idx1) { - PreparePair(histograms, idx1, clusters[i], - &histo_queue.queue[histo_queue.size]); - UpdateQueueFront(&histo_queue); + HistoQueuePush(&histo_queue, histograms, idx1, clusters[i], 0.); } } }