/// <summary>
/// Create a pair from indices "idx1" and "idx2" provided its cost is inferior to "threshold", a negative entropy.
/// </summary>
/// <returns>The cost of the pair, or 0 if it superior to threshold.</returns>
private static double HistoPriorityListPush(List <HistogramPair> histoList, int maxSize, List <Vp8LHistogram> histograms, int idx1, int idx2, double threshold, Vp8LStreaks stats, Vp8LBitEntropy bitsEntropy)
{
var pair = new HistogramPair();
if (histoList.Count == maxSize)
{
return(0.0d);
}
if (idx1 > idx2)
{
int tmp = idx2;
idx2 = idx1;
idx1 = tmp;
}
pair.Idx1 = idx1;
pair.Idx2 = idx2;
Vp8LHistogram h1 = histograms[idx1];
Vp8LHistogram h2 = histograms[idx2];
HistoListUpdatePair(h1, h2, stats, bitsEntropy, threshold, pair);
// Do not even consider the pair if it does not improve the entropy.
if (pair.CostDiff >= threshold)
{
return(0.0d);
}
histoList.Add(pair);
HistoListUpdateHead(histoList, pair);
return(pair.CostDiff);
}
/// <summary>
/// Check whether a pair in the list should be updated as head or not.
/// </summary>
private static void HistoListUpdateHead(List <HistogramPair> histoList, HistogramPair pair)
{
if (pair.CostDiff < histoList[0].CostDiff)
{
// Replace the best pair.
int oldIdx = histoList.IndexOf(pair);
histoList[oldIdx] = histoList[0];
histoList[0] = pair;
}
}
/// <summary>
/// Update the cost diff and combo of a pair of histograms. This needs to be called when the the histograms have been merged with a third one.
/// </summary>
private static void HistoListUpdatePair(Vp8LHistogram h1, Vp8LHistogram h2, Vp8LStreaks stats, Vp8LBitEntropy bitsEntropy, double threshold, HistogramPair pair)
{
double sumCost = h1.BitCost + h2.BitCost;
pair.CostCombo = 0.0d;
h1.GetCombinedHistogramEntropy(h2, stats, bitsEntropy, sumCost + threshold, costInitial: pair.CostCombo, out double cost);
pair.CostCombo = cost;
pair.CostDiff = pair.CostCombo - sumCost;
}
private static void HistogramCombineGreedy(List <Vp8LHistogram> histograms)
{
int histoSize = histograms.Count(h => h != null);
// Priority list of histogram pairs.
var histoPriorityList = new List <HistogramPair>();
int maxSize = histoSize * histoSize;
var stats = new Vp8LStreaks();
var bitsEntropy = new Vp8LBitEntropy();
for (int i = 0; i < histoSize; i++)
{
if (histograms[i] == null)
{
continue;
}
for (int j = i + 1; j < histoSize; j++)
{
if (histograms[j] == null)
{
continue;
}
HistoPriorityListPush(histoPriorityList, maxSize, histograms, i, j, 0.0d, stats, bitsEntropy);
}
}
while (histoPriorityList.Count > 0)
{
int idx1 = histoPriorityList[0].Idx1;
int idx2 = histoPriorityList[0].Idx2;
HistogramAdd(histograms[idx2], histograms[idx1], histograms[idx1]);
histograms[idx1].BitCost = histoPriorityList[0].CostCombo;
// Remove merged histogram.
histograms[idx2] = null;
// Remove pairs intersecting the just combined best pair.
for (int i = 0; i < histoPriorityList.Count;)
{
HistogramPair p = histoPriorityList.ElementAt(i);
if (p.Idx1 == idx1 || p.Idx2 == idx1 || p.Idx1 == idx2 || p.Idx2 == idx2)
{
// Replace item at pos i with the last one and shrinking the list.
histoPriorityList[i] = histoPriorityList[histoPriorityList.Count - 1];
histoPriorityList.RemoveAt(histoPriorityList.Count - 1);
}
else
{
HistoListUpdateHead(histoPriorityList, p);
i++;
}
}
// Push new pairs formed with combined histogram to the list.
for (int i = 0; i < histoSize; i++)
{
if (i == idx1 || histograms[i] == null)
{
continue;
}
HistoPriorityListPush(histoPriorityList, maxSize, histograms, idx1, i, 0.0d, stats, bitsEntropy);
}
}
}
/// <summary>
/// Perform histogram aggregation using a stochastic approach.
/// </summary>
/// <returns>true if a greedy approach needs to be performed afterwards, false otherwise.</returns>
private static bool HistogramCombineStochastic(List <Vp8LHistogram> histograms, int minClusterSize)
{
uint seed = 1;
int triesWithNoSuccess = 0;
int numUsed = histograms.Count(h => h != null);
int outerIters = numUsed;
int numTriesNoSuccess = outerIters / 2;
var stats = new Vp8LStreaks();
var bitsEntropy = new Vp8LBitEntropy();
if (numUsed < minClusterSize)
{
return(true);
}
// Priority list of histogram pairs. Its size impacts the quality of the compression and the speed:
// the smaller the faster but the worse for the compression.
var histoPriorityList = new List <HistogramPair>();
int maxSize = 9;
// Fill the initial mapping.
int[] mappings = new int[histograms.Count];
for (int j = 0, iter = 0; iter < histograms.Count; iter++)
{
if (histograms[iter] == null)
{
continue;
}
mappings[j++] = iter;
}
// Collapse similar histograms.
for (int iter = 0; iter < outerIters && numUsed >= minClusterSize && ++triesWithNoSuccess < numTriesNoSuccess; iter++)
{
double bestCost = histoPriorityList.Count == 0 ? 0.0d : histoPriorityList[0].CostDiff;
int numTries = numUsed / 2;
uint randRange = (uint)((numUsed - 1) * numUsed);
// Pick random samples.
for (int j = 0; numUsed >= 2 && j < numTries; j++)
{
// Choose two different histograms at random and try to combine them.
uint tmp = MyRand(ref seed) % randRange;
int idx1 = (int)(tmp / (numUsed - 1));
int idx2 = (int)(tmp % (numUsed - 1));
if (idx2 >= idx1)
{
idx2++;
}
idx1 = mappings[idx1];
idx2 = mappings[idx2];
// Calculate cost reduction on combination.
double currCost = HistoPriorityListPush(histoPriorityList, maxSize, histograms, idx1, idx2, bestCost, stats, bitsEntropy);
// Found a better pair?
if (currCost < 0)
{
bestCost = currCost;
if (histoPriorityList.Count == maxSize)
{
break;
}
}
}
if (histoPriorityList.Count == 0)
{
continue;
}
// Get the best histograms.
int bestIdx1 = histoPriorityList[0].Idx1;
int bestIdx2 = histoPriorityList[0].Idx2;
int mappingIndex = Array.IndexOf(mappings, bestIdx2);
Span <int> src = mappings.AsSpan(mappingIndex + 1, numUsed - mappingIndex - 1);
Span <int> dst = mappings.AsSpan(mappingIndex);
src.CopyTo(dst);
// Merge the histograms and remove bestIdx2 from the list.
HistogramAdd(histograms[bestIdx2], histograms[bestIdx1], histograms[bestIdx1]);
histograms.ElementAt(bestIdx1).BitCost = histoPriorityList[0].CostCombo;
histograms[bestIdx2] = null;
numUsed--;
for (int j = 0; j < histoPriorityList.Count;)
{
HistogramPair p = histoPriorityList[j];
bool isIdx1Best = p.Idx1 == bestIdx1 || p.Idx1 == bestIdx2;
bool isIdx2Best = p.Idx2 == bestIdx1 || p.Idx2 == bestIdx2;
bool doEval = false;
// The front pair could have been duplicated by a random pick so
// check for it all the time nevertheless.
if (isIdx1Best && isIdx2Best)
{
histoPriorityList[j] = histoPriorityList[histoPriorityList.Count - 1];
histoPriorityList.RemoveAt(histoPriorityList.Count - 1);
continue;
}
// Any pair containing one of the two best indices should only refer to
// bestIdx1. Its cost should also be updated.
if (isIdx1Best)
{
p.Idx1 = bestIdx1;
doEval = true;
}
else if (isIdx2Best)
{
p.Idx2 = bestIdx1;
doEval = true;
}
// Make sure the index order is respected.
if (p.Idx1 > p.Idx2)
{
int tmp = p.Idx2;
p.Idx2 = p.Idx1;
p.Idx1 = tmp;
}
if (doEval)
{
// Re-evaluate the cost of an updated pair.
HistoListUpdatePair(histograms[p.Idx1], histograms[p.Idx2], stats, bitsEntropy, 0.0d, p);
if (p.CostDiff >= 0.0d)
{
histoPriorityList[j] = histoPriorityList[histoPriorityList.Count - 1];
histoPriorityList.RemoveAt(histoPriorityList.Count - 1);
continue;
}
}
HistoListUpdateHead(histoPriorityList, p);
j++;
}
triesWithNoSuccess = 0;
}
bool doGreedy = numUsed <= minClusterSize;
return(doGreedy);
}
