/**
* Method to check if a prefix has a backward-extension (see Bide+ article for full details). This
* method do it a little bit differently than the BIDE+ article since we iterate with i on
* elements of the prefix instead of iterating with a i on the itemsets of the prefix. But the
* idea is the same!
*
* @param prefix the current prefix
* @return boolean true, if there is a backward extension
*/
private boolean checkBackwardExtension(SequentialPattern prefix, Set<Integer> sidset) {
// System.out.println("======" + prefix);
int totalOccurenceCount = prefix.getItemOccurencesTotalCount();
// For the ith item of the prefix
for (int i = 0; i < totalOccurenceCount; i++) {
Set<Integer> alreadyVisitedSID = new HashSet<Integer>();
// // SOME CODE USED BY "findAllFrequentPairsForBackwardExtensionCheck"
Integer itemI = prefix.getIthItem(i); // iPeriod
Integer itemIm1 = null; // iPeriod -1
if (i > 0) {
itemIm1 = prefix.getIthItem(i - 1);
}
// // END NEW
// Create a Map of pairs to count the support of items (represented by a pair)
// in the ith semi-maximum periods
Map<PairBIDE, PairBIDE> mapPaires = new HashMap<PairBIDE, PairBIDE>();
// (1) For each i, we build the list of maximum periods
// for each sequence in the original database
// int seqCount =0;
int highestSupportUntilNow = -1;
// 1703 pat - 9391 ms
for (int sequenceID : sidset) {
// OPTIMIZATION PART 1== DON'T CHECK THE BACK EXTENSION IF THERE IS NOT ENOUGH SEQUENCE
// LEFT TO FIND AN EXTENSION
// THIS CAN IMPROVE THE PERFORMANCE BY UP TO 30% on FIFA
int remainingSeqID = (sidset.size() - alreadyVisitedSID.size());
if (highestSupportUntilNow != -1
&& highestSupportUntilNow + remainingSeqID < sidset.size()) {
break;
}
alreadyVisitedSID.add(sequenceID);
// if(!alreadyVisitedSID.contains(sequenceID)) {
// seqCount++;
// alreadyVisitedSID.add(sequenceID);
// }
// END OF OPTIMIZATION PART 1 (IT CONTINUES A FEW LINES BELOW...)
PseudoSequenceBIDE sequence = initialDatabase.get(sequenceID);
PseudoSequenceBIDE period = sequence.getIthMaximumPeriodOfAPrefix(prefix.getItemsets(), i);
// if the period is not null
if (period != null) {
boolean hasBackwardExtension =
findAllFrequentPairsForBackwardExtensionCheck(
alreadyVisitedSID.size(), prefix, period, i, mapPaires, itemI, itemIm1);
if (hasBackwardExtension) {
// System.out.println(prefix + " has a backward extension from " + i + "th
// maxperiod in sequence from seq. " + sequenceID );
return true;
}
// ===== OPTIMIZATION PART 2
if ((sidset.size() - alreadyVisitedSID.size()) < minsuppAbsolute) {
for (PairBIDE pair : mapPaires.values()) {
int supportOfPair = pair.getSequenceIDs().size();
if (supportOfPair > highestSupportUntilNow) {
highestSupportUntilNow =
supportOfPair; // +1 because it may be raised for this sequence...
}
}
}
// ===== END OF OPTIMIZATION PART 2
}
}
}
// totaltimeForBackwardExtension += System.currentTimeMillis() - start;
return false; // no backward extension, we return false
}
