/**
* Method to recursively grow a given sequential pattern.
*
* @param prefix the current sequential pattern that we want to try to grow
* @param database the current projected sequence database
* @throws IOException exception if there is an error writing to the output file
*/
private void recursion(SequentialPattern prefix, List<PseudoSequence> database)
throws IOException {
// find frequent items of size 1 in the current projected database.
Set<Pair> pairs = findAllFrequentPairs(prefix, database);
// For each pair found (a pair is an item with a boolean indicating if it
// appears in an itemset that is cut (a postfix) or not, and the sequence IDs
// where it appears in the projected database).
for (Pair pair : pairs) {
// if the item is frequent in the current projected database
if (pair.getCount() >= minsuppAbsolute) {
// create the new postfix by appending this item to the prefix
SequentialPattern newPrefix;
// if the item is part of a postfix
if (pair.isPostfix()) {
// we append it to the last itemset of the prefix
newPrefix = appendItemToPrefixOfSequence(prefix, pair.getItem());
} else { // else, we append it as a new itemset to the sequence
newPrefix = appendItemToSequence(prefix, pair.getItem());
}
// build the projected database with this item
List<PseudoSequence> projectedDB =
buildProjectedContext(pair.getItem(), database, pair.isPostfix());
newPrefix.setSequencesID(pair.getSequencesID());
// save the pattern
savePattern(newPrefix);
// make a recursive call
recursion(newPrefix, projectedDB);
}
}
MemoryLogger.getInstance().checkMemory();
}
/**
* This is the main method for the PrefixSpan algorithm that is called to start the algorithm
*
* @param outputFilePath an output file path if the result should be saved to a file or null if
* the result should be saved to memory.
* @param database a sequence database
* @throws IOException exception if an error while writing the output file
*/
private void prefixSpan(SequenceDatabase database, String outputFilePath) throws IOException {
// if the user want to keep the result into memory
if (outputFilePath == null) {
writer = null;
patterns = new SequentialPatterns("FREQUENT SEQUENTIAL PATTERNS");
} else { // if the user want to save the result to a file
patterns = null;
writer = new BufferedWriter(new FileWriter(outputFilePath));
}
// We have to scan the database to find all frequent patterns of size 1.
// We note the sequences in which these patterns appear.
Map<String, Set<Integer>> mapSequenceID = findSequencesContainingItems(database);
// WE CONVERT THE DATABASE ITON A PSEUDO-DATABASE, AND REMOVE
// THE ITEMS OF SIZE 1 THAT ARE NOT FREQUENT, SO THAT THE ALGORITHM
// WILL NOT CONSIDER THEM ANYMORE. (OPTIMIZATION : OCTOBER-08 )
// Create a list of pseudosequence
List<PseudoSequence> initialContext = new ArrayList<PseudoSequence>();
// for each sequence in the database
for (Sequence sequence : database.getSequences()) {
// remove infrequent items
Sequence optimizedSequence = sequence.cloneSequenceMinusItems(mapSequenceID, minsuppAbsolute);
if (optimizedSequence.size() != 0) {
// if the size is > 0, create a pseudo sequence with this sequence
initialContext.add(new PseudoSequence(optimizedSequence, 0, 0));
}
}
// For each item
for (Entry<String, Set<Integer>> entry : mapSequenceID.entrySet()) {
// if the item is frequent (has a support >= minsup)
if (entry.getValue().size() >= minsuppAbsolute) { // if the item is frequent
// build the projected context
String item = entry.getKey();
List<PseudoSequence> projectedContext = buildProjectedContext(item, initialContext, false);
// Create the prefix for the projected context.
SequentialPattern prefix = new SequentialPattern(0);
prefix.addItemset(new Itemset(item));
prefix.setSequencesID(entry.getValue());
// The prefix is a frequent sequential pattern.
// We save it in the result.
savePattern(prefix); // we found a sequence.
// Recursive call !
recursion(prefix, projectedContext);
}
}
}
