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package be.ugent.rml;
import be.ugent.rml.extractor.Extractor;
import be.ugent.rml.extractor.ReferenceExtractor;
import be.ugent.rml.store.Quad;
import be.ugent.rml.store.QuadStore;
import be.ugent.rml.term.NamedNode;
import be.ugent.rml.term.Term;
import java.util.*;
import static be.ugent.rml.Utils.getObjectsFromQuads;
public class MappingOptimizer {
private final QuadStore rmlStore;
public MappingOptimizer(QuadStore rmlStore) {
this.rmlStore = rmlStore;
}
public QuadStore optimizeMapping() throws Exception {
renameSameLogicalSource();
eliminateSelfJoins();
return rmlStore;
}
private void renameSameLogicalSource() {
List logicalSources = Utils.getObjectsFromQuads(rmlStore.getQuads(null,new NamedNode(NAMESPACES.RML2 + "logicalSource"),null));
Map, Term> logicalSourcesDict = new HashMap<>();
for (Term logicalSource : logicalSources){
// two logical Sources are considered to be identical when they have the same objects at the leaves of their subgraph
Set allObjects = new HashSet<>();
List objects = Utils.getObjectsFromQuads(rmlStore.getQuads(logicalSource, null, null));
while (!objects.isEmpty()) {
Term object = objects.remove(objects.size()-1);
if (object.isBNode() || object.isIRI()) {
List newObjects = Utils.getObjectsFromQuads(rmlStore.getQuads(object, null, null));
if (!newObjects.isEmpty()) {
objects.addAll(newObjects);
} else {
//object is final, not subject of new quads
allObjects.add(object);
}
} else {
//object is final, not subject of new quads
allObjects.add(object);
}
}
Set finalObjectSet = Collections.unmodifiableSet(allObjects);
if (!logicalSourcesDict.keySet().contains(finalObjectSet)) {
logicalSourcesDict.put(finalObjectSet, logicalSource);
} else {
List triplesMaps = Utils.getSubjectsFromQuads(this.rmlStore.getQuads(null, new NamedNode(NAMESPACES.RML2 + "logicalSource"), logicalSource));
for (Term triplesMap : triplesMaps) {
rmlStore.removeQuads(triplesMap, new NamedNode(NAMESPACES.RML2 + "logicalSource"), logicalSource);
rmlStore.addQuad(triplesMap, new NamedNode(NAMESPACES.RML2 + "logicalSource"), logicalSourcesDict.get(finalObjectSet));
}
}
}
}
private void eliminateSelfJoins() {
List refObjectMapsQuads = rmlStore.getQuads(null, new NamedNode(NAMESPACES.RML2 + "parentTriplesMap"), null);
for (Quad refObjectMapQuad : refObjectMapsQuads) {
Term parentTriplesMap = refObjectMapQuad.getObject();
Term childObjectMap = refObjectMapQuad.getSubject();
Term parentLogicalSource = Utils.getObjectsFromQuads(rmlStore.getQuads(parentTriplesMap, new NamedNode(NAMESPACES.RML2 + "logicalSource"), null)).get(0);
Term childPredicateObjectMap = Utils.getSubjectsFromQuads(rmlStore.getQuads(null, new NamedNode(NAMESPACES.RML2 + "objectMap"), childObjectMap)).get(0);
Term childTriplesMap = Utils.getSubjectsFromQuads(rmlStore.getQuads(null, new NamedNode(NAMESPACES.RML2 + "predicateObjectMap"), childPredicateObjectMap)).get(0);
Term childLogicalSource = Utils.getObjectsFromQuads(rmlStore.getQuads(childTriplesMap, new NamedNode(NAMESPACES.RML2 + "logicalSource"), null)).get(0);
// check if the logical sources are the same
if (childLogicalSource.equals(parentLogicalSource)) {
List joinConditions = Utils.getObjectsFromQuads(rmlStore.getQuads(childObjectMap, new NamedNode(NAMESPACES.RML2 + "joinCondition"), null));
List parentSubjectMaps = Utils.getObjectsFromQuads(rmlStore.getQuads(parentTriplesMap, new NamedNode(NAMESPACES.RML2 + "subjectMap"), null));
Term parentSubjectMap = null;
if (!parentSubjectMaps.isEmpty()) {
parentSubjectMap = parentSubjectMaps.get(0);
}
boolean safeSelfJoinElimination = true;
// if no join condition, we can safely eliminate the self-join
// else we need more checks
if (parentSubjectMap != null && !joinConditions.isEmpty()) {
// we can eliminate a self-join when all join conditions have equal references and all references for the parent subject or all reference for the related child triple come back in the join conditions
// 1. check if all join references are equal
List joinReferences = new ArrayList<>();
for (Term joinCondition : joinConditions) {
String parent = getObjectsFromQuads(rmlStore.getQuads(joinCondition, new NamedNode(NAMESPACES.RML2 + "parent"), null)).get(0).getValue();
String child = getObjectsFromQuads(rmlStore.getQuads(joinCondition, new NamedNode(NAMESPACES.RML2 + "child"), null)).get(0).getValue();
if (child.equals(parent)) {
joinReferences.add(child);
} else {
safeSelfJoinElimination = false;
}
}
if (safeSelfJoinElimination) {
// 2. check if all references for the parent subject come back in the join conditions
boolean safeTerms = hasSafeReferences(parentSubjectMap, joinReferences);
if(!safeTerms) {
// if not all references for the parent subject come back in the join conditions,
// 3. check if all references for the related child terms come back in the join conditions
// 3.1 check child subject
List childSubjectMaps = Utils.getObjectsFromQuads(rmlStore.getQuads(parentTriplesMap, new NamedNode(NAMESPACES.RML2 + "subjectMap"), null));
if(!childSubjectMaps.isEmpty()) {
safeTerms = hasSafeReferences(childSubjectMaps.get(0), joinReferences);
} else {
safeTerms = true;
}
//3.2 check child predicate (only make sense if the child subject was safe, otherwise we cannot eliminate the-self join)
if (safeTerms) {
List childPredicateMaps = Utils.getObjectsFromQuads(rmlStore.getQuads(childPredicateObjectMap, new NamedNode(NAMESPACES.RML2 + "predicateMap"), null));
if(!childPredicateMaps.isEmpty()) {
safeTerms = hasSafeReferences(childPredicateMaps.get(0), joinReferences);
}
}
}
// 4. if parent subject or all child terms are safe, the self join can be eliminated, else not
if (!safeTerms) {
safeSelfJoinElimination = false;
}
}
}
if (safeSelfJoinElimination) {
// now we rewrite the mapping file to eliminate the self-join
boolean termTypeAdded = false;
List parentSubjectMapQuads = rmlStore.getQuads(parentSubjectMap, null, null);
for (Quad parentSubjectMapQuad : parentSubjectMapQuads) {
Term predicate = parentSubjectMapQuad.getPredicate();
if (predicate.equals(new NamedNode(NAMESPACES.FNML + "functionValue"))
|| predicate.equals(new NamedNode(NAMESPACES.RML2 + "termType"))
|| predicate.equals(new NamedNode(NAMESPACES.RML2 + "reference"))
|| predicate.equals(new NamedNode(NAMESPACES.RML2 + "template"))
|| predicate.equals(new NamedNode(NAMESPACES.RML2 + "constant"))) {
rmlStore.addQuad(childObjectMap, predicate, parentSubjectMapQuad.getObject());
}
if (predicate.equals(new NamedNode(NAMESPACES.RML2 + "termType"))) {
termTypeAdded = true;
}
}
rmlStore.removeQuads(childObjectMap, new NamedNode(NAMESPACES.RML2 + "parentTriplesMap"), parentTriplesMap);
if (!termTypeAdded) {
rmlStore.addQuad(childObjectMap, new NamedNode(NAMESPACES.RML2 + "termType"), new NamedNode(NAMESPACES.RML2 + "IRI"));
}
}
}
}
}
private Set getAllLinkedReferences(Term term){
Set references = new HashSet<>();
List linkedSubjects = new ArrayList<>();
linkedSubjects.add(term);
while(!linkedSubjects.isEmpty()) {
Term subject = linkedSubjects.get(0);
List linkedQuads = rmlStore.getQuads(subject, null, null);
for (Quad linkedQuad : linkedQuads) {
Term predicate = linkedQuad.getPredicate();
if (predicate.equals(new NamedNode(NAMESPACES.RML2 + "reference"))) {
references.add(linkedQuad.getObject().getValue());
} else if (predicate.equals(new NamedNode(NAMESPACES.RML2 + "template"))) {
String template = linkedQuad.getObject().getValue();
List extractors = Utils.parseTemplate(template, false);
for (Extractor extractor : extractors) {
if (extractor instanceof ReferenceExtractor) {
references.add(((ReferenceExtractor) extractor).getReference());
}
}
} else {
Term object = linkedQuad.getObject();
if (object.isBNode() || object.isIRI()) {
linkedSubjects.add(object);
}
}
}
linkedSubjects.remove(0);
}
return references;
}
private boolean hasSafeReferences(Term term, List joinReferences){
boolean isSafe = true;
Set termReferences = getAllLinkedReferences(term);
for (String parentReference : termReferences){
if (!joinReferences.contains(parentReference)){
isSafe = false;
}
}
return isSafe;
}
}
