/*
* Copyright Aduna (http://www.aduna-software.com/) (c) 1997-2007.
*
* Licensed under the Aduna BSD-style license.
*/
package org.openrdf.query.parser.serql;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import org.openrdf.query.algebra.BNodeGenerator;
import org.openrdf.query.algebra.Distinct;
import org.openrdf.query.algebra.EmptySet;
import org.openrdf.query.algebra.Extension;
import org.openrdf.query.algebra.ExtensionElem;
import org.openrdf.query.algebra.MultiProjection;
import org.openrdf.query.algebra.Projection;
import org.openrdf.query.algebra.ProjectionElem;
import org.openrdf.query.algebra.ProjectionElemList;
import org.openrdf.query.algebra.Reduced;
import org.openrdf.query.algebra.StatementPattern;
import org.openrdf.query.algebra.TupleExpr;
import org.openrdf.query.algebra.ValueConstant;
import org.openrdf.query.algebra.ValueExpr;
import org.openrdf.query.algebra.Var;
import org.openrdf.query.algebra.helpers.StatementPatternCollector;
class ConstructorBuilder {
public TupleExpr buildConstructor(TupleExpr bodyExpr, TupleExpr constructExpr, boolean distinct) {
return buildConstructor(bodyExpr, constructExpr, true, distinct);
}
public TupleExpr buildConstructor(TupleExpr bodyExpr, boolean distinct) {
return buildConstructor(bodyExpr, bodyExpr, false, distinct);
}
private TupleExpr buildConstructor(TupleExpr bodyExpr, TupleExpr constructExpr, boolean explicit,
boolean distinct)
{
TupleExpr result = bodyExpr;
// Retrieve all StatementPattern's from the construct expression
List<StatementPattern> statementPatterns = StatementPatternCollector.process(constructExpr);
Set<Var> constructVars = getConstructVars(statementPatterns);
// Note: duplicate elimination is a two-step process. The first step
// removes duplicates from the set of constructor variables. After this
// step, any bnodes that need to be generated are added to each solution
// and these solutions are projected to subject-predicate-object bindings.
// Finally, the spo-bindings are again filtered for duplicates.
if (distinct) {
// Create projection that removes all bindings that are not used in the
// constructor
ProjectionElemList projElemList = new ProjectionElemList();
for (Var var : constructVars) {
// Ignore anonymous and constant vars, these will be handled after
// the distinct
if (!var.isAnonymous() && !var.hasValue()) {
projElemList.addElement(new ProjectionElem(var.getName()));
}
}
result = new Projection(result, projElemList);
// Filter the duplicates from these projected bindings
result = new Distinct(result);
}
// Create BNodeGenerator's for all anonymous variables
Map<Var, ExtensionElem> extElemMap = new HashMap<Var, ExtensionElem>();
for (Var var : constructVars) {
if (var.isAnonymous() && !extElemMap.containsKey(var)) {
ValueExpr valueExpr = null;
if (var.hasValue()) {
valueExpr = new ValueConstant(var.getValue());
}
else if (explicit) {
// only generate bnodes in case of an explicit constructor
valueExpr = new BNodeGenerator();
}
if (valueExpr != null) {
extElemMap.put(var, new ExtensionElem(valueExpr, var.getName()));
}
}
}
if (!extElemMap.isEmpty()) {
result = new Extension(result, extElemMap.values());
}
// Create a Projection for each StatementPattern in the constructor
List<ProjectionElemList> projections = new ArrayList<ProjectionElemList>();
for (StatementPattern sp : statementPatterns) {
ProjectionElemList projElemList = new ProjectionElemList();
projElemList.addElement(new ProjectionElem(sp.getSubjectVar().getName(), "subject"));
projElemList.addElement(new ProjectionElem(sp.getPredicateVar().getName(), "predicate"));
projElemList.addElement(new ProjectionElem(sp.getObjectVar().getName(), "object"));
projections.add(projElemList);
}
if (projections.size() == 1) {
result = new Projection(result, projections.get(0));
// Note: no need to apply the second duplicate elimination step if
// there's just one projection
}
else if (projections.size() > 1) {
result = new MultiProjection(result, projections);
if (distinct) {
// Add another distinct to filter duplicate statements
result = new Distinct(result);
}
}
else {
// Empty constructor
result = new EmptySet();
}
if (!distinct) {
result = new Reduced(result);
}
return result;
}
/**
* Gets the set of variables that are relevant for the constructor. This
* method accumulates all subject, predicate and object variables from the
* supplied statement patterns, but ignores any context variables.
*/
private Set<Var> getConstructVars(Collection<StatementPattern> statementPatterns) {
Set<Var> vars = new LinkedHashSet<Var>(statementPatterns.size() * 2);
for (StatementPattern sp : statementPatterns) {
vars.add(sp.getSubjectVar());
vars.add(sp.getPredicateVar());
vars.add(sp.getObjectVar());
}
return vars;
}
}