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RDF model implementations.
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/*******************************************************************************
 * Copyright (c) 2015 Eclipse RDF4J contributors, Aduna, and others.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Distribution License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 *******************************************************************************/
package org.eclipse.rdf4j.model.util;

import java.io.Serializable;
import java.util.Comparator;
import java.util.Optional;

import org.eclipse.rdf4j.model.BNode;
import org.eclipse.rdf4j.model.IRI;
import org.eclipse.rdf4j.model.Literal;
import org.eclipse.rdf4j.model.Triple;
import org.eclipse.rdf4j.model.Value;
import org.eclipse.rdf4j.model.datatypes.XMLDatatypeUtil;
import org.eclipse.rdf4j.model.vocabulary.XSD;

/**
 * A lexical RDF Term Comparator, this class does not compare numerically and is therefore a bit faster than a SPARQL
 * compliant comparator.
 *
 * @author james
 * @author Arjohn Kampman
 */
public class LexicalValueComparator implements Serializable, Comparator {

	private static final long serialVersionUID = -7055973992568220217L;

	@Override
	public int compare(Value o1, Value o2) {
		// check equality
		if (o1 == o2) {
			return 0;
		}

		// 1. (Lowest) no value assigned to the variable
		if (o1 == null) {
			return -1;
		}
		if (o2 == null) {
			return 1;
		}

		// 2. Blank nodes
		boolean b1 = o1.isBNode();
		boolean b2 = o2.isBNode();
		if (b1 && b2) {
			return compareBNodes((BNode) o1, (BNode) o2);
		}
		if (b1) {
			return -1;
		}
		if (b2) {
			return 1;
		}

		// 3. IRIs
		boolean u1 = o1.isIRI();
		boolean u2 = o2.isIRI();
		if (u1 && u2) {
			return compareURIs((IRI) o1, (IRI) o2);
		}
		if (u1) {
			return -1;
		}
		if (u2) {
			return 1;
		}

		// 4. RDF literals
		boolean l1 = o1.isLiteral();
		boolean l2 = o2.isLiteral();
		if (l1 && l2) {
			return compareLiterals((Literal) o1, (Literal) o2);
		}
		if (l1) {
			return -1;
		}
		if (l2) {
			return 1;
		}

		// 5. RDF-star triples
		return compareTriples((Triple) o1, (Triple) o2);
	}

	private int compareBNodes(BNode leftBNode, BNode rightBNode) {
		return leftBNode.getID().compareTo(rightBNode.getID());
	}

	private int compareURIs(IRI leftURI, IRI rightURI) {
		return leftURI.toString().compareTo(rightURI.toString());
	}

	private int compareLiterals(Literal leftLit, Literal rightLit) {
		// Additional constraint for ORDER BY: "A plain literal is lower
		// than an RDF literal with type xsd:string of the same lexical
		// form."
		int result = 0;
		// FIXME: Confirm these rules work with RDF-1.1
		// Sort by datatype first, plain literals come before datatyped literals
		IRI leftDatatype = leftLit.getDatatype();
		IRI rightDatatype = rightLit.getDatatype();

		if (leftDatatype != null) {
			if (rightDatatype != null) {
				// Both literals have datatypes
				Optional leftXmlDatatype = Literals.getXsdDatatype(leftLit);
				Optional rightXmlDatatype = Literals.getXsdDatatype(rightLit);

				if (leftXmlDatatype.isPresent() && rightXmlDatatype.isPresent()) {
					result = compareDatatypes(leftXmlDatatype.get(), rightXmlDatatype.get());
				} else {
					result = compareDatatypes(leftDatatype, rightDatatype);
				}

			} else {
				result = 1;
			}
		} else if (rightDatatype != null) {
			result = -1;
		}

		if (result == 0) {
			// datatypes are equal or both literals are untyped; sort by language
			// tags, simple literals come before literals with language tags
			Optional leftLanguage = leftLit.getLanguage();
			Optional rightLanguage = rightLit.getLanguage();

			if (leftLanguage.isPresent()) {
				if (rightLanguage.isPresent()) {
					result = leftLanguage.get().compareTo(rightLanguage.get());
				} else {
					result = 1;
				}
			} else if (rightLanguage.isPresent()) {
				result = -1;
			}
		}

		if (result == 0) {
			// Literals are equal as fas as their datatypes and language tags are
			// concerned, compare their labels
			result = leftLit.getLabel().compareTo(rightLit.getLabel());
		}

		return result;
	}

	/**
	 * Compares two literal datatypes and indicates if one should be ordered after the other. This algorithm ensures
	 * that compatible ordered datatypes (numeric and date/time) are grouped together so that
	 * {@link QueryEvaluationUtil#compareLiterals(Literal, Literal, CompareOp)} is used in consecutive ordering steps.
	 */
	private int compareDatatypes(IRI leftDatatype, IRI rightDatatype) {
		if (XMLDatatypeUtil.isNumericDatatype(leftDatatype)) {
			if (XMLDatatypeUtil.isNumericDatatype(rightDatatype)) {
				// both are numeric datatypes
				return compareURIs(leftDatatype, rightDatatype);
			} else {
				return -1;
			}
		} else if (XMLDatatypeUtil.isNumericDatatype(rightDatatype)) {
			return 1;
		} else if (XMLDatatypeUtil.isCalendarDatatype(leftDatatype)) {
			if (XMLDatatypeUtil.isCalendarDatatype(rightDatatype)) {
				// both are calendar datatypes
				return compareURIs(leftDatatype, rightDatatype);
			} else {
				return -1;
			}
		} else if (XMLDatatypeUtil.isCalendarDatatype(rightDatatype)) {
			return 1;
		} else {
			// incompatible or unordered datatypes
			return compareURIs(leftDatatype, rightDatatype);
		}
	}

	private int compareDatatypes(XSD.Datatype leftDatatype, XSD.Datatype rightDatatype) {
		if (leftDatatype.isNumericDatatype()) {
			if (rightDatatype.isNumericDatatype()) {
				// both are numeric datatypes
				return compareURIs(leftDatatype.getIri(), rightDatatype.getIri());
			} else {
				return -1;
			}
		} else if (rightDatatype.isNumericDatatype()) {
			return 1;
		} else if (leftDatatype.isCalendarDatatype()) {
			if (rightDatatype.isCalendarDatatype()) {
				// both are calendar datatypes
				return compareURIs(leftDatatype.getIri(), rightDatatype.getIri());
			} else {
				return -1;
			}
		} else if (rightDatatype.isCalendarDatatype()) {
			return 1;
		} else {
			// incompatible or unordered datatypes
			return compareURIs(leftDatatype.getIri(), rightDatatype.getIri());
		}
	}

	private int compareTriples(Triple leftTriple, Triple rightTriple) {
		int c = compare(leftTriple.getSubject(), rightTriple.getSubject());
		if (c == 0) {
			c = compare(leftTriple.getPredicate(), rightTriple.getPredicate());
			if (c == 0) {
				c = compare(leftTriple.getObject(), rightTriple.getObject());
			}
		}
		return c;
	}
}