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Experimental Rio parser and writer implementation for the HDT file format.
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/*******************************************************************************
 * Copyright (c) 2020 Eclipse RDF4J contributors.
 *
 * 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.rio.hdt;

import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.StandardCharsets;
import java.util.zip.CheckedInputStream;

import org.eclipse.rdf4j.common.io.UncloseableInputStream;

/**
 * HDT Triples Part.
 * 
 * This part starts with $HDT, followed by a byte indicating the type of the part, the NULL-terminated URI
 * string for the format, and optionally one or more key=value; properties.
 * 

 * These properties may include the order (SPO, SOP...), and the number of triples.
 * 

 * Then a NULL byte, followed by the 16-bit CRC ($HDT and NULL included)
 * 

 * Structure:
 *
 * 
 * +------+------+-----+------+------------+------+-------+
 * | $HDT | type | URI | NULL | key=value; | NULL | CRC16 |
 * +------+------+-----+------+------------+------+-------+
 * 
 *
 * @author Bart Hanssens
 */
class HDTTriples extends HDTPart {
	enum Order {
		UNKNOWN(0),
		SPO(1),
		SOP(2),
		PSO(3),
		POS(4),
		OSP(5),
		OPS(6);

		private final int value;

		int getValue() {
			return value;
		}

		/**
		 * Constructor
		 *
		 * @param value integer value
		 */
		Order(int value) {
			this.value = value;
		}
	}

	final static byte[] FORMAT_LIST = ""
			.getBytes(StandardCharsets.US_ASCII);
	final static byte[] FORMAT_BITMAP = ""
			.getBytes(StandardCharsets.US_ASCII);
	final static String ORDER = "order";
	final static String NUM = "numTriples";

	private Order order;
	private int nrtriples;

	/**
	 * Return triple order
	 *
	 * @return enum
	 */
	Order getOrder() {
		return order;
	}

	@Override
	void parse(InputStream is) throws IOException {
		// don't close CheckedInputStream, as it will close the underlying inputstream
		try (UncloseableInputStream uis = new UncloseableInputStream(is);
				CheckedInputStream cis = new CheckedInputStream(uis, new CRC16())) {

			checkControl(cis, HDTPart.Type.TRIPLES);
			checkFormat(cis, FORMAT_BITMAP);

			properties = getProperties(cis);

			int i = getIntegerProperty(properties, ORDER, "order");
			if (i != HDTTriples.Order.SPO.getValue()) {
				throw new UnsupportedOperationException(
						"Triples section: order " + Integer.toString(i) + ", but only SPO order is supported");
			}

			checkCRC(cis, is, 2);
		}
	}
}