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

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

 * Structure:
 *
 * 
 * +------+------+--------+------+------------+------+-------+
 * | $HDT | type | format | NULL | key=value; | NULL | CRC16 |
 * +------+------+--------+------+------------+------+-------+
 * 
 *
 * @author Bart Hanssens
 */
class HDTHeader extends HDTPart {

	final static byte[] HEADER_FORMAT = "ntriples".getBytes(StandardCharsets.US_ASCII);
	final static String HEADER_LENGTH = "length";

	private byte[] headerData;

	@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.HEADER);
			checkFormat(cis, HEADER_FORMAT);
			properties = getProperties(cis);

			checkCRC(cis, is, 2);
		}
		int hlen = getIntegerProperty(properties, HEADER_LENGTH, "Header length");
		headerData = parseHeaderData(is, hlen);
	}

	/**
	 * Get raw header data (byte array data stored as NTriples)
	 *
	 * @return byte array
	 */
	byte[] getHeaderData() {
		return headerData;
	}

	/**
	 * Parse header data with metadata in NTriples format.
	 *
	 * @param is
	 * @param len
	 * @throws IOException
	 */
	private byte[] parseHeaderData(InputStream is, int len) throws IOException {
		byte[] b = new byte[len];
		is.read(b);
		return b;
	}
}