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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.util.zip.Checksum;

/**
 * CRC16-ANSI checksum
 *
 * @author Bart Hanssens
 */
class CRC16 implements Checksum {
	// note that Java considers short to be signed, hence the need to cast values larger than 32767
	private final static short[] TABLE = new short[] {
			(short) 0x0000, (short) 0xC0C1, (short) 0xC181, (short) 0x0140, (short) 0xC301, (short) 0x03C0,
			(short) 0x0280, (short) 0xC241,
			(short) 0xC601, (short) 0x06C0, (short) 0x0780, (short) 0xC741, (short) 0x0500, (short) 0xC5C1,
			(short) 0xC481, (short) 0x0440,
			(short) 0xCC01, (short) 0x0CC0, (short) 0x0D80, (short) 0xCD41, (short) 0x0F00, (short) 0xCFC1,
			(short) 0xCE81, (short) 0x0E40,
			(short) 0x0A00, (short) 0xCAC1, (short) 0xCB81, (short) 0x0B40, (short) 0xC901, (short) 0x09C0,
			(short) 0x0880, (short) 0xC841,
			(short) 0xD801, (short) 0x18C0, (short) 0x1980, (short) 0xD941, (short) 0x1B00, (short) 0xDBC1,
			(short) 0xDA81, (short) 0x1A40,
			(short) 0x1E00, (short) 0xDEC1, (short) 0xDF81, (short) 0x1F40, (short) 0xDD01, (short) 0x1DC0,
			(short) 0x1C80, (short) 0xDC41,
			(short) 0x1400, (short) 0xD4C1, (short) 0xD581, (short) 0x1540, (short) 0xD701, (short) 0x17C0,
			(short) 0x1680, (short) 0xD641,
			(short) 0xD201, (short) 0x12C0, (short) 0x1380, (short) 0xD341, (short) 0x1100, (short) 0xD1C1,
			(short) 0xD081, (short) 0x1040,
			(short) 0xF001, (short) 0x30C0, (short) 0x3180, (short) 0xF141, (short) 0x3300, (short) 0xF3C1,
			(short) 0xF281, (short) 0x3240,
			(short) 0x3600, (short) 0xF6C1, (short) 0xF781, (short) 0x3740, (short) 0xF501, (short) 0x35C0,
			(short) 0x3480, (short) 0xF441,
			(short) 0x3C00, (short) 0xFCC1, (short) 0xFD81, (short) 0x3D40, (short) 0xFF01, (short) 0x3FC0,
			(short) 0x3E80, (short) 0xFE41,
			(short) 0xFA01, (short) 0x3AC0, (short) 0x3B80, (short) 0xFB41, (short) 0x3900, (short) 0xF9C1,
			(short) 0xF881, (short) 0x3840,
			(short) 0x2800, (short) 0xE8C1, (short) 0xE981, (short) 0x2940, (short) 0xEB01, (short) 0x2BC0,
			(short) 0x2A80, (short) 0xEA41,
			(short) 0xEE01, (short) 0x2EC0, (short) 0x2F80, (short) 0xEF41, (short) 0x2D00, (short) 0xEDC1,
			(short) 0xEC81, (short) 0x2C40,
			(short) 0xE401, (short) 0x24C0, (short) 0x2580, (short) 0xE541, (short) 0x2700, (short) 0xE7C1,
			(short) 0xE681, (short) 0x2640,
			(short) 0x2200, (short) 0xE2C1, (short) 0xE381, (short) 0x2340, (short) 0xE101, (short) 0x21C0,
			(short) 0x2080, (short) 0xE041,
			(short) 0xA001, (short) 0x60C0, (short) 0x6180, (short) 0xA141, (short) 0x6300, (short) 0xA3C1,
			(short) 0xA281, (short) 0x6240,
			(short) 0x6600, (short) 0xA6C1, (short) 0xA781, (short) 0x6740, (short) 0xA501, (short) 0x65C0,
			(short) 0x6480, (short) 0xA441,
			(short) 0x6C00, (short) 0xACC1, (short) 0xAD81, (short) 0x6D40, (short) 0xAF01, (short) 0x6FC0,
			(short) 0x6E80, (short) 0xAE41,
			(short) 0xAA01, (short) 0x6AC0, (short) 0x6B80, (short) 0xAB41, (short) 0x6900, (short) 0xA9C1,
			(short) 0xA881, (short) 0x6840,
			(short) 0x7800, (short) 0xB8C1, (short) 0xB981, (short) 0x7940, (short) 0xBB01, (short) 0x7BC0,
			(short) 0x7A80, (short) 0xBA41,
			(short) 0xBE01, (short) 0x7EC0, (short) 0x7F80, (short) 0xBF41, (short) 0x7D00, (short) 0xBDC1,
			(short) 0xBC81, (short) 0x7C40,
			(short) 0xB401, (short) 0x74C0, (short) 0x7580, (short) 0xB541, (short) 0x7700, (short) 0xB7C1,
			(short) 0xB681, (short) 0x7640,
			(short) 0x7200, (short) 0xB2C1, (short) 0xB381, (short) 0x7340, (short) 0xB101, (short) 0x71C0,
			(short) 0x7080, (short) 0xB041,
			(short) 0x5000, (short) 0x90C1, (short) 0x9181, (short) 0x5140, (short) 0x9301, (short) 0x53C0,
			(short) 0x5280, (short) 0x9241,
			(short) 0x9601, (short) 0x56C0, (short) 0x5780, (short) 0x9741, (short) 0x5500, (short) 0x95C1,
			(short) 0x9481, (short) 0x5440,
			(short) 0x9C01, (short) 0x5CC0, (short) 0x5D80, (short) 0x9D41, (short) 0x5F00, (short) 0x9FC1,
			(short) 0x9E81, (short) 0x5E40,
			(short) 0x5A00, (short) 0x9AC1, (short) 0x9B81, (short) 0x5B40, (short) 0x9901, (short) 0x59C0,
			(short) 0x5880, (short) 0x9841,
			(short) 0x8801, (short) 0x48C0, (short) 0x4980, (short) 0x8941, (short) 0x4B00, (short) 0x8BC1,
			(short) 0x8A81, (short) 0x4A40,
			(short) 0x4E00, (short) 0x8EC1, (short) 0x8F81, (short) 0x4F40, (short) 0x8D01, (short) 0x4DC0,
			(short) 0x4C80, (short) 0x8C41,
			(short) 0x4400, (short) 0x84C1, (short) 0x8581, (short) 0x4540, (short) 0x8701, (short) 0x47C0,
			(short) 0x4680, (short) 0x8641,
			(short) 0x8201, (short) 0x42C0, (short) 0x4380, (short) 0x8341, (short) 0x4100, (short) 0x81C1,
			(short) 0x8081, (short) 0x4040
	};

	private long value = 0;

	@Override
	public void update(int b) {
		int j = (int) (value ^ b) & 0xFF;
		value = ((value >>> 8) ^ TABLE[j]) & 0xFFFF;
	}

	@Override
	public void update(byte[] b, int off, int len) {
		while (len-- > 0) {
			int j = (int) (value ^ b[off++]) & 0xFF;
			value = (TABLE[j] ^ value >>> 8) & 0xFFFF;
		}
	}

	@Override
	public long getValue() {
		return value & 0xFFFF;
	}

	@Override
	public void reset() {
		value = 0;
	}
}