# CRC64.java

    /*******************************************************************************
     * Copyright (c) 2009, 2013 Mountainminds GmbH & Co. KG and Contributors
     * All rights reserved. This program and the accompanying materials
     * are made available under the terms of the Eclipse Public License v1.0
     * which accompanies this distribution, and is available at
     * http://www.eclipse.org/legal/epl-v10.html
     *
     * Contributors:
     *    Marc R. Hoffmann - initial API and implementation
     *
     *******************************************************************************/
    package org.jacoco.core.internal.data;

    /**
     * CRC64 checksum calculator based on the polynom specified in ISO 3309. The
     * implementation is based on the following publications:
     *
     * <ul>
     * <li>http://en.wikipedia.org/wiki/Cyclic_redundancy_check</li>
     * <li>http://www.geocities.com/SiliconValley/Pines/8659/crc.htm</li>
     * </ul>
     */
    public final class CRC64 {

    	private static final long POLY64REV = 0xd800000000000000L;

    	private static final long[] LOOKUPTABLE;

    	static {
    		LOOKUPTABLE = new long[0x100];
    		for (int i = 0; i < 0x100; i++) {
    			long v = i;
    			for (int j = 0; j < 8; j++) {
    				if ((v & 1) == 1) {
    					v = (v >>> 1) ^ POLY64REV;
    				} else {
    					v = (v >>> 1);
    				}
    			}
    			LOOKUPTABLE[i] = v;
    		}
    	}

    	/**
    	 * Calculates the CRC64 checksum for the given data array.
    	 *
    	 * @param data
    	 *            data to calculate checksum for
    	 * @return checksum value
    	 */
    	public static long checksum(final byte[] data) {
    		long sum = 0;
    		for (final byte b : data) {
    			final int lookupidx = ((int) sum ^ b) & 0xff;
    			sum = (sum >>> 8) ^ LOOKUPTABLE[lookupidx];
    		}
    		return sum;
    	}

    	private CRC64() {
    	}

    }