/*

 * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

package java.util.zip;

import java.nio.ByteBuffer;

import java.nio.ByteOrder;

import sun.misc.Unsafe;

import sun.nio.ch.DirectBuffer;

/**

 * A class that can be used to compute the CRC-32C of a data stream.

 *

 * <p>

 * CRC-32C is defined in <a href="http://www.ietf.org/rfc/rfc3720.txt">RFC

 * 3720</a>: Internet Small Computer Systems Interface (iSCSI).

 * </p>

 *

 * <p>

 * Passing a {@code null} argument to a method in this class will cause a

 * {@link NullPointerException} to be thrown.

 * </p>

 *

 * @since 1.9

 */

public final class CRC32C implements Checksum {

    /*

     * This CRC-32C implementation uses the 'slicing-by-8' algorithm described

     * in the paper "A Systematic Approach to Building High Performance

     * Software-Based CRC Generators" by Michael E. Kounavis and Frank L. Berry,

     * Intel Research and Development

     */

    /**

     * CRC-32C Polynomial

     */

    private static final int CRC32C_POLY = 0x1EDC6F41;

    private static final int REVERSED_CRC32C_POLY = Integer.reverse(CRC32C_POLY);

    private static final Unsafe UNSAFE = Unsafe.getUnsafe();

    // Lookup tables

    // Lookup table for single byte calculations

    private static final int[] byteTable;

    // Lookup tables for bulk operations in 'slicing-by-8' algorithm

    private static final int[][] byteTables = new int[8][256];

    private static final int[] byteTable0 = byteTables[0];

    private static final int[] byteTable1 = byteTables[1];

    private static final int[] byteTable2 = byteTables[2];

    private static final int[] byteTable3 = byteTables[3];

    private static final int[] byteTable4 = byteTables[4];

    private static final int[] byteTable5 = byteTables[5];

    private static final int[] byteTable6 = byteTables[6];

    private static final int[] byteTable7 = byteTables[7];

    static {

        // Generate lookup tables

        // High-order polynomial term stored in LSB of r.

        for (int index = 0; index < byteTables[0].length; index++) {

           int r = index;

            for (int i = 0; i < Byte.SIZE; i++) {

                if ((r & 1) != 0) {

                    r = (r >>> 1) ^ REVERSED_CRC32C_POLY;

                } else {

                    r >>>= 1;

                }

            }

            byteTables[0][index] = r;

        }

        for (int index = 0; index < byteTables[0].length; index++) {

            int r = byteTables[0][index];

            for (int k = 1; k < byteTables.length; k++) {

                r = byteTables[0][r & 0xFF] ^ (r >>> 8);

                byteTables[k][index] = r;

            }

        }

        if (ByteOrder.nativeOrder() == ByteOrder.LITTLE_ENDIAN) {

            byteTable = byteTables[0];

        } else { // ByteOrder.BIG_ENDIAN

            byteTable = new int[byteTable0.length];

            System.arraycopy(byteTable0, 0, byteTable, 0, byteTable0.length);

            for (int[] table : byteTables) {

                for (int index = 0; index < table.length; index++) {

                    table[index] = Integer.reverseBytes(table[index]);

                }

            }

        }

    }

    /**

     * Calculated CRC-32C value

     */

    private int crc = 0xFFFFFFFF;

    /**

     * Creates a new CRC32C object.

     */

    public CRC32C() {

    }

    /**

     * Updates the CRC-32C checksum with the specified byte (the low eight bits

     * of the argument b).

     */

    @Override

    public void update(int b) {

        crc = (crc >>> 8) ^ byteTable[(crc ^ (b & 0xFF)) & 0xFF];

    }

    /**

     * Updates the CRC-32C checksum with the specified array of bytes.

     *

     * @throws ArrayIndexOutOfBoundsException

     *         if {@code off} is negative, or {@code len} is negative, or

     *         {@code off+len} is negative or greater than the length of

     *         the array {@code b}.

     */

    @Override

    public void update(byte[] b, int off, int len) {

        if (b == null) {

            throw new NullPointerException();

        }

        if (off < 0 || len < 0 || off > b.length - len) {

            throw new ArrayIndexOutOfBoundsException();

        }

        crc = updateBytes(crc, b, off, (off + len));

    }

    /**

     * Updates the CRC-32C checksum with the bytes from the specified buffer.

     *

     * The checksum is updated with the remaining bytes in the buffer, starting

     * at the buffer's position. Upon return, the buffer's position will be

     * updated to its limit; its limit will not have been changed.

     */

    @Override

    public void update(ByteBuffer buffer) {

        int pos = buffer.position();

        int limit = buffer.limit();

        assert (pos <= limit);

        int rem = limit - pos;

        if (rem <= 0) {

            return;

        }

        if (buffer instanceof DirectBuffer) {

            crc = updateDirectByteBuffer(crc, ((DirectBuffer) buffer).address(),

                                         pos, limit);

        } else if (buffer.hasArray()) {

            crc = updateBytes(crc, buffer.array(), pos + buffer.arrayOffset(),

                              limit + buffer.arrayOffset());

        } else {

            byte[] b = new byte[Math.min(buffer.remaining(), 4096)];

            while (buffer.hasRemaining()) {

                int length = Math.min(buffer.remaining(), b.length);

                buffer.get(b, 0, length);

                update(b, 0, length);

            }

        }

        buffer.position(limit);

    }

    /**

     * Resets CRC-32C to initial value.

     */

    @Override

    public void reset() {

        crc = 0xFFFFFFFF;

    }

    /**

     * Returns CRC-32C value.

     */

    @Override

    public long getValue() {

        return (~crc) & 0xFFFFFFFFL;

    }

    /**

     * Updates the CRC-32C checksum with the specified array of bytes.

     */

    private static int updateBytes(int crc, byte[] b, int off, int end) {

        // Do only byte reads for arrays so short they can't be aligned

        // or if bytes are stored with a larger witdh than one byte.,%

        if (end - off >= 8 && Unsafe.ARRAY_BYTE_INDEX_SCALE == 1) {

            // align on 8 bytes

            int alignLength

                    = (8 - ((Unsafe.ARRAY_BYTE_BASE_OFFSET + off) & 0x7)) & 0x7;

            for (int alignEnd = off + alignLength; off < alignEnd; off++) {

                crc = (crc >>> 8) ^ byteTable[(crc ^ b[off]) & 0xFF];

            }

            if (ByteOrder.nativeOrder() == ByteOrder.BIG_ENDIAN) {

                crc = Integer.reverseBytes(crc);

            }

            // slicing-by-8

            for (; off < (end - Long.BYTES); off += Long.BYTES) {

                int firstHalf;

                int secondHalf;

                if (Unsafe.ADDRESS_SIZE == 4) {

                    // On 32 bit platforms read two ints instead of a single 64bit long

                    firstHalf = UNSAFE.getInt(b, Unsafe.ARRAY_BYTE_BASE_OFFSET + off);

                    secondHalf = UNSAFE.getInt(b, Unsafe.ARRAY_BYTE_BASE_OFFSET + off

                                               + Integer.BYTES);

                } else {

                    long value = UNSAFE.getLong(b, Unsafe.ARRAY_BYTE_BASE_OFFSET + off);

                    if (ByteOrder.nativeOrder() == ByteOrder.LITTLE_ENDIAN) {

                        firstHalf = (int) value;

                        secondHalf = (int) (value >>> 32);

                    } else { // ByteOrder.BIG_ENDIAN

                        firstHalf = (int) (value >>> 32);

                        secondHalf = (int) value;

                    }

                }

                crc ^= firstHalf;

                if (ByteOrder.nativeOrder() == ByteOrder.LITTLE_ENDIAN) {

                    crc = byteTable7[crc & 0xFF]

                            ^ byteTable6[(crc >>> 8) & 0xFF]

                            ^ byteTable5[(crc >>> 16) & 0xFF]

                            ^ byteTable4[crc >>> 24]

                            ^ byteTable3[secondHalf & 0xFF]

                            ^ byteTable2[(secondHalf >>> 8) & 0xFF]

                            ^ byteTable1[(secondHalf >>> 16) & 0xFF]

                            ^ byteTable0[secondHalf >>> 24];

                } else { // ByteOrder.BIG_ENDIAN

                    crc = byteTable0[secondHalf & 0xFF]

                            ^ byteTable1[(secondHalf >>> 8) & 0xFF]

                            ^ byteTable2[(secondHalf >>> 16) & 0xFF]

                            ^ byteTable3[secondHalf >>> 24]

                            ^ byteTable4[crc & 0xFF]

                            ^ byteTable5[(crc >>> 8) & 0xFF]

                            ^ byteTable6[(crc >>> 16) & 0xFF]

                            ^ byteTable7[crc >>> 24];

                }

            }

            if (ByteOrder.nativeOrder() == ByteOrder.BIG_ENDIAN) {

                crc = Integer.reverseBytes(crc);

            }

        }

        // Tail

        for (; off < end; off++) {

            crc = (crc >>> 8) ^ byteTable[(crc ^ b[off]) & 0xFF];

        }

        return crc;

    }

    /**

     * Updates the CRC-32C checksum reading from the specified address.

     */

    private static int updateDirectByteBuffer(int crc, long address,

                                              int off, int end) {

        // Do only byte reads for arrays so short they can't be aligned

        if (end - off >= 8) {

            // align on 8 bytes

            int alignLength = (8 - (int) ((address + off) & 0x7)) & 0x7;

            for (int alignEnd = off + alignLength; off < alignEnd; off++) {

                crc = (crc >>> 8)

                        ^ byteTable[(crc ^ UNSAFE.getByte(address + off)) & 0xFF];

            }

            if (ByteOrder.nativeOrder() == ByteOrder.BIG_ENDIAN) {

                crc = Integer.reverseBytes(crc);

            }

            // slicing-by-8

            for (; off <= (end - Long.BYTES); off += Long.BYTES) {

                // Always reading two ints as reading a long followed by

                // shifting and casting was slower.

                int firstHalf = UNSAFE.getInt(address + off);

                int secondHalf = UNSAFE.getInt(address + off + Integer.BYTES);

                crc ^= firstHalf;

                if (ByteOrder.nativeOrder() == ByteOrder.LITTLE_ENDIAN) {

                    crc = byteTable7[crc & 0xFF]

                            ^ byteTable6[(crc >>> 8) & 0xFF]

                            ^ byteTable5[(crc >>> 16) & 0xFF]

                            ^ byteTable4[crc >>> 24]

                            ^ byteTable3[secondHalf & 0xFF]

                            ^ byteTable2[(secondHalf >>> 8) & 0xFF]

                            ^ byteTable1[(secondHalf >>> 16) & 0xFF]

                            ^ byteTable0[secondHalf >>> 24];

                } else { // ByteOrder.BIG_ENDIAN

                    crc = byteTable0[secondHalf & 0xFF]

                            ^ byteTable1[(secondHalf >>> 8) & 0xFF]

                            ^ byteTable2[(secondHalf >>> 16) & 0xFF]

                            ^ byteTable3[secondHalf >>> 24]

                            ^ byteTable4[crc & 0xFF]

                            ^ byteTable5[(crc >>> 8) & 0xFF]

                            ^ byteTable6[(crc >>> 16) & 0xFF]

                            ^ byteTable7[crc >>> 24];

                }

            }

            if (ByteOrder.nativeOrder() == ByteOrder.BIG_ENDIAN) {

                crc = Integer.reverseBytes(crc);

            }

        }

        // Tail

        for (; off < end; off++) {

            crc = (crc >>> 8)

                    ^ byteTable[(crc ^ UNSAFE.getByte(address + off)) & 0xFF];

        }

        return crc;

    }

}