http://netty.io/netty-all/: Netty is an asynchronous event-driven network application framework for rapid development of maintainable high performance protocol servers and clients. (The Netty Project) 
/*
 * Copyright 2012 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */
package io.netty.handler.codec.compression;

import io.netty.buffer.ByteBuf;


Uncompresses an input ByteBuf encoded with Snappy compression into an output ByteBuf. See snappy format.

/**
 * Uncompresses an input {@link ByteBuf} encoded with Snappy compression into an
 * output {@link ByteBuf}.
 *
 * See <a href="https://github.com/google/snappy/blob/master/format_description.txt">snappy format</a>.
 */
public final class Snappy {

    private static final int MAX_HT_SIZE = 1 << 14;
    private static final int MIN_COMPRESSIBLE_BYTES = 15;

    // used as a return value to indicate that we haven't yet read our full preamble
    private static final int PREAMBLE_NOT_FULL = -1;
    private static final int NOT_ENOUGH_INPUT = -1;

    // constants for the tag types
    private static final int LITERAL = 0;
    private static final int COPY_1_BYTE_OFFSET = 1;
    private static final int COPY_2_BYTE_OFFSET = 2;
    private static final int COPY_4_BYTE_OFFSET = 3;

    private State state = State.READY;
    private byte tag;
    private int written;

    private enum State {
        READY,
        READING_PREAMBLE,
        READING_TAG,
        READING_LITERAL,
        READING_COPY
    }

    public void reset() {
        state = State.READY;
        tag = 0;
        written = 0;
    }

    public void encode(final ByteBuf in, final ByteBuf out, final int length) {
        // Write the preamble length to the output buffer
        for (int i = 0;; i ++) {
            int b = length >>> i * 7;
            if ((b & 0xFFFFFF80) != 0) {
                out.writeByte(b & 0x7f | 0x80);
            } else {
                out.writeByte(b);
                break;
            }
        }

        int inIndex = in.readerIndex();
        final int baseIndex = inIndex;

        final short[] table = getHashTable(length);
        final int shift = Integer.numberOfLeadingZeros(table.length) + 1;

        int nextEmit = inIndex;

        if (length - inIndex >= MIN_COMPRESSIBLE_BYTES) {
            int nextHash = hash(in, ++inIndex, shift);
            outer: while (true) {
                int skip = 32;

                int candidate;
                int nextIndex = inIndex;
                do {
                    inIndex = nextIndex;
                    int hash = nextHash;
                    int bytesBetweenHashLookups = skip++ >> 5;
                    nextIndex = inIndex + bytesBetweenHashLookups;

                    // We need at least 4 remaining bytes to read the hash
                    if (nextIndex > length - 4) {
                        break outer;
                    }

                    nextHash = hash(in, nextIndex, shift);

                    candidate = baseIndex + table[hash];

                    table[hash] = (short) (inIndex - baseIndex);
                }
                while (in.getInt(inIndex) != in.getInt(candidate));

                encodeLiteral(in, out, inIndex - nextEmit);

                int insertTail;
                do {
                    int base = inIndex;
                    int matched = 4 + findMatchingLength(in, candidate + 4, inIndex + 4, length);
                    inIndex += matched;
                    int offset = base - candidate;
                    encodeCopy(out, offset, matched);
                    in.readerIndex(in.readerIndex() + matched);
                    insertTail = inIndex - 1;
                    nextEmit = inIndex;
                    if (inIndex >= length - 4) {
                        break outer;
                    }

                    int prevHash = hash(in, insertTail, shift);
                    table[prevHash] = (short) (inIndex - baseIndex - 1);
                    int currentHash = hash(in, insertTail + 1, shift);
                    candidate = baseIndex + table[currentHash];
                    table[currentHash] = (short) (inIndex - baseIndex);
                }
                while (in.getInt(insertTail + 1) == in.getInt(candidate));

                nextHash = hash(in, insertTail + 2, shift);
                ++inIndex;
            }
        }

        // If there are any remaining characters, write them out as a literal
        if (nextEmit < length) {
            encodeLiteral(in, out, length - nextEmit);
        }
    }

    
Hashes the 4 bytes located at index, shifting the resulting hash into
the appropriate range for our hash table.

Params:
  • in – The input buffer to read 4 bytes from
  • index – The index to read at
  • shift – The shift value, for ensuring that the resulting value is
    withing the range of our hash table size
Returns:A 32-bit hash of 4 bytes located at index
/**
     * Hashes the 4 bytes located at index, shifting the resulting hash into
     * the appropriate range for our hash table.
     *
     * @param in The input buffer to read 4 bytes from
     * @param index The index to read at
     * @param shift The shift value, for ensuring that the resulting value is
     *     withing the range of our hash table size
     * @return A 32-bit hash of 4 bytes located at index
     */
    private static int hash(ByteBuf in, int index, int shift) {
        return in.getInt(index) * 0x1e35a7bd >>> shift;
    }

    
Creates an appropriately sized hashtable for the given input size

Params:
  • inputSize – The size of our input, ie. the number of bytes we need to encode
Returns:An appropriately sized empty hashtable
/**
     * Creates an appropriately sized hashtable for the given input size
     *
     * @param inputSize The size of our input, ie. the number of bytes we need to encode
     * @return An appropriately sized empty hashtable
     */
    private static short[] getHashTable(int inputSize) {
        int htSize = 256;
        while (htSize < MAX_HT_SIZE && htSize < inputSize) {
            htSize <<= 1;
        }
        return new short[htSize];
    }

    
Iterates over the supplied input buffer between the supplied minIndex and
maxIndex to find how long our matched copy overlaps with an already-written
literal value.

Params:
  • in – The input buffer to scan over
  • minIndex – The index in the input buffer to start scanning from
  • inIndex – The index of the start of our copy
  • maxIndex – The length of our input buffer
Returns:The number of bytes for which our candidate copy is a repeat of
/**
     * Iterates over the supplied input buffer between the supplied minIndex and
     * maxIndex to find how long our matched copy overlaps with an already-written
     * literal value.
     *
     * @param in The input buffer to scan over
     * @param minIndex The index in the input buffer to start scanning from
     * @param inIndex The index of the start of our copy
     * @param maxIndex The length of our input buffer
     * @return The number of bytes for which our candidate copy is a repeat of
     */
    private static int findMatchingLength(ByteBuf in, int minIndex, int inIndex, int maxIndex) {
        int matched = 0;

        while (inIndex <= maxIndex - 4 &&
                in.getInt(inIndex) == in.getInt(minIndex + matched)) {
            inIndex += 4;
            matched += 4;
        }

        while (inIndex < maxIndex && in.getByte(minIndex + matched) == in.getByte(inIndex)) {
            ++inIndex;
            ++matched;
        }

        return matched;
    }

    
Calculates the minimum number of bits required to encode a value.  This can
then in turn be used to calculate the number of septets or octets (as
appropriate) to use to encode a length parameter.

Params:
  • value – The value to calculate the minimum number of bits required to encode
Returns:The minimum number of bits required to encode the supplied value
/**
     * Calculates the minimum number of bits required to encode a value.  This can
     * then in turn be used to calculate the number of septets or octets (as
     * appropriate) to use to encode a length parameter.
     *
     * @param value The value to calculate the minimum number of bits required to encode
     * @return The minimum number of bits required to encode the supplied value
     */
    private static int bitsToEncode(int value) {
        int highestOneBit = Integer.highestOneBit(value);
        int bitLength = 0;
        while ((highestOneBit >>= 1) != 0) {
            bitLength++;
        }

        return bitLength;
    }

    
Writes a literal to the supplied output buffer by directly copying from
the input buffer.  The literal is taken from the current readerIndex
up to the supplied length.

Params:
  • in – The input buffer to copy from
  • out – The output buffer to copy to
  • length – The length of the literal to copy
/**
     * Writes a literal to the supplied output buffer by directly copying from
     * the input buffer.  The literal is taken from the current readerIndex
     * up to the supplied length.
     *
     * @param in The input buffer to copy from
     * @param out The output buffer to copy to
     * @param length The length of the literal to copy
     */
    static void encodeLiteral(ByteBuf in, ByteBuf out, int length) {
        if (length < 61) {
            out.writeByte(length - 1 << 2);
        } else {
            int bitLength = bitsToEncode(length - 1);
            int bytesToEncode = 1 + bitLength / 8;
            out.writeByte(59 + bytesToEncode << 2);
            for (int i = 0; i < bytesToEncode; i++) {
                out.writeByte(length - 1 >> i * 8 & 0x0ff);
            }
        }

        out.writeBytes(in, length);
    }

    private static void encodeCopyWithOffset(ByteBuf out, int offset, int length) {
        if (length < 12 && offset < 2048) {
            out.writeByte(COPY_1_BYTE_OFFSET | length - 4 << 2 | offset >> 8 << 5);
            out.writeByte(offset & 0x0ff);
        } else {
            out.writeByte(COPY_2_BYTE_OFFSET | length - 1 << 2);
            out.writeByte(offset & 0x0ff);
            out.writeByte(offset >> 8 & 0x0ff);
        }
    }

    
Encodes a series of copies, each at most 64 bytes in length.

Params:
  • out – The output buffer to write the copy pointer to
  • offset – The offset at which the original instance lies
  • length – The length of the original instance
/**
     * Encodes a series of copies, each at most 64 bytes in length.
     *
     * @param out The output buffer to write the copy pointer to
     * @param offset The offset at which the original instance lies
     * @param length The length of the original instance
     */
    private static void encodeCopy(ByteBuf out, int offset, int length) {
        while (length >= 68) {
            encodeCopyWithOffset(out, offset, 64);
            length -= 64;
        }

        if (length > 64) {
            encodeCopyWithOffset(out, offset, 60);
            length -= 60;
        }

        encodeCopyWithOffset(out, offset, length);
    }

    public void decode(ByteBuf in, ByteBuf out) {
        while (in.isReadable()) {
            switch (state) {
            case READY:
                state = State.READING_PREAMBLE;
                // fall through
            case READING_PREAMBLE:
                int uncompressedLength = readPreamble(in);
                if (uncompressedLength == PREAMBLE_NOT_FULL) {
                    // We've not yet read all of the preamble, so wait until we can
                    return;
                }
                if (uncompressedLength == 0) {
                    // Should never happen, but it does mean we have nothing further to do
                    state = State.READY;
                    return;
                }
                out.ensureWritable(uncompressedLength);
                state = State.READING_TAG;
                // fall through
            case READING_TAG:
                if (!in.isReadable()) {
                    return;
                }
                tag = in.readByte();
                switch (tag & 0x03) {
                case LITERAL:
                    state = State.READING_LITERAL;
                    break;
                case COPY_1_BYTE_OFFSET:
                case COPY_2_BYTE_OFFSET:
                case COPY_4_BYTE_OFFSET:
                    state = State.READING_COPY;
                    break;
                }
                break;
            case READING_LITERAL:
                int literalWritten = decodeLiteral(tag, in, out);
                if (literalWritten != NOT_ENOUGH_INPUT) {
                    state = State.READING_TAG;
                    written += literalWritten;
                } else {
                    // Need to wait for more data
                    return;
                }
                break;
            case READING_COPY:
                int decodeWritten;
                switch (tag & 0x03) {
                case COPY_1_BYTE_OFFSET:
                    decodeWritten = decodeCopyWith1ByteOffset(tag, in, out, written);
                    if (decodeWritten != NOT_ENOUGH_INPUT) {
                        state = State.READING_TAG;
                        written += decodeWritten;
                    } else {
                        // Need to wait for more data
                        return;
                    }
                    break;
                case COPY_2_BYTE_OFFSET:
                    decodeWritten = decodeCopyWith2ByteOffset(tag, in, out, written);
                    if (decodeWritten != NOT_ENOUGH_INPUT) {
                        state = State.READING_TAG;
                        written += decodeWritten;
                    } else {
                        // Need to wait for more data
                        return;
                    }
                    break;
                case COPY_4_BYTE_OFFSET:
                    decodeWritten = decodeCopyWith4ByteOffset(tag, in, out, written);
                    if (decodeWritten != NOT_ENOUGH_INPUT) {
                        state = State.READING_TAG;
                        written += decodeWritten;
                    } else {
                        // Need to wait for more data
                        return;
                    }
                    break;
                }
            }
        }
    }

    
Reads the length varint (a series of bytes, where the lower 7 bits
are data and the upper bit is a flag to indicate more bytes to be
read).

Params:
  • in – The input buffer to read the preamble from
Returns:The calculated length based on the input buffer, or 0 if
  no preamble is able to be calculated
/**
     * Reads the length varint (a series of bytes, where the lower 7 bits
     * are data and the upper bit is a flag to indicate more bytes to be
     * read).
     *
     * @param in The input buffer to read the preamble from
     * @return The calculated length based on the input buffer, or 0 if
     *   no preamble is able to be calculated
     */
    private static int readPreamble(ByteBuf in) {
        int length = 0;
        int byteIndex = 0;
        while (in.isReadable()) {
            int current = in.readUnsignedByte();
            length |= (current & 0x7f) << byteIndex++ * 7;
            if ((current & 0x80) == 0) {
                return length;
            }

            if (byteIndex >= 4) {
                throw new DecompressionException("Preamble is greater than 4 bytes");
            }
        }

        return 0;
    }

    
Reads a literal from the input buffer directly to the output buffer.
A "literal" is an uncompressed segment of data stored directly in the
byte stream.

Params:
  • tag – The tag that identified this segment as a literal is also
           used to encode part of the length of the data
  • in – The input buffer to read the literal from
  • out – The output buffer to write the literal to
Returns:The number of bytes appended to the output buffer, or -1 to indicate "try again later"
/**
     * Reads a literal from the input buffer directly to the output buffer.
     * A "literal" is an uncompressed segment of data stored directly in the
     * byte stream.
     *
     * @param tag The tag that identified this segment as a literal is also
     *            used to encode part of the length of the data
     * @param in The input buffer to read the literal from
     * @param out The output buffer to write the literal to
     * @return The number of bytes appended to the output buffer, or -1 to indicate "try again later"
     */
    static int decodeLiteral(byte tag, ByteBuf in, ByteBuf out) {
        in.markReaderIndex();
        int length;
        switch(tag >> 2 & 0x3F) {
        case 60:
            if (!in.isReadable()) {
                return NOT_ENOUGH_INPUT;
            }
            length = in.readUnsignedByte();
            break;
        case 61:
            if (in.readableBytes() < 2) {
                return NOT_ENOUGH_INPUT;
            }
            length = in.readUnsignedShortLE();
            break;
        case 62:
            if (in.readableBytes() < 3) {
                return NOT_ENOUGH_INPUT;
            }
            length = in.readUnsignedMediumLE();
            break;
        case 63:
            if (in.readableBytes() < 4) {
                return NOT_ENOUGH_INPUT;
            }
            length = in.readIntLE();
            break;
        default:
            length = tag >> 2 & 0x3F;
        }
        length += 1;

        if (in.readableBytes() < length) {
            in.resetReaderIndex();
            return NOT_ENOUGH_INPUT;
        }

        out.writeBytes(in, length);
        return length;
    }

    
Reads a compressed reference offset and length from the supplied input
buffer, seeks back to the appropriate place in the input buffer and
writes the found data to the supplied output stream.

Params:
  • tag – The tag used to identify this as a copy is also used to encode
    the length and part of the offset
  • in – The input buffer to read from
  • out – The output buffer to write to
Throws:
Returns:The number of bytes appended to the output buffer, or -1 to indicate
    "try again later"
/**
     * Reads a compressed reference offset and length from the supplied input
     * buffer, seeks back to the appropriate place in the input buffer and
     * writes the found data to the supplied output stream.
     *
     * @param tag The tag used to identify this as a copy is also used to encode
     *     the length and part of the offset
     * @param in The input buffer to read from
     * @param out The output buffer to write to
     * @return The number of bytes appended to the output buffer, or -1 to indicate
     *     "try again later"
     * @throws DecompressionException If the read offset is invalid
     */
    private static int decodeCopyWith1ByteOffset(byte tag, ByteBuf in, ByteBuf out, int writtenSoFar) {
        if (!in.isReadable()) {
            return NOT_ENOUGH_INPUT;
        }

        int initialIndex = out.writerIndex();
        int length = 4 + ((tag & 0x01c) >> 2);
        int offset = (tag & 0x0e0) << 8 >> 5 | in.readUnsignedByte();

        validateOffset(offset, writtenSoFar);

        out.markReaderIndex();
        if (offset < length) {
            int copies = length / offset;
            for (; copies > 0; copies--) {
                out.readerIndex(initialIndex - offset);
                out.readBytes(out, offset);
            }
            if (length % offset != 0) {
                out.readerIndex(initialIndex - offset);
                out.readBytes(out, length % offset);
            }
        } else {
            out.readerIndex(initialIndex - offset);
            out.readBytes(out, length);
        }
        out.resetReaderIndex();

        return length;
    }

    
Reads a compressed reference offset and length from the supplied input
buffer, seeks back to the appropriate place in the input buffer and
writes the found data to the supplied output stream.

Params:
  • tag – The tag used to identify this as a copy is also used to encode
    the length and part of the offset
  • in – The input buffer to read from
  • out – The output buffer to write to
Throws:
Returns:The number of bytes appended to the output buffer, or -1 to indicate
    "try again later"
/**
     * Reads a compressed reference offset and length from the supplied input
     * buffer, seeks back to the appropriate place in the input buffer and
     * writes the found data to the supplied output stream.
     *
     * @param tag The tag used to identify this as a copy is also used to encode
     *     the length and part of the offset
     * @param in The input buffer to read from
     * @param out The output buffer to write to
     * @throws DecompressionException If the read offset is invalid
     * @return The number of bytes appended to the output buffer, or -1 to indicate
     *     "try again later"
     */
    private static int decodeCopyWith2ByteOffset(byte tag, ByteBuf in, ByteBuf out, int writtenSoFar) {
        if (in.readableBytes() < 2) {
            return NOT_ENOUGH_INPUT;
        }

        int initialIndex = out.writerIndex();
        int length = 1 + (tag >> 2 & 0x03f);
        int offset = in.readUnsignedShortLE();

        validateOffset(offset, writtenSoFar);

        out.markReaderIndex();
        if (offset < length) {
            int copies = length / offset;
            for (; copies > 0; copies--) {
                out.readerIndex(initialIndex - offset);
                out.readBytes(out, offset);
            }
            if (length % offset != 0) {
                out.readerIndex(initialIndex - offset);
                out.readBytes(out, length % offset);
            }
        } else {
            out.readerIndex(initialIndex - offset);
            out.readBytes(out, length);
        }
        out.resetReaderIndex();

        return length;
    }

    
Reads a compressed reference offset and length from the supplied input
buffer, seeks back to the appropriate place in the input buffer and
writes the found data to the supplied output stream.

Params:
  • tag – The tag used to identify this as a copy is also used to encode
    the length and part of the offset
  • in – The input buffer to read from
  • out – The output buffer to write to
Throws:
Returns:The number of bytes appended to the output buffer, or -1 to indicate
    "try again later"
/**
     * Reads a compressed reference offset and length from the supplied input
     * buffer, seeks back to the appropriate place in the input buffer and
     * writes the found data to the supplied output stream.
     *
     * @param tag The tag used to identify this as a copy is also used to encode
     *     the length and part of the offset
     * @param in The input buffer to read from
     * @param out The output buffer to write to
     * @return The number of bytes appended to the output buffer, or -1 to indicate
     *     "try again later"
     * @throws DecompressionException If the read offset is invalid
     */
    private static int decodeCopyWith4ByteOffset(byte tag, ByteBuf in, ByteBuf out, int writtenSoFar) {
        if (in.readableBytes() < 4) {
            return NOT_ENOUGH_INPUT;
        }

        int initialIndex = out.writerIndex();
        int length = 1 + (tag >> 2 & 0x03F);
        int offset = in.readIntLE();

        validateOffset(offset, writtenSoFar);

        out.markReaderIndex();
        if (offset < length) {
            int copies = length / offset;
            for (; copies > 0; copies--) {
                out.readerIndex(initialIndex - offset);
                out.readBytes(out, offset);
            }
            if (length % offset != 0) {
                out.readerIndex(initialIndex - offset);
                out.readBytes(out, length % offset);
            }
        } else {
            out.readerIndex(initialIndex - offset);
            out.readBytes(out, length);
        }
        out.resetReaderIndex();

        return length;
    }

    
Validates that the offset extracted from a compressed reference is within
the permissible bounds of an offset (0 < offset < Integer.MAX_VALUE), and does not
exceed the length of the chunk currently read so far.

Params:
  • offset – The offset extracted from the compressed reference
  • chunkSizeSoFar – The number of bytes read so far from this chunk
Throws:
/**
     * Validates that the offset extracted from a compressed reference is within
     * the permissible bounds of an offset (0 < offset < Integer.MAX_VALUE), and does not
     * exceed the length of the chunk currently read so far.
     *
     * @param offset The offset extracted from the compressed reference
     * @param chunkSizeSoFar The number of bytes read so far from this chunk
     * @throws DecompressionException if the offset is invalid
     */
    private static void validateOffset(int offset, int chunkSizeSoFar) {
        if (offset == 0) {
            throw new DecompressionException("Offset is less than minimum permissible value");
        }

        if (offset < 0) {
            // Due to arithmetic overflow
            throw new DecompressionException("Offset is greater than maximum value supported by this implementation");
        }

        if (offset > chunkSizeSoFar) {
            throw new DecompressionException("Offset exceeds size of chunk");
        }
    }

    
Computes the CRC32C checksum of the supplied data and performs the "mask" operation
on the computed checksum

Params:
  • data – The input data to calculate the CRC32C checksum of
/**
     * Computes the CRC32C checksum of the supplied data and performs the "mask" operation
     * on the computed checksum
     *
     * @param data The input data to calculate the CRC32C checksum of
     */
    static int calculateChecksum(ByteBuf data) {
        return calculateChecksum(data, data.readerIndex(), data.readableBytes());
    }

    
Computes the CRC32C checksum of the supplied data and performs the "mask" operation
on the computed checksum

Params:
  • data – The input data to calculate the CRC32C checksum of
/**
     * Computes the CRC32C checksum of the supplied data and performs the "mask" operation
     * on the computed checksum
     *
     * @param data The input data to calculate the CRC32C checksum of
     */
    static int calculateChecksum(ByteBuf data, int offset, int length) {
        Crc32c crc32 = new Crc32c();
        try {
            crc32.update(data, offset, length);
            return maskChecksum((int) crc32.getValue());
        } finally {
            crc32.reset();
        }
    }

    
Computes the CRC32C checksum of the supplied data, performs the "mask" operation
on the computed checksum, and then compares the resulting masked checksum to the
supplied checksum.

Params:
  • expectedChecksum – The checksum decoded from the stream to compare against
  • data – The input data to calculate the CRC32C checksum of
Throws:
/**
     * Computes the CRC32C checksum of the supplied data, performs the "mask" operation
     * on the computed checksum, and then compares the resulting masked checksum to the
     * supplied checksum.
     *
     * @param expectedChecksum The checksum decoded from the stream to compare against
     * @param data The input data to calculate the CRC32C checksum of
     * @throws DecompressionException If the calculated and supplied checksums do not match
     */
    static void validateChecksum(int expectedChecksum, ByteBuf data) {
        validateChecksum(expectedChecksum, data, data.readerIndex(), data.readableBytes());
    }

    
Computes the CRC32C checksum of the supplied data, performs the "mask" operation
on the computed checksum, and then compares the resulting masked checksum to the
supplied checksum.

Params:
  • expectedChecksum – The checksum decoded from the stream to compare against
  • data – The input data to calculate the CRC32C checksum of
Throws:
/**
     * Computes the CRC32C checksum of the supplied data, performs the "mask" operation
     * on the computed checksum, and then compares the resulting masked checksum to the
     * supplied checksum.
     *
     * @param expectedChecksum The checksum decoded from the stream to compare against
     * @param data The input data to calculate the CRC32C checksum of
     * @throws DecompressionException If the calculated and supplied checksums do not match
     */
    static void validateChecksum(int expectedChecksum, ByteBuf data, int offset, int length) {
        final int actualChecksum = calculateChecksum(data, offset, length);
        if (actualChecksum != expectedChecksum) {
            throw new DecompressionException(
                    "mismatching checksum: " + Integer.toHexString(actualChecksum) +
                            " (expected: " + Integer.toHexString(expectedChecksum) + ')');
        }
    }

    
From the spec:
"Checksums are not stored directly, but masked, as checksumming data and
then its own checksum can be problematic. The masking is the same as used
in Apache Hadoop: Rotate the checksum by 15 bits, then add the constant
0xa282ead8 (using wraparound as normal for unsigned integers)."

Params:
  • checksum – The actual checksum of the data
Returns:The masked checksum
/**
     * From the spec:
     *
     * "Checksums are not stored directly, but masked, as checksumming data and
     * then its own checksum can be problematic. The masking is the same as used
     * in Apache Hadoop: Rotate the checksum by 15 bits, then add the constant
     * 0xa282ead8 (using wraparound as normal for unsigned integers)."
     *
     * @param checksum The actual checksum of the data
     * @return The masked checksum
     */
    static int maskChecksum(int checksum) {
        return (checksum >> 15 | checksum << 17) + 0xa282ead8;
    }
}