Java example source code file (Snappy.java)
The Snappy.java Java example source code/*
* 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 {@link ByteBuf} encoded with Snappy compression into an
* output {@link ByteBuf}.
*
* See <a href="http://code.google.com/p/snappy/source/browse/trunk/format_description.txt">snappy format.
*/
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 = 32 - (int) Math.floor(Math.log(table.length) / Math.log(2));
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.
*
* @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
*
* @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;
}
short[] table;
if (htSize <= 256) {
table = new short[256];
} else {
table = new short[MAX_HT_SIZE];
}
return table;
}
/**
* 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.
*
* @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.
*
* @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.
*
* @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;
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;
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).
*
* @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.
*
* @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.readShortLE();
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.
*
* @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.
*
* @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.readShortLE();
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.
*
* @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 (4 <= offset <= 32768), 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 > Short.MAX_VALUE) {
throw new DecompressionException("Offset exceeds maximum permissible value");
}
if (offset <= 0) {
throw new DecompressionException("Offset is less than minimum permissible value");
}
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
*
* @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
*
* @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 {
if (data.hasArray()) {
crc32.update(data.array(), data.arrayOffset() + offset, length);
} else {
byte[] array = new byte[length];
data.getBytes(offset, array);
crc32.update(array, 0, 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.
*
* @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.
*
* @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)."
*
* @param checksum The actual checksum of the data
* @return The masked checksum
*/
static int maskChecksum(int checksum) {
return (checksum >> 15 | checksum << 17) + 0xa282ead8;
}
}
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