package FlexibleEncoding.Parquet;
/*
* adapted from Parquet*
*/
import java.io.IOException;
/**
* Encodes values using a combination of run length encoding and bit packing,
* according to the following grammar:
*
* <pre>
* {@code
* rle-bit-packed-hybrid: <length> <encoded-data>
* length := length of the <encoded-data> in bytes stored as 4 bytes little endian
* encoded-data := <run>*
* run := <bit-packed-run> | <rle-run>
* bit-packed-run := <bit-packed-header> <bit-packed-values>
* bit-packed-header := varint-encode(<bit-pack-count> << 1 | 1)
* // we always bit-pack a multiple of 8 values at a time, so we only store the number of values / 8
* bit-pack-count := (number of values in this run) / 8
* bit-packed-values := bit packed back to back, from LSB to MSB
* rle-run := <rle-header> <repeated-value>
* rle-header := varint-encode( (number of times repeated) << 1)
* repeated-value := value that is repeated, using a fixed-width of round-up-to-next-byte(bit-width)
* }
* </pre>
* NOTE: this class is only responsible for creating and returning the {@code <encoded-data>}
* portion of the above grammar. The {@code <length>} portion is done by
* {@link RunLengthBitPackingHybridValuesWriter}
* <p>
* Only supports values >= 0 // TODO: is that ok? Should we make a signed version?
*
* @author Alex Levenson
*/
public class RunLengthBitPackingHybridEncoder {
private static final Log LOG = Log.getLog(RunLengthBitPackingHybridEncoder.class);
private final BytePacker packer;
private final CapacityByteArrayOutputStream baos;
/**
* The bit width used for bit-packing and for writing
* the repeated-value
*/
private final int bitWidth;
/**
* Values that are bit packed 8 at at a time are packed into this
* buffer, which is then written to baos
*/
private final byte[] packBuffer;
/**
* Previous value written, used to detect repeated values
*/
private int previousValue;
/**
* We buffer 8 values at a time, and either bit pack them
* or discard them after writing a rle-run
*/
private final int[] bufferedValues;
private int numBufferedValues;
/**
* How many times a value has been repeated
*/
private int repeatCount;
/**
* How many groups of 8 values have been written
* to the current bit-packed-run
*/
private int bitPackedGroupCount;
/**
* A "pointer" to a single byte in baos,
* which we use as our bit-packed-header. It's really
* the logical index of the byte in baos.
*
* We are only using one byte for this header,
* which limits us to writing 504 values per bit-packed-run.
*
* MSB must be 0 for varint encoding, LSB must be 1 to signify
* that this is a bit-packed-header leaves 6 bits to write the
* number of 8-groups -> (2^6 - 1) * 8 = 504
*/
private long bitPackedRunHeaderPointer;
private boolean toBytesCalled;
public RunLengthBitPackingHybridEncoder(int bitWidth, int initialCapacity) {
if (Log.DEBUG) {
LOG.debug(String.format("Encoding: RunLengthBitPackingHybridEncoder with "
+ "bithWidth: %d initialCapacity %d", bitWidth, initialCapacity));
}
Preconditions.checkArgument(bitWidth >= 0 && bitWidth <= 32, "bitWidth must be >= 0 and <= 32");
this.bitWidth = bitWidth;
this.baos = new CapacityByteArrayOutputStream(initialCapacity);
this.packBuffer = new byte[bitWidth];
this.bufferedValues = new int[8];
this.packer = Packer.LITTLE_ENDIAN.newBytePacker(bitWidth);
reset(false);
}
private void reset(boolean resetBaos) {
if (resetBaos) {
this.baos.reset();
}
this.previousValue = 0;
this.numBufferedValues = 0;
this.repeatCount = 0;
this.bitPackedGroupCount = 0;
this.bitPackedRunHeaderPointer = -1;
this.toBytesCalled = false;
}
public void writeInt(int value) throws IOException {
if (value == previousValue) {
// keep track of how many times we've seen this value
// consecutively
++repeatCount;
if (repeatCount >= 8) {
// we've seen this at least 8 times, we're
// certainly going to write an rle-run,
// so just keep on counting repeats for now
return;
}
} else {
// This is a new value, check if it signals the end of
// an rle-run
if (repeatCount >= 8) {
// it does! write an rle-run
writeRleRun();
}
// this is a new value so we've only seen it once
repeatCount = 1;
// start tracking this value for repeats
previousValue = value;
}
// We have not seen enough repeats to justify an rle-run yet,
// so buffer this value in case we decide to write a bit-packed-run
bufferedValues[numBufferedValues] = value;
++numBufferedValues;
if (numBufferedValues == 8) {
// we've encountered less than 8 repeated values, so
// either start a new bit-packed-run or append to the
// current bit-packed-run
writeOrAppendBitPackedRun();
}
}
private void writeOrAppendBitPackedRun() throws IOException {
if (bitPackedGroupCount >= 63) {
// we've packed as many values as we can for this run,
// end it and start a new one
endPreviousBitPackedRun();
}
if (bitPackedRunHeaderPointer == -1) {
// this is a new bit-packed-run, allocate a byte for the header
// and keep a "pointer" to it so that it can be mutated later
baos.write(0); // write a sentinel value
bitPackedRunHeaderPointer = baos.getCurrentIndex();
}
packer.pack8Values(bufferedValues, 0, packBuffer, 0);
baos.write(packBuffer);
// empty the buffer, they've all been written
numBufferedValues = 0;
// clear the repeat count, as some repeated values
// may have just been bit packed into this run
repeatCount = 0;
++bitPackedGroupCount;
}
/**
* If we are currently writing a bit-packed-run, update the
* bit-packed-header and consider this run to be over
*
* does nothing if we're not currently writing a bit-packed run
*/
private void endPreviousBitPackedRun() {
if (bitPackedRunHeaderPointer == -1) {
// we're not currently in a bit-packed-run
return;
}
// create bit-packed-header, which needs to fit in 1 byte
byte bitPackHeader = (byte) ((bitPackedGroupCount << 1) | 1);
// update this byte
baos.setByte(bitPackedRunHeaderPointer, bitPackHeader);
// mark that this run is over
bitPackedRunHeaderPointer = -1;
// reset the number of groups
bitPackedGroupCount = 0;
}
private void writeRleRun() throws IOException {
// we may have been working on a bit-packed-run
// so close that run if it exists before writing this
// rle-run
endPreviousBitPackedRun();
// write the rle-header (lsb of 0 signifies a rle run)
BytesUtils.writeUnsignedVarInt(repeatCount << 1, baos);
// write the repeated-value
BytesUtils.writeIntLittleEndianPaddedOnBitWidth(baos, previousValue, bitWidth);
// reset the repeat count
repeatCount = 0;
// throw away all the buffered values, they were just repeats and they've been written
numBufferedValues = 0;
}
public BytesInput toBytes() throws IOException {
Preconditions.checkArgument(!toBytesCalled,
"You cannot call toBytes() more than once without calling reset()");
// write anything that is buffered / queued up for an rle-run
if (repeatCount >= 8) {
writeRleRun();
} else if(numBufferedValues > 0) {
for (int i = numBufferedValues; i < 8; i++) {
bufferedValues[i] = 0;
}
writeOrAppendBitPackedRun();
endPreviousBitPackedRun();
} else {
endPreviousBitPackedRun();
}
toBytesCalled = true;
return BytesInput.from(baos);
}
/**
* Reset this encoder for re-use
*/
public void reset() {
reset(true);
}
public long getBufferedSize() {
return baos.size();
}
public long getAllocatedSize() {
return baos.getCapacity();
}
}