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/*
* Licensed 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 ai.vespa.airlift.zstd;
import java.util.Arrays;
import static ai.vespa.airlift.zstd.Huffman.MAX_FSE_TABLE_LOG;
import static ai.vespa.airlift.zstd.Huffman.MAX_SYMBOL;
import static ai.vespa.airlift.zstd.Huffman.MAX_SYMBOL_COUNT;
import static ai.vespa.airlift.zstd.Huffman.MAX_TABLE_LOG;
import static ai.vespa.airlift.zstd.Huffman.MIN_TABLE_LOG;
import static ai.vespa.airlift.zstd.UnsafeUtil.UNSAFE;
import static ai.vespa.airlift.zstd.Util.checkArgument;
import static ai.vespa.airlift.zstd.Util.minTableLog;
final class HuffmanCompressionTable
{
private final short[] values;
private final byte[] numberOfBits;
private int maxSymbol;
private int maxNumberOfBits;
public HuffmanCompressionTable(int capacity)
{
this.values = new short[capacity];
this.numberOfBits = new byte[capacity];
}
public static int optimalNumberOfBits(int maxNumberOfBits, int inputSize, int maxSymbol)
{
if (inputSize <= 1) {
throw new IllegalArgumentException(); // not supported. Use RLE instead
}
int result = maxNumberOfBits;
result = Math.min(result, Util.highestBit((inputSize - 1)) - 1); // we may be able to reduce accuracy if input is small
// Need a minimum to safely represent all symbol values
result = Math.max(result, minTableLog(inputSize, maxSymbol));
result = Math.max(result, MIN_TABLE_LOG); // absolute minimum for Huffman
result = Math.min(result, MAX_TABLE_LOG); // absolute maximum for Huffman
return result;
}
public void initialize(int[] counts, int maxSymbol, int maxNumberOfBits, HuffmanCompressionTableWorkspace workspace)
{
checkArgument(maxSymbol <= MAX_SYMBOL, "Max symbol value too large");
workspace.reset();
NodeTable nodeTable = workspace.nodeTable;
nodeTable.reset();
int lastNonZero = buildTree(counts, maxSymbol, nodeTable);
// enforce max table log
maxNumberOfBits = setMaxHeight(nodeTable, lastNonZero, maxNumberOfBits, workspace);
checkArgument(maxNumberOfBits <= MAX_TABLE_LOG, "Max number of bits larger than max table size");
// populate table
int symbolCount = maxSymbol + 1;
for (int node = 0; node < symbolCount; node++) {
int symbol = nodeTable.symbols[node];
numberOfBits[symbol] = nodeTable.numberOfBits[node];
}
short[] entriesPerRank = workspace.entriesPerRank;
short[] valuesPerRank = workspace.valuesPerRank;
for (int n = 0; n <= lastNonZero; n++) {
entriesPerRank[nodeTable.numberOfBits[n]]++;
}
// determine starting value per rank
short startingValue = 0;
for (int rank = maxNumberOfBits; rank > 0; rank--) {
valuesPerRank[rank] = startingValue; // get starting value within each rank
startingValue += entriesPerRank[rank];
startingValue >>>= 1;
}
for (int n = 0; n <= maxSymbol; n++) {
values[n] = valuesPerRank[numberOfBits[n]]++; // assign value within rank, symbol order
}
this.maxSymbol = maxSymbol;
this.maxNumberOfBits = maxNumberOfBits;
}
private int buildTree(int[] counts, int maxSymbol, NodeTable nodeTable)
{
// populate the leaves of the node table from the histogram of counts
// in descending order by count, ascending by symbol value.
short current = 0;
for (int symbol = 0; symbol <= maxSymbol; symbol++) {
int count = counts[symbol];
// simple insertion sort
int position = current;
while (position > 1 && count > nodeTable.count[position - 1]) {
nodeTable.copyNode(position - 1, position);
position--;
}
nodeTable.count[position] = count;
nodeTable.symbols[position] = symbol;
current++;
}
int lastNonZero = maxSymbol;
while (nodeTable.count[lastNonZero] == 0) {
lastNonZero--;
}
// populate the non-leaf nodes
short nonLeafStart = MAX_SYMBOL_COUNT;
current = nonLeafStart;
int currentLeaf = lastNonZero;
// combine the two smallest leaves to create the first intermediate node
int currentNonLeaf = current;
nodeTable.count[current] = nodeTable.count[currentLeaf] + nodeTable.count[currentLeaf - 1];
nodeTable.parents[currentLeaf] = current;
nodeTable.parents[currentLeaf - 1] = current;
current++;
currentLeaf -= 2;
int root = MAX_SYMBOL_COUNT + lastNonZero - 1;
// fill in sentinels
for (int n = current; n <= root; n++) {
nodeTable.count[n] = 1 << 30;
}
// create parents
while (current <= root) {
int child1;
if (currentLeaf >= 0 && nodeTable.count[currentLeaf] < nodeTable.count[currentNonLeaf]) {
child1 = currentLeaf--;
}
else {
child1 = currentNonLeaf++;
}
int child2;
if (currentLeaf >= 0 && nodeTable.count[currentLeaf] < nodeTable.count[currentNonLeaf]) {
child2 = currentLeaf--;
}
else {
child2 = currentNonLeaf++;
}
nodeTable.count[current] = nodeTable.count[child1] + nodeTable.count[child2];
nodeTable.parents[child1] = current;
nodeTable.parents[child2] = current;
current++;
}
// distribute weights
nodeTable.numberOfBits[root] = 0;
for (int n = root - 1; n >= nonLeafStart; n--) {
short parent = nodeTable.parents[n];
nodeTable.numberOfBits[n] = (byte) (nodeTable.numberOfBits[parent] + 1);
}
for (int n = 0; n <= lastNonZero; n++) {
short parent = nodeTable.parents[n];
nodeTable.numberOfBits[n] = (byte) (nodeTable.numberOfBits[parent] + 1);
}
return lastNonZero;
}
// TODO: consider encoding 2 symbols at a time
// - need a table with 256x256 entries with
// - the concatenated bits for the corresponding pair of symbols
// - the sum of bits for the corresponding pair of symbols
// - read 2 symbols at a time from the input
public void encodeSymbol(BitOutputStream output, int symbol)
{
output.addBitsFast(values[symbol], numberOfBits[symbol]);
}
public int write(Object outputBase, long outputAddress, int outputSize, HuffmanTableWriterWorkspace workspace)
{
byte[] weights = workspace.weights;
long output = outputAddress;
int maxNumberOfBits = this.maxNumberOfBits;
int maxSymbol = this.maxSymbol;
// convert to weights per RFC 8478 section 4.2.1
for (int symbol = 0; symbol < maxSymbol; symbol++) {
int bits = numberOfBits[symbol];
if (bits == 0) {
weights[symbol] = 0;
}
else {
weights[symbol] = (byte) (maxNumberOfBits + 1 - bits);
}
}
// attempt weights compression by FSE
int size = compressWeights(outputBase, output + 1, outputSize - 1, weights, maxSymbol, workspace);
if (maxSymbol > 127 && size > 127) {
// This should never happen. Since weights are in the range [0, 12], they can be compressed optimally to ~3.7 bits per symbol for a uniform distribution.
// Since maxSymbol has to be <= MAX_SYMBOL (255), this is 119 bytes + FSE headers.
throw new AssertionError();
}
if (size != 0 && size != 1 && size < maxSymbol / 2) {
// Go with FSE only if:
// - the weights are compressible
// - the compressed size is better than what we'd get with the raw encoding below
// - the compressed size is <= 127 bytes, which is the most that the encoding can hold for FSE-compressed weights (see RFC 8478 section 4.2.1.1). This is implied
// by the maxSymbol / 2 check, since maxSymbol must be <= 255
UNSAFE.putByte(outputBase, output, (byte) size);
return size + 1; // header + size
}
else {
// Use raw encoding (4 bits per entry)
// #entries = #symbols - 1 since last symbol is implicit. Thus, #entries = (maxSymbol + 1) - 1 = maxSymbol
int entryCount = maxSymbol;
size = (entryCount + 1) / 2; // ceil(#entries / 2)
checkArgument(size + 1 /* header */ <= outputSize, "Output size too small"); // 2 entries per byte
// encode number of symbols
// header = #entries + 127 per RFC
UNSAFE.putByte(outputBase, output, (byte) (127 + entryCount));
output++;
weights[maxSymbol] = 0; // last weight is implicit, so set to 0 so that it doesn't get encoded below
for (int i = 0; i < entryCount; i += 2) {
UNSAFE.putByte(outputBase, output, (byte) ((weights[i] << 4) + weights[i + 1]));
output++;
}
return (int) (output - outputAddress);
}
}
/**
* Can this table encode all symbols with non-zero count?
*/
public boolean isValid(int[] counts, int maxSymbol)
{
if (maxSymbol > this.maxSymbol) {
// some non-zero count symbols cannot be encoded by the current table
return false;
}
for (int symbol = 0; symbol <= maxSymbol; ++symbol) {
if (counts[symbol] != 0 && numberOfBits[symbol] == 0) {
return false;
}
}
return true;
}
public int estimateCompressedSize(int[] counts, int maxSymbol)
{
int numberOfBits = 0;
for (int symbol = 0; symbol <= Math.min(maxSymbol, this.maxSymbol); symbol++) {
numberOfBits += this.numberOfBits[symbol] * counts[symbol];
}
return numberOfBits >>> 3; // convert to bytes
}
// http://fastcompression.blogspot.com/2015/07/huffman-revisited-part-3-depth-limited.html
private static int setMaxHeight(NodeTable nodeTable, int lastNonZero, int maxNumberOfBits, HuffmanCompressionTableWorkspace workspace)
{
int largestBits = nodeTable.numberOfBits[lastNonZero];
if (largestBits <= maxNumberOfBits) {
return largestBits; // early exit: no elements > maxNumberOfBits
}
// there are several too large elements (at least >= 2)
int totalCost = 0;
int baseCost = 1 << (largestBits - maxNumberOfBits);
int n = lastNonZero;
while (nodeTable.numberOfBits[n] > maxNumberOfBits) {
totalCost += baseCost - (1 << (largestBits - nodeTable.numberOfBits[n]));
nodeTable.numberOfBits[n ] = (byte) maxNumberOfBits;
n--;
} // n stops at nodeTable.numberOfBits[n + offset] <= maxNumberOfBits
while (nodeTable.numberOfBits[n] == maxNumberOfBits) {
n--; // n ends at index of smallest symbol using < maxNumberOfBits
}
// renormalize totalCost
totalCost >>>= (largestBits - maxNumberOfBits); // note: totalCost is necessarily a multiple of baseCost
// repay normalized cost
int noSymbol = 0xF0F0F0F0;
int[] rankLast = workspace.rankLast;
Arrays.fill(rankLast, noSymbol);
// Get pos of last (smallest) symbol per rank
int currentNbBits = maxNumberOfBits;
for (int pos = n; pos >= 0; pos--) {
if (nodeTable.numberOfBits[pos] >= currentNbBits) {
continue;
}
currentNbBits = nodeTable.numberOfBits[pos]; // < maxNumberOfBits
rankLast[maxNumberOfBits - currentNbBits] = pos;
}
while (totalCost > 0) {
int numberOfBitsToDecrease = Util.highestBit(totalCost) + 1;
for (; numberOfBitsToDecrease > 1; numberOfBitsToDecrease--) {
int highPosition = rankLast[numberOfBitsToDecrease];
int lowPosition = rankLast[numberOfBitsToDecrease - 1];
if (highPosition == noSymbol) {
continue;
}
if (lowPosition == noSymbol) {
break;
}
int highTotal = nodeTable.count[highPosition];
int lowTotal = 2 * nodeTable.count[lowPosition];
if (highTotal <= lowTotal) {
break;
}
}
// only triggered when no more rank 1 symbol left => find closest one (note : there is necessarily at least one !)
// HUF_MAX_TABLELOG test just to please gcc 5+; but it should not be necessary
while ((numberOfBitsToDecrease <= MAX_TABLE_LOG) && (rankLast[numberOfBitsToDecrease] == noSymbol)) {
numberOfBitsToDecrease++;
}
totalCost -= 1 << (numberOfBitsToDecrease - 1);
if (rankLast[numberOfBitsToDecrease - 1] == noSymbol) {
rankLast[numberOfBitsToDecrease - 1] = rankLast[numberOfBitsToDecrease]; // this rank is no longer empty
}
nodeTable.numberOfBits[rankLast[numberOfBitsToDecrease]]++;
if (rankLast[numberOfBitsToDecrease] == 0) { /* special case, reached largest symbol */
rankLast[numberOfBitsToDecrease] = noSymbol;
}
else {
rankLast[numberOfBitsToDecrease]--;
if (nodeTable.numberOfBits[rankLast[numberOfBitsToDecrease]] != maxNumberOfBits - numberOfBitsToDecrease) {
rankLast[numberOfBitsToDecrease] = noSymbol; // this rank is now empty
}
}
}
while (totalCost < 0) { // Sometimes, cost correction overshoot
if (rankLast[1] == noSymbol) { /* special case : no rank 1 symbol (using maxNumberOfBits-1); let's create one from largest rank 0 (using maxNumberOfBits) */
while (nodeTable.numberOfBits[n] == maxNumberOfBits) {
n--;
}
nodeTable.numberOfBits[n + 1]--;
rankLast[1] = n + 1;
totalCost++;
continue;
}
nodeTable.numberOfBits[rankLast[1] + 1]--;
rankLast[1]++;
totalCost++;
}
return maxNumberOfBits;
}
/**
* All elements within weightTable must be <= Huffman.MAX_TABLE_LOG
*/
private static int compressWeights(Object outputBase, long outputAddress, int outputSize, byte[] weights, int weightsLength, HuffmanTableWriterWorkspace workspace)
{
if (weightsLength <= 1) {
return 0; // Not compressible
}
// Scan input and build symbol stats
int[] counts = workspace.counts;
Histogram.count(weights, weightsLength, counts);
int maxSymbol = Histogram.findMaxSymbol(counts, MAX_TABLE_LOG);
int maxCount = Histogram.findLargestCount(counts, maxSymbol);
if (maxCount == weightsLength) {
return 1; // only a single symbol in source
}
if (maxCount == 1) {
return 0; // each symbol present maximum once => not compressible
}
short[] normalizedCounts = workspace.normalizedCounts;
int tableLog = FiniteStateEntropy.optimalTableLog(MAX_FSE_TABLE_LOG, weightsLength, maxSymbol);
FiniteStateEntropy.normalizeCounts(normalizedCounts, tableLog, counts, weightsLength, maxSymbol);
long output = outputAddress;
long outputLimit = outputAddress + outputSize;
// Write table description header
int headerSize = FiniteStateEntropy.writeNormalizedCounts(outputBase, output, outputSize, normalizedCounts, maxSymbol, tableLog);
output += headerSize;
// Compress
FseCompressionTable compressionTable = workspace.fseTable;
compressionTable.initialize(normalizedCounts, maxSymbol, tableLog);
int compressedSize = FiniteStateEntropy.compress(outputBase, output, (int) (outputLimit - output), weights, weightsLength, compressionTable);
if (compressedSize == 0) {
return 0;
}
output += compressedSize;
return (int) (output - outputAddress);
}
}
