ReservoirLongsUnion.java
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* to you under the Apache License, Version 2.0 (the
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* http://www.apache.org/licenses/LICENSE-2.0
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package org.apache.datasketches.sampling;
import static org.apache.datasketches.common.Util.LS;
import static org.apache.datasketches.sampling.PreambleUtil.EMPTY_FLAG_MASK;
import static org.apache.datasketches.sampling.PreambleUtil.FAMILY_BYTE;
import static org.apache.datasketches.sampling.PreambleUtil.RESERVOIR_SER_VER;
import static org.apache.datasketches.sampling.PreambleUtil.extractEncodedReservoirSize;
import static org.apache.datasketches.sampling.PreambleUtil.extractFlags;
import static org.apache.datasketches.sampling.PreambleUtil.extractMaxK;
import static org.apache.datasketches.sampling.PreambleUtil.extractPreLongs;
import static org.apache.datasketches.sampling.PreambleUtil.extractSerVer;
import org.apache.datasketches.common.Family;
import org.apache.datasketches.common.SketchesArgumentException;
import org.apache.datasketches.memory.Memory;
import org.apache.datasketches.memory.WritableMemory;
/**
* Class to union reservoir samples of longs.
*
* <p>
* For efficiency reasons, the unioning process picks one of the two sketches to use as the base. As
* a result, we provide only a stateful union. Using the same approach for a merge would result in
* unpredictable side effects on the underlying sketches.
* </p>
*
* <p>
* A union object is created with a maximum value of <code>k</code>, represented using the ReservoirSize
* class. The unioning process may cause the actual number of samples to fall below that maximum
* value, but never to exceed it. The result of a union will be a reservoir where each item from the
* global input has a uniform probability of selection, but there are no claims about higher order
* statistics. For instance, in general all possible permutations of the global input are not
* equally likely.
* </p>
*
* @author Jon Malkin
* @author Kevin Lang
*/
public final class ReservoirLongsUnion {
private ReservoirLongsSketch gadget_;
private final int maxK_;
/**
* Empty constructor using ReservoirSize-encoded maxK value
*
* @param maxK Maximum allowed reservoir capacity for this union
*/
private ReservoirLongsUnion(final int maxK) {
maxK_ = maxK;
}
/**
* Creates an empty Union with a maximum reservoir capacity of size k.
*
* @param maxK The maximum allowed reservoir capacity for any sketches in the union
* @return A new ReservoirLongsUnion
*/
public static ReservoirLongsUnion newInstance(final int maxK) {
return new ReservoirLongsUnion(maxK);
}
/**
* Instantiates a Union from Memory
*
* @param srcMem Memory object containing a serialized union
* @return A ReservoirLongsUnion created from the provided Memory
*/
public static ReservoirLongsUnion heapify(final Memory srcMem) {
Family.RESERVOIR_UNION.checkFamilyID(srcMem.getByte(FAMILY_BYTE));
final int numPreLongs = extractPreLongs(srcMem);
final int serVer = extractSerVer(srcMem);
final boolean isEmpty = (extractFlags(srcMem) & EMPTY_FLAG_MASK) != 0;
int maxK = extractMaxK(srcMem);
final boolean preLongsEqMin = (numPreLongs == Family.RESERVOIR_UNION.getMinPreLongs());
final boolean preLongsEqMax = (numPreLongs == Family.RESERVOIR_UNION.getMaxPreLongs());
if (!preLongsEqMin && !preLongsEqMax) {
throw new SketchesArgumentException("Possible corruption: Non-empty union with only "
+ Family.RESERVOIR_UNION.getMinPreLongs() + "preLongs");
}
if (serVer != RESERVOIR_SER_VER) {
if (serVer == 1) {
final short encMaxK = extractEncodedReservoirSize(srcMem);
maxK = ReservoirSize.decodeValue(encMaxK);
} else {
throw new SketchesArgumentException(
"Possible Corruption: Ser Ver must be " + RESERVOIR_SER_VER + ": " + serVer);
}
}
final ReservoirLongsUnion rlu = new ReservoirLongsUnion(maxK);
if (!isEmpty) {
final int preLongBytes = numPreLongs << 3;
final Memory sketchMem =
srcMem.region(preLongBytes, srcMem.getCapacity() - preLongBytes);
rlu.update(sketchMem);
}
return rlu;
}
/**
* Returns the maximum allowed reservoir capacity in this union. The current reservoir capacity
* may be lower.
*
* @return The maximum allowed reservoir capacity in this union.
*/
public int getMaxK() { return maxK_; }
/**
* Union the given sketch.
* <p>
* This method can be repeatedly called. If the given sketch is null it is interpreted as an empty
* sketch.
* </p>
*
* @param sketchIn The incoming sketch.
*/
public void update(final ReservoirLongsSketch sketchIn) {
if (sketchIn == null) {
return;
}
final ReservoirLongsSketch rls =
(sketchIn.getK() <= maxK_ ? sketchIn : sketchIn.downsampledCopy(maxK_));
// can modify the sketch if we downsampled, otherwise may need to copy it
final boolean isModifiable = (sketchIn != rls);
if (gadget_ == null) {
createNewGadget(rls, isModifiable);
} else {
twoWayMergeInternal(rls, isModifiable);
}
}
/**
* Union the given Memory image of the sketch.
*
* <p>
* This method can be repeatedly called. If the given sketch is null it is interpreted as an empty
* sketch.
* </p>
*
* @param mem Memory image of sketch to be merged
*/
public void update(final Memory mem) {
if (mem == null) {
return;
}
ReservoirLongsSketch rls = ReservoirLongsSketch.heapify(mem);
rls = (rls.getK() <= maxK_ ? rls : rls.downsampledCopy(maxK_));
if (gadget_ == null) {
createNewGadget(rls, true);
} else {
twoWayMergeInternal(rls, true);
}
}
/**
* Present this union with a long.
*
* @param datum The given long datum.
*/
public void update(final long datum) {
if (gadget_ == null) {
gadget_ = ReservoirLongsSketch.newInstance(maxK_);
}
gadget_.update(datum);
}
/**
* Resets this Union. MaxK remains intact, otherwise reverts back to its virgin state.
*/
void reset() {
gadget_.reset();
}
/**
* Returns a sketch representing the current state of the union.
*
* @return The result of any unions already processed.
*/
public ReservoirLongsSketch getResult() {
return (gadget_ != null ? gadget_.copy() : null);
}
/**
* Returns a human-readable summary of the sketch, without items.
*
* @return A string version of the sketch summary
*/
@Override
public String toString() {
final StringBuilder sb = new StringBuilder();
final String thisSimpleName = this.getClass().getSimpleName();
sb.append(LS);
sb.append("### ").append(thisSimpleName).append(" SUMMARY: ").append(LS);
sb.append("Max k: ").append(maxK_).append(LS);
if (gadget_ == null) {
sb.append("Gadget is null").append(LS);
} else {
sb.append("Gadget summary: ").append(gadget_.toString());
}
sb.append("### END UNION SUMMARY").append(LS);
return sb.toString();
}
/**
* Returns a byte array representation of this union
*
* @return a byte array representation of this union
*/
// gadgetBytes will be null only if gadget_ == null AND empty == true
public byte[] toByteArray() {
final int preLongs, outBytes;
final boolean empty = gadget_ == null;
final byte[] gadgetBytes = (gadget_ != null ? gadget_.toByteArray() : null);
if (empty) {
preLongs = Family.RESERVOIR_UNION.getMinPreLongs();
outBytes = 8;
} else {
preLongs = Family.RESERVOIR_UNION.getMaxPreLongs();
outBytes = (preLongs << 3) + gadgetBytes.length; // longs, so we know the size
}
final byte[] outArr = new byte[outBytes];
final WritableMemory mem = WritableMemory.writableWrap(outArr);
// construct header
PreambleUtil.insertPreLongs(mem, preLongs); // Byte 0
PreambleUtil.insertSerVer(mem, RESERVOIR_SER_VER); // Byte 1
PreambleUtil.insertFamilyID(mem, Family.RESERVOIR_UNION.getID()); // Byte 2
if (empty) {
PreambleUtil.insertFlags(mem, EMPTY_FLAG_MASK); // Byte 3
} else {
PreambleUtil.insertFlags(mem, 0);
}
PreambleUtil.insertMaxK(mem, maxK_); // Bytes 4-7
if (!empty) {
final int preBytes = preLongs << 3;
mem.putByteArray(preBytes, gadgetBytes, 0, gadgetBytes.length);
}
return outArr;
}
private void createNewGadget(final ReservoirLongsSketch sketchIn,
final boolean isModifiable) {
if ((sketchIn.getK() < maxK_) && (sketchIn.getN() <= sketchIn.getK())) {
// incoming sketch is in exact mode with sketch's k < maxK,
// so we can create a gadget at size maxK and keep everything
// NOTE: assumes twoWayMergeInternal first checks if sketchIn is in exact mode
gadget_ = ReservoirLongsSketch.newInstance(maxK_);
twoWayMergeInternal(sketchIn, isModifiable); // isModifiable could be fixed to false here
} else {
// use the input sketch as gadget, copying if needed
gadget_ = (isModifiable ? sketchIn : sketchIn.copy());
}
}
// We make a three-way classification of sketch states.
// "uni" when (n < k); source of unit weights, can only accept unit weights
// "mid" when (n == k); source of unit weights, can accept "light" general weights.
// "gen" when (n > k); source of general weights, can accept "light" general weights.
// source target status update notes
// ----------------------------------------------------------------------------------------------
// uni,mid uni okay standard target might transition to mid and gen
// uni,mid mid,gen okay standard target might transition to gen
// gen uni must swap N/A
// gen mid,gen maybe swap weighted N assumes fractional values during merge
// ----------------------------------------------------------------------------------------------
// Here is why in the (gen, gen) merge case, the items will be light enough in at least one
// direction:
// Obviously either (n_s/k_s <= n_t/k_t) OR (n_s/k_s >= n_t/k_t).
// WLOG say its the former, then (n_s/k_s < n_t/(k_t - 1)) provided n_t > 0 and k_t > 1
/**
* This either merges sketchIn into gadget_ or gadget_ into sketchIn. If merging into sketchIn
* with isModifiable set to false, copies elements from sketchIn first, leaving original
* unchanged.
*
* @param sketchIn Sketch with new samples from which to draw
* @param isModifiable Flag indicating whether sketchIn can be modified (e.g. if it was rebuild
* from Memory)
*/
private void twoWayMergeInternal(final ReservoirLongsSketch sketchIn,
final boolean isModifiable) {
if (sketchIn.getN() <= sketchIn.getK()) {
twoWayMergeInternalStandard(sketchIn);
} else if (gadget_.getN() < gadget_.getK()) {
// merge into sketchIn, so swap first
final ReservoirLongsSketch tmpSketch = gadget_;
gadget_ = (isModifiable ? sketchIn : sketchIn.copy());
twoWayMergeInternalStandard(tmpSketch);
} else if (sketchIn.getImplicitSampleWeight() < (gadget_.getN()
/ ((double) (gadget_.getK() - 1)))) {
// implicit weights in sketchIn are light enough to merge into gadget
twoWayMergeInternalWeighted(sketchIn);
} else {
// Use next next line for an assert/exception?
// gadget_.getImplicitSampleWeight() < sketchIn.getN() / ((double) (sketchIn.getK() - 1)))
// implicit weights in gadget are light enough to merge into sketchIn, so swap first
final ReservoirLongsSketch tmpSketch = gadget_;
gadget_ = (isModifiable ? sketchIn : sketchIn.copy());
twoWayMergeInternalWeighted(tmpSketch);
}
}
// should be called ONLY by twoWayMergeInternal
private void twoWayMergeInternalStandard(final ReservoirLongsSketch source) {
assert (source.getN() <= source.getK());
final int numInputSamples = source.getNumSamples();
for (int i = 0; i < numInputSamples; ++i) {
gadget_.update(source.getValueAtPosition(i));
}
}
// should be called ONLY by twoWayMergeInternal
private void twoWayMergeInternalWeighted(final ReservoirLongsSketch source) {
// gadget_ capable of accepting (light) general weights
assert (gadget_.getN() >= gadget_.getK());
final int numSourceSamples = source.getK();
final double sourceItemWeight = (source.getN() / (double) numSourceSamples);
final double rescaled_prob = gadget_.getK() * sourceItemWeight; // K * weight
double targetTotal = gadget_.getN(); // assumes fractional values during merge
final int tgtK = gadget_.getK();
for (int i = 0; i < numSourceSamples; ++i) {
// inlining the update procedure, using targetTotal for the fractional N values
// similar to ReservoirLongsSketch.update()
// p(keep_new_item) = (k * w) / newTotal
// require p(keep_new_item) < 1.0, meaning strict lightness
targetTotal += sourceItemWeight;
final double rescaled_one = targetTotal;
assert (rescaled_prob < rescaled_one); // Use an exception to enforce strict lightness?
final double rescaled_flip = rescaled_one * SamplingUtil.rand().nextDouble();
if (rescaled_flip < rescaled_prob) {
// Intentionally NOT doing optimization to extract slot number from rescaled_flip.
// Grabbing new random bits to ensure all slots in play
final int slotNo = SamplingUtil.rand().nextInt(tgtK);
gadget_.insertValueAtPosition(source.getValueAtPosition(i), slotNo);
} // end of inlined weight update
} // end of loop over source samples
// targetTotal was fractional but should now be an integer again. Could validate with
// low tolerance, but for now just round to check.
final long checkN = (long) Math.floor(0.5 + targetTotal);
gadget_.forceIncrementItemsSeen(source.getN());
assert (checkN == gadget_.getN());
}
}