/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF 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 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());
  }
}
