src/main/java/org/apache/datasketches/sampling/ReservoirItemsUnion.java [399:435]:
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  private void twoWayMergeInternalWeighted(final ReservoirItemsSketch<T> 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());
  }
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src/main/java/org/apache/datasketches/sampling/ReservoirLongsUnion.java [350:386]:
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  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());
  }
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