static void downSamplingMergeInto()

in src/main/java/org/apache/datasketches/quantiles/DoublesMergeImpl.java [143:221]
64 lines of code
15 McCabe index (conditional complexity)

  static void downSamplingMergeInto(final DoublesSketch src, final UpdateDoublesSketch tgt) {
    final int sourceK = src.getK();
    final int targetK = tgt.getK();
    final long tgtN = tgt.getN();

    if ((sourceK % targetK) != 0) {
      throw new SketchesArgumentException(
          "source.getK() must equal target.getK() * 2^(nonnegative integer).");
    }

    final int downFactor = sourceK / targetK;
    checkIfPowerOf2(downFactor, "source.getK()/target.getK() ratio");
    final int lgDownFactor = Integer.numberOfTrailingZeros(downFactor);

    if (src.isEmpty()) { return; }

    final DoublesSketchAccessor srcSketchBuf = DoublesSketchAccessor.wrap(src);
    final long nFinal = tgtN + src.getN();

    for (int i = 0; i < srcSketchBuf.numItems(); i++) { // update only the base buffer
      tgt.update(srcSketchBuf.get(i));
    }

    final int spaceNeeded = DoublesUpdateImpl.getRequiredItemCapacity(targetK, nFinal);
    final int curCombBufCap = tgt.getCombinedBufferItemCapacity();
    if (spaceNeeded > curCombBufCap) { //copies base buffer plus current levels
      tgt.growCombinedBuffer(curCombBufCap, spaceNeeded);
    }

    //working scratch buffers
    final DoublesArrayAccessor scratch2KAcc = DoublesArrayAccessor.initialize(2 * targetK);
    final DoublesArrayAccessor downScratchKAcc = DoublesArrayAccessor.initialize(targetK);

    final DoublesSketchAccessor tgtSketchBuf = DoublesSketchAccessor.wrap(tgt, true);

    long srcBitPattern = src.getBitPattern();
    long newTgtBitPattern = tgt.getBitPattern();
    for (int srcLvl = 0; srcBitPattern != 0L; srcLvl++, srcBitPattern >>>= 1) {
      if ((srcBitPattern & 1L) > 0L) {
        justZipWithStride(
            srcSketchBuf.setLevel(srcLvl),
            downScratchKAcc,
            targetK,
            downFactor
        );
        newTgtBitPattern = DoublesUpdateImpl.inPlacePropagateCarry(
            srcLvl + lgDownFactor,    //starting level
            downScratchKAcc,       //optSrcKBuf,
            scratch2KAcc,          //size2KBuf,
            false,                    //do mergeInto version
            targetK,
            tgtSketchBuf,
            newTgtBitPattern
        );

        tgt.putBitPattern(newTgtBitPattern); //off-heap is a no-op
      }
    }
    if (tgt.hasMemory() && (nFinal > 0)) {
      final WritableMemory mem = tgt.getMemory();
      mem.clearBits(FLAGS_BYTE, (byte) EMPTY_FLAG_MASK);
    }
    tgt.putN(nFinal);

    assert (tgt.getN() / (2L * targetK)) == newTgtBitPattern; // internal consistency check

    double srcMax = src.getMaxItem();
    srcMax = Double.isNaN(srcMax) ? Double.NEGATIVE_INFINITY : srcMax;
    double srcMin = src.getMinItem();
    srcMin = Double.isNaN(srcMin) ? Double.POSITIVE_INFINITY : srcMin;

    double tgtMax = tgt.getMaxItem();
    tgtMax = Double.isNaN(tgtMax) ? Double.NEGATIVE_INFINITY : tgtMax;
    double tgtMin = tgt.getMinItem();
    tgtMin = Double.isNaN(tgtMin) ? Double.POSITIVE_INFINITY : tgtMin;

    if (srcMax > tgtMax) { tgt.putMaxItem(srcMax); }
    if (srcMin < tgtMin) { tgt.putMinItem(srcMin); }
  }