private static HllSketchImpl unionImpl()

in src/main/java/org/apache/datasketches/hll/Union.java [353:485]
120 lines of code
37 McCabe index (conditional complexity)

  private static HllSketchImpl unionImpl(final HllSketch source, final HllSketch gadget,
      final int lgMaxK) {
    assert gadget.getTgtHllType() == HLL_8;
    if ((source == null) || source.isEmpty()) {
      return gadget.hllSketchImpl;
    }

    final CurMode srcMode = source.getCurMode();
    if (srcMode == CurMode.LIST ) {
      source.mergeTo(gadget);
      return gadget.hllSketchImpl;
    }

    final int srcLgK = source.getLgConfigK();
    final int gadgetLgK = gadget.getLgConfigK();
    final boolean srcIsMem = source.isMemory();
    final boolean gdtIsMem = gadget.isMemory();
    final boolean gdtEmpty = gadget.isEmpty();

    if (srcMode == CurMode.SET ) {
      if (gdtEmpty && (srcLgK == gadgetLgK) && (!srcIsMem) && (!gdtIsMem)) {
        gadget.hllSketchImpl = source.copyAs(HLL_8).hllSketchImpl;
        return gadget.hllSketchImpl;
      }
      source.mergeTo(gadget);
      return gadget.hllSketchImpl;
    }

    //Hereafter, the source is in HLL mode.
    final int bit0 = gdtIsMem ? 1 : 0;
    final int bits1_2 = (gdtEmpty ? 3 : gadget.getCurMode().ordinal()) << 1;
    final int bit3 = (srcLgK < gadgetLgK) ? 8 : 0;
    final int bit4 = (srcLgK > lgMaxK) ? 16 : 0;
    final int sw = bit4 | bit3 | bits1_2 | bit0;
    HllSketchImpl hllSketchImpl = null; //never returned as null

    switch (sw) {
      case 0: //src <= max, src >= gdt, gdtLIST, gdtHeap
      case 8: //src <= max, src <  gdt, gdtLIST, gdtHeap
      case 2: //src <= max, src >= gdt, gdtSET,  gdtHeap
      case 10://src <= max, src <  gdt, gdtSET,  gdtHeap
      { //Action: copy src, reverse merge w/autofold, ooof=src
        final HllSketch srcHll8Heap = source.copyAs(HLL_8);
        gadget.mergeTo(srcHll8Heap); //merge gdt(Hll8,heap,list/set) -> src(Hll8,heap,hll)
        hllSketchImpl = srcHll8Heap.hllSketchImpl;
        break;
      }
      case 16://src >  max, src >= gdt, gdtList, gdtHeap
      case 18://src >  max, src >= gdt, gdtSet,  gdtHeap
      { //Action: downsample src to MaxLgK, reverse merge w/autofold, ooof=src
        final HllSketch srcHll8Heap = downsample(source, lgMaxK);
        gadget.mergeTo(srcHll8Heap); //merge gdt(Hll8,heap,list/set) -> src(Hll8,heap,hll)
        hllSketchImpl = srcHll8Heap.hllSketchImpl;
        break;
      }

      case 1: //src <= max, src >= gdt, gdtLIST, gdtMemory
      case 9: //src <= max, src <  gdt, gdtLIST, gdtMemory
      case 3: //src <= max, src >= gdt, gdtSET,  gdtMemory
      case 11://src <= max, src <  gdt, gdtSET,  gdtMemory
      { //Action: copy src, reverse merge w/autofold, use gdt memory, ooof=src
        final HllSketch srcHll8Heap = source.copyAs(HLL_8);
        gadget.mergeTo(srcHll8Heap);  //merge gdt(Hll8,mem,list/set) -> src(Hll8,heap,hll)
        hllSketchImpl = useGadgetMemory(gadget, srcHll8Heap, false).hllSketchImpl;
        break;
      }
      case 17://src >  max, src >= gdt, gdtList, gdtMemory
      case 19://src >  max, src >= gdt, gdtSet,  gdtMemory
      { //Action: downsample src to MaxLgK, reverse merge w/autofold, use gdt memory, ooof=src
        final HllSketch srcHll8Heap = downsample(source, lgMaxK);
        gadget.mergeTo(srcHll8Heap); //merge gdt(Hll8,mem,list/set) -> src(Hll8,heap,hll), autofold
        hllSketchImpl = useGadgetMemory(gadget, srcHll8Heap, false).hllSketchImpl;
        break;
      }

      case 4: //src <= max, src >= gdt, gdtHLL, gdtHeap
      case 20://src >  max, src >= gdt, gdtHLL, gdtHeap
      case 5: //src <= max, src >= gdt, gdtHLL, gdtMemory
      case 21://src >  max, src >= gdt, gdtHLL, gdtMemory
      { //Action: forward HLL merge w/autofold, ooof=True
        //merge src(Hll4,6,8,heap/mem,Mode=HLL) -> gdt(Hll8,heap,Mode=HLL)
        mergeHlltoHLLmode(source, gadget, srcLgK, gadgetLgK, srcIsMem, gdtIsMem);
        hllSketchImpl = gadget.putOutOfOrderFlag(true).hllSketchImpl;
        break;
      }
      case 12://src <= max, src <  gdt, gdtHLL, gdtHeap
      { //Action: downsample gdt to srcLgK, forward HLL merge w/autofold, ooof=True
        final HllSketch gdtHll8Heap = downsample(gadget, srcLgK);
        //merge src(Hll4,6,8;heap/mem,Mode=HLL) -> gdt(Hll8,heap,hll)
        mergeHlltoHLLmode(source, gdtHll8Heap, srcLgK, gadgetLgK, srcIsMem, false);
        hllSketchImpl = gdtHll8Heap.putOutOfOrderFlag(true).hllSketchImpl;
        break;
      }
      case 13://src <= max, src < gdt, gdtHLL, gdtMemory
      { //Action: downsample gdt to srcLgK, forward HLL merge w/autofold, use gdt memory, ooof=True
        final HllSketch gdtHll8Heap = downsample(gadget, srcLgK);
        //merge src(Hll4,6,8;heap/mem;Mode=HLL) -> gdt(Hll8,heap,Mode=HLL)
        mergeHlltoHLLmode(source, gdtHll8Heap, srcLgK, gadgetLgK, srcIsMem, false);
        hllSketchImpl = useGadgetMemory(gadget, gdtHll8Heap, true).hllSketchImpl;
        break;
      }

      case 6: //src <= max, src >= gdt, gdtEmpty, gdtHeap
      case 14://src <= max, src <  gdt, gdtEmpty, gdtHeap
      { //Action: copy src, replace gdt, ooof=src
        final HllSketch srcHll8Heap = source.copyAs(HLL_8);
        hllSketchImpl = srcHll8Heap.hllSketchImpl;
        break;
      }
      case 22://src >  max, src >= gdt, gdtEmpty, gdtHeap
      { //Action: downsample src to lgMaxK, replace gdt, ooof=src
        final HllSketch srcHll8Heap = downsample(source, lgMaxK);
        hllSketchImpl = srcHll8Heap.hllSketchImpl;
        break;
      }

      case 7: //src <= max, src >= gdt, gdtEmpty, gdtMemory
      case 15://src <= max, src <  gdt, gdtEmpty, gdtMemory
      { //Action: copy src, use gdt memory, ooof=src
        final HllSketch srcHll8Heap = source.copyAs(HLL_8);
        hllSketchImpl = useGadgetMemory(gadget, srcHll8Heap, false).hllSketchImpl;
        break;
      }
      case 23://src >  max, src >= gdt, gdtEmpty, gdtMemory, replace mem, downsample src, ooof=src
      { //Action: downsample src to lgMaxK, use gdt memory, ooof=src
        final HllSketch srcHll8Heap = downsample(source, lgMaxK);
        hllSketchImpl = useGadgetMemory(gadget, srcHll8Heap, false).hllSketchImpl;
        break;
      }
      default: return gadget.hllSketchImpl; //not possible
    }
    return hllSketchImpl;
  }