[stojkomilos]

kll_sketch::merge() now throws std::invalid_argument("A sketch cannot merge with itself.") when the other sketch is this sketch.

Java

The Java KLL sketches throw in the same case, since apache/datasketches-java#516. KllDoublesSketch, KllFloatsSketch, KllLongsSketch and KllItemsSketch all start merge() with this check, where SELF_MERGE_MSG in QuantilesAPI is "A sketch cannot merge with itself. ":

public final void merge(final KllSketch other) {
  if (readOnly || (sketchStructure != UPDATABLE)) { throw new SketchesArgumentException(TGT_IS_READ_ONLY_MSG); }
  if (this == other) { throw new SketchesArgumentException(SELF_MERGE_MSG); }
  final KllDoublesSketch othDblSk = (KllDoublesSketch)other;
  if (othDblSk.isEmpty()) { return; }

Master

On master, a self-merge sometimes works and sometimes throws "Total weight does not match N", depending on n and k. A minimal repro with the default k = 200:

#include <iostream>
#include <kll_sketch.hpp>

int main() {
  for (int n : {100, 101, 1000}) {
    datasketches::kll_sketch<float> sketch;  // k = 200
    for (int i = 0; i < n; ++i) sketch.update(static_cast<float>(i));
    try {
      sketch.merge(sketch);
      std::cout << n << " items: merged, n = " << sketch.get_n() << std::endl;
    } catch (const std::exception& e) {
      std::cout << n << " items: threw \"" << e.what() << "\"" << std::endl;
    }
  }
}

On master (c22f2b1) it prints:

100 items: merged, n = 200
101 items: threw "Total weight does not match N"
1000 items: merged, n = 2000

With this PR, all three throw "A sketch cannot merge with itself.".

Tests

• Added a test that verifies this new throwing behaviour.

All 17 CTest suites pass with GCC 10.5 and C++11, and kll_test also passes with ASan and UBSan. Without the check, the new section fails.