fix: kll_sketch throws when a sketch is merged into itself, as in Java

kll/include/kll_sketch.hpp

@@ -241,6 +241,7 @@ class kll_sketch {
      * Merges another sketch into this one.
      * If sketches contain strings, callers are responsible for ensuring that
      * both sketches were built using compatible string encodings.
+     * If other is this sketch, this throws std::invalid_argument.
      * @param other sketch to merge into this one
      */
     template<typename FwdSk>

kll/include/kll_sketch_impl.hpp

@@ -208,6 +208,7 @@ uint32_t kll_sketch<T, C, A>::internal_update() {
 template<typename T, typename C, typename A>
 template<typename FwdSk>
 void kll_sketch<T, C, A>::merge(FwdSk&& other) {
+  if (this == &other) { throw std::invalid_argument("A sketch cannot merge with itself."); }
   if (other.is_empty()) { return; }
   if (m_ != other.m_) {
     throw std::invalid_argument("incompatible M: " + std::to_string(m_) + " and " + std::to_string(other.m_));

kll/test/kll_sketch_test.cpp

@@ -566,6 +566,16 @@ TEST_CASE("kll sketch", "[kll_sketch]") {
     REQUIRE(sketch2.get_max_item() == 999999.0f);
   }
 
+  SECTION("merge into itself") {
+    kll_float_sketch sketch(200, std::less<float>(), 0);
+    REQUIRE_THROWS_WITH(sketch.merge(sketch), "A sketch cannot merge with itself.");
+    for (int i = 0; i < 150; i++) sketch.update(static_cast<float>(i));
+    const auto bytes = sketch.serialize();
+    REQUIRE_THROWS_WITH(sketch.merge(sketch), "A sketch cannot merge with itself.");
+    REQUIRE_THROWS_WITH(sketch.merge(static_cast<const kll_float_sketch&>(sketch)), "A sketch cannot merge with itself.");
+    REQUIRE(sketch.serialize() == bytes);
+  }
+
   SECTION("sketch of ints") {
     kll_sketch<int> sketch;
     REQUIRE_THROWS_AS(sketch.get_quantile(0), std::runtime_error);