fix: density_sketch did not work with a custom allocator

density/include/density_sketch.hpp

@@ -32,7 +32,9 @@ namespace datasketches {
 
 template<typename T>
 struct gaussian_kernel {
-  T operator()(const std::vector<T>& v1, const std::vector<T>& v2) const {
+  // any two vectors of T, so the sketch's points can use its allocator
+  template<typename V1, typename V2>
+  T operator()(const V1& v1, const V2& v2) const {
     return exp(-std::inner_product(v1.begin(), v1.end(), v2.begin(), 0.0, std::plus<T>(), [](T a, T b){return (a-b)*(a-b);}));
   }
 };
@@ -203,7 +205,6 @@ class density_sketch {
   static const uint8_t SERIAL_VERSION = 1;
   static const size_t LEVELS_ARRAY_START = 5;
 
-  Allocator allocator_;
   Kernel kernel_;
   uint16_t k_;
   uint32_t dim_;

density/test/density_sketch_test.cpp

@@ -21,6 +21,7 @@
 #include <catch2/catch.hpp>
 
 #include <density_sketch.hpp>
+#include <test_allocator.hpp>
 
 namespace datasketches {
 
@@ -241,4 +242,33 @@ TEST_CASE("serialize stream", "[density_sketch]") {
   }
 }
 
+TEST_CASE("density sketch: custom allocator", "[density_sketch]") {
+  using alloc = test_allocator<double>;
+  using sketch = density_sketch<double, gaussian_kernel<double>, alloc>;
+  using point = std::vector<double, alloc>;
+  test_allocator_total_bytes = 0;
+  test_allocator_net_allocations = 0;
+  {
+    sketch s(10, 3, gaussian_kernel<double>(), alloc(0));
+    // Keep every retained point close to the query so the Gaussian kernel cannot underflow to zero.
+    for (int i = 0; i < 1000; ++i) s.update(point({i / 1000.0, 1.0, 2.0}, alloc(0)));
+    REQUIRE(s.get_n() == 1000);
+    REQUIRE(s.is_estimation_mode());
+    REQUIRE(s.get_estimate({0.5, 1.0, 2.0}) > 0);
+    REQUIRE(test_allocator_total_bytes > 0);
+
+    sketch other(10, 3, gaussian_kernel<double>(), alloc(0));
+    for (int i = 0; i < 100; ++i) other.update(point({i / 1000.0, 1.0, 2.0}, alloc(0)));
+    s.merge(other);
+    REQUIRE(s.get_n() == 1100);
+
+    auto bytes = s.serialize();
+    auto copy = sketch::deserialize(bytes.data(), bytes.size(), gaussian_kernel<double>(), alloc(0));
+    REQUIRE(copy.get_n() == s.get_n());
+    REQUIRE(copy.get_estimate({0.5, 1.0, 2.0}) == s.get_estimate({0.5, 1.0, 2.0}));
+  }
+  REQUIRE(test_allocator_total_bytes == 0);
+  REQUIRE(test_allocator_net_allocations == 0);
+}
+
 } /* namespace datasketches */