fix invalid pointer when running with two points

cpp/dbscan.cpp

@@ -44,8 +44,9 @@ auto Dbscan::fit_predict(std::vector<Dbscan::Point> const& points) -> std::vecto
     // derive num_bins out of it
     float const range_x{max[0] - min[0]};
     float const range_y{max[1] - min[1]};
-    auto const num_bins_x{static_cast<std::uint32_t>(std::ceil(range_x / eps_))};
-    auto const num_bins_y{static_cast<std::uint32_t>(std::ceil(range_y / eps_))};
+    // add 1e-7 to handle the case where range is exactly divisible by eps_
+    auto const num_bins_x{static_cast<std::uint32_t>(std::ceil((range_x + 1e-7f) / eps_))};
+    auto const num_bins_y{static_cast<std::uint32_t>(std::ceil((range_y + 1e-7f) / eps_))};
 
     // count number of points in every bin
     counts_.assign(num_bins_x * num_bins_y, 0);

python/dbscan_test.py

@@ -61,5 +61,16 @@ def test_blobs():
     assert all(np.count_nonzero(y_pred == label) == 500 for label in range(n_blobs))
 
 
+def test_points_on_border():
+    """Test using two points on the border with an eps which divides (point - min_point) without remainder."""
+    X = np.array([[0, 0], [1, 1]])
+
+    dbscan = py_dbscan.DBSCAN(0.5, 2)
+    y_pred = dbscan.fit_predict(X)
+
+    assert y_pred.shape[0] == 2
+    np.testing.assert_equal(y_pred, np.array([-1, -1]))
+
+
 if __name__ == "__main__":
     sys.exit(pytest.main([__file__, "-rP"]))