Enable setting attribute phase change thresholds.

include/highfive/H5PropertyList.hpp

@@ -660,6 +660,41 @@ class LinkCreationOrder {
     unsigned _flags;
 };
 
+
+///
+/// \brief Set threshold for attribute storage.
+///
+/// HDF5 can store Attributes in the object header (compact) or in the B-tree
+/// (dense). This property sets the threshold when attributes are moved to one
+/// or the other storage format.
+///
+/// Please refer to the upstream documentation of `H5Pset_attr_phase_change` or
+/// Section 8 (Attributes) in the User Guide, in particular Subsection 8.5.
+///
+class AttributePhaseChange {
+  public:
+    ///
+    /// \brief Create the property from the threshold values.
+    ///
+    /// When the number of attributes hits `max_compact` the attributes are
+    /// moved to dense storage, once the number drops to below `min_dense` the
+    /// attributes are moved to compact storage.
+    AttributePhaseChange(unsigned max_compact, unsigned min_dense);
+
+    /// \brief Extract threshold values from property list.
+    explicit AttributePhaseChange(const GroupCreateProps& gcpl);
+
+    unsigned max_compact() const;
+    unsigned min_dense() const;
+
+  private:
+    friend GroupCreateProps;
+    void apply(hid_t hid) const;
+
+    unsigned _max_compact;
+    unsigned _min_dense;
+};
+
 /// @}
 
 }  // namespace HighFive

include/highfive/bits/H5PropertyList_misc.hpp

@@ -543,4 +543,32 @@ inline void LinkCreationOrder::fromPropertyList(hid_t hid) {
             "Error getting property for link creation order");
     }
 }
+
+inline AttributePhaseChange::AttributePhaseChange(unsigned max_compact, unsigned min_dense)
+    : _max_compact(max_compact)
+    , _min_dense(min_dense) {}
+
+inline AttributePhaseChange::AttributePhaseChange(const GroupCreateProps& gcpl) {
+    if (H5Pget_attr_phase_change(gcpl.getId(), &_max_compact, &_min_dense) < 0) {
+        HDF5ErrMapper::ToException<PropertyException>(
+            "Error getting property for attribute phase change");
+    }
+}
+
+inline unsigned AttributePhaseChange::max_compact() const {
+    return _max_compact;
+}
+
+inline unsigned AttributePhaseChange::min_dense() const {
+    return _min_dense;
+}
+
+inline void AttributePhaseChange::apply(hid_t hid) const {
+    if (H5Pset_attr_phase_change(hid, _max_compact, _min_dense) < 0) {
+        HDF5ErrMapper::ToException<PropertyException>(
+            "Error getting property for attribute phase change");
+    }
+}
+
+
 }  // namespace HighFive

src/examples/create_large_attribute.cpp

@@ -0,0 +1,20 @@
+#include <H5Fpublic.h>
+#include <highfive/H5File.hpp>
+#include <highfive/H5Group.hpp>
+#include <numeric>
+#include <vector>
+
+int main() {
+    std::vector<double> large_attr(16000, 0.0);
+
+    auto fapl = HighFive::FileAccessProps::Default();
+    fapl.add(HighFive::FileVersionBounds(H5F_LIBVER_LATEST, H5F_LIBVER_LATEST));
+    HighFive::File file("create_large_attribute.h5", HighFive::File::Truncate, fapl);
+    auto gcpl = HighFive::GroupCreateProps::Default();
+    gcpl.add(HighFive::AttributePhaseChange(0, 0));
+
+    auto group = file.createGroup("grp", gcpl);
+    group.createAttribute("attr", large_attr);
+
+    return 0;
+}