IO: Reversed persistent ID order in exports to Alembic and USD

Each duplicated (a.k.a. instanced) object has a Persistent ID, which
identifies a dupli within the context of its duplicator. This ID
consists of several numbers when there are nested duplis (for example a
mesh instancing empties on its vertices, where each empty instances a
collection). When exporting to Alembic/USD, these are used to uniquely
name the duplicated objects in the export.

This commit reverses the order of the persistent ID numbers, so that the
first number identifies the first level of recursion. This produces
trees like this:

    ABC
     `--Triangle
         |--Triangle
         |--Empty-1
         |    `--Pole-1-0
         |        |--Pole
         |        `--Block-1-1
         |            `--Block
         |--Empty
         |    `--Pole-0
         |        |--Pole
         |        `--Block-1
         |            `--Block
         |--Empty-2
         |    `--Pole-2-0
         |        |--Pole
         |        `--Block-2-1
         |            `--Block
         `--Empty-0
             `--Pole-0-0
                 |--Pole
                 `--Block-0-1
                     `--Block

It is now clearer that `Pole-2-0` and `Block-2-1` are instanced by
`Empty-2`. Before this commit, they would have been named `Pole-0-2` and
`Block-1-2`.

3 files changed, 33 insertions, 4 deletions

diff --git a/source/blender/io/common/IO_dupli_persistent_id.hh b/source/blender/io/common/IO_dupli_persistent_id.hh
index dc70c1cdf31..5dc54164684 100644
--- a/source/blender/io/common/IO_dupli_persistent_id.hh
+++ b/source/blender/io/common/IO_dupli_persistent_id.hh
@@ -48,6 +48,13 @@ class PersistentID {
   /* Construct the persistent ID of this instance's instancer. */
   PersistentID instancer_pid() const;
 
+  /* Construct a string representation by reversing the persistent ID.
+   * In case of a duplicator that is duplicated itself as well, this
+   * results in strings like:
+   * "3" for the duplicated duplicator, and
+   * "3-0", "3-1", etc. for its duplis. */
+  std::string as_object_name_suffix() const;
+
   friend bool operator==(const PersistentID &persistent_id_a, const PersistentID &persistent_id_b);
   friend bool operator<(const PersistentID &persistent_id_a, const PersistentID &persistent_id_b);
   friend std::ostream &operator<<(std::ostream &os, const PersistentID &persistent_id);
diff --git a/source/blender/io/common/intern/abstract_hierarchy_iterator.cc b/source/blender/io/common/intern/abstract_hierarchy_iterator.cc
index c8d916c0950..3622c1eb7cd 100644
--- a/source/blender/io/common/intern/abstract_hierarchy_iterator.cc
+++ b/source/blender/io/common/intern/abstract_hierarchy_iterator.cc
@@ -427,10 +427,10 @@ void AbstractHierarchyIterator::visit_dupli_object(DupliObject *dupli_object,
   copy_m4_m4(context->matrix_world, dupli_object->mat);
 
   // Construct export name for the dupli-instance.
-  std::stringstream suffix_stream;
-  suffix_stream << std::hex;
-  suffix_stream << "-" << context->persistent_id;
-  context->export_name = make_valid_name(get_object_name(context->object) + suffix_stream.str());
+  std::stringstream export_name_stream;
+  export_name_stream << get_object_name(context->object) << "-"
+                     << context->persistent_id.as_object_name_suffix();
+  context->export_name = make_valid_name(export_name_stream.str());
 
   ExportGraph::key_type graph_index = determine_graph_index_dupli(
       context, dupli_object, dupli_parent_finder);
diff --git a/source/blender/io/common/intern/dupli_persistent_id.cc b/source/blender/io/common/intern/dupli_persistent_id.cc
index 8563fd5cea6..f0f3079a75c 100644
--- a/source/blender/io/common/intern/dupli_persistent_id.cc
+++ b/source/blender/io/common/intern/dupli_persistent_id.cc
@@ -22,6 +22,7 @@
 #include <climits>
 #include <cstring>
 #include <ostream>
+#include <sstream>
 
 namespace blender::io {
 
@@ -84,6 +85,27 @@ PersistentID PersistentID::instancer_pid() const
   return PersistentID(new_pid_values);
 }
 
+std::string PersistentID::as_object_name_suffix() const
+{
+  std::stringstream stream;
+
+  /* Find one past the last index. */
+  int index;
+  for (index = 0; index < array_length_ && persistent_id_[index] < INT_MAX; ++index)
+    ;
+
+  /* Iterate backward to construct the string. */
+  --index;
+  for (; index >= 0; --index) {
+    stream << persistent_id_[index];
+    if (index > 0) {
+      stream << "-";
+    }
+  }
+
+  return stream.str();
+}
+
 bool operator<(const PersistentID &persistent_id_a, const PersistentID &persistent_id_b)
 {
   const PersistentID::PIDArray &pid_a = persistent_id_a.persistent_id_;