Optimise IfcPatch recipe: make toposort backend configurable

aothms asked for the toposort dependency ordering used by the dedup
walk to try igraph's C-backed topological_sorting() first, since it
should shave off additional time on top of the non-recursive
get_info fix. Falls back to the pure python toposort package with a
warning if igraph is not installed.

Generated with the assistance of an AI coding tool.

src/ifcpatch/ifcpatch/recipes/Optimise.py

@@ -16,9 +16,40 @@
 # You should have received a copy of the GNU Lesser General Public License
 # along with IfcPatch.  If not, see <http://www.gnu.org/licenses/>.
 
+import logging
+
 import ifcopenshell
 
 
+def _toposort(graph: dict[int, set[int]], logger: logging.Logger) -> list[int]:
+    """Flatten a dependency graph of entity ids into dependency order.
+
+    Uses igraph's C-backed topological sort when available, otherwise falls
+    back to the pure python toposort package with a warning.
+    """
+    try:
+        import igraph
+    except ImportError:
+        logger.warning(
+            "igraph is not installed, falling back to the slower pure python toposort. "
+            "Install python-igraph for better performance."
+        )
+        from toposort import toposort_flatten
+
+        return toposort_flatten(graph)
+
+    ids = list(graph)
+    index = {id_: i for i, id_ in enumerate(ids)}
+    for references in graph.values():
+        for reference in references:
+            if reference not in index:
+                index[reference] = len(ids)
+                ids.append(reference)
+    edges = [(index[reference], index[id_]) for id_, references in graph.items() for reference in references]
+    order = igraph.Graph(n=len(ids), edges=edges, directed=True).topological_sorting(mode="out")
+    return [ids[i] for i in order]
+
+
 class Patcher:
     def __init__(self, file, logger):
         """Optimise the filesize of an IFC model
@@ -52,8 +83,6 @@ class Patcher:
         self.optimized_file = ifcopenshell.file(schema=self.file.schema)
 
     def patch(self):
-        from toposort import toposort_flatten as toposort
-
         def generate_instances_and_references():
             """
             Generator which yields an entity id and
@@ -90,7 +119,7 @@ class Patcher:
 
         info_to_id = {}
 
-        for id in toposort(dict(generate_instances_and_references())):
+        for id in _toposort(dict(generate_instances_and_references()), self.logger):
             inst = self.file[id]
             info = map_value(inst.get_info(include_identifier=False, recursive=False, return_type=tuple), as_key=True)
             if info in info_to_id:

src/ifcpatch/test/test_Optimise.py

@@ -18,11 +18,18 @@
 
 # This file was generated with the assistance of an AI coding tool.
 
+import logging
+import sys
+from unittest import mock
+
+import pytest
+
 import ifcopenshell
 import ifcopenshell.guid
 
 import ifcpatch
 import test.bootstrap
+from ifcpatch.recipes.Optimise import _toposort
 
 
 def add_context(f: ifcopenshell.file) -> ifcopenshell.entity_instance:
@@ -71,3 +78,42 @@ class TestOptimise(test.bootstrap.IFC4):
         assert len(output.by_type("IfcPolyline")) == 2
         walls_after = output.by_type("IfcWall")
         assert curve_of(walls_after[0]) != curve_of(walls_after[1])
+
+    def test_folding_still_works_on_the_pure_python_fallback(self):
+        context = add_context(self.file)
+        coords = [(0.0, 0.0), (1.0, 0.0), (1.0, 1.0), (0.0, 0.0)]
+        add_wall_with_curve(self.file, context, coords)
+        add_wall_with_curve(self.file, context, coords)
+
+        with mock.patch.dict(sys.modules, {"igraph": None}):
+            output = ifcpatch.execute({"file": self.file, "recipe": "Optimise", "arguments": []})
+
+        assert len(output.by_type("IfcPolyline")) == 1
+
+
+GRAPH = {4: {2, 3}, 2: {1}, 3: {1}, 1: set()}
+
+
+def assert_dependency_order(order: list[int], graph: dict[int, set[int]]) -> None:
+    position = {id_: i for i, id_ in enumerate(order)}
+    assert sorted(order) == sorted(graph)
+    for id_, references in graph.items():
+        for reference in references:
+            assert position[reference] < position[id_]
+
+
+class TestToposortBackends:
+    def test_igraph_backend_orders_dependencies_first(self):
+        pytest.importorskip("igraph")
+        assert_dependency_order(_toposort(GRAPH, logging.getLogger(__name__)), GRAPH)
+
+    def test_igraph_backend_includes_references_missing_from_the_keys(self):
+        pytest.importorskip("igraph")
+        assert _toposort({2: {1}}, logging.getLogger(__name__)) == [1, 2]
+
+    def test_pure_python_fallback_warns_when_igraph_is_unavailable(self, caplog):
+        with mock.patch.dict(sys.modules, {"igraph": None}):
+            with caplog.at_level(logging.WARNING):
+                order = _toposort(GRAPH, logging.getLogger(__name__))
+        assert_dependency_order(order, GRAPH)
+        assert "python-igraph" in caplog.text