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pipeline/transform/postcode_boundaries/output.py

@@ -63,10 +63,34 @@ def to_wgs84_geojson(
     }
 
 
+def _fill_holes(geom):
+    """Remove all interior rings (holes) from a polygon or multipolygon."""
+    if geom.geom_type == "Polygon":
+        return Polygon(geom.exterior)
+    elif geom.geom_type == "MultiPolygon":
+        return MultiPolygon([Polygon(p.exterior) for p in geom.geoms])
+    return geom
+
+
+def _largest_polygon(geom):
+    """Extract the largest polygon from a MultiPolygon."""
+    if geom.geom_type == "MultiPolygon":
+        return max(geom.geoms, key=lambda g: g.area)
+    return geom
+
+
 def merge_fragments(
     all_fragments: list[tuple[str, Polygon | MultiPolygon]],
+    greenspace_tree=None,
+    greenspace_geoms=None,
 ) -> dict[str, Polygon | MultiPolygon]:
-    """Merge cross-OA fragments for postcodes spanning multiple OAs."""
+    """Merge cross-OA fragments for postcodes spanning multiple OAs.
+
+    Args:
+        all_fragments: List of (postcode, geometry) pairs.
+        greenspace_tree: Optional STRtree of park/water polygons.
+        greenspace_geoms: Optional list of park/water geometries (indexed by tree).
+    """
     by_postcode: dict[str, list] = defaultdict(list)
     for pc, geom in all_fragments:
         by_postcode[pc].append(geom)
@@ -80,13 +104,25 @@ def merge_fragments(
             combined = make_valid(combined)
         # Close tiny gaps between adjacent OA boundary edges (float mismatches)
         if combined.geom_type == "MultiPolygon":
-            combined = combined.buffer(1.0).buffer(-1.0)
+            combined = combined.buffer(5.0).buffer(-5.0)
             if not combined.is_valid:
                 combined = make_valid(combined)
         # Postcodes are contiguous delivery routes — keep only the largest
         # polygon; small detached fragments are algorithm artifacts
-        if combined.geom_type == "MultiPolygon":
-            combined = max(combined.geoms, key=lambda g: g.area)
+        combined = _largest_polygon(combined)
+        # Remove artifact interior holes from INSPIRE+Voronoi+make_valid chain
+        combined = _fill_holes(combined)
+        # Subtract parks/water if provided
+        if greenspace_tree is not None and greenspace_geoms is not None:
+            from .greenspace import subtract_greenspace
+
+            pre_green = combined
+            combined = subtract_greenspace(combined, greenspace_tree, greenspace_geoms)
+            combined = _largest_polygon(combined)
+            combined = _fill_holes(combined)
+            # Revert if subtraction + fragment selection lost >90% of area
+            if pre_green.area > 0 and combined.area / pre_green.area < 0.1:
+                combined = pre_green
         merged[pc] = combined
     return merged