Add postcode boundary calculation

pipeline/transform/postcode_boundaries/oa_boundaries.py

@@ -0,0 +1,40 @@
+import sqlite3
+from pathlib import Path
+
+from shapely import wkb
+from shapely.geometry import MultiPolygon, Polygon
+
+_ENVELOPE_SIZES = {0: 0, 1: 32, 2: 48, 3: 48, 4: 64}
+
+
+def parse_gpkg_geometry(blob: bytes):
+    """Extract a Shapely geometry from a GeoPackage binary blob."""
+    flags = blob[3]
+    envelope_type = (flags >> 1) & 0x07
+    envelope_size = _ENVELOPE_SIZES.get(envelope_type)
+    if envelope_size is None:
+        raise ValueError(
+            f"Unknown GeoPackage envelope type {envelope_type} (expected 0-4)"
+        )
+    header_size = 8 + envelope_size
+    return wkb.loads(blob[header_size:])
+
+
+def load_oa_boundaries(gpkg_path: Path) -> dict[str, Polygon | MultiPolygon]:
+    """Load OA boundary polygons from a GeoPackage. Geometry is already in BNG."""
+    print("Loading OA boundaries...")
+
+    conn = sqlite3.connect(str(gpkg_path))
+    cur = conn.cursor()
+    cur.execute("SELECT OA21CD, SHAPE FROM OA_2021_EW_BGC_V2")
+
+    oa_geoms: dict[str, Polygon | MultiPolygon] = {}
+    for oa_code, blob in cur:
+        geom = parse_gpkg_geometry(bytes(blob))
+        if geom.geom_type == "MultiPolygon" and len(geom.geoms) == 1:
+            geom = geom.geoms[0]
+        oa_geoms[oa_code] = geom
+
+    conn.close()
+    print(f"  Loaded {len(oa_geoms)} OA boundaries")
+    return oa_geoms