Slice S0380.128: extractor §14.0 closure falls back to "14.1 Community Heating"

Elmhurst Summary §14.0 Main Heating1 normally closes at "14.1 Main
Heating2", but community-heated dwellings and "no system" certs lodge
§14.0 followed directly by "14.1 Community Heating/Heat Network" (no
second main system exists on a community-heated dwelling). Pre-slice
the extractor's `_between("14.0 Main Heating1", "14.1 Main Heating2")`
returned an empty string for these shapes — every §14.0 field
(including `Main Heating SAP Code`) came back None, then the mapper
strict-raised `UnmappedElmhurstLabel` with "§14.0 Main Heating1 has
neither PCDF boiler reference (None) nor SAP code (None)".

The fix adds a `_section_lines_first_end(start, ends)` helper that
accepts a tuple of end-marker candidates and uses whichever appears
first after `start`. `_extract_main_heating` now closes §14.0 at
either "14.1 Main Heating2" or "14.1 Community Heating" — whichever
Summary lodges.

Impact on heating-systems corpus 001431 at `sap worksheets/heating
systems examples/`:

  Variant                  Pre-S0380.128 -> Post-S0380.128
  ------------------------ ------------------ -----------------
  community heating 1      mapper-raise   ->  SAP code 301 OK
  community heating 2      mapper-raise   ->  SAP code 302 OK
  community heating 3      mapper-raise   ->  SAP code 304 OK
  community heating 4      mapper-raise   ->  SAP code 302 OK
  community heating 6      mapper-raise   ->  SAP code 302 OK
  no system                mapper-raise   ->  SAP code 699 OK

Corpus tally: **35/41 -> 41/41 cascade-OK**. With all populated
variants now executing, the cascade-vs-worksheet residual cluster is
fully visible for the first time. Notably community heating 6 surfaces
the FIRST negative ΔSAP in the corpus (-6.87 — cascade undershooting
the worksheet rather than overshooting), a distinct diagnostic shape
worth investigating next.

The fix is structural (extractor section bracketing) — no spec rule
to cite. RdSAP 10 §17 page 85 row 1.0 ("Main Heating") + §17 row
10-1a ("Community Heat Source") confirm that community-heated certs
have only one main heating system (no Main 2 block).

Extended handover suite at HEAD post-slice: **832 pass, 0 fail**
(was 831 + 1 new AAA test).

Pyright net-zero on touched files (13 → 13 — pre-existing errors
unrelated).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/documents_parser/elmhurst_extractor.py

@@ -128,6 +128,32 @@ class ElmhurstSiteNotesExtractor:
         text = self._between(start, end)
         return [l.strip() for l in text.splitlines() if l.strip()]
 
+    def _section_lines_first_end(
+        self, start: str, ends: tuple[str, ...],
+    ) -> List[str]:
+        """Like `_section_lines` but accepts multiple end-marker candidates
+        and uses whichever appears first after `start`. Defends against
+        Summary-shape variants where the next-section heading differs
+        (e.g. §14.0 Main Heating1 closes at "14.1 Main Heating2" on
+        boiler/HP certs but at "14.1 Community Heating" on community-
+        heated certs)."""
+        try:
+            s = self._text.index(start) + len(start)
+        except ValueError:
+            return []
+        earliest: int | None = None
+        for end in ends:
+            try:
+                idx = self._text.index(end, s)
+            except ValueError:
+                continue
+            if earliest is None or idx < earliest:
+                earliest = idx
+        if earliest is None:
+            return []
+        text = self._text[s:earliest]
+        return [l.strip() for l in text.splitlines() if l.strip()]
+
     def _local_val(self, lines: List[str], label: str) -> Optional[str]:
         lb = label.rstrip(":")
         lc = lb + ":"
@@ -1171,7 +1197,18 @@ class ElmhurstSiteNotesExtractor:
         )
 
     def _extract_main_heating(self) -> MainHeating:
-        lines = self._section_lines("14.0 Main Heating1", "14.1 Main Heating2")
+        # Community-heated dwellings (e.g. SAP code 301 "Community heating
+        # scheme" per SAP10.2 Table 4a category 6) and "no system" certs
+        # (SAP code 699 "Electric heaters assumed where no system lodged")
+        # lodge §14.0 Main Heating1 directly followed by §14.1 Community
+        # Heating/Heat Network rather than §14.1 Main Heating2 — there is
+        # no second main system on a community-heated dwelling. Close the
+        # §14.0 block at whichever §14.1 form appears first so every
+        # Summary shape surfaces the SAP code.
+        lines = self._section_lines_first_end(
+            "14.0 Main Heating1",
+            ("14.1 Main Heating2", "14.1 Community Heating"),
+        )
         pct_raw = self._local_val(lines, "Percentage of Heat")
         pct = int(pct_raw.split()[0]) if pct_raw else 0
         # §14.0 "Main Heating SAP Code" identifies Main 1 by SAP 10.2

backend/documents_parser/tests/test_summary_pdf_mapper_chain.py

@@ -236,6 +236,40 @@ def test_summary_001479_mapper_extensions_count_matches_extension_bps() -> None:
     assert len(epc.sap_building_parts) == 3
 
 
+def test_summary_001431_community_heating_1_main_heating_sap_code_extracted_when_no_main_heating_2_block() -> None:
+    # Arrange — Heating-systems corpus fixture 001431 / "community heating 1"
+    # lodges §14.0 Main Heating1 directly followed by §14.1 Community
+    # Heating/Heat Network (no §14.1 Main Heating2 block, since community-
+    # heated dwellings don't have a second main system to lodge). The §14.0
+    # block carries `Main Heating SAP Code: 301` (Community heating per
+    # SAP10.2 Table 4a category 6 — "Heat networks").
+    #
+    # Pre-slice the extractor's `_section_lines("14.0 Main Heating1",
+    # "14.1 Main Heating2")` returned an empty list because the end marker
+    # was missing, so every §14.0 field (incl. `Main Heating SAP Code`)
+    # came back as None. The mapper then raised `UnmappedElmhurstLabel`
+    # with "§14.0 Main Heating1 has neither PCDF boiler reference (None)
+    # nor SAP code (None)" — blocking all 6 community-heated + "no system"
+    # corpus variants from cascade execution.
+    #
+    # The fix closes the §14.0 block at whichever §14.1 marker appears
+    # first ("14.1 Main Heating2" or "14.1 Community Heating"), so the
+    # SAP code surfaces correctly on every Summary shape.
+    summary_pdf = (
+        Path(__file__).parents[3]
+        / "sap worksheets/heating systems examples/community heating 1/Summary_001431.pdf"
+    )
+    pages = _summary_pdf_to_textract_style_pages(summary_pdf)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    main_1 = epc.sap_heating.main_heating_details[0]
+    assert main_1.sap_main_heating_code == 301
+
+
 def test_summary_001431_pcdb_1_inaccessible_cylinder_resolves_to_normal_per_rdsap_10_table_28() -> None:
     # Arrange — Heating-systems corpus fixture 001431 / "pcdb 1" lodges
     # §15.1 "Cylinder Size: No Access" (the Elmhurst inaccessible-cylinder