# SAP calculator — agent guide (start here)

This is the **canonical onboarding doc** for working on the SAP 10.2 /
RdSAP 10 calculator. It is meant to get you productive **without reading
any historical handover**. The `HANDOVER_*.md` files in this directory
are point-in-time session notes (useful for the specific residual they
chase, ignore otherwise). For deep architecture/API see
[`SAP_CALCULATOR.md`](SAP_CALCULATOR.md).

Three things this doc gives you: (1) the **accuracy bar** for the two
input paths, (2) the **debugging loop**, (3) the **tools & pipeline**.

---

## 0. The one-paragraph mental model

A cert's data comes in via one of two front-ends — an **Elmhurst Summary
PDF** (site-notes path) or an **EPC-register API JSON** (API path). Both
map to the same typed `EpcPropertyData`, which feeds a deterministic
cascade that reproduces the RdSAP10 engine. Our **ground truth is the
Elmhurst worksheet PDF** (U985 / P960 / dr87) — the per-line `(1)..(286)`
calculation, not the rounded values the EPC register lodges. We pin the
cascade against the worksheet to **abs = 1e-4 on every line ref**.

---

## 1. Accuracy expectations — site-notes vs API

The worksheet PDF is **always** the target. The EPC register's lodged
SAP/CO2/PE are rounded *and* carry Elmhurst's own residual, so matching
the lodged values is not the goal — matching the worksheet is.

| Path | Input | When a worksheet PDF exists for the cert | API/site-notes-only (no worksheet) |
|---|---|---|---|
| **Site-notes** | Elmhurst Summary PDF → extractor → `from_elmhurst_site_notes` | **abs = 1e-4** on continuous SAP **and every populated line ref** and cost / CO2 / PE | n/a (we always have the worksheet for site-notes fixtures) |
| **API** | register JSON → `from_api_response` | **abs = 1e-4** on continuous SAP vs the worksheet (same bar as site-notes — the two paths must converge) | **±0.5** SAP vs the lodged register value (fallback only) |

Three rules that fall out of this:

- **Cross-mapper parity.** For a cert that has both an API JSON and an
  Elmhurst Summary, the two paths must produce SAP within **1e-4 of each
  other** *and* of the worksheet. The cascade output (not a structural
  EPC diff) is the equivalence check. A divergence localises to one
  mapper.
- **No tolerance widening.** A failing 1e-4 pin is a real cascade bug or
  a fixture defect — diagnose it, don't relax it. No `rel=`, no `xfail`,
  no adaptive ceilings. ΔSAP = 0.07 is **not** "closed".
- **±0.5 is a fallback, not a destination.** It's only for API-only
  certs with no worksheet to check against. If you can get a worksheet,
  the bar is 1e-4.

Two documented, deliberate exceptions to "match the spec literal" live
in [`SAP_CALCULATOR.md` §8](SAP_CALCULATOR.md) ("Elmhurst-mirrored spec
divergences") — cases where the BRE-approved Elmhurst engine diverges
from the SAP 10.2 text and we mirror the engine. Add a §8 row only with
≥2-cert evidence.

---

## 2. The tools & pipeline

### 2.1 The two PDFs per cert

- **`Summary_NNNNNN.pdf`** — the Elmhurst **site notes / input**. This is
  what the assessor lodged: dimensions, fabric, heating system, controls,
  cylinder, etc. It is the INPUT, equivalent to the API JSON.
- **The worksheet** — the **ground truth output**, every line ref
  `(1)..(286)` to 4 d.p. Three families, all the same format:
  - `U985-0001-NNNNNN.pdf` — the 6 gas-combi conformance fixtures.
  - `P960-0001-NNNNNN.pdf` — the heating-systems corpus + community heating.
  - `dr87-0001-NNNNNN.pdf` — the API-paired cohort ("Additional data with api").

### 2.2 The cascade pipeline (site-notes path)

```python
import subprocess, re
from pathlib import Path
from backend.documents_parser.elmhurst_extractor import ElmhurstSiteNotesExtractor
from datatypes.epc.domain.mapper import EpcPropertyDataMapper
from domain.sap10_calculator.rdsap.cert_to_inputs import (
    cert_to_inputs, cert_to_demand_inputs, local_climate_for_cert,
)
from domain.sap10_calculator.calculator import calculate_sap_from_inputs

# 1. Summary PDF -> per-page text (pdftotext -layout, one string per page)
def summary_pdf_to_pages(pdf: Path) -> list[str]:
    n = int(re.search(r"Pages:\s+(\d+)",
            subprocess.run(["pdfinfo", str(pdf)], capture_output=True, text=True).stdout).group(1))
    pages = []
    for i in range(1, n + 1):
        layout = subprocess.run(
            ["pdftotext", "-layout", "-f", str(i), "-l", str(i), str(pdf), "-"],
            capture_output=True, text=True).stdout
        pages.append("\n".join(
            tok for line in layout.splitlines() for tok in re.split(r"\s{2,}", line.strip()) if tok))
    return pages

pages      = summary_pdf_to_pages(Path("sap worksheets/.../Summary_NNNNNN.pdf"))
site_notes = ElmhurstSiteNotesExtractor(pages).extract()         # -> ElmhurstSiteNotes
epc        = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)  # -> EpcPropertyData

# 2. Two cascades. RATING = SAP/EI rating (UK-avg climate, region 0).
#    DEMAND = Current Carbon / Current PE / Fuel Bill (postcode climate, PCDB Table 172).
rating = calculate_sap_from_inputs(cert_to_inputs(epc))
demand = calculate_sap_from_inputs(cert_to_demand_inputs(epc))   # climate = local_climate_for_cert(epc)

rating.sap_score_continuous   # un-rounded SAP — pin THIS, not the integer
rating.total_fuel_cost_gbp
rating.co2_kg_per_yr
demand.primary_energy_kwh_per_yr
```

Shortcut: `Sap10Calculator().calculate(epc)` runs the rating cascade
(`cert_to_inputs` → `calculate_sap_from_inputs`) in one call.

### 2.3 The API path

Identical from `EpcPropertyData` onward — only the front-end changes:

```python
import json
data = json.loads(Path("tests/domain/sap10_calculator/rdsap/fixtures/golden/<cert>.json").read_text())
epc  = EpcPropertyDataMapper.from_api_response(data)   # -> EpcPropertyData
# ... same cert_to_inputs / calculate_sap_from_inputs as above
```

### 2.4 Section helpers — intermediate line refs

Every worksheet section has a `<section>_section_from_cert(epc)` helper
returning a typed result with the line-ref values. Use these to inspect
where a residual originates **without** running the whole cascade
(`postcode_climate=` selects rating vs demand):

```python
from domain.sap10_calculator.rdsap.cert_to_inputs import (
    water_heating_section_from_cert,        # §4  (42)..(65)m
    heat_transmission_section_from_cert,    # §3  (26)..(37)
    internal_gains_section_from_cert,       # §5  (66)..(73)
    mean_internal_temperature_section_from_cert,  # §7 (85)..(94)
    space_heating_section_from_cert,        # §8  (95)..(99)
    fuel_cost_section_from_cert,            # §10a (240)..(255)
    environmental_section_from_cert,        # §12  (261)..(274)
    primary_energy_section_from_cert,       # §13a (275)..(286)
)
wh = water_heating_section_from_cert(epc)
wh.energy_content_monthly_kwh   # (45)m ;  wh.output_kwh_per_yr  # (62)/(64)
```

(Full table of helpers + line refs is in [`SAP_CALCULATOR.md` §1.3](SAP_CALCULATOR.md).)

### 2.5 Reading the worksheet from the shell

```bash
# Dump a worksheet line ref (e.g. (217)m water-heater monthly efficiency):
pdftotext -layout "sap worksheets/.../P960-0001-NNNNNN.pdf" - | grep -nE "\(217\)|\(62\)|\(210\)"
# Read a Summary input field (controls, cylinder, fuel):
pdftotext -layout "sap worksheets/.../Summary_NNNNNN.pdf" - | grep -niE "cylinder|control|interlock|fuel"
```

### 2.6 Where the test vectors live

| Set | Location | What |
|---|---|---|
| 6 U985 conformance fixtures | `tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_NNNNNN.py` (+ Summary PDFs in `backend/documents_parser/tests/fixtures/`) | Gas-combi certs, every line ref transcribed as `LINE_*` / `DEMAND_LINE_*` constants. Pinned in `worksheet/test_section_cascade_pins.py` + `worksheet/test_e2e_elmhurst_sap_score.py`. |
| Heating-systems corpus | `sap worksheets/heating systems examples/<variant>/` (Summary + P960) | 41 variants of **one property** with only the heating system changed → any residual is attributable to the heating subsystem. Pinned in `backend/documents_parser/tests/test_heating_systems_corpus.py`. |
| API golden fixtures | `tests/domain/sap10_calculator/rdsap/fixtures/golden/<cert>.json` | Register JSON for the API path. |
| API + worksheet pairs | `sap worksheets/Additional data with api/<cert>/` (Summary + dr87) | Certs that have BOTH an API JSON and a worksheet → cross-mapper parity checks. |

---

## 3. The debugging loop

When a cert's SAP/cost/CO2/PE is off, **never guess a fix** — walk it.

1. **Reproduce & decompose.** Build the epc (extractor+mapper, or a
   fixture's `build_epc()`), run both cascades, and see **which of the
   four outputs** drifts. Cost/CO2/PE drift with the same sign as energy;
   isolate the carrier.
2. **Find the section.** Walk the four metrics back to a worksheet
   section: SAP off but cost EXACT often means a demand/gains issue;
   cost off but energy EXACT means a price/factor issue; CO2/PE off but
   cost EXACT means a factor issue. Use the §2.4 section helpers to get
   the cascade's intermediate line refs.
3. **Per-line compare vs the worksheet.** `pdftotext -layout` the
   worksheet and compare the cascade's `(45)/(56)/(62)/(210)/(217)m/...`
   line-by-line against the PDF. The first diverging line ref is the bug.
4. **Localise to a layer.**
   - cascade value present in worksheet but cascade has 0 / wrong → **calculator** gap (a spec rule not wired, or a dispatch gate).
   - the input field the worksheet used isn't in `epc` → **mapper** (mis-mapped) or **extractor** (didn't capture the Summary field). Audit the Summary PDF for the field first — many lodgements are incomplete and the fixture, not the calculator, is wrong.
5. **Cite the spec.** Find the SAP 10.2 / RdSAP 10 rule (page + line) that
   produces the worksheet's number. Confirm the worksheet matches the
   spec literal; if it diverges, it's a candidate §8 Elmhurst-mirror
   (needs ≥2-cert evidence). **SAP 10.2 only — never 10.3.**
6. **Cross-check vs API (when available).** If the cert has an API JSON
   too, run `from_api_response` through the same cascade. If the API path
   matches the worksheet but the site-notes path doesn't (or vice-versa),
   the bug is in **that mapper**, not the calculator. If both diverge
   identically, it's the **calculator/cascade**.
7. **Fix one cause, re-pin smaller.** TDD: one failing AAA test → one
   impl → re-pin the (now smaller) residual. A spec-correct fix often
   **exposes** the next residual that an offsetting bug was masking —
   that's the next slice, not a regression. Don't conflate
   `main_heating_category` (often `None` on Elmhurst Table 4b boilers)
   with `sap_main_heating_code`.

### Worked shape (real example: oil 6)

Residual +3.05 SAP. (1) HW + space both off. (2) §4 HW efficiency. (3)
worksheet (210) space eff = 75 but Table 4b code 126 = 80; (217)m summer
= 63 = 68−5 → a −5pp penalty. (4) the Summary lodges control `2101` ("no
thermostatic control of room temperature") → no room thermostat → P960
header "Boiler Interlock: No". (5) RdSAP 10 §3 + SAP 10.2 Table 4c(2):
no room thermostat ⇒ not interlocked ⇒ −5pp Space+DHW. Fix the
`no_interlock` gate → space+HW fuel EXACT, residual collapses to a single
exposed pump cause (Table 4f footnote a) ×1.3) → next slice. Two slices,
fully closed.

---

## 4. Run the suite

```bash
PYTHONPATH=/workspaces/model python -m pytest \
    tests/domain/sap10_calculator/ \
    backend/documents_parser/tests/ \
    --no-cov -q -p no:cacheprovider
```

Conformance pins only:

```bash
PYTHONPATH=/workspaces/model python -m pytest \
    tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py \
    tests/domain/sap10_calculator/worksheet/test_e2e_elmhurst_sap_score.py \
    backend/documents_parser/tests/test_heating_systems_corpus.py \
    --no-cov -q
```

Notes:
- `load_cells` tests pin against the gitignored `*.xlsx` reference
  worksheet at repo root; they **skip** when it's absent (CI), run
  locally when present.
- All new code passes `pyright` strict, zero errors. Tests use literal
  `# Arrange / # Act / # Assert` headers and `abs(x - y) <= tol` (not
  `pytest.approx`, which strict-pyright flags).
- Commit one slice per change, with the spec citation in the message.

---

## 5. Spec PDFs on disk

```
domain/sap10_calculator/docs/specs/
  sap-10-2-full-specification-2025-03-14.pdf   # SAP 10.2 (the methodology)
  RdSAP 10 Specification 10-06-2025.pdf         # RdSAP 10 (the reduced-data rules)
  pcdb10.dat / pcdb_table_*.jsonl               # PCDB (boilers, HPs, postcode weather)
```

Pages worth bookmarking: SAP 10.2 §7 MIT (p.28-32), Table 4b boiler eff
(p.168), Table 4c efficiency adjustments (p.169), Table 4e controls
(p.171-174), Table 4f auxiliary energy (p.175), Table 12 factors (p.191),
Appendix U region tables (p.124-127). RdSAP 10 §10 water heating (p.54-56,
incl. §10.7 no-water-heating default), Table 28/29 cylinder defaults,
Table 32 prices (p.95).
```