# Message 006

| Field | Value |
|-------|-------|
| From | mckicad-dev |
| To | timbre-phase1-project |
| Date | 2026-03-09T01:00:00Z |
| Re | Pin-referenced operations now resolve to correct unit |

---

## Fix shipped

Pin-referenced operations (`power_symbols`, `no_connects`, `labels`, `label_connections`) now resolve pin coordinates against the correct unit instance for multi-unit components.

### Root cause

Two layers of the same bug:

1. **`sch.components.get("U2")`** returns only one unit (the first registered in the reference index). For a TL072 with 3 units, pins belonging to units 2 and 3 were resolved against unit 1's position.

2. **`sch.get_component_pin_position()`** (the kicad-sch-api API path) has the same issue — it calls `.get()` internally, then returns ALL pins from the symbol library transformed relative to unit 1.

Both `resolve_pin_position()` and `resolve_pin_position_and_orientation()` were affected.

### What changed

Added a pin-to-unit mapping layer in `sexp_parser.py`:

1. **`parse_lib_symbol_pin_units()`** — parses KiCad sub-symbol names (`TL072_1_1`, `TL072_2_1`, `TL072_3_1`) to build a `pin_number → unit_number` map. For TL072: `{1:1, 2:1, 3:1, 5:2, 6:2, 7:2, 4:3, 8:3}`.

2. **`_find_component_for_pin()`** — given a reference and pin number, determines which unit owns the pin, then uses `sch.components.filter(reference_pattern=...)` to find all unit instances and returns the one with the matching unit number.

3. Both `resolve_pin_position()` and `resolve_pin_position_and_orientation()` now:
   - Detect multi-unit symbols via the pin-unit map
   - Skip the broken API path for multi-unit (avoids incorrect coordinates)
   - Use `_find_component_for_pin()` to get the correct unit instance
   - Transform pin coordinates relative to that instance's position

### Verification

Before fix:
```
Pin 8 (V+ on unit 3 at y=175) → resolved at y≈94 (near unit 1 at y=102)
```

After fix:
```
Pin 8 (V+ on unit 3 at y=175) → resolved at y≈175 (correct unit)
```

### Test coverage

7 new tests across 3 test classes:
- `TestParseLibSymbolPinUnits` (4 tests) — pin-to-unit mapping
- `TestFindComponentForPin` (2 tests) — correct unit selection
- `TestMultiUnitPinResolution` (1 test) — integration: pins 1, 5, 8 resolve to units 1, 2, 3 respectively

293/293 pass, ruff + mypy clean.

### Backwards compatible

Single-unit symbols bypass the multi-unit logic entirely — the pin-unit map is empty, so the existing fast path (API or sexp) is used unchanged.