perf(pfs): code→family index for anchor_metadata (refs #688)

code_family_index(families) builds a code -> family key map once so a
restamp run of ~1.15M chunks doesn't rescan every family per code.
anchor_metadata gains an optional code_index kwarg that looks family
membership up through it instead.

Ruling A5: families with fewer than two codes are excluded from the
index and from the default unindexed scan — the derived registry is
~17,187 keys and all but a handful are single-code stem families,
where a 'families' anchor would just echo the code already in
'codes'. Only real groupings (today: ACP, CCM, PCM, TCM, APCM) get a
families anchor.

src/pfs/anchors.py

@@ -11,26 +11,67 @@ from pfs.elements import parse_descriptor
 from pfs.families import FAMILIES, Family, find_codes
 
 
+def code_family_index(families: Mapping[str, Family]) -> dict[str, str]:
+    """Code -> family key, built once so ``anchor_metadata`` doesn't scan
+    every family for every chunk (a restamp run calls it ~1.15M times).
+
+    Single-code families are excluded (Ruling A5): the derived registry
+    is ~17,187 keys, all but a handful stem families with exactly one
+    code, and a 'families' anchor on one of those would just echo the
+    code already in 'codes'. Only families with two or more codes (today:
+    ACP, CCM, PCM, TCM, APCM) are real groupings worth a separate anchor.
+
+    A code that belongs to two qualifying families keeps the first family
+    reached in *families*' iteration (dict) order — same tie-break a
+    caller would see from scanning in that order."""
+    out: dict[str, str] = {}
+    for key, fam in families.items():
+        if len(fam.codes) < 2:
+            continue
+        for code in fam.codes:
+            out.setdefault(code, key)
+    return out
+
+
 def anchor_metadata(
-    text: str, *, families: Mapping[str, Family] | None = None
+    text: str,
+    *,
+    families: Mapping[str, Family] | None = None,
+    code_index: Mapping[str, str] | None = None,
 ) -> dict[str, str]:
     """Extract codes, families, and elements from text.
 
     Returns a dict with keys:
     - 'codes': space-joined sorted unique codes (from find_codes)
-    - 'families': space-joined sorted unique family keys
+    - 'families': space-joined sorted unique family keys (families with
+      only one code are excluded — see ``code_family_index``)
     - 'elements': space-joined sorted unique element slugs (type=value, no detail)
 
     Args:
         text: The text to analyze
         families: Optional mapping of family keys to Family objects. Defaults to pfs.families.FAMILIES
+        code_index: Optional precomputed ``code_family_index(families)``. When
+            given, family lookup goes through it instead of scanning every
+            family for every code — same result, built once per run by a
+            caller like the restamp backfill.
 
     Returns:
         dict with 'codes', 'families', and 'elements' keys
     """
     fams = FAMILIES if families is None else families
     codes = find_codes(text)
-    keys = sorted({k for k, f in fams.items() for c in codes if c in f.codes})
+    if code_index is not None:
+        keys = sorted({code_index[c] for c in codes if c in code_index})
+    else:
+        keys = sorted(
+            {
+                k
+                for k, f in fams.items()
+                if len(f.codes) >= 2
+                for c in codes
+                if c in f.codes
+            }
+        )
     elements = sorted({f"{e.type.value}={e.value}" for e in parse_descriptor(text)})
     return {
         "codes": " ".join(codes),

tests/pfs/test_anchors.py

@@ -1,7 +1,7 @@
 """pfs.anchors — metadata for chunks: codes, families, and element slugs."""
 
-from pfs.anchors import anchor_metadata
-from pfs.families import Family
+from pfs.anchors import anchor_metadata, code_family_index
+from pfs.families import FAMILIES, Family
 
 
 def test_codes_families_elements():
@@ -23,5 +23,33 @@ def test_empty_and_unknown():
         "families": "",
         "elements": "",
     }
-    fams = {"X": Family("X", "X fam", ("12345",), ("x fam",))}
+    fams = {"X": Family("X", "X fam", ("12345", "23456"), ("x fam",))}
     assert anchor_metadata("code 12345", families=fams)["families"] == "X"
+
+
+def test_code_index_matches_unindexed_scan():
+    text = "Use 99439 in conjunction with 99490; consent; per calendar month"
+    index = code_family_index(FAMILIES)
+    assert anchor_metadata(text, code_index=index) == anchor_metadata(text)
+
+
+def test_code_index_keeps_first_family_on_collision():
+    fams = {
+        "A": Family("A", "A fam", ("12345", "23456"), ("a fam",)),
+        "B": Family("B", "B fam", ("12345", "34567"), ("b fam",)),
+    }
+    index = code_family_index(fams)
+    assert index["12345"] == "A"
+
+
+def test_single_code_family_never_stamped():
+    """A stem-derived family of exactly one code (~17,182 of 17,187
+    derived keys) is not a grouping worth its own anchor — it would just
+    echo the code already in 'codes' (Ruling A5)."""
+    fams = {"SOLO": Family("SOLO", "Solo", ("54321",), ("solo",))}
+    assert "SOLO" not in code_family_index(fams)
+    assert anchor_metadata("code 54321", families=fams)["families"] == ""
+    index = code_family_index(fams)
+    assert (
+        anchor_metadata("code 54321", families=fams, code_index=index)["families"] == ""
+    )