Reindex into the ASU: follow the change of hand, not the hand landed on

ReindexMergedIntoAsu swapped I(+)/I(-) whenever the reindexed index came out on
the minus side of its Friedel pair. That is right only for a row that started on
the plus side, which is every row when merge_friedel is set - and none of the
problem cases.

With the mates kept apart, the merge stores the plus mate at +hkl_asu and the
minus mate at -hkl_asu (Merge.cpp), while RotationScaleMerge attaches I_plus /
I_minus in the plus convention on both rows alike, keying the lookup at
plus=true. An alternative-indexing operator is rotation-type, so op(-x) == -op(x)
and the two mates always land on opposite sides: testing key.plus alone therefore
swaps exactly one row of every pair, whichever way the operator went. The pair
ends up self-contradictory, and the mmCIF's pdbx_I_plus / pdbx_I_minus and the
F(+)/F(-) columns are swapped on half the rows.

Reached by "rugnux -A --model model.pdb" - -A clears MergeFriedel - on a crystal
whose indexing ambiguity the model resolves, which is the one path that both
keeps the mates apart and reindexes.

The swap now fires on was_plus != key.plus. With merge_friedel every row is
stored at the ASU representative, so was_plus is always true and this reduces to
the previous test; the existing change-of-hand case is unchanged.

The test that covered the merge_friedel=false path checked only I, so it passed
either way. The new one builds both mates of a pair and asserts they still agree
about which intensity is which afterwards - an invariant that holds however the
operator moves the hand, so it fails on the old code for either direction.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016L1qig74oYQzfUJJZbbxFh

tests/ReindexAmbiguityTest.cpp

@@ -142,3 +142,51 @@ TEST_CASE("Reindex into the ASU: the change of hand swaps the Bijvoet halves", "
     CHECK(anom[0].l == -asu.l);
     CHECK(anom[0].I == 100.0f);
 }
+
+TEST_CASE("Reindex into the ASU: both mates of an anomalous pair follow the same hand", "[reindex]") {
+    // With the mates kept apart, the merge stores the plus mate at +hkl_asu and the minus mate at
+    // -hkl_asu, while I_plus/I_minus are attached in the plus convention on BOTH rows alike. An
+    // alternative-indexing operator is rotation-type, so op(-x) == -op(x): whatever it does to one
+    // mate's hand it does to the other's, and the two rows must still agree afterwards about which
+    // intensity is which. Deciding the swap from the hand the new index lands on instead of from the
+    // CHANGE of hand moves exactly one of the two, and the pair ends up self-contradictory.
+    const gemmi::SpaceGroup *sg = gemmi::find_spacegroup_by_number(75);   // P4, Laue 4/m
+    REQUIRE(sg != nullptr);
+    const HKLKeyGenerator key_gen(/*merge_friedel=*/false, *sg);
+    const HKLKey asu = key_gen(3, 1, 2);
+    REQUIRE(asu.plus);
+
+    // The alternative indexing of a P4 lattice: 4/mmm holds it, 4/m does not.
+    const gemmi::Op op = gemmi::parse_triplet("k,h,-l");
+
+    MergedReflection plus;
+    plus.h = asu.h; plus.k = asu.k; plus.l = asu.l;
+    plus.I = 100.0f; plus.sigma = 1.0f; plus.d = 10.0f;
+    plus.I_plus = 110.0f; plus.sigma_plus = 2.0f; plus.I_minus = 90.0f; plus.sigma_minus = 3.0f;
+    plus.F_plus = 10.5f; plus.F_minus = 9.5f;
+
+    MergedReflection minus = plus;                   // same anomalous split, stored at -hkl_asu
+    minus.h = -asu.h; minus.k = -asu.k; minus.l = -asu.l;
+
+    const auto out = ReindexMergedIntoAsu({plus, minus}, op, 75, /*merge_friedel=*/false);
+    REQUIRE(out.size() == 2);
+
+    // The two rows are still one pair: same Laue label up to the Friedel sign, opposite hands.
+    CHECK(out[0].h == -out[1].h);
+    CHECK(out[0].k == -out[1].k);
+    CHECK(out[0].l == -out[1].l);
+
+    // ... and they still tell the same story about which hand carries which intensity.
+    CHECK(out[0].I_plus == out[1].I_plus);
+    CHECK(out[0].I_minus == out[1].I_minus);
+    CHECK(out[0].sigma_plus == out[1].sigma_plus);
+    CHECK(out[0].sigma_minus == out[1].sigma_minus);
+    CHECK(out[0].F_plus == out[1].F_plus);
+    CHECK(out[0].F_minus == out[1].F_minus);
+
+    // And the swap did have to happen: "k,h,-l" takes (3,1,2) to (1,3,-2), whose Laue class has its
+    // ASU representative at l > 0, so the row that was the plus mate of its pair is the minus mate of
+    // the new one. The intensity now standing at the plus index is the one that was at -hkl before.
+    CHECK(out[0].I_plus == 90.0f);
+    CHECK(out[0].I_minus == 110.0f);
+}