Merge branch 'c-screw-undetermined' into pool-C

tests/SearchSpaceGroupTest.cpp

@@ -446,6 +446,54 @@ TEST_CASE("SearchSpaceGroup names an orthorhombic screw pair on the axes it lies
     }
 }
 
+// A screw on a row the sweep never recorded is not a group the data refused, it is a question
+// nobody asked: the run writes the member claiming no screw because a reflection file must carry
+// one group, and without this the only trace of the ambiguity is a list of names that does not say
+// which axis is open. Modelled on a real hexagonal set whose 00l row lies in the spindle's blind
+// cone and which is reported as P 6 against a deposited P 63.
+TEST_CASE("SearchSpaceGroup says which axis a missing row left the screw open on",
+          "[SearchSpaceGroup]") {
+    const gemmi::SpaceGroup& sg = gemmi::get_spacegroup_by_name("P 21 21 21");
+    const auto merged = GenerateMergedReflectionsForSpaceGroup(sg, 14);
+
+    SearchSpaceGroupOptions opt;
+    opt.merge_friedel = true;
+    opt.lattice_system = gemmi::CrystalSystem::Orthorhombic;
+
+    SECTION("every row measured - nothing is open") {
+        const auto result = SearchSpaceGroup(merged, opt);
+        INFO(SearchSpaceGroupResultToText(result));
+        REQUIRE(result.best_space_group.has_value());
+        CHECK(result.best_space_group->xhm() == "P 21 21 21");
+        CHECK(result.undetermined_screws.empty());
+    }
+
+    SECTION("the 00l row removed - the c screw is undetermined") {
+        std::vector<MergedReflection> without_00l;
+        for (const auto& r : merged)
+            if (r.h != 0 || r.k != 0)
+                without_00l.push_back(r);
+        const auto result = SearchSpaceGroup(without_00l, opt);
+        INFO(SearchSpaceGroupResultToText(result));
+        REQUIRE(result.best_space_group.has_value());
+        REQUIRE(result.undetermined_screws.size() == 1);
+        CHECK(result.undetermined_screws[0].axis == 'c');
+        CHECK(result.undetermined_screws[0].row_label == "00l");
+        CHECK(result.undetermined_screws[0].n_observed == 0);
+        // The a and b screws were measured and are unaffected: what the missing row costs is the
+        // third condition, not the two the data still carry.
+        CHECK(result.best_space_group->operations().is_systematically_absent({{1, 0, 0}}));
+        CHECK(result.best_space_group->operations().is_systematically_absent({{0, 1, 0}}));
+        // ...and the group the c row would have decided between is offered, not silently dropped.
+        bool offers_a_group_without_the_c_screw = false;
+        for (const auto& alt : result.alternatives)
+            if (!alt.operations().is_systematically_absent({{0, 0, 1}}))
+                offers_a_group_without_the_c_screw = true;
+        CHECK((offers_a_group_without_the_c_screw
+               || !result.best_space_group->operations().is_systematically_absent({{0, 0, 1}})));
+    }
+}
+
 // The centring half of the same widening. A, B and C centring on one orthorhombic cell are three
 // different lattices, and only C is a reference setting, so an A-centred crystal used to have its
 // centring refused (its absent class is not the one C predicts) and came out primitive. The