test(#285) regression test for horizontal-bisecting on trigonal-rhombohedral

tests/test_regression_issue_285.py

@@ -0,0 +1,119 @@
+# Copyright (c) 2026 UChicago Argonne, LLC
+# SPDX-License-Identifier: LicenseRef-ANL-Open-Source-License
+"""
+Regression tests for issue #285.
+
+Issue #285 reported that three horizontal-bisecting modes returned
+zero forward solutions for the trigonal-rhombohedral reflection
+``(1, 1, 0)`` on a single-parameter rhombohedral cell
+(``a = b = c = 7.0``, ``alpha = beta = gamma = 72°``):
+
+* ``fourch  / bisecting``
+* ``psic    / bisecting_vertical``
+* ``kappa6c / bisecting_horizontal``
+
+Investigation summary
+---------------------
+
+Replaying the issue's self-contained reproducer (verbatim) in a
+clean Python 3.14 / numpy 2.4 environment across released versions
+shows that two of the three claims do not reproduce on any version
+of the package, and the third was already fixed before the issue
+was filed:
+
++------------------+--------------------+---------------------------+--------------------------------+
+|     version      | fourch / bisecting | psic / bisecting_vertical | kappa6c / bisecting_horizontal |
++==================+====================+===========================+================================+
+| v0.10.0          | 2 valid solutions  | 2 valid solutions         | 0 solutions (real failure)     |
++------------------+--------------------+---------------------------+--------------------------------+
+| v0.10.1          | 2 valid solutions  | 2 valid solutions         | 0 solutions (real failure)     |
++------------------+--------------------+---------------------------+--------------------------------+
+| v0.11.0 (PyPI)   | 2 valid solutions  | 2 valid solutions         | 2 valid solutions              |
++------------------+--------------------+---------------------------+--------------------------------+
+| main (post-#284) | 2 valid solutions  | 2 valid solutions         | 2 valid solutions              |
++------------------+--------------------+---------------------------+--------------------------------+
+
+Every returned solution round-trips correctly through ``inverse()``
+back to ``(1, 1, 0)``.
+
+The ``kappa6c bisecting_horizontal`` zero-solution failure on
+v0.10.x was eliminated by the issue #280 composition-order overhaul
+(merged in v0.11.0) — most likely the B-matrix correction in phase 3,
+which changed the reciprocal-lattice geometry for non-orthogonal
+cells.  The ``fourch bisecting`` and ``psic bisecting_vertical``
+claims in the issue body could not be reproduced on any released
+version; the most plausible explanation is that the issue's
+"self-contained reproducer" was synthesized from a downstream
+cross-validation harness that uses a *bootstrapped* UB (built from
+``calc_UB`` of two seed reflections at chosen positions) rather than
+``ub_identity``, and the simplification to ``ub_identity`` lost the
+specific UB orientation that produced the zero-solution branch
+selection.
+
+No code change was required to close issue #285.  These tests pin
+down the working behavior so any future regression on the rhombo-
+hedral path will be caught immediately.
+"""
+
+from __future__ import annotations
+
+from contextlib import nullcontext as does_not_raise
+
+import numpy as np
+import pytest
+
+import ad_hoc_diffractometer as ahd
+
+# Trigonal-rhombohedral single-parameter cell from the issue's
+# reproducer block.
+TRIGONAL_RHOMBOHEDRAL = dict(a=7.0, b=7.0, c=7.0, alpha=72.0, beta=72.0, gamma=72.0)
+WAVELENGTH = 1.54
+
+
+# ---------------------------------------------------------------------------
+# Issue #285 reproducer: the three (geometry, mode) cases solve.
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.parametrize(
+    "geom_name, mode_name, context",
+    [
+        pytest.param("fourch", "bisecting", does_not_raise(), id="fourch-bisecting"),
+        pytest.param(
+            "psic",
+            "bisecting_vertical",
+            does_not_raise(),
+            id="psic-bisecting_vertical",
+        ),
+        pytest.param(
+            "kappa6c",
+            "bisecting_horizontal",
+            does_not_raise(),
+            id="kappa6c-bisecting_horizontal",
+        ),
+    ],
+)
+def test_horizontal_bisecting_solves_trigonal_rhombohedral_110(
+    geom_name, mode_name, context
+):
+    """``forward(1, 1, 0)`` returns at least one solution that round-trips.
+
+    Direct copy of the reproducer block in the issue body.  Pre-fix
+    (v0.10.x) ``kappa6c bisecting_horizontal`` returned ``n_sols=0``
+    here; the other two cases returned valid solutions on every
+    released version.
+    """
+    g = ahd.make_geometry(geom_name)
+    g.sample.lattice = ahd.Lattice(**TRIGONAL_RHOMBOHEDRAL)
+    g.wavelength = WAVELENGTH
+    ahd.ub_identity(g.sample)
+    g.mode_name = mode_name
+    with context:
+        sols = g.forward(1, 1, 0)
+        assert len(sols) >= 1, (
+            f"{geom_name} / {mode_name}: expected at least one solution "
+            f"for trigonal-rhombohedral (1, 1, 0), got {len(sols)}."
+        )
+        for sol in sols:
+            hkl_back = g.inverse(sol)
+            np.testing.assert_allclose(hkl_back, (1.0, 1.0, 0.0), atol=1e-6)