report: name the direction the anisotropy warning is about

The warning said the diffraction limit "runs from 3.34 to 2.45 A depending on
direction" and stopped there, so a reader was told the crystal is anisotropic
and given no direction to act on. It fires on 19 of 113 datasets in the battery.

The eigenvectors are already measured and already written to the merged mmCIF
as `_reflns.pdbx_aniso_B_tensor_eigenvector_N_ortho`; they had simply never
reached the human report. The tensor is fitted on s = frac.mat * (h,k,l), so in
that Cartesian frame a*, b*, c* are the rows of frac.mat and naming the axis is
one dot product per eigenvector. The label is exact in every Laue class that
has a free tensor direction except triclinic, and the cosine is printed so a
loose fit shows as one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EFEJG6WBQv8th4UJFNe53N

docs/CHANGELOG.md

@@ -17,6 +17,7 @@
 * `rugnux` fits the direction of the goniometer axis and not its length, so the cell chosen by the first pass is the one its own refinement scored.
 * `rugnux` reports the detector geometry it measured - the direct beam, the tilt and the beam centre - and what a single sweep can and cannot determine.
 * `rugnux` reports the resolution the CC1/2 fit reached and the strong-direction diffraction limit.
+* The rugnux anisotropy warning says which reciprocal axis each of the two diffraction limits it quotes belongs to.
 * `rugnux` reports twinning measured before and after the space group was decided, and no longer reports it when the L-test contradicts it.
 * `rugnux --mode calibration` writes `<prefix>.json` beside the `.poni`, holding the geometry as a `jfjoch_broker` `dataset_settings` body.
 * `rugnux --mode calibration` says when a fit is not a measurement: it writes no `.poni`, exits non-zero, and records `converged` in the `.json`.

rugnux/ResultReport.cpp

@@ -37,6 +37,30 @@ namespace {
         return fmt::format("{:.3f} {:.3f} {:.3f} {:.3f} {:.3f} {:.3f}", c.a, c.b, c.c,
                            c.alpha, c.beta, c.gamma);
     }
+
+    // Which reciprocal axis a principal anisotropy direction lies along. The tensor is fitted on
+    // s = frac.mat * (h,k,l), so in that Cartesian frame a*, b*, c* are the rows of frac.mat and the
+    // eigenvector is named by whichever it makes the smallest angle with. The label is exact in every
+    // Laue class the tensor has a free direction in except triclinic; the cosine says how well it fits.
+    std::string ReciprocalAxisLabel(const UnitCell &cell, const double v[3]) {
+        static const char *NAME[3] = {"a*", "b*", "c*"};
+        const gemmi::UnitCell gc = cell;
+        int best = 0;
+        double best_cos = -1.0;
+        for (int i = 0; i < 3; ++i) {
+            const gemmi::Vec3 axis = gc.frac.mat.left_multiply(
+                    gemmi::Vec3(i == 0 ? 1.0 : 0.0, i == 1 ? 1.0 : 0.0, i == 2 ? 1.0 : 0.0));
+            const double len = axis.length();
+            if (!(len > 0.0))
+                continue;
+            const double c = std::fabs((axis.x * v[0] + axis.y * v[1] + axis.z * v[2]) / len);
+            if (c > best_cos) {
+                best_cos = c;
+                best = i;
+            }
+        }
+        return fmt::format("{} (cos {:.2f})", NAME[best], best_cos);
+    }
 }
 
 std::string RenderResultReport(const std::string &output_prefix,
@@ -521,13 +545,24 @@ std::string RenderResultReport(const std::string &output_prefix,
                         "fall-off the resolution cut is read off does not describe these data",
                         sh[rose_after].d_max, sh[rose_after].d_min));
         }
-        if (an.verdict == AnisotropyVerdict::Detected && an.d_min_spread > 0.5)
+        if (an.verdict == AnisotropyVerdict::Detected && an.d_min_spread > 0.5) {
+            // Say WHICH direction each limit belongs to. Without it the warning states that the crystal
+            // is anisotropic and leaves the reader no way to act on it; the eigenvectors are measured
+            // here and reach the mmCIF, so the name costs nothing.
+            const double *worst = std::max_element(an.d_min_axis, an.d_min_axis + 3);
+            const double *best = std::min_element(an.d_min_axis, an.d_min_axis + 3);
+            const auto along = [&](const double *it) {
+                const int n = static_cast<int>(it - an.d_min_axis);
+                return result.consensus_cell
+                       ? ReciprocalAxisLabel(*result.consensus_cell, an.eigenvector[n])
+                       : fmt::format("principal direction {}", n + 1);
+            };
             warnings.emplace_back(fmt::format(
-                    "Diffraction is anisotropic (deltaB {:.1f} A^2; the diffraction limit runs from "
-                    "{:.2f} to {:.2f} A depending on direction) - refinement and map interpretation "
-                    "should allow for it; no intensity has been corrected for it here",
-                    an.delta_b, *std::max_element(an.d_min_axis, an.d_min_axis + 3),
-                    *std::min_element(an.d_min_axis, an.d_min_axis + 3)));
+                    "Diffraction is anisotropic (deltaB {:.1f} A^2; the diffraction limit is {:.2f} A "
+                    "along {} and {:.2f} A along {}) - refinement and map interpretation should allow "
+                    "for it; no intensity has been corrected for it here",
+                    an.delta_b, *worst, along(worst), *best, along(best)));
+        }
     }
 
     // ------------------------------------------------------- 10. MODEL VALIDATION