Jungfraujoch/image_analysis/scale_merge/SearchSpaceGroup.h

// SPDX-FileCopyrightText: 2025 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
// SPDX-License-Identifier: GPL-3.0-only

#pragma once

#include <array>
#include <limits>
#include <optional>
#include <string>
#include <vector>

#include "../../common/Reflection.h"
#include "gemmi/symmetry.hpp"
#include "gemmi/unitcell.hpp"

// Determine the likely space group of a dataset from its P1-merged intensities, in the spirit
// of POINTLESS (Evans 2006):
//
//   Stage A - point group (Laue) symmetry. Every candidate rotation operator is scored once by
//             the correlation of E^2(h) with E^2(Rh), on resolution-normalised intensities as
//             POINTLESS does. The chosen point group is the largest one all
//             of whose operators are confirmed (high CC). A wrong operator scores ~0, so this is
//             self-pruning - the unit cell is not needed.
//
//   Stage B - space group within that point group. Each Sohncke space group of the point group
//             predicts a set of systematically absent reflections (centering + screw axes). The
//             chosen space group is the one that explains the MOST absences while every reflection
//             it predicts absent is in fact weak. The symmorphic group (no absences) is the
//             fallback when no screw/centering is supported. Enantiomorphic pairs (e.g. P4_1 vs
//             P4_3) are indistinguishable from intensities and are reported as a pair.

struct SpaceGroupOperatorScore {
    std::string op_triplet_hkl;   // reciprocal-space triplet of the rotation, e.g. "-h,-k,l"
    double cc = 0.0;              // correlation of E^2(h) with E^2(Rh) - resolution-normalised
    int n_pairs = 0;             // independent reflection pairs the CC was computed from
    bool present = false;        // operator confirmed as a real symmetry of the intensities
    // MEDIAN |I1-I2|/(I1+I2) over this operator's pairs - the disagreement the operator implies, with no
    // sigma in it. Unlike chi^2 and the systematic-b, which are ratios to a merge error model that
    // drifts with multiplicity, this is a property of the intensities alone; compared against an
    // operator already confirmed on the same reflections it is what separates a real symmetry from a
    // twin law (see max_operator_h_ratio). The median rather than the mean because a merohedral twin
    // perturbs EVERY pair while a badly integrated minority perturbs only the tail.
    double h_stat = 0.0;
    // INTENSITY-WEIGHTED disagreement across this operator's pairs: sum|I1-I2| / sum(I1+I2), an
    // R-factor form. Unlike h_stat (a median) it is dominated by the STRONG reflections, which is where
    // a false operator relating unequal intensities shows the largest contrast and where a genuine one
    // agrees best - the same reflections a median under-weights. Sigma-free, so it does not saturate
    // with the merge's ISa the way the chi^2 and systematic-b gates do on a starved search merge. Used
    // as the full-resolution merge-degradation gate (see max_operator_r_over_floor).
    double r_stat = 0.0;
};

struct SpaceGroupCandidateScore {
    gemmi::SpaceGroup space_group;
    int absent_observed = 0;          // observed reflections this SG predicts systematically absent
    int absent_violations = 0;        // of those, how many are nonetheless strongly present
    int centering_absent = 0;         // of absent_observed, the ones extinguished by the CENTERING
    int screw_absent = 0;             // ... and the ones extinguished by a SCREW (i.e. on axial rows)
    // How much likelier the screw-absent class is if the screw exists than if it does not, in nats -
    // the quantity that decides whether a screw may be claimed at all (see min_screw_absence_evidence,
    // and ScrewZoneEvidence for the statistic). 0 when there are no screw absences, or when no axial
    // row carried a control class to judge them against.
    double screw_absence_evidence = 0.0;
    // How unlikely the CENTERING-absent class would be without the centering, judged against the
    // present class as its control (see AbsenceEvidence). 0 for a primitive group, and for a centred
    // one whose class this merge never sampled. This is what ranks the candidates, in place of a net
    // absence COUNT.
    double centering_absence_evidence = 0.0;
    // One entry per axial ROW (zone) the group's screws extinguish, each scored on its own evidence.
    // A zone whose row carries no control reflections in this merge cannot be judged: it ABSTAINS,
    // neither confirming the screw nor refusing it, rather than being folded into one pooled verdict
    // where an unmeasured row can outweigh a confirmed one.
    struct ScrewZone {
        std::array<int, 3> row{};   // the row's direction, e.g. {0,1,0} for 0k0
        int n_absent = 0;
        int n_control = 0;          // 0 = undetermined: this row has no control class in the merge
        double evidence = 0.0;      // nats; meaningless when n_control is 0
    };
    std::vector<ScrewZone> screw_zones;
    // Of the three axial rows, how many this group's screws extinguish while the merge holds no
    // control class to judge them - claims the data can neither confirm nor refuse. Zero unless at
    // least one screw zone WAS judged, so the tie-break it feeds (the sort in SearchSpaceGroup) is
    // only ever spent by a crystal that has already shown a screw; centring-driven axial conditions
    // are not counted.
    int unjudged_screw_claims = 0;

    // A GLIDE plane extinguishes a two-dimensional ZONE (h0l with l odd for a c-glide normal to b),
    // where a screw extinguishes a one-dimensional row. The zone is keyed by the mirror direction -
    // by the ROTATION part of the improper operator, so a glide and its centring partner (the c and
    // the n of C2/c, same plane, different translation) are ONE zone and not two, which would score
    // the same reflections twice.
    //
    // Every member here is zero for a Sohncke group, and by construction rather than by measurement:
    // a Sohncke group has no improper operator at all, so the loop that fills these never has a zone
    // to fill. That is what makes glide detection inert on chiral (protein) data.
    struct GlideZone {
        std::string label;          // the zone as a crystallographer names it: "h0l", "hk0", "hhl", ...
        int n_absent = 0;
        int n_control = 0;          // the rest of the same zone; 0 = the zone is not measurable here
        double evidence = 0.0;      // nats, ScrewZoneEvidence on the zone's own control mean
        // evidence / n_absent. THIS is what a glide is judged on, not `evidence`. The statistic is
        // linear in the number of absences at fixed deadness, and a zone is a PLANE - hundreds to
        // thousands of reflections where a screw row holds tens - so a whole-zone sum reaches
        // hundreds of nats on a class that is merely a few times weak. Measured over 140 protein
        // datasets: the largest false zone SUM is 331 nats, against 422 for the smallest genuine
        // glide in the corpus - a factor of 1.3, no separation. Per reflection the same two are
        // 0.65 and 5.95 - a factor of 9. The rate is the scale the two populations separate on.
        double evidence_per_reflection = 0.0;
        // The absent class in units of its zone's control mean - what the evidence is computed from,
        // and the number a crystallographer reads: 0.005 means the extinguished class sits at half a
        // percent of the rest of its own plane. Reported because the evidence is a log and a report
        // that only prints nats cannot be sanity-checked against the diffraction pattern.
        double mean_u = 0.0;
    };
    std::vector<GlideZone> glide_zones;
    int glide_absent = 0;             // observed reflections extinguished by a glide (not a screw)
    // The WEAKEST zone's evidence per reflection - the minimum, not the maximum a screw takes. A
    // screw's zones are separate conditions and the best one may speak for the group; a candidate
    // that claims two glides claims BOTH planes are dead, so the one that is not dead refuses it.
    // The safe direction: a false glide is an over-call, and over-calls are the dangerous ones.
    double glide_absence_evidence = 0.0;
    // A zone this merge cannot judge - too few absences, or no control class of its own. For a screw
    // an unjudgeable zone ABSTAINS; for a glide it REFUSES. A screw is being weighed against its own
    // absence in a group that otherwise fits; a glide is an extra claim on top of a group that
    // already fits without it, and an extra claim with no evidence behind it is not a tie, it is a no.
    bool glide_unmeasurable = false;

    double absent_mean_i_over_sigma = 0.0;
    double present_mean_i_over_sigma = 0.0;
    bool consistent = false;          // absent class confirmed weak (few violations)
    bool selected = false;            // chosen result (or its enantiomorph)
    // The centering could not be TESTED: the group is centred, but this merge holds not one
    // reflection of the class its centering extinguishes. That happens when the data were indexed
    // and integrated on the primitive sub-cell rather than on the conventional centred one - the
    // interstitial nodes were never predicted, so there is nothing here to confirm or disprove.
    // The candidate then scores zero absences, which TIES it with the primitive group rather than
    // supporting it, and a centering adopted in that state comes from the lattice METRIC and not
    // from these intensities. Reported so that case is visible instead of silent.
    bool centering_untested = false;
};

// How unlikely a predicted-absent class would be if the condition producing it did not exist, in
// nats: the -log lower tail of Beta(n_absent, n_control) at T = sum_u / (sum_u + n_control), where
// sum_u is the absent intensities expressed in units of their control class's mean. Used for a SCREW
// against the rest of its own axial row and for a CENTRING against the present class, so the two are
// in the same units and add. The control's own strength cancels, which is the property a count of
// absences does not have. Declared here because it is the statistic the candidate ranking is built
// on, and the comments below name it.
double AbsenceEvidence(double sum_u, int n_absent, int n_control);

// The same for a SCREW zone, with the control COUNT dropped (the b -> infinity limit), which makes it
// a likelihood ratio for the absent class rather than a tail probability. A screw's control class is
// the complement of its absent class on one axial row, so a candidate that predicts more of the row
// absent shrinks its own control and was charged for it - an equally dead SUPERSET of another
// candidate's absences could score lower. See SearchSpaceGroup.cpp.
double ScrewZoneEvidence(double sum_u, int n_absent);

struct SearchSpaceGroupOptions {
    // Lattice (metric) symmetry from LatticeSearch. When set, the point-group search is limited to
    // the Sohncke subgroups of this system's holohedry - the metric is an upper bound on the
    // intensity symmetry, so e.g. a tetragonal metric tests {1,2,222,4,422} and never 3/6/23. All
    // subgroups are still tested (down to P1), so a pseudo-symmetric metric never forces a higher
    // symmetry than the intensities support. Unset = search every Sohncke system.
    std::optional<gemmi::CrystalSystem> lattice_system;

    // Centering is NOT taken from the lattice metric: an indexer that returns the conventional cell
    // (e.g. cubic 'P'-looking axes for a body-centered lattice) hides the centering, which lives only
    // in the systematic absences. Stage B therefore tests every centering allowed by the point group
    // and confirms it from the data (h+k+l etc. absent), rather than trusting a geometric hint.

    // The conventional cell the merged reflections are indexed on. Only needed with the two
    // enumeration options below: a setting names the symmetry directions by AXIS, so a candidate in a
    // setting the cell does not support (a c-unique monoclinic group against a b-unique cell) is
    // refused before it is scored. Unset with either option on means no non-reference setting is
    // offered at all - the check is a precondition, not an optional extra.
    std::optional<gemmi::UnitCell> cell;

    // Enumerate the non-reference SETTINGS of the chosen point group in Stage B - P 2_1 2 2 and
    // P 2 2_1 2 beside P 2 2 2_1, A 2 2 2 beside C 2 2 2, I 1 2 1 beside C 1 2 1. These are
    // alternative namings at the same group ORDER, so this cannot promote the point group; what it
    // adds is the ability to put a screw or a centering on the axis the data actually show it on.
    // Without it a crystal whose screws are on b and c is reported as P 2 2 2_1 - the wrong group,
    // not a lower one, because the only candidate that predicts the 00l absences and nothing else
    // wins on zero extra evidence.
    bool enumerate_all_settings = false;

    // Enumerate the rotation sets that only a non-reference setting carries in Stage A - the a-unique
    // and c-unique monoclinic 2-folds. Unlike the option above this DOES add promotions: a crystal
    // whose only 2-fold lies on a or c currently has no rung to stand on and falls to P1. Requires a
    // cell, and requires enumerate_all_settings for those point groups to be nameable in Stage B
    // (no reference setting carries them), which this option turns on for them by itself.
    bool enumerate_all_rotation_sets = false;

    // Friedel mates are treated as equivalent when matching HKLs (i.e. anomalous signal ignored).
    bool merge_friedel = true;

    // Ignore reflections beyond this resolution (smaller d = higher resolution). 0 disables.
    double d_min_limit_A = 0.0;

    // Drop reflections weaker than this from the correlation stage only (the absence stage must
    // keep weak reflections - that is where the screw-axis signal lives).
    double min_i_over_sigma = 0.0;

    // Drop reflections STRONGER than this resolution-normalised E^2 = I/<I>(shell) from the correlation
    // stage only. A second lattice deposits intensity on one reciprocal position but not its symmetry
    // mate, so an overlap-contaminated reflection is a one-sided E^2 outlier that poisons an operator's
    // I(h)/I(Rh) correlation (it flipped a pseudo-merohedral P2_1 case to P1 under -A: excluding the E^2>9 tail, ~0.4%
    // of reflections, lifted the 2-fold CC 0.33->0.53 back over the gate - values on the raw-I scale the
    // operator CC used before it was normalised). Clean Wilson-distributed data
    // almost never reaches E^2=9 (P(E^2>9) ~ 0.01-0.3%), so this removes essentially nothing there and
    // only trims the overlap tail. 0 disables. Absences are unaffected (they need the weak tail).
    // This E^2 is the one normalised over pass_absence, NOT the one the correlation is scored on: the
    // cut helps DEFINE pass_cc, so normalising it over pass_cc would be circular.
    double max_e_squared_for_cc = 9.0;

    // --- Stage A: point group ---
    // A rotation is accepted as a real symmetry when its E^2(h)/E^2(Rh) correlation reaches this over
    // at least min_pairs_per_operator independent pairs.
    //
    // The correlation is on RESOLUTION-NORMALISED intensity (see SearchSpaceGroup.cpp), which is what
    // sets this value. On raw I the two arms of every pair share the whole resolution fall-off, so a
    // completely false operator still scores a large positive CC - measured over the rotation battery
    // by shell-matched random pairing, that noise floor runs 0.05 to 0.53 across crystals, a SPREAD
    // (0.46) wider than the entire true/false gap (0.38). One absolute number therefore meant a
    // different test on every crystal: the old bound of 0.5 on raw I corresponds, crystal by crystal,
    // to a normalised threshold anywhere from -0.03 to 0.47. On the normalised statistic the floor has
    // a median of 0.015 and never exceeds 0.06, so a single number finally means the same thing on
    // every crystal.
    //
    // 0.30 is the midpoint of the gap the battery leaves: the weakest genuine conjugacy-class mean is
    // 0.35 and the strongest false candidate 0.23 (both on the pessimistic arm, which carries rugnux's
    // own search merge onto the normalised scale). Scanning the constant against the battery: 0.20 lets
    // a cubic over-promotion in, 0.35 loses a genuine monoclinic 2-fold, and 0.25-0.30 changes nothing.
    // It is also the median of the per-crystal threshold the old 0.5 already imposed (0.309), so it is
    // the behaviour-preserving choice at the median crystal and only redistributes strictness at the
    // tails - away from the strong crystals, which were being handed margin out of their own noise
    // floor, and towards the weak ones, which had none.
    double min_operator_cc = 0.30;
    // KNOWN-WEAK, DELIBERATELY LEFT AT 20 - do not raise it without reading this.
    //
    // 20 pairs is not a test. The null of the normalised CC - shell-matched random pairing over
    // pass_cc with symmetry-related pairs vetoed, i.e. what a metrically-allowed but FALSE operator
    // looks like - has a mean of -0.02 to +0.05 and a standard deviation C/sqrt(n) with C = 1.08
    // (0.98 to 1.23 across crystals), flat from 20 to 5000 pairs. Its upper tail is far heavier than
    // normal theory, because E^2 is exponentially distributed: measured over 142 (crystal, n) points
    // the exceedance is log10 P = -1.03 z + 0.69 with z = (min_operator_cc - null mean)/sd, i.e. ONE
    // DECADE PER STANDARD DEVIATION where the normal law would give five. At 20 pairs a false
    // operator therefore clears 0.30 on 12.4% of draws (and cleared the old bound of 0.5 on 3.2%).
    // A bound that held that to 1e-3 on the worst merge measured would be 251 pairs.
    //
    // It is left at 20 because a pair-count FLOOR is the wrong shape for this problem, and 251 was
    // measured to do harm. Nothing in the rotation battery is confirmed on noise: over its 314
    // candidate operators the sparsest carries 511 pairs and the weakest confirmed one stands 9.1
    // sigma above its own null, so there is no live case to fix. Meanwhile SearchSpaceGroup is run
    // twice per data set (see Rugnux.cpp) - once on all observations, which decides, and once on a
    // Lorentz-filtered merge that is deliberately starved and only reports - and on that second arm
    // genuine operators reach 98 pairs. At 251 the sparsest crystal's filtered arm loses 19 of its
    // 23 operators and the run prints "the Lorentz-filtered one supports 1" for an arm whose own
    // correlations are 0.91-0.97, every one of them 9 sigma clear. A floor discards an operator on
    // pair count alone however decisive its CC is, and that is what makes it the wrong instrument.
    //
    // The right fix, when it is worth its own calibration and battery, is the pair-count-aware bound
    // this null hands over directly: require cc >= max(min_operator_cc, mu + C*z/sqrt(n)). That
    // rejects the 20-pair noise the floor is aimed at while keeping every operator above, because it
    // asks how large the correlation is and not only how many pairs it came from.
    int min_pairs_per_operator = 20;

    // Per-operator CC alone cannot tell a real weak operator from a false strong one (a noisy crystal's
    // genuine 2-fold can score below a pseudo-symmetric crystal's near-perfect false one). So a point
    // group is also required to be SELF-CONSISTENT: merging the intensities under it must not inflate the
    // reduced chi^2 (within-orbit scatter / sigma^2) beyond this factor times the most-consistent
    // candidate. A false operator forces non-equivalent reflections together so they disagree by many
    // sigma and chi^2 blows up; a real one leaves it ~flat even when the operator CC is only moderate.
    // Calibrated on the rotation-test battery: every correct point group stays within ~1.7x the best
    // subgroup even on weak / badly-integrated data (worst real case a P41212 at 1.71), while a twin
    // law or pseudo-symmetry lands clearly higher (a merohedral R3->R32 twin 2-fold at 2.01). 1.85 sits
    // between the two, so a partial merohedral twin is kept in its true lower symmetry (R3), not
    // over-promoted to the holohedral R32.
    double max_merge_chi2_ratio = 1.85;

    // A genuine high-symmetry merge can drift just past max_merge_chi2_ratio when its data are only
    // imperfectly scaled: each real symmetry step then adds a little systematic scatter, so a weakly-
    // scaled cubic case lands at ratio ~2.0 - right where a merohedral twin (an R3->R32 case at ~1.95)
    // also lands, so the chi^2 ratio alone cannot separate them. A candidate whose chi^2 is
    // only this far past the best subgroup is therefore rescued if the SYSTEMATIC part of the extra
    // scatter stayed small (max_systematic_b_ratio): merging under a genuine operator gains multiplicity
    // without intensity-proportional disagreement, so the merge error model's b barely moves, whereas a
    // twin forces non-equivalent reflections together and b balloons. Both bounds sit in the gap; the
    // rescue only ever promotes, and only in this narrow chi^2 band.
    double max_merge_chi2_rescue = 2.30;
    // 1.78, and this bound is load-bearing in a way it was not when it was first set. Where a genuine
    // high-symmetry step and a merohedral twin law BOTH fail the chi^2 ratio, the rescue is the only
    // thing still choosing between them, and on a correctly-scaled search merge it is the only one of
    // the four Stage-A statistics that can. Measured over the rotation battery on merges whose scaling
    // loop runs the iterations it was asked for, a weak tetragonal 4 -> 422 step that is genuine and a
    // trigonal 3 -> 32 step that is a twin law read
    //
    //     chi^2 ratio  2.05 genuine vs 1.99 twin      H ratio  1.82 genuine vs 1.67 twin
    //     b ratio      1.71 genuine vs 1.85 twin      R_meas   1.31 genuine vs 1.36 twin
    //
    // - chi^2 and H INVERTED, the genuine step looking worse than the twin on both. That is what b is
    // for and why it survives here: a twin's disagreement is proportional to I, and b is the statistic
    // that isolates the intensity-proportional part, where chi^2 is normalised by a sigma the weak
    // genuine case has too little of and H is a per-operator median that case's angular coverage
    // inflates. 1.78 is the midpoint of the gap b leaves.
    //
    // It is a 4% margin on each side, narrower than the shift a change in how the search merge is
    // scaled has already been measured to produce on these same four numbers. Re-measure it whenever
    // that scaling changes rather than trusting it through such a change.
    //
    // 1.44, not the 1.78 those four numbers were read in. This bound is a bound on a RATIO of two b
    // values, so its scale is set by how merge_systematic_b normalises its reduced chi^2 - and that
    // normalisation changed, from the observation count N to the fit's degrees of freedom N - G.
    // Every ratio in the corpus fell with it: a measured -23.7% median over three disjoint corpora,
    // lower in 42 of 42 promotions, because the low-multiplicity parent loses a larger fraction of
    // its dof than the candidate and so gains more b. Measured on
    // one trigonal crystal, same merge, only the divisor differing: the parent's b 0.126 -> 0.181
    // (+43%) against the candidate's 0.219 -> 0.254 (+16%), and the ratio 1.74 -> 1.40.
    //
    // Leaving the bound at 1.78 through that was a change of units without a conversion, and it
    // loosened the RESCUE below onto a merohedral twin the old convention refused. Rescaling by the
    // same factor restores every verdict exactly - b_new > 0.805 x 1.78 is the same test as
    // b_old > 1.78 - which is what the divisor change intended and did not deliver. Derived three
    // ways that agree: 1.78 x 0.808; the midpoint of the calibration pair above re-read in the
    // corrected convention (genuine 1.71 -> 1.38, twin 1.85 -> 1.49); and a population-median
    // derivation that argued 1.38.
    double max_systematic_b_ratio = 1.44;

    // Promotion gate on the operator disagreement H = median|I1-I2|/(I1+I2), taken as the ratio of the
    // operators the promotion ADDS to the operators of the parent group already confirmed on the same
    // reflections. A real symmetry operator relates equal intensities, so its H matches the parent's
    // (ratio ~1); a merohedral twin law relates DIFFERENT reflections mixed in proportion alpha, so its
    // H is systematically larger. Unlike the chi^2 and systematic-b ratios (genuine 1.00-3.47 /
    // 1.09-3.89 vs twin 1.35-3.32 / 1.77-4.76, fully interleaved) it does not drift with multiplicity,
    // because there is no sigma in it and the parent normalisation cancels data quality.
    //
    // A MEDIAN, not a mean. A merohedral twin mixes every reflection with its twin mate, so it shifts the
    // whole distribution of |I1-I2|/(I1+I2); a minority of badly measured reflections shifts only the
    // tail. Measured on the same real crystals, moving from the mean to the median leaves genuine
    // promotions where they are (1.016 -> 1.013, 1.051 -> 1.067, 1.238 -> 1.231) and pushes every real
    // twin UP (1.280 -> 1.622, 1.427 -> 2.010, 1.272 -> 1.447, 1.441 -> 1.522), widening the margin
    // around this bound from 2.7% to 17.5%. Battery-neutral: 33 crystals, no point group changed.
    //
    // The parent normalisation is what makes this work, and it is not optional. Symmetry-related
    // reflections never agree exactly on real data - absorption, illumination and partiality differ
    // between them - and that systematic floor varies by crystal AND by operator (a cubic 3-fold permutes
    // axes, relating far-apart parts of reciprocal space, so it disagrees more than the 2-folds of its
    // parent even when the symmetry is perfectly real). Measuring an added operator against the parent's
    // own operators, on the same reflections, is what divides that floor out. An ABSOLUTE bound on any
    // per-operator agreement statistic cannot: measured absolute values for genuine symmetry span the
    // whole range from 0.99 on strong data down to 0.71 on weak, straddling every twin.
    //
    // Its known limit is angular coverage, not data quality. What the parent normalisation cannot divide
    // out is the part of the systematic floor that differs BETWEEN the added operators and the parent's,
    // and that part grows as the measurement gets less uniform: on a full sweep a real tetragonal crystal
    // reads the same H on all seven operators of 422 (0.051-0.057, ratio 1.03), but on a quarter of the
    // frames of the same crystal the seven spread over 0.082-0.168 and the ratio reaches 1.45, the two
    // diagonal 2-folds - left with the fewest surviving pairs - carrying the whole excess. So a refusal
    // from a merge built on a small or lopsided part of the sweep says as much about the coverage as about
    // the symmetry; the fix for that is upstream, in getting the sweep indexed.
    //
    // The bound is set from the gap between the two populations, measured over the rotation battery:
    // genuine promotions read 0.85-1.57 (and 2.48 on a genuine orthorhombic step in a pair-starved arm),
    // real merohedral twins 1.82 and 4.01. 1.25 sat INSIDE the genuine range - four genuine promotions
    // already exceeded it and survived only because the two-arm rule happened to cover them, and one
    // cubic case with no such cover was refused outright by a margin of 0.4%.
    //
    // Re-measured over ~780 reports (three batteries and four de-novo integration-radius arms), H is the
    // only Stage-A guard that ever fires: 14 refusals on three crystals, no chi-squared-ratio refusal and
    // no b veto anywhere. Two of the three are the genuine trigonal twins, reading 1.90/1.91 on every arm
    // and 3.61-4.80; the third is a weak tetragonal crystal reading 1.73-1.80 whose 422 is confirmed by
    // every operator correlation and which three independent tests - our own L test on the merge it was
    // demoted into, xtriage on the same merge ("most likely due to an NCS axis parallel to the twin
    // axis"), and the arithmetic that a twin at the implied fraction caps pair correlation at 0.55 while
    // the added operators score 0.84-0.90 - agree is not twinned. Its excess is the coverage limit named
    // above: the added 422 operators exchange h and k and carry a systematic floor the parent's
    // sign-flipping 2-folds do not. So the populations collide at 1.80 and 1.90 rather than at 1.70, and
    // the bound sits at the midpoint. Degrading the data drives the ratio towards 1, never up, and the
    // estimator is precise (bootstrap standard error 0.2-3.7% of the ratio), so the collision is a real
    // overlap and not sampling noise - which is also why scaling the bound by that error cannot work.
    //
    // Those two populations were measured on search merges whose scaling loop had been over-iterated,
    // and on correctly-scaled ones they SWAP: the weak tetragonal case rises to 1.82 and the trigonal
    // twin falls to 1.67, so no value of this bound keeps the one and refuses the other. H is kept as a
    // necessary condition because it still refuses the twins that read far above either (3.4 and up),
    // but it no longer decides the close case - max_systematic_b_ratio does.
    double max_operator_h_ratio = 1.85;

    // The H test needs at least this many pairs on both sides to mean anything.
    int min_pairs_for_h = 200;

    // Full-resolution merge-degradation gate. The operators a promotion ADDS are scored by their
    // intensity-weighted R (r_stat: a SUM, so the strong low-resolution reflections that carry the
    // contrast dominate it, where the median H under-weights them), and that R is judged against a
    // CLEAN reference. This is the one Stage-A gate whose reference cannot be contaminated by false
    // operators: the H ratio normalises against the PARENT group, and a parent can itself be a false
    // promotion (a pseudo-tetragonal 2 -> 222 -> 422 cascade confirms 222 by pooling the one real
    // 2-fold with two false ones, so the 422 step is then measured against an already-contaminated
    // 222, and every ratio reads ~1.4 and passes). It also acts on the first step out of P1, where
    // there is no parent to normalise against at all and the operator CC floor stood alone. Sigma-free, so unlike the chi^2 and b gates it does not saturate with the
    // merge's ISa on a starved search merge.
    //
    // Two references, each scale-free so the bound means the same on every crystal (the property an
    // absolute R bound lacks):
    //
    //  - max_operator_r_over_best: when the data confirm more than one operator, the added operators'
    //    mean R over the GLOBALLY best-agreeing operator (the smallest r_stat anywhere). A genuine
    //    operator relates equal intensities, so a small R is itself the signature of a real operator
    //    and the smallest one is the cleanest reference the data hold. On a genuine group EVERY
    //    operator agrees about as well as the best (ratio ~1.0-1.2 measured, cubic 3-folds and a
    //    genuine 6 included); a false operator sits far above it, INCLUDING one whose own group's
    //    members agree among themselves (a pseudo-tetragonal 4 reads ~3.8, because the reference is
    //    the true 2-fold OUTSIDE that group). Measured over the probe crystals: genuine promotions
    //    reach 1.2, the false 2-fold/4-fold/6-fold-holohedry steps 2.2-3.8. 2.0 sits in that gap.
    double max_operator_r_over_best = 2.0;
    //
    //  - max_operator_r_over_floor: the first step out of P1 confirms only the one operator, so there
    //    is no other to be the best - fall back to the merge's own random-noise R floor (merge_r_floor,
    //    from the half-dataset merges). The floor is a genuine-symmetry disagreement (two halves of
    //    the SAME reflections) at the same multiplicity, so it too is a clean scale-free reference.
    //    Measured: a genuine first 2-fold sits at ~1.3-2.3x the floor, a false one (a pseudo-C-centred
    //    metric coincidence) at ~4.7. 3.5 sits in that gap. Wider than the best-operator bound because
    //    the floor is a noisier reference than a confirmed operator (it can also be inflated by an
    //    indexing ambiguity that mixes hands), and the safe direction here is to stay in P1.
    double max_operator_r_over_floor = 3.5;

    // Above this reduced chi^2 for the best subgroup (chi2_ref), the merged error model is treated as
    // badly miscalibrated (weak, low-resolution data whose sigmas are far too small): the fixed-sigma
    // chi^2 ratio then grows with point-group order for genuine high symmetry too and can no longer
    // arbitrate, so a promotion is confirmed on the systematic-b test alone (which re-fits its own error
    // and stays valid). A well-calibrated merge sits near 1; 3.0 (sigmas ~1.7x too small) marks the point
    // where the ratio stops being trustworthy. The H and added-operator-R gates below still guard
    // against a twin: both are necessary conditions no rescue can lift.
    double chi2_ref_reliable = 3.0;

    // Adopt this space group's point group instead of the one Stage A would choose, and go straight to
    // Stage B with it. Used when one merge decides the point group and a different merge has to decide
    // the absences: a caller cannot pass the point group by name, because gemmi reports both P321 and
    // P312 as "32", so the group itself is passed and its rotations are taken from it. Stage A still
    // runs (its operator scores and its refusal report are still wanted); only the choice is overridden.
    std::optional<gemmi::SpaceGroup> fixed_point_group;

    // --- Stage B: space group (screw axes / centering) ---
    bool determine_space_group = true;     // false: stop at the symmorphic representative

    // Signed I/sigma above which a reflection counts as genuinely present. Signed, so negative
    // noise is never mistaken for a real reflection. Used both to spot reflections that violate a
    // wrongly-assumed absence, and to keep the correlation stage from pairing near-zero reflections.
    // Read against the reflection's COUNTING sigma, not against the merged one - see merge_isa, which
    // is what converts it into a cut on the quantity the merge actually exports.
    double present_i_over_sigma = 3.0;

    // ISa = 1/b of the error model the merge handed in was scaled with: the I/sigma one observation
    // of an arbitrarily strong reflection can reach. 0 = not known, or b = 0 (no systematic term at
    // all), and then present_i_over_sigma is used as it stands.
    //
    // It is needed because a merged I/sigma is not a measure of how strong a reflection is. The merge
    // carries the error model's intensity-proportional term, sigma^2 = a*sigma0^2 + (b*I)^2, so
    // I/sigma saturates: at ISa*sqrt(n) for a reflection observed n times, and at ISa exactly for one
    // observed once. Above that knee the quantity stops rising with the intensity - measured over the
    // rotation battery's space-group search merges, the median I/sigma of the top E^2 decile sits at
    // 0.97 of ISa on the weakest merge, and its log-log slope against E^2 over the strong half of the
    // merge is 0.008, i.e. flat. An absolute cut on it is therefore a different test on every crystal,
    // and on a merge whose ISa is near the cut it is close to unreachable: 3% of reflections clear 3.0
    // there, against a battery median of 53%.
    //
    // So the cut is converted to the counting scale once, using the reflection's own significance.
    // For a reflection observed once sigma_counting^2 = sigma^2 - (b*I)^2, hence
    // I/sigma_counting = t/sqrt(1-(t/ISa)^2) with t = I/sigma, and "I/sigma_counting >= T" is exactly
    // "t >= T/sqrt(1+(T/ISa)^2)". That converted cut lies strictly below ISa for every ISa, so it is
    // always reachable, and it is within 1% of T on any merge with ISa >= 21, so it leaves a healthy
    // merge where it was. Multiplicity is taken as 1 deliberately and not estimated: the cut has to be
    // reachable by the reflection the ceiling binds hardest, which is the one measured once.
    double merge_isa = 0.0;

    // Extra intensity gate for the systematic-absence test: a reflection also has to reach this
    // resolution-normalised intensity E^2 = I / <I>(shell) to count as violating a predicted absence.
    // The error model can under-estimate sigma on weak axial reflections and fake a high I/sigma, so a
    // reflection at a few percent of the shell-mean intensity is judged absent regardless of its sigma
    // (e.g. the monoclinic 2_1: 0k0-odd at ~1% of 0k0-even). 0 disables the gate (I/sigma only).
    //
    // For a SCREW the threshold is this fraction of the median E^2 of the axial row the screw
    // constrains (floored at the plain value), not of an average reflection at that resolution - an
    // axial row is often far stronger than the shell mean, and only the row is a fair comparison.
    double present_e_squared = 0.3;

    // A candidate's SCREW/glide absence conditions are accepted when at most this fraction of the
    // reflections it predicts absent are in fact strongly present.
    double max_absent_violation_fraction = 0.10;

    // A CENTERING is accepted when its systematically-absent class is this much weaker than the present
    // class - mean signed I/sigma of the centering-absent reflections <= this fraction of the present
    // mean. A real centering cancels structure factors so its absent class sits near zero (ratio ~0-0.3
    // across the test battery) even on noisy or obverse/reverse-twinned data; a false centering leaves it
    // as strong as the present class (ratio ~1.0). 0.5 separates the two with wide margin. This strength
    // test replaces a per-reflection violation-count gate for centering, which was brittle when noise
    // pushed genuinely-absent reflections over I/sigma>3 (a true R3 was lost at 13.5% violations).
    double max_absent_present_ratio = 0.5;

    // Need at least this many observed reflections in the CENTERING-absent class before a centering is
    // claimed (guards against deciding from a handful of reflections). A real centering extinguishes a
    // third to a half of every reflection in the data set, so this is never the binding constraint on a
    // centering that exists - it only refuses one the data barely sampled.
    //
    // It does NOT apply to screws. A screw's absent class is one row of reciprocal space by
    // construction, and that row is often the one a rotation sweep records least: it lies near the
    // spindle, where the blind cusp maps onto itself and symmetry cannot fill it in. Counting it
    // measures the geometry of the sweep, not the strength of the evidence. Applied to screws this
    // bound cost a monoclinic crystal its 2_1 - six 0k0-odd reflections, every one of them measured at
    // |E^2| <= 0.013 with zero violations, against a 0k0 row averaging 1.44x the shell mean, refused
    // for being six rather than eight. Screws answer to min_screw_absence_evidence below instead.
    int min_absent_observed = 8;

    // Evidence, in nats, that a screw's predicted-absent class really is absent, required before the
    // screw may be claimed. In the spirit of the POINTLESS zone test (Evans, Acta Cryst D67, 282-292
    // (2011), Appendix A3), which likewise scores an absence against the rest of its own axial row
    // rather than against a global mean or a fixed cut, and likewise lets the confidence fall away with
    // the number of axial reflections instead of refusing outright below a count.
    //
    // The statistic (see ScrewZoneEvidence) is the likelihood ratio of the absent class against its
    // row's control class - the -log Beta tail AbsenceEvidence uses for a centring, with the control
    // COUNT dropped so that candidates judged on the same row stay comparable. Two properties are
    // what a violation count lacks: the row's own strength cancels, so a uniformly weak axial row
    // decides nothing rather than reading as "absent"; and the scale is set by the number of
    // reflections, so few-but-decisive and many-but-marginal are told apart. It is sigma-free by
    // design - the merged sigma carries the error model's intensity-proportional term and so shrinks
    // with I, reading much the same on an absent reflection as on a present one (the <I/s> columns of
    // the candidate table show this directly).
    //
    // Measured over the probe crystals, an adopted screw reads 24-600 nats and the false ones (the
    // 4_1/4_3 conditions of a crystal that has no screw at all, whose predicted-absent class is as
    // strong as its control row) read -3.8 to -38.5. The bound sits in that gap - loose enough that
    // three well-measured dead axial reflections clear it, tight enough that two do not.
    //
    // That last sentence is no longer true, and the reason is worth recording rather than re-tuning
    // blind. MIN_U_PER_ABSENT_REFLECTION floors sum_u at 1e-3 per absent reflection, which caps a
    // zone's evidence at -a*log(a*1e-3) + log(a!): 13.1 nats at two absences, 19.2 at three, 25.3 at
    // four. A zone of three therefore CANNOT clear 20 however dead its reflections are, so the gate
    // is in practice "four absences in one zone" - a count, which is the measure the whole statistic
    // exists to avoid. A thin monoclinic sweep reaching only three 0k0-odd reflections loses its
    // 2(1) to this. Lowering the floor to 7.7e-4 would let three through; that is a recalibration
    // and needs its own battery.
    double min_screw_absence_evidence = 20.0;

    // ---- Glide planes (small-molecule space groups) -------------------------------------------
    // Offer the NON-SOHNCKE space groups of the same proper-rotation set as candidates, so a glide
    // plane can be named. Protein crystals are built from L-amino acids and are therefore chiral:
    // their groups are Sohncke and contain rotations and screws only, so this can never change a
    // protein answer except through a false positive - which is why it is measured rather than
    // assumed (see min_glide_evidence_per_reflection). Small molecules are where it earns its place:
    // three corpus datasets get the right cell and the right Sohncke subgroup and stop one c-glide
    // short of the deposited group.
    //
    // Only a group whose ABSENCES differ from a Sohncke candidate's is offered. That is not an
    // optimisation, it is the honest limit of the measurement: P2/m predicts exactly what P2
    // predicts, so the inversion centre buys no candidate and is never claimed. See the note on
    // Friedel's law at the top of SearchSpaceGroup.cpp.
    bool enumerate_non_sohncke = true;

    // How dead a glide zone must be, per reflection, in nats, before the glide may be claimed.
    // ScrewZoneEvidence is asymptotically n * (log(1/ubar) - 1) where ubar is the absent class in
    // units of its zone's control mean, so this is a bound on ubar and not on a sum: 2.0 nats is
    // ubar <= exp(-3) = 0.050, i.e. the zone's absent class must be 20x weaker than the rest of its
    // own plane. Measured over 140 protein datasets and 6 small-molecule ones: the largest protein
    // zone reads 0.65 nats/reflection (ubar 0.19) and the next 0.12, while genuine glides read
    // 4.24-5.95 (ubar 0.0003-0.0054). The bar sits between the two populations with about a factor
    // of 4 in ubar to the false side and 10 to the true side.
    double min_glide_evidence_per_reflection = 2.0;

    // A zone with fewer absences than this, or fewer than half as many control reflections, cannot
    // be judged and refuses the candidate that needs it. Small numbers are where a RATE is noisy:
    // the corpus's one Sohncke small molecule shows an 8-reflection zone at 1.9 nats/reflection, on
    // a crystal whose true group has no glide at all.
    int min_glide_absent = 20;

    // Workers for the operator-correlation stage, which is the bulk of the search: one pass over the
    // whole merge per candidate rotation, and the rotations are independent of each other. 1 = serial.
    size_t nthreads = 1;
};

// One row of the FINALIST LEDGER: what the intensities say about ONE point-group hypothesis, on the
// merge this search was handed. Every candidate the operator CCs confirmed gets a row, adopted or
// refused, so a run can show what the rival was and how far behind it came - the current flow keeps
// only the winner and the single highest refusal.
//
// Every number here is PAIRED or a RATIO TO A REFERENCE THE HYPOTHESIS CANNOT MOVE, never an absolute
// per-operator statistic: r_over_best divides by the best-agreeing operator anywhere in the data,
// r_over_floor by the merge's own random-noise floor, chi2_over_best and b_over_parent by the same
// quantity under a subgroup of this very candidate. An absolute score would be the joint-likelihood
// design that was already refuted; a ratio to a clean within-crystal reference is the form thread1_A
// measured to separate genuine operators (1.06-1.15x) from false ones (2.4-4.66x).
//
// REPORT-ONLY. Nothing in the search reads these back; the adoption is made exactly as before.
struct PointGroupLedgerEntry {
    std::string point_group_hm;
    int order = 0;
    double min_class_cc = 0.0;    // weakest operator CC among the group's own operators
    double chi2 = std::numeric_limits<double>::quiet_NaN();            // merge chi^2 folding under it
    double chi2_over_best = std::numeric_limits<double>::quiet_NaN();  // ... over the most consistent subgroup's
    double b_extra = 0.0;                                              // systematic-b refit under it
    double b_over_parent = std::numeric_limits<double>::quiet_NaN();   // ... over its largest confirmed subgroup's
    double h_ratio = std::numeric_limits<double>::quiet_NaN();
    double r_added = std::numeric_limits<double>::quiet_NaN();         // added operators' mean intensity-weighted R
    double r_over_best = std::numeric_limits<double>::quiet_NaN();     // ... over the best-agreeing operator anywhere
    double r_over_floor = std::numeric_limits<double>::quiet_NaN();    // ... over the merge's random-noise R floor
    bool adopted = false;         // this is the group the search took
    bool eligible = false;        // it passed every consistency test (so it COULD have been taken)
    std::string refused_reason;   // why it did not, when it was the highest refusal
};

struct SearchSpaceGroupResult {
    std::optional<gemmi::SpaceGroup> best_space_group;
    // Other space groups that fit the data equally well (same systematic absences): enantiomorphic
    // partners (P4_1 vs P4_3), origin-ambiguous pairs (I222 vs I2_12_12_1), or groups left
    // undetermined by incomplete data. The data cannot choose between best_space_group and these.
    std::vector<gemmi::SpaceGroup> alternatives;

    std::string point_group_hm;                            // chosen point group, e.g. "422"
    // The symmorphic space group representing that point group. This, not the name, identifies it:
    // gemmi reports both P321 and P312 as "32", so two searches that disagree about which 2-folds are
    // real look identical by name. Also what a caller passes back as fixed_point_group.
    std::optional<gemmi::SpaceGroup> point_group_representative;
    // Order of that point group (its number of rotations). Reported separately because Stage B can
    // leave best_space_group unset - no candidate had enough absences to be eligible - while Stage A
    // has confirmed the point group perfectly well, and a caller comparing two searches has to see
    // the symmetry that was found either way. 0 only when no point group was chosen at all.
    int point_group_order = 0;
    std::vector<SpaceGroupOperatorScore> operator_scores;  // Stage A, all distinct operators tested
    std::vector<SpaceGroupCandidateScore> candidates;      // Stage B, ranked

    // The twin-law H ratio of the point-group promotion that was ADOPTED, and the bound it was
    // judged against (max_operator_h_ratio). NaN when there is no ratio to form: P1, the first step
    // out of it (no parent group to normalise against), or too few pairs on either side.
    //
    // Reported on every run, not only when the bound refuses a promotion, which is all it used to
    // be. The two populations that bound sits between are 1.80 and 1.90 apart, so where a CONFIRMED
    // promotion falls inside that window is exactly what a recalibration needs - and printing the
    // number only on refusal made it unreadable from every run that came out right.
    double h_ratio = std::numeric_limits<double>::quiet_NaN();
    double h_ratio_bound = 0.0;

    // The merge's own RANDOM-noise R floor: sum|Ihalf0-Ihalf1| / sum(Ihalf0+Ihalf1) over the present
    // reflections, from the two half-dataset merges. This is the intensity-weighted disagreement two
    // GENUINELY equivalent groups of observations of the SAME reflection show - i.e. the value the
    // r_stat of a real symmetry operator would tend to (plus a modest systematic floor), measured on
    // this very crystal at this very multiplicity. It is the clean, uncontaminated reference the
    // operator R is judged against (see max_operator_r_over_floor), the one a false operator cannot
    // move because it is built without applying any candidate symmetry. NaN if the merge carried no
    // half-dataset intensities. Reported so a run shows the margin the gate had.
    double merge_r_floor = std::numeric_limits<double>::quiet_NaN();

    // The point group the CONFIRMED OPERATORS GENERATE, when that group is not itself one of the
    // candidates the operator bar admitted. A candidate is admitted only if EVERY one of its
    // operators clears min_operator_cc, so a confirmed set whose products are not all confirmed
    // leaves the search standing on a subgroup while holding evidence for something larger, and
    // nothing says so. Where that happens the generated group - or, when the basis the merge is
    // indexed on names no such group, the smallest candidate containing it - is offered as a
    // candidate and judged by the same consistency tests as every other. Empty on the usual run,
    // where the confirmed operators already form one of the candidates.
    std::string generated_point_group_hm;
    bool generated_point_group_adopted = false;

    // The half-integer pseudo-translation a screw zone was judged against, per axial row, in halves
    // ({1,1,1} is (1/2,1/2,1/2)), with the depth of the modulation it puts on that row. Empty when
    // the merge carries none, which is the usual case. A translation like this suppresses one parity
    // class of every reflection, the axial rows included, so an absence test that expects every
    // reflection to be equally strong reads the suppressed class as extinct and claims a screw axis
    // that is not there.
    struct RowPseudoTranslation {
        std::array<int, 3> row{};
        std::array<int, 3> halves{};
        double ratio = 0.0;   // <E^2>(suppressed)/<E^2>(control) at the lowest resolution measurable
    };
    std::vector<RowPseudoTranslation> pseudo_translations;
    // The intensity-weighted operator R of the promotion that was ADOPTED (mean over the operators it
    // adds), that R over merge_r_floor, and that R over the globally best-agreeing operator - the
    // numbers the merge-degradation gate acts on (best-operator where two or more operators are
    // confirmed, floor on the first step out of P1). Reported so a run shows the margin the gate had.
    double r_added = std::numeric_limits<double>::quiet_NaN();
    double r_over_floor = std::numeric_limits<double>::quiet_NaN();
    double r_over_best = std::numeric_limits<double>::quiet_NaN();
    double global_best_operator_r = std::numeric_limits<double>::quiet_NaN();

    // A HIGHER point group whose operators the intensities confirmed (Stage A) but whose promotion the
    // consistency tests refused, with the reason. Processing continues in the lower group, which is the
    // safe direction: merging a twinned crystal in the twin's holohedry averages non-equivalent
    // reflections into each other and is unrecoverable from the output (and makes the run report that no
    // twin law exists), whereas keeping the subgroup costs only redundancy and can be promoted later.
    // Empty when nothing was refused. Surfaced to the user - a silent demotion is how a twin gets missed.
    std::string refused_point_group_hm;
    std::string refused_reason;
    // The symmorphic representative of that group - the same thing point_group_representative is, and
    // for the same reason (a name cannot tell P321 from P312). It is what a caller hands back as
    // fixed_point_group to ask what merging under the refused group would give. Unset when nothing
    // was refused.
    std::optional<gemmi::SpaceGroup> refused_point_group_representative;

    // Every operator-confirmed point-group hypothesis with its full evidence vector, ranked by order.
    // Report-only (see PointGroupLedgerEntry); the adoption above is unchanged by it. Empty on a search
    // that confirmed no operator at all, which the report renders as "no finalists" rather than as an
    // empty table - a blank one reads as "nothing was wrong" when it means "nothing was asked".
    std::vector<PointGroupLedgerEntry> point_group_ledger;
    // The refusal bounds the ledger's own channels were judged against, copied from the options so the
    // report can print the MARGIN the adopted row had rather than only its value. The accept-side
    // headroom is thin (measured: the largest genuine adopted r_over_best on a 63-dataset corpus is
    // 1.90 against a bound of 2.00), and a margin is the only form in which that is visible per run.
    double r_over_best_bound = 0.0;
    double r_over_floor_bound = 0.0;

    // The best SOHNCKE candidate, always, even when a glide plane was confirmed and best_space_group
    // therefore names a non-Sohncke group. Both are reported on every run: a chiral (protein) crystal
    // cannot have a glide, so a user who knows their sample is a protein must be able to see the
    // Sohncke answer without re-running, and a user working on a small molecule must be able to see
    // what the glide bought. Unset only when no Sohncke candidate was eligible.
    std::optional<gemmi::SpaceGroup> sohncke_space_group;
    // The best NON-SOHNCKE candidate - the "small molecule" answer, the group named with its glide
    // planes. Unset when no glide zone in this merge cleared min_glide_evidence_per_reflection,
    // which is the expected and measured outcome on every protein dataset.
    std::optional<gemmi::SpaceGroup> glide_space_group;
    // That candidate's glide zones, so a run shows WHICH plane was found dead and how dead it was.
    std::vector<SpaceGroupCandidateScore::GlideZone> glide_zones;
};

SearchSpaceGroupResult SearchSpaceGroup(
    const std::vector<MergedReflection>& merged,
    const SearchSpaceGroupOptions& opt = {});

// The finalist ledger as a ranked table, highest point group first. Report-only; printed behind
// --finalist-ledger so the numbers cannot be read as a decision before they have been calibrated.
std::string FinalistLedgerToText(const SearchSpaceGroupResult& result);

// Stage A's operator statistic for ONE rotation, given by its Miller-index matrix (h -> M h) in the
// basis the merge is indexed on: the correlation of resolution-normalised E^2(h) with E^2(Mh), over
// the same reflection population and with the same Friedel-folded pairing Stage A uses. For a caller
// that wants to ask about a single operator without enumerating a point group - a metric two-fold, say.
// Nothing when the operator makes too few pairs to judge (min_pairs_per_operator); the returned
// `present` flag is the same min_operator_cc test Stage A applies.
std::optional<SpaceGroupOperatorScore> OperatorCorrelation(const std::vector<MergedReflection>& merged,
                                                           const gemmi::Mat33& hkl_matrix,
                                                           const SearchSpaceGroupOptions& opt = {});

std::string SearchSpaceGroupResultToText(
    const SearchSpaceGroupResult& result,
    size_t max_candidates_to_print = 20);