Jungfraujoch/image_analysis/scale_merge/TwinningAnalysis.h

// SPDX-FileCopyrightText: 2026 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
// SPDX-License-Identifier: GPL-3.0-only

#pragma once

#include <array>
#include <string>
#include <vector>

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

// Intensity-statistics test for merohedral twinning, in the spirit of phenix.xtriage. Two
// indicators on the merged intensities, both normalised per resolution shell:
//
//   - Padilla & Yeates L-test (Acta Cryst D59, 2003): for pairs of nearby but symmetry-independent
//     reflections, L = (I1 - I2)/(I1 + I2). Untwinned acentric data give <|L|> = 0.5, <L^2> = 1/3;
//     a perfect merohedral twin narrows this to <|L|> = 0.375, <L^2> = 0.2. Insensitive to
//     anisotropy and overall scale because it is a local ratio of shell-normalised intensities.
//
//   - Second moment <I^2>/<I>^2 of acentric intensities, normalised per resolution shell so it is
//     2.0 for untwinned data and 1.5 for a perfect twin (= 2(1 - a + a^2) for twin fraction a).
//
// These flag the *presence* of twinning; they do not by themselves identify the twin law. In a
// holohedral Laue class, where no twin law exists, the same low reading says instead that an operator
// of the group averages unequal intensities - the group is too high, or a twin law was absorbed into
// it (adopted_operators_suspect).

enum class TwinFractionSource { None, LTest, SecondMoment };

struct TwinningAnalysisResult {
    int l_test_pairs = 0;
    double mean_abs_l = 0.0;   // <|L|>: 0.5 untwinned, 0.375 perfect twin
    double mean_l_squared = 0.0;   // <L^2>: 0.333 untwinned, 0.2 perfect twin

    int moment_reflections = 0;
    double second_moment = 0.0;   // <I^2>/<I>^2 (shell-normalised): 2.0 untwinned, 1.5 perfect twin

    // Twin fraction in [0, 0.5], from the statistic named by twin_fraction_source: the L-test unless
    // the verdict rests on the second moment alone, a = (1 - sqrt(2M-3))/2. None where it cannot be
    // measured - merged under a false operator, or a second moment under a pseudo-translation.
    double estimated_twin_fraction = 0.0;
    TwinFractionSource twin_fraction_source = TwinFractionSource::None;

    // An indicator reads twin-like (<|L|> < 0.44 or second moment < 1.85, and <|L|> not at or above
    // 0.5). Set in every Laue class; what it means depends on the next two fields.
    bool twinning_suspected = false;

    // False when the Laue class is holohedral (4/mmm, 6/mmm, m-3m, rhombohedral -3m): no merohedral
    // twin law can exist WITHIN it.
    bool merohedral_twinning_possible = true;

    // twinning_suspected in a holohedral class: an operator of the group averages intensities that
    // are not equal, so the group is too high or it has absorbed a twin law. A warning about the
    // symmetry, not a twin call - and the twin fraction is not measurable on such a merge.
    bool adopted_operators_suspect = false;

    // How the L-test coped with a translational pseudo-symmetry, when one was declared (see the
    // partner-step note in the .cpp). Exactly one of these can be set:
    //   - `l_test_tncs_step_restricted`: partners were taken only along steps that preserve the
    //     pseudo-translation's class, so <|L|> above is unbiased and is read normally. Costs a few
    //     per mille of the pairs.
    //   - `l_test_contaminated_by_tncs`: NO available step preserves the class, so <|L|> is biased
    //     upwards and is not read in either direction - neither as evidence of twinning nor as the
    //     proof of its absence that the >= 0.50 branch would otherwise take it for.
    bool l_test_tncs_step_restricted = false;
    bool l_test_contaminated_by_tncs = false;

    // Set when this analysis ran on a merge whose space group the pipeline CHOSE by promoting past a
    // subgroup. The "holohedral Laue class, so no twin law exists" conclusion is then circular - the
    // promotion is exactly what a twin would have caused - so the report must not state it as a fact.
    bool laue_class_was_chosen_by_promotion = false;
};

// The space group (when known) is used to drop centric reflections, which follow different
// statistics (<|L|>, <I^2>/<I>^2 = 3 not 2); pass nullptr for P1 data, where none are centric.
// `lattice_centring` drops the reflections the lattice centring extinguishes, for a P1 merge of a
// centred lattice (whose space_group is nullptr and so says nothing about them).
// `tncs_vector` is the fractional pseudo-translation of a DETECTED translational pseudo-symmetry on
// this same merge (TranslationalNCSVectorForLTest), or nullptr. It changes which partner reflections
// the L-test pairs with, so it has to be an argument rather than a flag set on the result afterwards:
// the verdict and the rendered text both depend on it.
TwinningAnalysisResult AnalyzeTwinning(const std::vector<MergedReflection>& merged,
                                       const gemmi::SpaceGroup* space_group = nullptr,
                                       int resolution_shells = 20,
                                       const std::array<double, 3>* tncs_vector = nullptr,
                                       char lattice_centring = 'P');

std::string TwinningAnalysisToText(const TwinningAnalysisResult& result);

// The verdict in one line - "INDICATED (...)", "SYMMETRY SUSPECT (...)" or "no indication (...)" -
// so the report's summary row, its prose and the viewer all say the same thing.
std::string TwinningVerdictLine(const TwinningAnalysisResult& result);

// Twin-immune zone evidence for the operators a point group adds over each of its index-2 subgroups,
// read on a P1 merge. REPORT-ONLY.
//
// Under a twin law T, I_obs(h) = (1-a) I(h) + a I(Th); a reflection whose twin mate Th is h itself, up
// to the true group and Friedel, is untouched by twinning at every fraction a. For an index-2 subgroup
// H of the adopted group G those are the reflections centric in G but acentric in H: centric if the
// operators G adds over H are real, acentric if they are a twin law or a pseudo-symmetry the promotion
// took for real. So their intensity distribution says which, and does so where every operator
// statistic is blind - at a = 0.5, where I(h) = I(Th) exactly. Read ABSOLUTELY, <|E^2-1|> against
// 0.968 (centric) and 0.736 (acentric); never as the difference to the control, because noise, a
// pseudo-translation and anisotropy all inflate every class towards centric, and a perfect twin's
// control (0.541) plus that inflation looks like true symmetry's.
struct TwinImmuneZone {
    std::string operators;              // the operators G adds over H, as h,k,l triplets
    int n = 0;
    double mean_abs_e2_minus_1 = 0.0;   // 0.968 centric (operator real), 0.736 acentric (not)
    double standard_error = 0.0;
    // Log-likelihood ratio, centric over acentric Wilson (each convolved with the reflection's error),
    // summed over the zone: > 0 favours the operators being real symmetry, < 0 a twin law or
    // pseudo-symmetry. About +0.2 nats per reflection for a centric zone and -0.13 for an acentric one
    // on error-free data; less either way as the errors grow.
    double evidence_nats = 0.0;
};

struct TwinImmuneZoneResult {
    std::vector<TwinImmuneZone> zones;  // one per index-2 subgroup of the group
    TwinImmuneZone control;             // the reflections acentric in the group
    double d_max_A = 0.0;               // resolution range of the shells read
    double d_min_A = 0.0;
    bool tncs_normalised = false;       // normalised within the pseudo-translation's two phase classes
};

TwinImmuneZoneResult AnalyzeTwinImmuneZones(const std::vector<MergedReflection>& p1_merged,
                                            const gemmi::SpaceGroup& group,
                                            const std::array<double, 3>* tncs_vector = nullptr);

std::string TwinImmuneZonesToText(const TwinImmuneZoneResult& result);