Jungfraujoch/tests/SearchSpaceGroupTest.cpp

#include <catch2/catch_all.hpp>

#include "../image_analysis/scale_merge/SearchSpaceGroup.h"
#include "gemmi/symmetry.hpp"

#include <algorithm>
#include <cmath>
#include <cstdint>
#include <string>
#include <tuple>
#include <unordered_set>
#include <vector>

namespace {
    struct HKL {
        int h = 0;
        int k = 0;
        int l = 0;

        bool operator==(const HKL& o) const noexcept {
            return h == o.h && k == o.k && l == o.l;
        }
    };

    struct HKLHash {
        size_t operator()(const HKL& x) const noexcept {
            auto mix = [](uint64_t v) {
                v ^= v >> 33;
                v *= 0xff51afd7ed558ccdULL;
                v ^= v >> 33;
                v *= 0xc4ceb9fe1a85ec53ULL;
                v ^= v >> 33;
                return v;
            };
            return static_cast<size_t>(
                mix(static_cast<uint64_t>(x.h)) ^
                (mix(static_cast<uint64_t>(x.k)) << 1) ^
                (mix(static_cast<uint64_t>(x.l)) << 2));
        }
    };

    double CalcSyntheticD(int h, int k, int l) {
        const double q2 = static_cast<double>(h * h + k * k + l * l);
        return 40.0 / std::sqrt(q2 + 1.0);
    }

    double SyntheticIntensityFromAsu(const gemmi::Op::Miller& asu) {
        uint64_t x = static_cast<uint64_t>((asu[0] + 31) * 73856093u) ^
                     static_cast<uint64_t>((asu[1] + 37) * 19349663u) ^
                     static_cast<uint64_t>((asu[2] + 41) * 83492791u);
        x ^= x >> 13;
        x *= 0x9e3779b97f4a7c15ULL;
        x ^= x >> 17;
        return 100.0 + static_cast<double>(x % 500);
    }

    std::vector<MergedReflection> GenerateMergedReflectionsForSpaceGroup(
        const gemmi::SpaceGroup& sg,
        int hmax = 8) {

        std::vector<MergedReflection> merged;
        std::unordered_set<HKL, HKLHash> added;

        const gemmi::GroupOps gops = sg.operations();
        const gemmi::ReciprocalAsu rasu(&sg);

        for (int h = -hmax; h <= hmax; ++h) {
            for (int k = -hmax; k <= hmax; ++k) {
                for (int l = -hmax; l <= hmax; ++l) {
                    if (h == 0 && k == 0 && l == 0)
                        continue;

                    bool absent = false;
                    gemmi::Op::Miller hkl{{h, k, l}};
                    if (gops.is_systematically_absent(hkl))
                        absent = true;

                    const auto [asu, sign_plus] = rasu.to_asu_sign(hkl, gops);
                    if (!sign_plus)
                        continue;

                    const HKL key{h, k, l};
                    if (added.find(key) != added.end())
                        continue;
                    added.insert(key);

                    merged.push_back(MergedReflection{
                        .h = h,
                        .k = k,
                        .l = l,
                        .I = absent ? 0.0 : SyntheticIntensityFromAsu(asu),
                        .sigma = 1.0,
                        .d = CalcSyntheticD(h, k, l)
                    });
                }
            }
        }

        return merged;
    }
}

TEST_CASE("SearchSpaceGroup detects synthetic space groups") {
    struct Case {
        std::string input_name;
        std::string expected_short_name;
    };

    const std::vector<Case> cases = {
        {"P 1", "P1"},
        {"P 1 2 1", "P2"},
        {"P 3 2 1", "P321"},
        {"P 4 2 2", "P422"},
        {"P 4 3 2", "P432"},
        {"P 43 21 2", "P43212"},
        {"P 6 2 2", "P622"},
        {"C 1 2 1", "C2"},
        {"C 2 2 2", "C222"},
        {"I 4 3 2", "I432"},
        {"I 21 21 21", "I212121"},
        {"I 2 1 3", "I213"},
    };

    for (const auto& tc : cases) {
        DYNAMIC_SECTION(tc.expected_short_name) {
            const gemmi::SpaceGroup& sg = gemmi::get_spacegroup_by_name(tc.input_name);
            const auto merged = GenerateMergedReflectionsForSpaceGroup(sg);

            SearchSpaceGroupOptions opt;
            opt.merge_friedel = true;

            const auto result = SearchSpaceGroup(merged, opt);

            // Several inputs cannot be told apart from intensities alone: enantiomorphic partners
            // (P4_3 vs P4_1) and origin-ambiguous pairs (I2_12_12_1 vs I222, I2_13 vs I2_3) share
            // the same systematic absences. The search reports those as alternatives, so the
            // expected group must appear among the best group and its alternatives.
            std::vector<std::string> accepted;
            if (result.best_space_group.has_value())
                accepted.push_back(result.best_space_group->short_name());
            for (const auto& alt : result.alternatives)
                accepted.push_back(alt.short_name());

            INFO(SearchSpaceGroupResultToText(result));
            REQUIRE(result.best_space_group.has_value());
            CHECK(std::find(accepted.begin(), accepted.end(), tc.expected_short_name) != accepted.end());
        }
    }
}

// Regression: a real screw axis whose systematically-absent reflections carry a genuinely weak
// intensity but an UNDER-estimated sigma (so their I/sigma clears the "present" cut) must still be
// found. Reproduces a monoclinic 2_1 miss on weakly-diffracting monoclinic data, where the merged sigmas on
// the 0k0-odd reflections were ~2x too small and faked screw-axis violations. The E^2 intensity gate
// (present_e_squared) is what keeps those reflections classified absent.
TEST_CASE("SearchSpaceGroup finds a screw axis despite under-estimated sigmas on absent reflections") {
    const gemmi::SpaceGroup& sg = gemmi::get_spacegroup_by_name("P 1 21 1");
    auto merged = GenerateMergedReflectionsForSpaceGroup(sg, 18);

    // Every systematically-absent (0k0, k odd) reflection: small-but-nonzero intensity (~2% of a
    // normal reflection) with a far-too-small sigma, so I/sigma ~ 27 fakes a "present" reflection.
    const gemmi::GroupOps gops = sg.operations();
    int absent_count = 0;
    for (auto& r : merged) {
        const gemmi::Op::Miller hkl{{r.h, r.k, r.l}};
        if (gops.is_systematically_absent(hkl)) {
            r.I = 8.0f;
            r.sigma = 0.3f;
            ++absent_count;
        }
    }
    REQUIRE(absent_count >= 8);   // enough predicted-absent reflections to be trusted

    SearchSpaceGroupOptions opt;
    opt.merge_friedel = true;

    SECTION("intensity gate on (default): screw recovered") {
        const auto result = SearchSpaceGroup(merged, opt);
        INFO(SearchSpaceGroupResultToText(result));
        REQUIRE(result.best_space_group.has_value());
        CHECK(result.best_space_group->short_name() == "P21");
    }

    SECTION("intensity gate off (I/sigma only): the screw is missed") {
        // Documents the failure the gate fixes: with I/sigma alone the too-small sigmas fake
        // violations and the search falls back to the symmorphic group.
        opt.present_e_squared = 0.0;
        const auto result = SearchSpaceGroup(merged, opt);
        INFO(SearchSpaceGroupResultToText(result));
        REQUIRE(result.best_space_group.has_value());
        CHECK(result.best_space_group->short_name() == "P2");
    }
}

// Regression: the E^2 gate above compares a reflection to the mean of its RESOLUTION SHELL, which
// falls off with resolution, while a systematically-absent reflection keeps a small non-decaying
// residual (background / profile leakage). On a crystal whose axial rows are much stronger than an
// average reflection, that turns the high-resolution residuals into screw-axis violations and the
// screw is lost, although the reflections beside them in the same row are tens of times stronger.
// A tetragonal 42_12 case failed exactly this way (18 of 47 absent 00l over the cut, all beyond
// 3.7 A, at 1-2% of the l=4n reflections next to them). The threshold is therefore taken relative to
// the axial row the screw constrains, not to the shell.
TEST_CASE("SearchSpaceGroup finds a screw axis whose absent class is weak only within its own row") {
    const gemmi::SpaceGroup& sg = gemmi::get_spacegroup_by_name("P 43 21 2");
    auto merged = GenerateMergedReflectionsForSpaceGroup(sg, 12);

    // Axial rows 40x stronger than a general reflection, and an absent class carrying ~2% of its own
    // row - but half of a general reflection, so a threshold set against the shell calls every one of
    // them a violation while a threshold set against the row calls none.
    const gemmi::GroupOps gops = sg.operations();
    int absent_on_axis = 0;
    for (auto& r : merged) {
        const gemmi::Op::Miller hkl{{r.h, r.k, r.l}};
        if (gops.epsilon_factor_without_centering(hkl) <= 1)
            continue;
        if (gops.is_systematically_absent(hkl)) {
            r.I = 300.0f;
            r.sigma = 1.0f;
            ++absent_on_axis;
        } else {
            r.I *= 40.0f;
        }
    }
    REQUIRE(absent_on_axis >= 8);

    SearchSpaceGroupOptions opt;
    opt.merge_friedel = true;

    const auto result = SearchSpaceGroup(merged, opt);
    INFO(SearchSpaceGroupResultToText(result));
    REQUIRE(result.best_space_group.has_value());
    // P4_1 2_1 2 and P4_3 2_1 2 are enantiomorphs and indistinguishable from intensities.
    std::vector<std::string> accepted{result.best_space_group->short_name()};
    for (const auto& alt : result.alternatives)
        accepted.push_back(alt.short_name());
    CHECK(std::find(accepted.begin(), accepted.end(), "P43212") != accepted.end());
}