Jungfraujoch/image_analysis/scale_merge/ScaleAndMerge.cpp

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

#include "ScaleAndMerge.h"

#include <ceres/ceres.h>

#include <thread>
#include <iostream>
#include <algorithm>
#include <cmath>
#include <limits>
#include <stdexcept>
#include <tuple>
#include <unordered_map>
#include <utility>
#include <vector>

#include "../../common/ResolutionShells.h"

namespace {
    struct HKLKey {
        int64_t h = 0;
        int64_t k = 0;
        int64_t l = 0;
        bool is_positive = true; // only relevant if opt.merge_friedel == false

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

    struct HKLKeyHash {
        size_t operator()(const HKLKey &key) const noexcept {
            auto mix = [](uint64_t x) {
                x ^= x >> 33;
                x *= 0xff51afd7ed558ccdULL;
                x ^= x >> 33;
                x *= 0xc4ceb9fe1a85ec53ULL;
                x ^= x >> 33;
                return x;
            };
            const uint64_t a = static_cast<uint64_t>(key.h);
            const uint64_t b = static_cast<uint64_t>(key.k);
            const uint64_t c = static_cast<uint64_t>(key.l);
            const uint64_t d = static_cast<uint64_t>(key.is_positive ? 1 : 0);
            return static_cast<size_t>(mix(a) ^ (mix(b) << 1) ^ (mix(c) << 2) ^ (mix(d) << 3));
        }
    };

    inline int RoundImageId(float image_number, double rounding_step) {
        if (!(rounding_step > 0.0))
            rounding_step = 1.0;
        const double x = static_cast<double>(image_number) / rounding_step;
        const double r = std::round(x) * rounding_step;
        return static_cast<int>(std::llround(r / rounding_step));
    }

    inline double SafeSigma(double s, double min_sigma) {
        if (!std::isfinite(s) || s <= 0.0)
            return min_sigma;
        return std::max(s, min_sigma);
    }

    inline double SafeD(double d) {
        if (!std::isfinite(d) || d <= 0.0)
            return std::numeric_limits<double>::quiet_NaN();
        return d;
    }

    inline int SafeToInt(int64_t x) {
        if (x < std::numeric_limits<int>::min() || x > std::numeric_limits<int>::max())
            throw std::out_of_range("HKL index out of int range for Gemmi");
        return static_cast<int>(x);
    }

    inline double SafeInv(double x, double fallback) {
        if (!std::isfinite(x) || x == 0.0)
            return fallback;
        return 1.0 / x;
    }

    inline HKLKey CanonicalizeHKLKey(const Reflection &r, const ScaleMergeOptions &opt) {
        HKLKey key{};
        key.h = r.h;
        key.k = r.k;
        key.l = r.l;
        key.is_positive = true;

        if (!opt.space_group.has_value()) {
            if (!opt.merge_friedel) {
                const HKLKey neg{-r.h, -r.k, -r.l, true};
                const bool pos = std::tie(key.h, key.k, key.l) >= std::tie(neg.h, neg.k, neg.l);
                if (!pos) {
                    key.h = -key.h;
                    key.k = -key.k;
                    key.l = -key.l;
                    key.is_positive = false;
                }
            }
            return key;
        }

        const gemmi::SpaceGroup &sg = *opt.space_group;
        const gemmi::GroupOps gops = sg.operations();
        const gemmi::ReciprocalAsu rasu(&sg);

        const gemmi::Op::Miller in{{SafeToInt(r.h), SafeToInt(r.k), SafeToInt(r.l)}};
        const auto [asu_hkl, sign_plus] = rasu.to_asu_sign(in, gops);

        key.h = asu_hkl[0];
        key.k = asu_hkl[1];
        key.l = asu_hkl[2];
        key.is_positive = opt.merge_friedel ? true : sign_plus;
        return key;
    }

    struct IntensityResidual {
        IntensityResidual(const Reflection &r, double sigma_obs, double wedge_deg, bool refine_partiality)
            : Iobs_(static_cast<double>(r.I)),
              weight_(SafeInv(sigma_obs, 1.0)),
              delta_phi_(r.delta_phi_deg),
              lp_(SafeInv(static_cast<double>(r.rlp), 1.0)),
              half_wedge_(wedge_deg / 2.0),
              c1_(r.zeta / std::sqrt(2.0)),
              partiality_(r.partiality),
              refine_partiality_(refine_partiality) {
        }

        template<typename T>
        bool operator()(const T *const G,
                        const T *const mosaicity,
                        const T *const Itrue,
                        T *residual) const {
            T partiality;
            if (refine_partiality_ && mosaicity[0] != 0.0) {
                const T arg_plus = T(delta_phi_ + half_wedge_) * T(c1_) / mosaicity[0];
                const T arg_minus = T(delta_phi_ - half_wedge_) * T(c1_) / mosaicity[0];
                partiality = (ceres::erf(arg_plus) - ceres::erf(arg_minus)) / T(2.0);
            } else
                partiality = T(partiality_);

            const T Ipred = G[0] * partiality * T(lp_) * Itrue[0];
            residual[0] = (Ipred - T(Iobs_)) * T(weight_);
            return true;
        }

        double Iobs_;
        double weight_;
        double delta_phi_;
        double lp_;
        double half_wedge_;
        double c1_;
        double partiality_;
        bool refine_partiality_;
    };

    struct ScaleRegularizationResidual {
        explicit ScaleRegularizationResidual(double sigma_k)
            : inv_sigma_(SafeInv(sigma_k, 1.0)) {
        }

        template<typename T>
        bool operator()(const T *const k, T *residual) const {
            residual[0] = (k[0] - T(1.0)) * T(inv_sigma_);
            return true;
        }

        double inv_sigma_;
    };

    struct SmoothnessRegularizationResidual {
        explicit SmoothnessRegularizationResidual(double sigma)
            : inv_sigma_(SafeInv(sigma, 1.0)) {
        }

        template<typename T>
        bool operator()(const T *const k0,
                        const T *const k1,
                        const T *const k2,
                        T *residual) const {
            residual[0] = (ceres::log(k0[0]) + ceres::log(k2[0]) - T(2.0) * ceres::log(k1[0])) * T(inv_sigma_);
            return true;
        }

        double inv_sigma_;
    };

    struct ObsRef {
        const Reflection *r = nullptr;
        int img_id = 0;
        int hkl_slot = -1;
        double sigma = 0.0;
    };

    struct CorrectedObs {
        int hkl_slot;
        double I_corr;
        double sigma_corr;
    };

    void scale(const ScaleMergeOptions &opt,
               std::vector<double> &g,
               std::vector<double> &mosaicity,
               const std::vector<uint8_t> &image_slot_used,
               bool rotation_crystallography,
               size_t nhkl,
               const std::vector<ObsRef> &obs) {
        ceres::Problem problem;

        std::vector<double> Itrue(nhkl, 0.0);

        // Initialize Itrue from per-HKL median of observed intensities
        {
            std::vector<std::vector<double> > per_hkl_I(nhkl);
            for (const auto &o: obs) {
                per_hkl_I[o.hkl_slot].push_back(static_cast<double>(o.r->I));
            }
            for (int h = 0; h < nhkl; ++h) {
                auto &v = per_hkl_I[h];
                if (v.empty()) {
                    Itrue[h] = std::max(opt.min_sigma, 1e-6);
                    continue;
                }
                std::nth_element(v.begin(), v.begin() + static_cast<long>(v.size() / 2), v.end());
                double med = v[v.size() / 2];
                if (!std::isfinite(med) || med <= opt.min_sigma)
                    med = opt.min_sigma;
                Itrue[h] = med;
            }
        }

        std::vector<bool> is_valid_hkl_slot(nhkl, false);

        for (const auto &o: obs) {
            auto *cost = new ceres::AutoDiffCostFunction<IntensityResidual, 1, 1, 1, 1>(
                new IntensityResidual(*o.r, o.sigma, opt.wedge_deg.value_or(0.0), rotation_crystallography));
            problem.AddResidualBlock(cost,
                                     nullptr,
                                     &g[o.img_id],
                                     &mosaicity[o.img_id],
                                     &Itrue[o.hkl_slot]);
            is_valid_hkl_slot[o.hkl_slot] = true;
        }

        for (int i = 0; i < g.size(); ++i) {
            if (image_slot_used[i]) {
                auto *cost = new ceres::AutoDiffCostFunction<ScaleRegularizationResidual, 1, 1>(
                    new ScaleRegularizationResidual(0.05));
                problem.AddResidualBlock(cost, nullptr, &g[i]);
            }
        }

        if (rotation_crystallography) {
            if (opt.smoothen_g) {
                for (int i = 0; i < g.size() - 2; ++i) {
                    if (image_slot_used[i] && image_slot_used[i + 1] && image_slot_used[i + 2]) {
                        auto *cost = new ceres::AutoDiffCostFunction<SmoothnessRegularizationResidual, 1, 1, 1, 1>(
                            new SmoothnessRegularizationResidual(0.05));

                        problem.AddResidualBlock(cost, nullptr, &g[i], &g[i + 1], &g[i + 2]);
                    }
                }
            }

            if (opt.smoothen_mos && opt.refine_mosaicity) {
                for (int i = 0; i < mosaicity.size() - 2; ++i) {
                    if (image_slot_used[i] && image_slot_used[i + 1] && image_slot_used[i + 2]) {
                        auto *cost = new ceres::AutoDiffCostFunction<SmoothnessRegularizationResidual, 1, 1, 1, 1>(
                            new SmoothnessRegularizationResidual(0.05));

                        problem.AddResidualBlock(cost, nullptr, &mosaicity[i], &mosaicity[i + 1], &mosaicity[i + 2]);
                    }
                }
            }
        }

        // Scaling factors must be always positive
        for (int i = 0; i < g.size(); i++) {
            if (image_slot_used[i])
                problem.SetParameterLowerBound(&g[i], 0, 1e-12);
        }

        // Mosaicity refinement + bounds
        if (!opt.refine_mosaicity) {
            for (int i = 0; i < mosaicity.size(); ++i) {
                if (image_slot_used[i])
                    problem.SetParameterBlockConstant(&mosaicity[i]);
            }
        } else {
            for (int i = 0; i < mosaicity.size(); ++i) {
                if (image_slot_used[i]) {
                    problem.SetParameterLowerBound(&mosaicity[i], 0, opt.mosaicity_min_deg);
                    problem.SetParameterUpperBound(&mosaicity[i], 0, opt.mosaicity_max_deg);
                }
            }
        }

        // use all available threads
        unsigned int hw = std::thread::hardware_concurrency();
        if (hw == 0)
            hw = 1; // fallback

        ceres::Solver::Options options;

        options.linear_solver_type = ceres::SPARSE_NORMAL_CHOLESKY;
        options.minimizer_progress_to_stdout = true;
        options.max_num_iterations = opt.max_num_iterations;
        options.max_solver_time_in_seconds = opt.max_solver_time_s;
        options.num_threads = static_cast<int>(hw);

        ceres::Solver::Summary summary;
        ceres::Solve(options, &problem, &summary);
        std::cout << summary.FullReport() << std::endl;
    }

    void merge(size_t nhkl, ScaleMergeResult &out, const std::vector<CorrectedObs> &corr_obs) {
        // ---- Merge (XDS/XSCALE style: inverse-variance weighted mean) ----

        struct HKLAccum {
            double sum_wI = 0.0;
            double sum_w = 0.0;
            double sum_wI2 = 0.0;
            int count = 0;
        };
        std::vector<HKLAccum> accum(nhkl);

        for (const auto &co: corr_obs) {
            const double w = 1.0 / (co.sigma_corr * co.sigma_corr);
            auto &a = accum[co.hkl_slot];
            a.sum_wI += w * co.I_corr;
            a.sum_w += w;
            a.sum_wI2 += w * co.I_corr * co.I_corr;
            a.count++;
        }

        for (int h = 0; h < nhkl; ++h) {
            const auto &a = accum[h];
            if (a.sum_w <= 0.0)
                continue;

            out.merged[h].I = a.sum_wI / a.sum_w;
            const double sigma_stat = std::sqrt(1.0 / a.sum_w);

            if (a.count >= 2) {
                const double var_internal = a.sum_wI2 - (a.sum_wI * a.sum_wI) / a.sum_w;
                const double sigma_int = std::sqrt(var_internal / (a.sum_w * (a.count - 1)));
                out.merged[h].sigma = std::max(sigma_stat, sigma_int);
            } else {
                out.merged[h].sigma = sigma_stat;
            }
        }
    }

    void stats(const ScaleMergeOptions &opt, size_t nhkl, ScaleMergeResult &out, const std::vector<CorrectedObs> &corr_obs)
    // ---- Compute per-shell merging statistics ----
    {
        constexpr int kStatShells = 10;

        float stat_d_min = std::numeric_limits<float>::max();
        float stat_d_max = 0.0f;
        for (int h = 0; h < nhkl; ++h) {
            const auto d = static_cast<float>(out.merged[h].d);
            if (std::isfinite(d) && d > 0.0f) {
                if (opt.d_min_limit_A > 0.0 && d < static_cast<float>(opt.d_min_limit_A))
                    continue;
                stat_d_min = std::min(stat_d_min, d);
                stat_d_max = std::max(stat_d_max, d);
            }
        }

        if (stat_d_min < stat_d_max && stat_d_min > 0.0f) {
            const float d_min_pad = stat_d_min * 0.999f;
            const float d_max_pad = stat_d_max * 1.001f;
            ResolutionShells stat_shells(d_min_pad, d_max_pad, kStatShells);

            const auto shell_mean_1_d2 = stat_shells.GetShellMeanOneOverResSq();
            const auto shell_min_res = stat_shells.GetShellMinRes();

            // Assign each unique reflection to a shell
            std::vector<int32_t> hkl_shell(nhkl, -1);
            for (int h = 0; h < nhkl; ++h) {
                const auto d = static_cast<float>(out.merged[h].d);
                if (std::isfinite(d) && d > 0.0f) {
                    if (opt.d_min_limit_A > 0.0 && d < static_cast<float>(opt.d_min_limit_A))
                        continue;
                    auto s = stat_shells.GetShell(d);
                    if (s.has_value())
                        hkl_shell[h] = s.value();
                }
            }

            // Per-HKL: collect corrected intensities for Rmeas
            struct HKLStats {
                double sum_I = 0.0;
                int n = 0;
                std::vector<double> I_list;
            };
            std::vector<HKLStats> per_hkl(nhkl);

            for (const auto &co: corr_obs) {
                if (hkl_shell[co.hkl_slot] < 0)
                    continue;
                auto &hs = per_hkl[co.hkl_slot];
                hs.sum_I += co.I_corr;
                hs.n += 1;
                hs.I_list.push_back(co.I_corr);
            }

            // Accumulators per shell
            struct ShellAccum {
                int total_obs = 0;
                std::unordered_set<int> unique_hkls;
                double rmeas_num = 0.0;
                double rmeas_den = 0.0;
                double sum_i_over_sig = 0.0;
                int n_merged_with_sigma = 0;
            };
            std::vector<ShellAccum> shell_acc(kStatShells);

            for (int h = 0; h < nhkl; ++h) {
                if (hkl_shell[h] < 0)
                    continue;
                const int s = hkl_shell[h];
                auto &sa = shell_acc[s];
                const auto &hs = per_hkl[h];
                if (hs.n == 0)
                    continue;

                sa.unique_hkls.insert(h);
                sa.total_obs += hs.n;

                const double mean_I = hs.sum_I / static_cast<double>(hs.n);

                if (hs.n >= 2) {
                    double sum_abs_dev = 0.0;
                    for (double Ii: hs.I_list)
                        sum_abs_dev += std::abs(Ii - mean_I);
                    sa.rmeas_num += std::sqrt(static_cast<double>(hs.n) / (hs.n - 1.0)) * sum_abs_dev;
                }

                for (double Ii: hs.I_list)
                    sa.rmeas_den += std::abs(Ii);

                if (out.merged[h].sigma > 0.0) {
                    sa.sum_i_over_sig += out.merged[h].I / out.merged[h].sigma;
                    sa.n_merged_with_sigma += 1;
                }
            }

            // Completeness (not yet available without unit cell)
            std::vector<int> possible_per_shell(kStatShells, 0);
            int total_possible = 0;
            bool have_completeness = false;

            // Fill output statistics
            out.statistics.shells.resize(kStatShells);
            for (int s = 0; s < kStatShells; ++s) {
                auto &ss = out.statistics.shells[s];
                const auto &sa = shell_acc[s];

                ss.mean_one_over_d2 = shell_mean_1_d2[s];
                ss.d_min = shell_min_res[s];
                ss.d_max = (s == 0) ? d_max_pad : shell_min_res[s - 1];

                ss.total_observations = sa.total_obs;
                ss.unique_reflections = static_cast<int>(sa.unique_hkls.size());
                ss.rmeas = (sa.rmeas_den > 0.0) ? (sa.rmeas_num / sa.rmeas_den) : 0.0;
                ss.mean_i_over_sigma = (sa.n_merged_with_sigma > 0)
                                           ? (sa.sum_i_over_sig / sa.n_merged_with_sigma)
                                           : 0.0;
                ss.possible_reflections = possible_per_shell[s];
                ss.completeness = (have_completeness && possible_per_shell[s] > 0)
                                      ? static_cast<double>(sa.unique_hkls.size()) / possible_per_shell[s]
                                      : 0.0;
            }

            // Overall statistics
            {
                auto &ov = out.statistics.overall;
                ov.d_min = stat_d_min;
                ov.d_max = stat_d_max;
                ov.mean_one_over_d2 = 0.0f;

                int total_obs_all = 0;
                std::unordered_set<int> all_unique;
                double rmeas_num_all = 0.0, rmeas_den_all = 0.0;
                double sum_isig_all = 0.0;
                int n_isig_all = 0;

                for (int s = 0; s < kStatShells; ++s) {
                    const auto &sa = shell_acc[s];
                    total_obs_all += sa.total_obs;
                    all_unique.insert(sa.unique_hkls.begin(), sa.unique_hkls.end());
                    rmeas_num_all += sa.rmeas_num;
                    rmeas_den_all += sa.rmeas_den;
                    sum_isig_all += sa.sum_i_over_sig;
                    n_isig_all += sa.n_merged_with_sigma;
                }

                ov.total_observations = total_obs_all;
                ov.unique_reflections = static_cast<int>(all_unique.size());
                ov.rmeas = (rmeas_den_all > 0.0) ? (rmeas_num_all / rmeas_den_all) : 0.0;
                ov.mean_i_over_sigma = (n_isig_all > 0) ? (sum_isig_all / n_isig_all) : 0.0;
                ov.possible_reflections = total_possible;
                ov.completeness = (have_completeness && total_possible > 0)
                                      ? static_cast<double>(all_unique.size()) / total_possible
                                      : 0.0;
            }
        }
    }
    void calc_obs(const ScaleMergeOptions &opt,
               std::vector<double> &g,
               std::vector<double> &mosaicity,
               bool rotation_crystallography,
               const std::vector<ObsRef> &obs,
               std::vector<CorrectedObs> &corr_obs) {

        // ---- Compute corrected observations once (used for both merging and statistics) ----
        const double half_wedge = opt.wedge_deg.value_or(0.0) / 2.0;

        for (const auto &o: obs) {
            const Reflection &r = *o.r;
            const double lp = SafeInv(static_cast<double>(r.rlp), 1.0);
            const double G_i = g[o.img_id];

            // Compute partiality with refined mosaicity
            double partiality;
            if (rotation_crystallography && mosaicity[o.img_id] > 0.0) {
                const double c1 = r.zeta / std::sqrt(2.0);
                const double arg_plus = (r.delta_phi_deg + half_wedge) * c1 / mosaicity[o.img_id];
                const double arg_minus = (r.delta_phi_deg - half_wedge) * c1 / mosaicity[o.img_id];
                partiality = (std::erf(arg_plus) - std::erf(arg_minus)) / 2.0;
            } else {
                partiality = r.partiality;
            }

            if (partiality <= opt.min_partiality_for_merge)
                continue;
            const double correction = G_i * partiality * lp;
            if (correction <= 0.0)
                continue;

            corr_obs.push_back({
                o.hkl_slot,
                static_cast<double>(r.I) / correction,
                o.sigma / correction
            });
        }
    }

    void proc_obs(const std::vector<std::vector<Reflection> > &observations,
        const ScaleMergeOptions &opt,
        std::vector<uint8_t> &image_slot_used,
        std::vector<ObsRef> &obs,
        std::unordered_map<HKLKey, int, HKLKeyHash> &hklToSlot
        ) {
        for (int i = 0; i < observations.size(); i++) {
            for (const auto &r: observations[i]) {
                const double d = SafeD(r.d);
                if (!std::isfinite(d))
                    continue;
                if (!std::isfinite(r.I))
                    continue;

                if (opt.d_min_limit_A > 0.0 && d < opt.d_min_limit_A)
                    continue;

                if (!std::isfinite(r.zeta) || r.zeta <= 0.0f)
                    continue;
                if (!std::isfinite(r.rlp) || r.rlp == 0.0f)
                    continue;

                const double sigma = SafeSigma(r.sigma, opt.min_sigma);

                const int img_id = i / opt.image_cluster;
                image_slot_used[img_id] = 1;

                int hkl_slot;
                try {
                    const HKLKey key = CanonicalizeHKLKey(r, opt);
                    auto it = hklToSlot.find(key);
                    if (it == hklToSlot.end()) {
                        hkl_slot = static_cast<int>(hklToSlot.size());
                        hklToSlot.emplace(key, hkl_slot);
                    } else {
                        hkl_slot = it->second;
                    }
                } catch (...) {
                    continue;
                }

                ObsRef o;
                o.r = &r;
                o.img_id = img_id;
                o.hkl_slot = hkl_slot;
                o.sigma = sigma;
                obs.push_back(o);
            }
        }

    }
} // namespace

ScaleMergeResult ScaleAndMergeReflectionsCeres(const std::vector<std::vector<Reflection> > &observations,
                                               const ScaleMergeOptions &opt) {
    if (opt.image_cluster <= 0)
        throw std::invalid_argument("image_cluster must be positive");

    const bool rotation_crystallography = opt.wedge_deg.has_value();

    size_t nrefl = 0;
    for (const auto &i: observations)
        nrefl += i.size();

    std::vector<ObsRef> obs;
    std::vector<CorrectedObs> corr_obs;

    obs.reserve(nrefl);
    corr_obs.reserve(nrefl);

    std::unordered_map<HKLKey, int, HKLKeyHash> hklToSlot;
    hklToSlot.reserve(nrefl);

    size_t n_image_slots = observations.size() / opt.image_cluster +
        (observations.size() % opt.image_cluster > 0 ? 1 : 0);

    std::vector<uint8_t> image_slot_used(n_image_slots, 0);

    proc_obs(observations, opt, image_slot_used, obs, hklToSlot);

    const int nhkl = static_cast<int>(hklToSlot.size());

    std::vector<double> g(n_image_slots, 1.0);
    std::vector<double> mosaicity(n_image_slots, opt.mosaicity_init_deg);
    for (int i = 0; i < n_image_slots; i++) {
        if (!image_slot_used[i]) {
            mosaicity[i] = NAN;
            g[i] = NAN;
        } else if (opt.mosaicity_init_deg_vec.size() > i && std::isfinite(opt.mosaicity_init_deg_vec[i])) {
            mosaicity[i] = opt.mosaicity_init_deg_vec[i];
        }
    }

    scale(opt, g, mosaicity, image_slot_used, rotation_crystallography, nhkl, obs);

    ScaleMergeResult out;

    out.image_scale_g.resize(observations.size(), NAN);
    out.mosaicity_deg.resize(observations.size(), NAN);
    for (int i = 0; i < observations.size(); i++) {
        size_t img_slot = i / opt.image_cluster;
        if (image_slot_used[img_slot]) {
            out.image_scale_g[i] = g[img_slot];
            out.mosaicity_deg[i] = mosaicity[img_slot];
        }
    }

    std::vector<HKLKey> slotToHKL(nhkl);
    for (const auto &kv: hklToSlot)
        slotToHKL[kv.second] = kv.first;

    out.merged.resize(nhkl);
    for (int h = 0; h < nhkl; ++h) {
        out.merged[h].h = slotToHKL[h].h;
        out.merged[h].k = slotToHKL[h].k;
        out.merged[h].l = slotToHKL[h].l;
        out.merged[h].I = 0.0;
        out.merged[h].sigma = 0.0;
        out.merged[h].d = 0.0;
    }

    // Populate d from median of observations per HKL
    {
        std::vector<std::vector<double> > per_hkl_d(nhkl);
        for (const auto &o: obs) {
            const double d_val = static_cast<double>(o.r->d);
            if (std::isfinite(d_val) && d_val > 0.0)
                per_hkl_d[o.hkl_slot].push_back(d_val);
        }
        for (int h = 0; h < nhkl; ++h) {
            auto &v = per_hkl_d[h];
            if (!v.empty()) {
                std::nth_element(v.begin(), v.begin() + static_cast<long>(v.size() / 2), v.end());
                out.merged[h].d = v[v.size() / 2];
            }
        }
    }

    calc_obs(opt, g, mosaicity, rotation_crystallography, obs, corr_obs);
    merge(nhkl, out, corr_obs);
    stats(opt, nhkl, out, corr_obs);

    return out;
}