gfield_asciimap.cc

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// dladdr (used to locate this plugin's own path) needs _GNU_SOURCE on glibc.
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
 
// plugin header
#include "gfield_asciimap.h"
 
// gemc gfield framework
#include "gfieldConventions.h"
 
// geant4 / CLHEP
#include "G4ThreeVector.hh"
#include "CLHEP/Units/SystemOfUnits.h"
 
// c++
#include <cmath>
#include <cstdlib>
#include <dlfcn.h>
#include <fstream>
#include <sstream>
 
using namespace CLHEP;
 
// Tells the loader how to create a GField in this plugin .so/.dylib.
extern "C" GField* GFieldFactory(const std::shared_ptr<GOptions>& g) {
    return static_cast<GField*>(new GField_AsciiMapFactory(g));
}
 
 
// ---------------------------------------------------------------------------------------------------
// Configuration helpers
// ---------------------------------------------------------------------------------------------------
 
std::string GField_AsciiMapFactory::param_string(const std::string& key, const std::string& dflt) const {
    auto it = gfield_definitions.field_parameters.find(key);
    return (it != gfield_definitions.field_parameters.end() && !it->second.empty()) ? it->second : dflt;
}
 
double GField_AsciiMapFactory::param_g4number(const std::string& key, const std::string& dflt) const {
    return gutilities::getG4Number(param_string(key, dflt));
}
 
std::string GField_AsciiMapFactory::field_maps_directory() const {
    // Locate this plugin's own shared object on disk and point at the sibling "fields" directory
    // (<plugin_dir>/../fields), the layout produced by `meson install`. No environment variable is used.
    Dl_info info;
    if (dladdr(reinterpret_cast<const void*>(&GFieldFactory), &info) != 0 && info.dli_fname != nullptr) {
        const std::string plugin_path = info.dli_fname;
        const auto        slash       = plugin_path.find_last_of('/');
        if (slash != std::string::npos) { return plugin_path.substr(0, slash) + "/../fields"; }
    }
    log->warning("GField_AsciiMapFactory: could not resolve plugin path; using ./fields for the map.");
    return "fields";
}
 
std::optional<unsigned> GField_AsciiMapFactory::axis_of_coordinate(const std::string& name) const {
    switch (symmetry) {
    case Symmetry::dipole_x:
    case Symmetry::dipole_y:
    case Symmetry::dipole_z:
        if (name == "longitudinal") return 0;
        if (name == "transverse") return 1;
        return std::nullopt;
    case Symmetry::cyl_x:
    case Symmetry::cyl_y:
    case Symmetry::cyl_z:
        if (name == "transverse") return 0;
        if (name == "longitudinal") return 1;
        return std::nullopt;
    case Symmetry::phi_segmented:
        if (name == "azimuthal") return 0;
        if (name == "transverse") return 1;
        if (name == "longitudinal") return 2;
        return std::nullopt;
    case Symmetry::cartesian_3d:
    case Symmetry::cartesian_3d_quadrant:
        if (name == "X") return 0;
        if (name == "Y") return 1;
        if (name == "Z") return 2;
        return std::nullopt;
    }
    return std::nullopt;
}
 
 
// ---------------------------------------------------------------------------------------------------
// Loading
// ---------------------------------------------------------------------------------------------------
 
void GField_AsciiMapFactory::load_coordinate(const std::string& key) {
    const std::string value = param_string(key, "");
    if (value.empty()) {
        log->error(
            gfields::ERR_WRONG_COORDINATE_DEF,
            "GField_AsciiMapFactory: missing coordinate <%1> for field <%2>.", key, gfield_definitions.name);
    }
 
    // Split "name, npoints, min, max" on commas.
    const auto tokens = gutilities::getStringVectorFromStringWithDelimiter(value, ",");
    if (tokens.size() != 4) {
        log->error(
            gfields::ERR_WRONG_COORDINATE_DEF,
            "GField_AsciiMapFactory: coordinate <%1> must be \"name, npoints, min, max\". Got <%2>.", key,
            value);
    }
 
    const std::string name    = tokens[0];
    const int         npoints = std::stoi(tokens[1]);
    const double      min     = gutilities::getG4Number(tokens[2]);
    const double      max     = gutilities::getG4Number(tokens[3]);
 
    const auto axis = axis_of_coordinate(name);
    if (!axis) {
        log->error(
            gfields::ERR_WRONG_COORDINATE_DEF,
            "GField_AsciiMapFactory: coordinate name <%1> is not valid for symmetry <%2>.", name,
            gfield_definitions.field_parameters["symmetry"]);
        return;
    }
    if (npoints < 2) {
        log->error(
            gfields::ERR_WRONG_COORDINATE_DEF,
            "GField_AsciiMapFactory: coordinate <%1> needs at least 2 points; got %2.", name, npoints);
    }
 
    np[*axis]       = static_cast<unsigned>(npoints);
    startMap[*axis] = min;
    endMap[*axis]   = max;
    cellSize[*axis] = (max - min) / (npoints - 1);
 
    // The map-file column for this coordinate is read in the same unit used in the min/max expression,
    // so a bare column value is converted back to Geant4 units with that factor.
    const std::string unit = [&]() {
        for (const std::string& expr : {tokens[3], tokens[2]}) {
            const auto star = expr.find('*');
            if (star != std::string::npos) {
                return gutilities::removeLeadingAndTrailingSpacesFromString(expr.substr(star + 1));
            }
        }
        return std::string("mm");
    }();
 
    columns.push_back(Column{static_cast<int>(*axis), gutilities::getG4Number(1.0, unit)});
}
 
void GField_AsciiMapFactory::load_map_file() {
    const std::string map_name = param_string("map", "");
    if (map_name.empty()) {
        log->error(
            gfields::ERR_MAP_FILE_NOT_FOUND, "GField_AsciiMapFactory: no 'map' file given for field <%1>.",
            gfield_definitions.name);
    }
 
    // Resolve the map file. An explicit path (containing '/') is used as-is. Otherwise the lookup order
    // is: the explicit `dir` parameter, then the directory of the YAML that defined the field (so a plain
    // .yaml run from its own directory or referenced by absolute path just works), then the `fields`
    // directory installed next to the plugin.
    std::string path = map_name;
    if (map_name.find('/') == std::string::npos) {
        std::vector<std::string> candidates;
        if (const std::string dir = param_string("dir", ""); !dir.empty()) { candidates.push_back(dir); }
        if (!gfield_definitions.config_dir.empty()) { candidates.push_back(gfield_definitions.config_dir); }
        candidates.push_back(field_maps_directory());
 
        path.clear();
        for (const auto& dir : candidates) {
            const std::string trial = dir + "/" + map_name;
            if (std::ifstream(trial).good()) { path = trial; break; }
        }
        if (path.empty()) {
            log->error(
                gfields::ERR_MAP_FILE_NOT_FOUND,
                "GField_AsciiMapFactory: cannot find map <%1> for field <%2> in dir/config/plugin locations.",
                map_name, gfield_definitions.name);
        }
    }
 
    std::ifstream in(path);
    if (!in.good()) {
        log->error(gfields::ERR_MAP_FILE_NOT_FOUND, "GField_AsciiMapFactory: cannot open map file <%1>.", path);
    }
    log->info(
        1, "Loading ASCII field map <%1> with symmetry <%2>.", path,
        gfield_definitions.field_parameters["symmetry"]);
 
    // Total grid points and field-value scaling.
    std::size_t total = np[0];
    for (int d = 1; d < ndim; ++d) { total *= np[d]; }
 
    const double field_scale = param_g4number("scale", "1") *
                               gutilities::getG4Number(1.0, param_string("field_unit", "gauss"));
 
    B1.assign(total, 0.0f);
    if (ncomp >= 2) { B2.assign(total, 0.0f); }
    if (ncomp >= 3) { B3.assign(total, 0.0f); }
 
    const double tolerance = 0.001; // relative cell-position tolerance for the grid-consistency check
 
    std::size_t read_points = 0;
    std::string line;
    int         line_number = 0;
    while (std::getline(in, line)) {
        ++line_number;
        const auto trimmed = gutilities::removeLeadingAndTrailingSpacesFromString(line);
        if (trimmed.empty() || trimmed[0] == '#') { continue; }
 
        std::istringstream tokens(trimmed);
 
        // Read the coordinate columns (file-column order), convert them to Geant4 units and turn each
        // into its canonical axis index.
        unsigned grid_index[3] = {0, 0, 0};
        bool     line_ok       = true;
        for (int c = 0; c < ndim; ++c) {
            double raw = 0.0;
            if (!(tokens >> raw)) { line_ok = false; break; }
            const double coord = raw * columns[c].unitFactor;
            const int    axis  = columns[c].axis;
            const unsigned i    = static_cast<unsigned>(
                std::floor((coord - startMap[axis] + cellSize[axis] / 2) / cellSize[axis]));
            grid_index[axis] = i;
 
            const double expected = startMap[axis] + i * cellSize[axis];
            if (cellSize[axis] != 0.0 && std::fabs(expected - coord) > tolerance * std::fabs(cellSize[axis])) {
                log->warning(
                    "GField_AsciiMapFactory: %1:%2 axis %3 value %4 does not match grid point %5.", path,
                    line_number, axis, coord, expected);
            }
        }
 
        double comp[3] = {0.0, 0.0, 0.0};
        for (int k = 0; k < ncomp && line_ok; ++k) {
            if (!(tokens >> comp[k])) { line_ok = false; }
        }
        if (!line_ok) {
            log->error(
                gfields::ERR_MAP_FILE_NOT_FOUND, "GField_AsciiMapFactory: %1:%2 has fewer than %3 numbers.",
                path, line_number, ndim + ncomp);
        }
 
        std::size_t flat = (ndim == 2) ? idx2(grid_index[0], grid_index[1])
                                        : idx3(grid_index[0], grid_index[1], grid_index[2]);
        if (flat >= total) {
            log->warning("GField_AsciiMapFactory: %1:%2 maps outside the grid; skipped.", path, line_number);
            continue;
        }
 
        B1[flat] = static_cast<float>(comp[0] * field_scale);
        if (ncomp >= 2) { B2[flat] = static_cast<float>(comp[1] * field_scale); }
        if (ncomp >= 3) { B3[flat] = static_cast<float>(comp[2] * field_scale); }
        ++read_points;
    }
 
    if (read_points != total) {
        log->warning(
            "GField_AsciiMapFactory: %1 provided %2 points but the grid expects %3.", path, read_points, total);
    }
    log->info(1, "ASCII field map <%1> loaded: %2 points.", gfield_definitions.name, read_points);
}
 
void GField_AsciiMapFactory::load_field_definitions(GFieldDefinition gfd) {
    gfield_definitions = gfd;
 
    // Decode the symmetry once. ndim/ncomp follow from it.
    const std::string sym = param_string("symmetry", "");
    if (sym == "dipole-x") { symmetry = Symmetry::dipole_x; ndim = 2; ncomp = 1; }
    else if (sym == "dipole-y") { symmetry = Symmetry::dipole_y; ndim = 2; ncomp = 1; }
    else if (sym == "dipole-z") { symmetry = Symmetry::dipole_z; ndim = 2; ncomp = 1; }
    else if (sym == "cylindrical-x") { symmetry = Symmetry::cyl_x; ndim = 2; ncomp = 2; }
    else if (sym == "cylindrical-y") { symmetry = Symmetry::cyl_y; ndim = 2; ncomp = 2; }
    else if (sym == "cylindrical-z") { symmetry = Symmetry::cyl_z; ndim = 2; ncomp = 2; }
    else if (sym == "phi-segmented") { symmetry = Symmetry::phi_segmented; ndim = 3; ncomp = 3; }
    else if (sym == "cartesian_3D") { symmetry = Symmetry::cartesian_3d; ndim = 3; ncomp = 3; }
    else if (sym == "cartesian_3D_quadrant") { symmetry = Symmetry::cartesian_3d_quadrant; ndim = 3; ncomp = 3; }
    else {
        log->error(
            gfields::ERR_WRONG_FIELD_SYMMETRY, "GField_AsciiMapFactory: unknown symmetry <%1> for field <%2>.",
            sym, gfield_definitions.name);
    }
 
    // Grid coordinates, in map-file column order.
    columns.clear();
    columns.reserve(ndim);
    load_coordinate("coordinate1");
    load_coordinate("coordinate2");
    if (ndim == 3) { load_coordinate("coordinate3"); }
 
    // Interpolation.
    linear = (param_string("interpolation", "linear") != "none");
 
    // Overall placement.
    mapOrigin[0]   = param_g4number("vx", "0");
    mapOrigin[1]   = param_g4number("vy", "0");
    mapOrigin[2]   = param_g4number("vz", "0");
    mapRotation[0] = param_g4number("rx", "0*deg");
    mapRotation[1] = param_g4number("ry", "0*deg");
    mapRotation[2] = param_g4number("rz", "0*deg");
    sinAlpha = std::sin(mapRotation[0]); cosAlpha = std::cos(mapRotation[0]);
    sinBeta  = std::sin(mapRotation[1]); cosBeta  = std::cos(mapRotation[1]);
    sinGamma = std::sin(mapRotation[2]); cosGamma = std::cos(mapRotation[2]);
 
    load_map_file();
}
 
 
// ---------------------------------------------------------------------------------------------------
// Field evaluation
// ---------------------------------------------------------------------------------------------------
 
void GField_AsciiMapFactory::GetFieldValue(const double pos[3], G4double* bfield) const {
    bfield[0] = bfield[1] = bfield[2] = 0.0;
 
    if (std::isnan(pos[0]) || std::isnan(pos[1]) || std::isnan(pos[2])) {
        log->warning("GField_AsciiMapFactory: field coordinates requested are nan.");
        return;
    }
 
    // Shift to the map frame.
    const double x[3] = {pos[0] - mapOrigin[0], pos[1] - mapOrigin[1], pos[2] - mapOrigin[2]};
 
    switch (symmetry) {
    case Symmetry::dipole_x:
    case Symmetry::dipole_y:
    case Symmetry::dipole_z:
        value_dipole(x, bfield);
        break;
    case Symmetry::cyl_x:
    case Symmetry::cyl_y:
    case Symmetry::cyl_z:
        value_cylindrical(x, bfield);
        break;
    case Symmetry::phi_segmented:
        value_phi_segmented(x, bfield);
        break;
    case Symmetry::cartesian_3d:
    case Symmetry::cartesian_3d_quadrant:
        value_cartesian3d(x, bfield);
        break;
    }
 
    log->info(
        2, "ASCII field <%1> at (%2, %3, %4) cm = (%5, %6, %7) gauss", gfield_definitions.name, pos[0] / cm,
        pos[1] / cm, pos[2] / cm, bfield[0] / gauss, bfield[1] / gauss, bfield[2] / gauss);
}
 
 
void GField_AsciiMapFactory::value_dipole(const double x[3], double* bfield) const {
    // Axis 0 = longitudinal, axis 1 = transverse.
    double LC = 0.0, TC = 0.0;
    if (symmetry == Symmetry::dipole_z) { TC = std::fabs(x[0]); LC = x[1]; }
    else if (symmetry == Symmetry::dipole_x) { TC = std::fabs(x[1]); LC = x[2]; }
    else /* dipole_y */ { TC = std::fabs(x[0]); LC = x[2]; }
 
    if (LC < startMap[0] || TC < startMap[1]) { return; }
    unsigned IL = static_cast<unsigned>(std::floor((LC - startMap[0]) / cellSize[0]));
    unsigned IT = static_cast<unsigned>(std::floor((TC - startMap[1]) / cellSize[1]));
    if (IL >= np[0] - 1 || IT >= np[1] - 1) { return; }
 
    double b = 0.0;
    if (!linear) {
        if (std::fabs(startMap[0] + IL * cellSize[0] - LC) > std::fabs(startMap[0] + (IL + 1) * cellSize[0] - LC)) IL++;
        if (std::fabs(startMap[1] + IT * cellSize[1] - TC) > std::fabs(startMap[1] + (IT + 1) * cellSize[1] - TC)) IT++;
        b = B1[idx2(IL, IT)];
    }
    else {
        const double xlr = (LC - (startMap[0] + IL * cellSize[0])) / cellSize[0];
        const double xtr = (TC - (startMap[1] + IT * cellSize[1])) / cellSize[1];
        const double b10 = B1[idx2(IL, IT)] * (1.0 - xtr) + B1[idx2(IL, IT + 1)] * xtr;
        const double b11 = B1[idx2(IL + 1, IT)] * (1.0 - xtr) + B1[idx2(IL + 1, IT + 1)] * xtr;
        b = b10 * (1.0 - xlr) + b11 * xlr;
    }
 
    if (symmetry == Symmetry::dipole_x) { bfield[0] = b; }
    else if (symmetry == Symmetry::dipole_y) { bfield[1] = b; }
    else { bfield[2] = b; }
 
    rotate_field(bfield);
}
 
 
void GField_AsciiMapFactory::value_cylindrical(const double x[3], double* bfield) const {
    // Axis 0 = transverse (radial), axis 1 = longitudinal.
    double LC = 0.0, TC = 0.0, phi = 0.0;
    if (symmetry == Symmetry::cyl_z) {
        LC = x[2]; TC = std::sqrt(x[0] * x[0] + x[1] * x[1]); phi = G4ThreeVector(x[0], x[1], x[2]).phi();
    }
    else if (symmetry == Symmetry::cyl_x) {
        LC = x[0]; TC = std::sqrt(x[1] * x[1] + x[2] * x[2]); phi = G4ThreeVector(x[1], x[2], x[0]).phi();
    }
    else /* cyl_y */ {
        LC = x[1]; TC = std::sqrt(x[0] * x[0] + x[2] * x[2]); phi = G4ThreeVector(x[2], x[0], x[1]).phi();
    }
 
    if (TC < startMap[0] || LC < startMap[1]) { return; }
    unsigned IT = static_cast<unsigned>(std::floor((TC - startMap[0]) / cellSize[0]));
    unsigned IL = static_cast<unsigned>(std::floor((LC - startMap[1]) / cellSize[1]));
    if (IT >= np[0] - 1 || IL >= np[1] - 1) { return; }
 
    double b1 = 0.0, b2 = 0.0;
    if (!linear) {
        if (std::fabs(startMap[0] + IT * cellSize[0] - TC) > std::fabs(startMap[0] + (IT + 1) * cellSize[0] - TC)) IT++;
        if (std::fabs(startMap[1] + IL * cellSize[1] - LC) > std::fabs(startMap[1] + (IL + 1) * cellSize[1] - LC)) IL++;
        b1 = B1[idx2(IT, IL)];
        b2 = B2[idx2(IT, IL)];
    }
    else {
        const double xtr = (TC - (startMap[0] + IT * cellSize[0])) / cellSize[0];
        const double xlr = (LC - (startMap[1] + IL * cellSize[1])) / cellSize[1];
        const double b10 = B1[idx2(IT, IL)] * (1.0 - xtr) + B1[idx2(IT + 1, IL)] * xtr;
        const double b11 = B1[idx2(IT, IL + 1)] * (1.0 - xtr) + B1[idx2(IT + 1, IL + 1)] * xtr;
        b1 = b10 * (1.0 - xlr) + b11 * xlr;
        const double b20 = B2[idx2(IT, IL)] * (1.0 - xtr) + B2[idx2(IT + 1, IL)] * xtr;
        const double b21 = B2[idx2(IT, IL + 1)] * (1.0 - xtr) + B2[idx2(IT + 1, IL + 1)] * xtr;
        b2 = b20 * (1.0 - xlr) + b21 * xlr;
    }
 
    if (symmetry == Symmetry::cyl_z) {
        bfield[0] = b1 * std::cos(phi); bfield[1] = b1 * std::sin(phi); bfield[2] = b2;
    }
    else if (symmetry == Symmetry::cyl_x) {
        bfield[0] = b2; bfield[1] = b1 * std::cos(phi); bfield[2] = b1 * std::sin(phi);
    }
    else /* cyl_y */ {
        bfield[1] = b2; bfield[0] = b1 * std::sin(phi); bfield[2] = b1 * std::cos(phi);
    }
 
    rotate_field(bfield);
}
 
 
void GField_AsciiMapFactory::value_phi_segmented(const double x[3], double* bfield) const {
    // Axis 0 = azimuthal, axis 1 = transverse, axis 2 = longitudinal. Fields are stored in the local
    // (first-segment) frame and rotated back to the query phi.
    double aC = std::atan2(x[1], x[0]) * rad; // phi
    if (aC < 0) { aC += 360 * deg; }
    const double tC = std::sqrt(x[0] * x[0] + x[1] * x[1]); // R
    const double lC = x[2];                                 // Z
 
    // Fold into the first 60-degree segment, keeping the rotation back to the lab.
    double aLC = aC;
    while (aLC / deg > 30) { aLC -= 60 * deg; }
    const double dphi = aC - aLC;
    const double aaLC = std::fabs(aLC);
    const int    sign = (aLC >= 0 ? 1 : -1);
 
    if (aC < startMap[0] || tC < startMap[1] || lC < startMap[2]) { return; }
    unsigned aI = static_cast<unsigned>(std::floor((aaLC - startMap[0]) / cellSize[0]));
    unsigned tI = static_cast<unsigned>(std::floor((tC - startMap[1]) / cellSize[1]));
    unsigned lI = static_cast<unsigned>(std::floor((lC - startMap[2]) / cellSize[2]));
    if (aI >= np[0] - 1 || tI >= np[1] - 1 || lI >= np[2] - 1) { return; }
 
    double mfield[3] = {0.0, 0.0, 0.0};
    if (!linear) {
        if (std::fabs(startMap[0] + aI * cellSize[0] - aaLC) > std::fabs(startMap[0] + (aI + 1) * cellSize[0] - aaLC)) aI++;
        if (std::fabs(startMap[1] + tI * cellSize[1] - tC) > std::fabs(startMap[1] + (tI + 1) * cellSize[1] - tC)) tI++;
        if (std::fabs(startMap[2] + lI * cellSize[2] - lC) > std::fabs(startMap[2] + (lI + 1) * cellSize[2] - lC)) lI++;
        mfield[0] = B1[idx3(aI, tI, lI)];
        mfield[1] = B2[idx3(aI, tI, lI)];
        mfield[2] = B3[idx3(aI, tI, lI)];
    }
    else {
        const double xaz = (aaLC - (startMap[0] + aI * cellSize[0])) / cellSize[0];
        const double xtr = (tC - (startMap[1] + tI * cellSize[1])) / cellSize[1];
        const double xlr = (lC - (startMap[2] + lI * cellSize[2])) / cellSize[2];
 
        const std::vector<float>* comps[3] = {&B1, &B2, &B3};
        for (int k = 0; k < 3; ++k) {
            const std::vector<float>& B = *comps[k];
            const double b00 = B[idx3(aI, tI, lI)] * (1 - xaz) + B[idx3(aI + 1, tI, lI)] * xaz;
            const double b01 = B[idx3(aI, tI, lI + 1)] * (1 - xaz) + B[idx3(aI + 1, tI, lI + 1)] * xaz;
            const double b10 = B[idx3(aI, tI + 1, lI)] * (1 - xaz) + B[idx3(aI + 1, tI + 1, lI)] * xaz;
            const double b11 = B[idx3(aI, tI + 1, lI + 1)] * (1 - xaz) + B[idx3(aI + 1, tI + 1, lI + 1)] * xaz;
            const double b0  = b00 * (1 - xtr) + b10 * xtr;
            const double b1  = b01 * (1 - xtr) + b11 * xtr;
            mfield[k]        = b0 * (1 - xlr) + b1 * xlr;
        }
    }
 
    // Rotate the local field back to the query azimuth.
    bfield[0] = sign * mfield[0] * std::cos(dphi / rad) - mfield[1] * std::sin(dphi / rad);
    bfield[1] = sign * mfield[0] * std::sin(dphi / rad) + mfield[1] * std::cos(dphi / rad);
    bfield[2] = sign * mfield[2];
 
    rotate_field(bfield);
}
 
 
void GField_AsciiMapFactory::value_cartesian3d(const double x[3], double* bfield) const {
    // Axis 0 = X, axis 1 = Y, axis 2 = Z.
    double XX = x[0], YY = x[1], ZZ = x[2];
 
    if (symmetry == Symmetry::cartesian_3d_quadrant) {
        // Fold the query point into the stored first quadrant (x>=0, y>=0).
        if (x[0] >= 0 && x[1] >= 0) { XX = x[0]; YY = x[1]; }
        else if (x[0] >= 0 && x[1] < 0) { XX = -x[1]; YY = x[0]; }
        else if (x[0] < 0 && x[1] < 0) { XX = -x[0]; YY = -x[1]; }
        else /* x[0] < 0 && x[1] >= 0 */ { XX = x[1]; YY = -x[0]; }
        ZZ = x[2];
        if (XX < 0 || YY < 0) { return; }
    }
 
    if (XX < startMap[0] || YY < startMap[1] || ZZ < startMap[2]) { return; }
    if (XX >= endMap[0] || YY >= endMap[1] || ZZ >= endMap[2]) { return; }
    const unsigned IXX = static_cast<unsigned>(std::floor((XX - startMap[0]) / cellSize[0]));
    const unsigned IYY = static_cast<unsigned>(std::floor((YY - startMap[1]) / cellSize[1]));
    const unsigned IZZ = static_cast<unsigned>(std::floor((ZZ - startMap[2]) / cellSize[2]));
 
    double B[3] = {0.0, 0.0, 0.0};
    if (!linear) {
        unsigned ix = IXX, iy = IYY, iz = IZZ;
        if (std::fabs(startMap[0] + ix * cellSize[0] - XX) > std::fabs(startMap[0] + (ix + 1) * cellSize[0] - XX)) ix++;
        if (std::fabs(startMap[1] + iy * cellSize[1] - YY) > std::fabs(startMap[1] + (iy + 1) * cellSize[1] - YY)) iy++;
        if (std::fabs(startMap[2] + iz * cellSize[2] - ZZ) > std::fabs(startMap[2] + (iz + 1) * cellSize[2] - ZZ)) iz++;
        B[0] = B1[idx3(ix, iy, iz)];
        B[1] = B2[idx3(ix, iy, iz)];
        B[2] = B3[idx3(ix, iy, iz)];
    }
    else {
        const double Xd = (XX - (startMap[0] + IXX * cellSize[0])) / cellSize[0];
        const double Yd = (YY - (startMap[1] + IYY * cellSize[1])) / cellSize[1];
        const double Zd = (ZZ - (startMap[2] + IZZ * cellSize[2])) / cellSize[2];
 
        const std::vector<float>* comps[3] = {&B1, &B2, &B3};
        for (int k = 0; k < 3; ++k) {
            const std::vector<float>& Bk = *comps[k];
            const double c00 = Bk[idx3(IXX, IYY, IZZ)] * (1 - Xd) + Bk[idx3(IXX + 1, IYY, IZZ)] * Xd;
            const double c01 = Bk[idx3(IXX, IYY, IZZ + 1)] * (1 - Xd) + Bk[idx3(IXX + 1, IYY, IZZ + 1)] * Xd;
            const double c10 = Bk[idx3(IXX, IYY + 1, IZZ)] * (1 - Xd) + Bk[idx3(IXX + 1, IYY + 1, IZZ)] * Xd;
            const double c11 = Bk[idx3(IXX, IYY + 1, IZZ + 1)] * (1 - Xd) + Bk[idx3(IXX + 1, IYY + 1, IZZ + 1)] * Xd;
            const double c0  = c00 * (1 - Yd) + c10 * Yd;
            const double c1  = c01 * (1 - Yd) + c11 * Yd;
            B[k]             = c0 * (1 - Zd) + c1 * Zd;
        }
    }
 
    if (symmetry == Symmetry::cartesian_3d_quadrant) {
        // Mirror the field components back to the query quadrant.
        if (x[0] >= 0 && x[1] >= 0) { bfield[0] = B[0]; bfield[1] = B[1]; }
        else if (x[0] >= 0 && x[1] < 0) { bfield[0] = B[1]; bfield[1] = -B[0]; }
        else if (x[0] < 0 && x[1] < 0) { bfield[0] = -B[0]; bfield[1] = -B[1]; }
        else /* x[0] < 0 && x[1] >= 0 */ { bfield[0] = -B[1]; bfield[1] = B[0]; }
        bfield[2] = B[2];
    }
    else {
        bfield[0] = B[0]; bfield[1] = B[1]; bfield[2] = B[2];
    }
 
    rotate_field(bfield);
}
 
 
void GField_AsciiMapFactory::rotate_field(double* bfield) const {
    // Rotate the field axes, not the point: each rotation is the inverse of the point rotation.
    if (mapRotation[0] != 0) {
        const double yPrime =  bfield[1] * cosAlpha + bfield[2] * sinAlpha;
        const double zPrime = -bfield[1] * sinAlpha + bfield[2] * cosAlpha;
        bfield[1] = yPrime; bfield[2] = zPrime;
    }
    if (mapRotation[1] != 0) {
        const double xPrime = bfield[0] * cosBeta - bfield[2] * sinBeta;
        const double zPrime = bfield[0] * sinBeta + bfield[2] * cosBeta;
        bfield[0] = xPrime; bfield[2] = zPrime;
    }
    if (mapRotation[2] != 0) {
        const double xPrime =  bfield[0] * cosGamma + bfield[1] * sinGamma;
        const double yPrime = -bfield[0] * sinGamma + bfield[1] * cosGamma;
        bfield[0] = xPrime; bfield[1] = yPrime;
    }
}