Method factory

src/AbstractState.cpp:168–288

// BICUBIC already use.

AbstractState* AbstractState::factory(const std::string& backend, const std::vector<std::string>& fluid_names) {
    if (get_debug_level() > 0) {
        std::cout << "AbstractState::factory(" << backend << "," << stringvec_to_string(fluid_names) << ")" << '\n';
    }

    // Split off any "?<options>" suffix once at the entry-point so the
    // rest of the dispatch operates on the unadorned backend / fluid
    // strings.  PropsSI calls extract_backend() first, which splits
    // "BACKEND::FLUID?<options>" on "::" without touching '?'  — so
    // the suffix arrives on the *fluid* side.  Direct factory()
    // callers may put it on either side.  Strip from wherever it
    // lives (rejecting the ambiguous both-sides case) and thread the
    // raw options JSON through to the backend generator via the
    // options-aware overload below.  See
    // docs/superpowers/specs/2026-05-16-backend-options-string-design.md.
    auto parsed = parse_factory_options(backend);
    std::string clean_backend = parsed.clean_string;
    std::string options_json = parsed.options_json;
    std::vector<std::string> clean_fluid_names = fluid_names;
    // The `?<options>` suffix on the fluid side can land on ANY
    // element of fluid_names — for mixtures, the
    // factory(string, string) convenience overload calls
    // strsplit('&') before forwarding, so e.g.
    // "HEOS::R32&R125?{...}" arrives as ["R32", "R125?{...}"] with
    // the suffix on the LAST token.  Scan the whole vector; throw
    // if two different tokens carry a suffix (typo); always strip
    // the trailing '?' so downstream fluid lookup sees the bare
    // component name.
    std::size_t fluid_options_index = clean_fluid_names.size();  // sentinel = none
    for (std::size_t i = 0; i < clean_fluid_names.size(); ++i) {
        if (clean_fluid_names[i].find('?') == std::string::npos) {
            continue;
        }
        if (fluid_options_index != clean_fluid_names.size()) {
            throw ValueError("factory: '?<options>' supplied in multiple fluid components; encode it once.");
        }
        fluid_options_index = i;
    }
    if (fluid_options_index != clean_fluid_names.size()) {
        auto parsed_fluid = parse_factory_options(clean_fluid_names[fluid_options_index]);
        if (!parsed_fluid.options_json.empty() && !options_json.empty()) {
            throw ValueError("factory: '?<options>' supplied on both the backend and fluid sides; pick one side.");
        }
        // Always replace the affected fluid name with the stripped
        // form — even when the suffix carried no options (bare '?'
        // or whitespace) — so the downstream fluid lookup doesn't
        // see a trailing '?'.
        clean_fluid_names[fluid_options_index] = parsed_fluid.clean_string;
        if (!parsed_fluid.options_json.empty()) {
            options_json = parsed_fluid.options_json;
        }
    }

    backend_families f1;
    std::string f2;
    extract_backend_families_string(clean_backend, f1, f2);

    std::map<backend_families, shared_ptr<AbstractStateGenerator>>::const_iterator gen, end;