multifluid.hpp

#pragma once

#include "nlohmann/json.hpp"

#include <Eigen/Dense>
#include <string>
#include <cmath>
#include <optional>
#include <variant>

#include "teqp/types.hpp"
#include "teqp/constants.hpp"
#include "teqp/filesystem.hpp"
#include "teqp/json_tools.hpp"
#include "teqp/exceptions.hpp"

#if defined(TEQP_MULTICOMPLEX_ENABLED)
#include "MultiComplex/MultiComplex.hpp"
#endif 

#include "multifluid_eosterms.hpp"
#include "multifluid_reducing.hpp"

#include <boost/algorithm/string/join.hpp>

#if defined(TEQP_MULTICOMPLEX_ENABLED)
// See https://eigen.tuxfamily.org/dox/TopicCustomizing_CustomScalar.html
namespace Eigen {
    template<typename TN> struct NumTraits<mcx::MultiComplex<TN>> : NumTraits<double> // permits to get the epsilon, dummy_precision, lowest, highest functions
    {
        enum {
            IsComplex = 1,
            IsInteger = 0,
            IsSigned = 1,
            RequireInitialization = 1,
            ReadCost = 1,
            AddCost = 3,
            MulCost = 3
        };
    };
}
#endif

namespace teqp{

template<typename EOSCollection>
class CorrespondingStatesContribution {

private:
    const EOSCollection EOSs;
public:
    CorrespondingStatesContribution(EOSCollection&& EOSs) : EOSs(EOSs) {};
    
    auto size() const { return EOSs.size(); }

    template<typename TauType, typename DeltaType, typename MoleFractions>
    auto alphar(const TauType& tau, const DeltaType& delta, const MoleFractions& molefracs) const {
        using resulttype = std::common_type_t<decltype(tau), decltype(molefracs[0]), decltype(delta)>; // Type promotion, without the const-ness
        resulttype alphar = 0.0;
        auto N = molefracs.size();
        for (auto i = 0; i < N; ++i) {
            alphar = alphar + molefracs[i] * EOSs[i].alphar(tau, delta);
        }
        return forceeval(alphar);
    }

    template<typename TauType, typename DeltaType>
    auto alphari(const TauType& tau, const DeltaType& delta, std::size_t i) const {
        return EOSs[i].alphar(tau, delta);
    }

    auto get_EOS(std::size_t i) const{
        return EOSs[i];
    }
};

template<typename FCollection, typename DepartureFunctionCollection>
class DepartureContribution {

private:
    const FCollection F;
    const DepartureFunctionCollection funcs;
public:
    DepartureContribution(FCollection&& F, DepartureFunctionCollection&& funcs) : F(F), funcs(funcs) {};

    template<typename TauType, typename DeltaType, typename MoleFractions>
    auto alphar(const TauType& tau, const DeltaType& delta, const MoleFractions& molefracs) const {
        using resulttype = std::common_type_t<decltype(tau), decltype(molefracs[0]), decltype(delta)>; // Type promotion, without the const-ness
        resulttype alphar = 0.0;
        auto N = molefracs.size();
        for (auto i = 0; i < N; ++i) {
            for (auto j = i+1; j < N; ++j) {
                alphar = alphar + molefracs[i] * molefracs[j] * F(i, j) * funcs[i][j].alphar(tau, delta);
            }
        }
        return forceeval(alphar);
    }

    /// Call a single departure term at i,j 
    template<typename TauType, typename DeltaType>
    auto get_alpharij(const int i, const int j,     const TauType& tau, const DeltaType& delta) const {
        int N = funcs.size();
        if (i < 0 || j < 0){
            throw teqp::InvalidArgument("i or j is negative");
        }
        if (i >= N || j >= N){
            throw teqp::InvalidArgument("i or j is invalid; size is " + std::to_string(N));
        }
        return forceeval(funcs[i][j].alphar(tau, delta));
    }
};

template<typename CorrespondingTerm, typename DepartureTerm>
class MultiFluid {  

private:
    std::string meta = ""; ///< A string that can be used to store arbitrary metadata as needed
public:
    const ReducingFunctions redfunc;
    const CorrespondingTerm corr;
    const DepartureTerm dep;

    template<class VecType>
    auto R(const VecType& molefrac) const {
        return get_R_gas<decltype(molefrac[0])>();
    }

    /// Store some sort of metadata in string form (perhaps a JSON representation of the model?)
    void set_meta(const std::string& m) { meta = m; }
    /// Get the metadata stored in string form
    auto get_meta() const { return meta; }

    MultiFluid(ReducingFunctions&& redfunc, CorrespondingTerm&& corr, DepartureTerm&& dep) : redfunc(redfunc), corr(corr), dep(dep) {};

    template<typename TType, typename RhoType>
    auto alphar(TType T,
        const RhoType& rhovec,
        const std::optional<typename RhoType::value_type> rhotot = std::nullopt) const
    {
        typename RhoType::value_type rhotot_ = (rhotot.has_value()) ? rhotot.value() : std::accumulate(std::begin(rhovec), std::end(rhovec), (decltype(rhovec[0]))0.0);
        auto molefrac = rhovec / rhotot_;
        return alphar(T, rhotot_, molefrac);
    }

    template<typename TType, typename RhoType, typename MoleFracType>
    auto alphar(const TType &T,
        const RhoType &rho,
        const MoleFracType& molefrac) const
    {
        if (molefrac.size() != corr.size()){
            throw teqp::InvalidArgument("Wrong size of mole fractions; "+std::to_string(corr.size()) + " are loaded but "+std::to_string(molefrac.size()) + " were provided");
        }
        auto Tred = forceeval(redfunc.get_Tr(molefrac));
        auto rhored = forceeval(redfunc.get_rhor(molefrac));
        auto delta = forceeval(rho / rhored);
        auto tau = forceeval(Tred / T);
        auto val = corr.alphar(tau, delta, molefrac) + dep.alphar(tau, delta, molefrac);
        return forceeval(val);
    }
};


/***
* \brief Get the JSON data structure for a given departure function
* \param name The name (or alias) of the departure function to be looked up
* \parm path The root path to the fluid data, or alternatively, the path to the json file directly
*/
inline auto get_departure_json(const std::string& name, const std::string& path) {
    std::string filepath = std::filesystem::is_regular_file(path) ? path : path + "/dev/mixtures/mixture_departure_functions.json";
    nlohmann::json j = load_a_JSON_file(filepath);
    std::string js = j.dump(2);
    // First pass, direct name lookup
    for (auto& el : j) {
        if (el.at("Name") == name) {
            return el;
        }
    }
    // Second pass, iterate over aliases
    for (auto& el : j) {
        for (auto &alias : el.at("aliases")) {
            if (alias == name) {
                return el;
            }
        }
    }
    throw std::invalid_argument("Could not match the name: " + name + "when looking up departure function");
}
    
inline auto build_departure_function(const nlohmann::json& j) {
    auto build_power = [&](auto term, auto& dep) {
        std::size_t N = term["n"].size();

        // Don't add a departure function if there are no coefficients provided
        if (N == 0) {
            return;
        }

        PowerEOSTerm eos;

        auto eigorzero = [&term, &N](const std::string& name) -> Eigen::ArrayXd {
            if (!term[name].empty()) {
                return toeig(term[name]);
            }
            else {
                return Eigen::ArrayXd::Zero(N);
            }
        };


        eos.n = eigorzero("n");
        eos.t = eigorzero("t");
        eos.d = eigorzero("d");

        Eigen::ArrayXd c(N), l(N); c.setZero();

        int Nlzero = 0, Nlnonzero = 0;
        bool contiguous_lzero;

        if (term["l"].empty()) {
            // exponential part not included
            l.setZero();
            if (!all_same_length(term, { "n","t","d" })) {
                throw std::invalid_argument("Lengths are not all identical in polynomial-like term");
            }
        }
        else {
            if (!all_same_length(term, { "n","t","d","l"})) {
                throw std::invalid_argument("Lengths are not all identical in exponential term");
            }
            l = toeig(term["l"]);
            // l is included, use it to build c; c_i = 1 if l_i > 0, zero otherwise
            for (auto i = 0; i < c.size(); ++i) {
                if (l[i] > 0) {
                    c[i] = 1.0;
                }
            }

            // See how many of the first entries have zero values for l_i
            contiguous_lzero = (l[0] == 0);
            for (auto i = 0; i < c.size(); ++i) {
                if (l[i] == 0) {
                    Nlzero++;
                }
            }
        }
        Nlnonzero = static_cast<int>(l.size()) - Nlzero;

        if ((l[0] != 0) && (l[l.size() - 1] == 0)) {
            throw std::invalid_argument("If l_i has zero and non-zero values, the zero values need to come first");
        }

        eos.c = c;
        eos.l = l;

        eos.l_i = eos.l.cast<int>();

        if (Nlzero + Nlnonzero != l.size()) {
            throw std::invalid_argument("Somehow the l lengths don't add up");
        }


        if (((eos.l_i.cast<double>() - eos.l).cwiseAbs() > 0.0).any()) {
            throw std::invalid_argument("Non-integer entry in l found");
        }

        // If a contiguous portion of the terms have values of l_i that are zero
        // it is computationally advantageous to break up the evaluation into 
        // part that has just the n_i*tau^t_i*delta^d_i and the part with the
        // exponential term exp(-delta^l_i)
        if (l.sum() == 0) {
            // No l term at all, just polynomial
            JustPowerEOSTerm poly;
            poly.n = eos.n;
            poly.t = eos.t;
            poly.d = eos.d;
            dep.add_term(poly);
        }
        else if (l.sum() > 0 && contiguous_lzero){
            JustPowerEOSTerm poly; 
            poly.n = eos.n.head(Nlzero);
            poly.t = eos.t.head(Nlzero);
            poly.d = eos.d.head(Nlzero);
            dep.add_term(poly);

            PowerEOSTerm e;
            e.n = eos.n.tail(Nlnonzero);
            e.t = eos.t.tail(Nlnonzero);
            e.d = eos.d.tail(Nlnonzero);
            e.c = eos.c.tail(Nlnonzero);
            e.l = eos.l.tail(Nlnonzero);
            e.l_i = eos.l_i.tail(Nlnonzero);
            dep.add_term(e);
        }
        else {
            // Don't try to get too clever, just add the departure term
            dep.add_term(eos);
        }
    };

    auto build_doubleexponential = [&](auto& term, auto& dep) {
        if (!all_same_length(term, { "n","t","d","ld","gd","lt","gt" })) {
            throw std::invalid_argument("Lengths are not all identical in double exponential term");
        }
        DoubleExponentialEOSTerm eos;
        eos.n = toeig(term.at("n"));
        eos.t = toeig(term.at("t"));
        eos.d = toeig(term.at("d"));
        eos.ld = toeig(term.at("ld"));
        eos.gd = toeig(term.at("gd"));
        eos.lt = toeig(term.at("lt"));
        eos.gt = toeig(term.at("gt"));
        eos.ld_i = eos.ld.cast<int>();
        dep.add_term(eos);
    }; 
    auto build_Chebyshev2D = [&](auto& term, auto& dep) {
        Chebyshev2DEOSTerm eos;
        int Ntau = term.at("Ntau"); // Degree in tau (there will be Ntau+1 coefficients in the tau direction)
        int Ndelta = term.at("Ndelta"); // Degree in delta (there will be Ndelta+1 coefficients in the delta direction)
        Eigen::ArrayXd c = toeig(term.at("a"));
        if ((Ntau + 1)*(Ndelta + 1) != c.size()){
            throw std::invalid_argument("Provided length [" + std::to_string(c.size()) + "] is not equal to (Ntau+1)*(Ndelta+1)");
        }
        eos.a = c.reshaped(Ntau+1, Ndelta+1).eval(); // All in one long array, then reshaped
        eos.taumin = term.at("taumin");
        eos.taumax = term.at("taumax");
        eos.deltamin = term.at("deltamin");
        eos.deltamax = term.at("deltamax");
        dep.add_term(eos);
    };
    //auto build_gaussian = [&](auto& term) {
    //    GaussianEOSTerm eos;
    //    eos.n = toeig(term["n"]);
    //    eos.t = toeig(term["t"]);
    //    eos.d = toeig(term["d"]);
    //    eos.eta = toeig(term["eta"]);
    //    eos.beta = toeig(term["beta"]);
    //    eos.gamma = toeig(term["gamma"]);
    //    eos.epsilon = toeig(term["epsilon"]);
    //    if (!all_same_length(term, { "n","t","d","eta","beta","gamma","epsilon" })) {
    //        throw std::invalid_argument("Lengths are not all identical in Gaussian term");
    //    }
    //    return eos;
    //};
    auto build_GERG2004 = [&](const auto& term, auto& dep) {
        if (!all_same_length(term, { "n","t","d","eta","beta","gamma","epsilon" })) {
            throw std::invalid_argument("Lengths are not all identical in GERG term");
        }
        int Npower = term["Npower"];
        auto NGERG = static_cast<int>(term["n"].size()) - Npower;

        PowerEOSTerm eos;
        eos.n = toeig(term["n"]).head(Npower);
        eos.t = toeig(term["t"]).head(Npower);
        eos.d = toeig(term["d"]).head(Npower);
        if (term.contains("l")) {
            eos.l = toeig(term["l"]).head(Npower);
        }
        else {
            eos.l = 0.0 * eos.n;
        }
        eos.c = (eos.l > 0).cast<int>().cast<double>();
        eos.l_i = eos.l.cast<int>();
        dep.add_term(eos);

        GERG2004EOSTerm e;
        e.n = toeig(term["n"]).tail(NGERG);
        e.t = toeig(term["t"]).tail(NGERG);
        e.d = toeig(term["d"]).tail(NGERG);
        e.eta = toeig(term["eta"]).tail(NGERG);
        e.beta = toeig(term["beta"]).tail(NGERG);
        e.gamma = toeig(term["gamma"]).tail(NGERG);
        e.epsilon = toeig(term["epsilon"]).tail(NGERG);
        dep.add_term(e);
    };
    auto build_GaussianExponential = [&](const auto& term, auto& dep) {
        if (!all_same_length(term, { "n","t","d","eta","beta","gamma","epsilon" })) {
            throw std::invalid_argument("Lengths are not all identical in Gaussian+Exponential term");
        }
        int Npower = term["Npower"];
        auto NGauss = static_cast<int>(term["n"].size()) - Npower;

        PowerEOSTerm eos;
        eos.n = toeig(term["n"]).head(Npower);
        eos.t = toeig(term["t"]).head(Npower);
        eos.d = toeig(term["d"]).head(Npower);
        if (term.contains("l")) {
            eos.l = toeig(term["l"]).head(Npower);
        }
        else {
            eos.l = 0.0 * eos.n;
        }
        eos.c = (eos.l > 0).cast<int>().cast<double>();
        eos.l_i = eos.l.cast<int>();
        dep.add_term(eos);

        GaussianEOSTerm e;
        e.n = toeig(term["n"]).tail(NGauss);
        e.t = toeig(term["t"]).tail(NGauss);
        e.d = toeig(term["d"]).tail(NGauss);
        e.eta = toeig(term["eta"]).tail(NGauss);
        e.beta = toeig(term["beta"]).tail(NGauss);
        e.gamma = toeig(term["gamma"]).tail(NGauss);
        e.epsilon = toeig(term["epsilon"]).tail(NGauss);
        dep.add_term(e);
    };

    std::string type = j.at("type");
    DepartureTerms dep;
    if (type == "Exponential") {
        build_power(j, dep);
    }
    else if (type == "DoubleExponential") {
        build_doubleexponential(j, dep);
    }
    else if (type == "GERG-2004" || type == "GERG-2008") {
        build_GERG2004(j, dep);
    }
    else if (type == "Gaussian+Exponential") {
        build_GaussianExponential(j, dep);
    }
    else if (type == "Chebyshev2D") {
        build_Chebyshev2D(j, dep);
    }
    else if (type == "none") {
        dep.add_term(NullEOSTerm());
    }
    else {
        
        std::vector<std::string> options = { "Exponential","GERG-2004","GERG-2008","Gaussian+Exponential", "none", "DoubleExponential","Chebyshev2D"};
        throw std::invalid_argument("Bad departure term type: " + type + ". Options are {" + boost::algorithm::join(options, ",") + "}");
    }
    return dep;
}

inline auto get_departure_function_matrix(const nlohmann::json& depcollection, const nlohmann::json& BIPcollection, const std::vector<std::string>& components, const nlohmann::json& flags) {

    // Allocate the matrix with default models
    std::vector<std::vector<DepartureTerms>> funcs(components.size()); for (auto i = 0; i < funcs.size(); ++i) { funcs[i].resize(funcs.size()); }

    // Load the collection of data on departure functions

    auto get_departure_json = [&depcollection](const std::string& Name) {
        for (auto& el : depcollection) {
            if (el["Name"] == Name) { return el; }
        }
        throw std::invalid_argument("Bad departure function name: "+Name);
    };

    auto funcsmeta = nlohmann::json::object();

    for (auto i = 0; i < funcs.size(); ++i) {
        std::string istr = std::to_string(i);
        if (funcsmeta.contains(istr)) { funcsmeta[istr] = {}; }
        for (auto j = i + 1; j < funcs.size(); ++j) {
            std::string jstr = std::to_string(j);
            auto [BIP, swap_needed] = reducing::get_BIPdep(BIPcollection, { components[i], components[j] }, flags);
            std::string funcname = BIP.contains("function") ? BIP["function"] : "";
            if (!funcname.empty()) {
                auto jj = get_departure_json(funcname);
                funcsmeta[istr][jstr] = { {"departure", jj}, {"BIP", BIP} };
                funcsmeta[istr][jstr]["BIP"]["swap_needed"] = swap_needed;
                funcs[i][j] = build_departure_function(jj);
                funcs[j][i] = build_departure_function(jj);
            }
            else {
                funcs[i][j].add_term(NullEOSTerm());
                funcs[j][i].add_term(NullEOSTerm());
            }
        }
    }
    return std::make_tuple(funcs, funcsmeta);
}

inline auto get_EOS_terms(const nlohmann::json& j)
{
    auto alphar = j["EOS"][0]["alphar"];

    // First check whether term type is allowed
    const std::vector<std::string> allowed_types = { "ResidualHelmholtzPower", "ResidualHelmholtzGaussian", "ResidualHelmholtzNonAnalytic","ResidualHelmholtzGaoB", "ResidualHelmholtzLemmon2005", "ResidualHelmholtzExponential", "ResidualHelmholtzDoubleExponential" };

    auto isallowed = [&](const auto& conventional_types, const std::string& name) {
        for (auto& a : conventional_types) { if (name == a) { return true; }; } return false;
    };

    for (auto& term : alphar) {
        std::string type = term["type"];
        if (!isallowed(allowed_types, type)) {
            std::string a = allowed_types[0]; for (auto i = 1; i < allowed_types.size(); ++i) { a += "," + allowed_types[i]; }
            throw std::invalid_argument("Bad type:" + type + "; allowed types are: {" + a + "}");
        }
    }

    EOSTerms container;

    auto build_power = [&](auto term, auto & container) {
        std::size_t N = term["n"].size();

        PowerEOSTerm eos;

        auto eigorzero = [&term, &N](const std::string& name) -> Eigen::ArrayXd {
            if (!term[name].empty()) {
                return toeig(term[name]);
            }
            else {
                return Eigen::ArrayXd::Zero(N);
            }
        };


        eos.n = eigorzero("n");
        eos.t = eigorzero("t");
        eos.d = eigorzero("d");

        Eigen::ArrayXd c(N), l(N); c.setZero();
        int Nlzero = 0, Nlnonzero = 0;
        bool contiguous_lzero;
        if (term["l"].empty()) {
            // exponential part not included
            l.setZero();
        }
        else {
            l = toeig(term["l"]);
            // l is included, use it to build c; c_i = 1 if l_i > 0, zero otherwise
            for (auto i = 0; i < c.size(); ++i) {
                if (l[i] > 0) {
                    c[i] = 1.0;
                }
            }

            // See how many of the first entries have zero values for l_i
            contiguous_lzero = (l[0] == 0);
            for (auto i = 0; i < c.size(); ++i) {
                if (l[i] == 0) {
                    Nlzero++;
                }
            }
        }
        Nlnonzero = static_cast<int>(l.size()) - Nlzero;
        
        eos.c = c;
        eos.l = l;

        eos.l_i = eos.l.cast<int>();

        if (Nlzero + Nlnonzero != l.size()) {
            throw std::invalid_argument("Somehow the l lengths don't add up");
        }

        if (((eos.l_i.cast<double>() - eos.l).cwiseAbs() > 0.0).any()) {
            throw std::invalid_argument("Non-integer entry in l found");
        }
        
        // If a contiguous portion of the terms have values of l_i that are zero
        // it is computationally advantageous to break up the evaluation into 
        // part that has just the n_i*tau^t_i*delta^d_i and the part with the
        // exponential term exp(-delta^l_i)
        if (l.sum() == 0) {
            // No l term at all, just polynomial
            JustPowerEOSTerm poly;
            poly.n = eos.n;
            poly.t = eos.t;
            poly.d = eos.d;
            container.add_term(poly);
        }
        else if (l.sum() > 0 && contiguous_lzero) {
            JustPowerEOSTerm poly;
            poly.n = eos.n.head(Nlzero);
            poly.t = eos.t.head(Nlzero);
            poly.d = eos.d.head(Nlzero);
            container.add_term(poly);

            PowerEOSTerm e;
            e.n = eos.n.tail(Nlnonzero);
            e.t = eos.t.tail(Nlnonzero);
            e.d = eos.d.tail(Nlnonzero);
            e.c = eos.c.tail(Nlnonzero);
            e.l = eos.l.tail(Nlnonzero);
            e.l_i = eos.l_i.tail(Nlnonzero);
            container.add_term(e);
        }
        else {
            // Don't try to get too clever, just add the term
            container.add_term(eos);
        }
    };

    auto build_Lemmon2005 = [&](auto term) {
        Lemmon2005EOSTerm eos;
        eos.n = toeig(term["n"]);
        eos.t = toeig(term["t"]);
        eos.d = toeig(term["d"]);
        eos.m = toeig(term["m"]);
        eos.l = toeig(term["l"]);
        eos.l_i = eos.l.cast<int>();
        if (!all_same_length(term, { "n","t","d","m","l" })) {
            throw std::invalid_argument("Lengths are not all identical in Lemmon2005 term");
        }
        if (((eos.l_i.cast<double>() - eos.l).cwiseAbs() > 0.0).any()) {
            throw std::invalid_argument("Non-integer entry in l found");
        }
        return eos;
    };

    auto build_gaussian = [&](auto term) {
        GaussianEOSTerm eos;
        eos.n = toeig(term["n"]);
        eos.t = toeig(term["t"]);
        eos.d = toeig(term["d"]);
        eos.eta = toeig(term["eta"]);
        eos.beta = toeig(term["beta"]);
        eos.gamma = toeig(term["gamma"]);
        eos.epsilon = toeig(term["epsilon"]);
        if (!all_same_length(term, { "n","t","d","eta","beta","gamma","epsilon" })) {
            throw std::invalid_argument("Lengths are not all identical in Gaussian term");
        }
        return eos;
    };

    auto build_exponential = [&](auto term) {
        ExponentialEOSTerm eos;
        eos.n = toeig(term["n"]);
        eos.t = toeig(term["t"]);
        eos.d = toeig(term["d"]);
        eos.g = toeig(term["g"]);
        eos.l = toeig(term["l"]);
        eos.l_i = eos.l.cast<int>();
        if (!all_same_length(term, { "n","t","d","g","l" })) {
            throw std::invalid_argument("Lengths are not all identical in exponential term");
        }
        return eos;
    };
    
    auto build_doubleexponential = [&](auto& term) {
        if (!all_same_length(term, { "n","t","d","ld","gd","lt","gt" })) {
            throw std::invalid_argument("Lengths are not all identical in double exponential term");
        }
        DoubleExponentialEOSTerm eos;
        eos.n = toeig(term.at("n"));
        eos.t = toeig(term.at("t"));
        eos.d = toeig(term.at("d"));
        eos.ld = toeig(term.at("ld"));
        eos.gd = toeig(term.at("gd"));
        eos.lt = toeig(term.at("lt"));
        eos.gt = toeig(term.at("gt"));
        eos.ld_i = eos.ld.cast<int>();
        return eos;
    };

    auto build_GaoB = [&](auto term) {
        GaoBEOSTerm eos;
        eos.n = toeig(term["n"]);
        eos.t = toeig(term["t"]);
        eos.d = toeig(term["d"]);
        eos.eta = -toeig(term["eta"]); // Watch out for this sign flip!!
        eos.beta = toeig(term["beta"]);
        eos.gamma = toeig(term["gamma"]);
        eos.epsilon = toeig(term["epsilon"]);
        eos.b = toeig(term["b"]);
        if (!all_same_length(term, { "n","t","d","eta","beta","gamma","epsilon","b" })) {
            throw std::invalid_argument("Lengths are not all identical in GaoB term");
        }
        return eos;
    };

    /// lambda function for adding non-analytic terms
    auto build_na = [&](auto& term) {
        NonAnalyticEOSTerm eos;
        eos.n = toeig(term["n"]);
        eos.A = toeig(term["A"]);
        eos.B = toeig(term["B"]);
        eos.C = toeig(term["C"]);
        eos.D = toeig(term["D"]);
        eos.a = toeig(term["a"]);
        eos.b = toeig(term["b"]);
        eos.beta = toeig(term["beta"]);
        if (!all_same_length(term, { "n","A","B","C","D","a","b","beta" })) {
            throw std::invalid_argument("Lengths are not all identical in nonanalytic term");
        }
        return eos;
    };
    
    for (auto& term : alphar) {
        std::string type = term["type"];
        if (type == "ResidualHelmholtzPower") {
            build_power(term, container);
        }
        else if (type == "ResidualHelmholtzGaussian") {
            container.add_term(build_gaussian(term));
        }
        else if (type == "ResidualHelmholtzNonAnalytic") {
            container.add_term(build_na(term));
        }
        else if (type == "ResidualHelmholtzLemmon2005") {
            container.add_term(build_Lemmon2005(term));
        }
        else if (type == "ResidualHelmholtzGaoB") {
            container.add_term(build_GaoB(term));
        }
        else if (type == "ResidualHelmholtzExponential") {
            container.add_term(build_exponential(term));
        }
        else if (type == "ResidualHelmholtzDoubleExponential") {
            container.add_term(build_doubleexponential(term));
        }
        else {
            throw std::invalid_argument("Bad term type: "+type);
        }
    }
    return container;
}

inline auto get_EOSs(const std::vector<nlohmann::json>& pureJSON) {
    std::vector<EOSTerms> EOSs;
    for (auto& j : pureJSON) {
        auto term = get_EOS_terms(j);
        EOSs.emplace_back(term);
    }
    return EOSs;
}

inline auto collect_component_json(const std::vector<std::string>& components, const std::string& root) 
{
    std::vector<nlohmann::json> out;
    for (auto c : components) {
        // First we try to lookup the name as a path, which can be on the filesystem, or relative to the root for default name lookup
        std::vector<std::filesystem::path> candidates = { c, root + "/dev/fluids/" + c + ".json" };
        std::filesystem::path selected_path = "";
        for (auto candidate : candidates) {
            if (std::filesystem::is_regular_file(candidate)) {
                selected_path = candidate;
                break;
            }
        }
        if (selected_path != "") {
            out.push_back(load_a_JSON_file(selected_path.string()));
        }
        else {
            throw std::invalid_argument("Could not load any of the candidates:" + c);
        }
    }
    return out;
}

inline auto collect_identifiers(const std::vector<nlohmann::json>& pureJSON)
{
    std::vector<std::string> CAS, Name, REFPROP;
    for (auto j : pureJSON) {
        Name.push_back(j.at("INFO").at("NAME"));
        CAS.push_back(j.at("INFO").at("CAS"));
        REFPROP.push_back(j.at("INFO").at("REFPROP_NAME"));
    }
    return std::map<std::string, std::vector<std::string>>{
        {"CAS", CAS},
        {"Name", Name},
        {"REFPROP", REFPROP}
    };
}

/// Iterate over the possible options for identifiers to determine which one will satisfy all the binary pairs
template<typename mapvecstring>
inline auto select_identifier(const nlohmann::json& BIPcollection, const mapvecstring& identifierset, const nlohmann::json& flags){
    for (const auto &ident: identifierset){
        std::string key; std::vector<std::string> identifiers;
        std::tie(key, identifiers) = ident;
        try{
            for (auto i = 0; i < identifiers.size(); ++i){
                for (auto j = i+1; j < identifiers.size(); ++j){
                    const std::vector<std::string> pair = {identifiers[i], identifiers[j]};
                    reducing::get_BIPdep(BIPcollection, pair, flags);
                }
            }
            return key;
        }
        catch(...){
            
        }
    }
    throw std::invalid_argument("Unable to match any of the identifier options");
}

/// Build a reverse-lookup map for finding a fluid JSON structure given a backup identifier
inline auto build_alias_map(const std::string& root) {
    std::map<std::string, std::string> aliasmap;
    for (auto path : get_files_in_folder(root + "/dev/fluids", ".json")) {
        auto j = load_a_JSON_file(path.string());
        std::string REFPROP_name = j.at("INFO").at("REFPROP_NAME"); 
        std::string name = j.at("INFO").at("NAME");
        for (std::string k : {"NAME", "CAS", "REFPROP_NAME"}) {
            std::string val = j.at("INFO").at(k);
            // Skip REFPROP names that match the fluid itself
            if (k == "REFPROP_NAME" && val == name) {
                continue;
            }
            // Skip invalid REFPROP names
            if (k == "REFPROP_NAME" && val == "N/A") {
                continue;
            }
            if (aliasmap.count(val) > 0) {
                throw std::invalid_argument("Duplicated reverse lookup identifier ["+k+"] found in file:" + path.string());
            }
            else {
                aliasmap[val] = std::filesystem::absolute(path).string();
            }
        }
        std::vector<std::string> aliases = j.at("INFO").at("ALIASES");
        
        for (std::string alias : aliases) {
            if (alias != REFPROP_name && alias != name) { // Don't add REFPROP name or base name, were already above to list of aliases
                if (aliasmap.count(alias) > 0) {
                    throw std::invalid_argument("Duplicated alias [" + alias + "] found in file:" + path.string());
                }
                else {
                    aliasmap[alias] = std::filesystem::absolute(path).string();
                }
            }
        }
    }
    return aliasmap;
}

/// Internal method for actually constructing the model with the provided JSON data structures
inline auto _build_multifluid_model(const std::vector<nlohmann::json> &pureJSON, const nlohmann::json& BIPcollection, const nlohmann::json& depcollection, const nlohmann::json& flags = {}) {

    auto [Tc, vc] = reducing::get_Tcvc(pureJSON);
    auto EOSs = get_EOSs(pureJSON);

    // Extract the set of possible identifiers to be used to match parameters
    auto identifierset = collect_identifiers(pureJSON);
    // Decide which identifier is to be used (Name, CAS, REFPROP name)
    auto identifiers = identifierset[select_identifier(BIPcollection, identifierset, flags)];

    // Things related to the mixture
    auto F = reducing::get_F_matrix(BIPcollection, identifiers, flags);
    auto [funcs, funcsmeta] = get_departure_function_matrix(depcollection, BIPcollection, identifiers, flags);
    auto [betaT, gammaT, betaV, gammaV] = reducing::get_BIP_matrices(BIPcollection, identifiers, flags, Tc, vc);

    nlohmann::json meta = {
        {"pures", pureJSON},
        {"mix", funcsmeta},
    };

    auto redfunc = ReducingFunctions(std::move(MultiFluidReducingFunction(betaT, gammaT, betaV, gammaV, Tc, vc)));

    auto model = MultiFluid(
        std::move(redfunc),
        CorrespondingStatesContribution(std::move(EOSs)),
        DepartureContribution(std::move(F), std::move(funcs))
    );
    model.set_meta(meta.dump(1));
    return model;
}

/// A builder function where the JSON-formatted strings are provided explicitly rather than file paths
inline auto build_multifluid_JSONstr(const std::vector<std::string>& componentJSON, const std::string& BIPJSON, const std::string& departureJSON, const nlohmann::json& flags = {}) {

    // Mixture things
    const auto BIPcollection = nlohmann::json::parse(BIPJSON);
    const auto depcollection = nlohmann::json::parse(departureJSON);

    // Pure fluids
    std::vector<nlohmann::json> pureJSON;
    for (auto& c : componentJSON) {
        pureJSON.emplace_back(nlohmann::json::parse(c));
    }
    return _build_multifluid_model(pureJSON, BIPcollection, depcollection, flags);
}

/**
 There are 4 options:
 
 1. Absolute paths to fluid files in the JSON format
 2. Names of fluid fluids that can all be looked up in the dev/fluids folder relative to the root
 3. Fluid data that is already in the JSON format
 4. Names that all resolve to absolute paths when looking up in the alias map
*/
inline auto make_pure_components_JSON(const nlohmann::json& components, const std::string& root){
    
    std::vector<nlohmann::json> pureJSON;
    if (!components.is_array()){
        throw std::invalid_argument("Must be an array");
    }
    
    // Check if are possibly valid paths (JSON should not be)
    bool all_valid_paths = true;
    bool all_abspath_exist = true;
    bool all_fluids_exist = true;
    bool might_be_JSON = true;
    for (auto s: components){
        if (!s.is_object()){
            might_be_JSON = false;
        }
        try{
            std::filesystem::path p = s.get<std::string>();
            if (!std::filesystem::exists(p)){
                all_abspath_exist = false;
            }
            if (!std::filesystem::exists(root+"/dev/fluids/"+s.get<std::string>()+".json")){
                all_fluids_exist = false;
            }
        }
        catch(...){
            all_valid_paths = false;
        }
    }
    
    // Normal treatment if:
    // a) Absolute paths were provided, to files that exist
    // b) Fluid names in the dev/fluids/ folder relative to the root
    if (all_valid_paths && (all_fluids_exist || all_abspath_exist)){
        return collect_component_json(components.get<std::vector<std::string>>(), root);
    }
    else if (might_be_JSON){
        // Data is already in JSON format, just turn it into vector of nlohmann
        for (auto c : components){
            pureJSON.push_back(c);
        }
        return pureJSON;
    }
    else{
        // Lookup the absolute paths for each component
        auto aliasmap = build_alias_map(root);
        std::vector<std::string> abspaths;
        for (auto c : components) {
            auto cstr = c.get<std::string>();
            // Allow matching of absolute paths first
            if (std::filesystem::is_regular_file(cstr)) {
                abspaths.push_back(cstr);
            }
            else {
                abspaths.push_back(aliasmap[cstr]);
            }
        }
        // Backup lookup with absolute paths resolved for each component
        pureJSON = collect_component_json(abspaths, root);
    }
    return pureJSON;
}

inline auto build_multifluid_model(const std::vector<std::string>& components, const std::string& coolprop_root, const std::string& BIPcollectionpath = {}, const nlohmann::json& flags = {}, const std::string& departurepath = {}) {

    
    auto is_valid_path = [](const std::string & s){
        try{
            std::filesystem::is_regular_file(s);
            return true;
        }
        catch(...){
            return false;
        }
    };
    
    nlohmann::json BIPcollection = nlohmann::json::array();
    // If not provided, default values
    if (BIPcollectionpath.empty()){
        auto BIPpath = coolprop_root + "/dev/mixtures/mixture_binary_pairs.json";
        BIPcollection = load_a_JSON_file(BIPpath);
    }
    // If path to existing file, use it
    else if (is_valid_path(BIPcollectionpath) && std::filesystem::is_regular_file(BIPcollectionpath)){
        BIPcollection = load_a_JSON_file(BIPcollectionpath);
    }
    // Or assume it is a string in JSON format
    else{
        BIPcollection = nlohmann::json::parse(BIPcollectionpath);
    }
    
    nlohmann::json depcollection = nlohmann::json::array();
    // If not provided, default values
    if (departurepath.empty()){
        std::string deppath = coolprop_root + "/dev/mixtures/mixture_departure_functions.json";
        depcollection = load_a_JSON_file(deppath);
    }
    // If path to existing file, use it
    else if (is_valid_path(departurepath) && std::filesystem::is_regular_file(departurepath)){
        depcollection = load_a_JSON_file(departurepath);
    }
    // Or assume it is a string in JSON format
    else{
        depcollection = nlohmann::json::parse(departurepath);
    }

    return _build_multifluid_model(make_pure_components_JSON(components, coolprop_root), BIPcollection, depcollection, flags);
}

/**
* \brief Load a model from a JSON data structure
* 
* Required fields are: components, BIP, departure
* 
* BIP and departure can be either the data in JSON format, or a path to file with those contents
* components is an array, which either contains the paths to the JSON data, or the file path
*/
inline auto multifluidfactory(const nlohmann::json& spec) {
    
    std::string root = (spec.contains("root")) ? spec.at("root") : "";
    
    auto get_json = [](const std::string& path1, const std::string& default_path){
        try{
            return load_a_JSON_file(path1);
        }