expression.h

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#ifndef EXPRESSION_H
#define EXPRESSION_H
 
#include <algorithm>
#include <memory>
#include <stdexcept>
#include <string>
#include <vector>
 
#include <exprtk.hpp>
 
class Expression {
    public:
        Expression(const std::string & name, const std::string & text) :
            vars(std::make_unique<Vars>()) {
            vars->table.add_variable("x", vars->x);
            vars->table.add_variable("y", vars->y);
            vars->table.add_variable("z", vars->z);
            vars->table.add_variable("t", vars->t);
            vars->table.add_constants();
            vars->expr.register_symbol_table(vars->table);
            exprtk::parser<double> parser;
            if (!parser.compile(text, vars->expr)) {
                throw std::runtime_error("Failed to parse expression for " + name + ": " + parser.error());
            }
            std::vector<std::string> used;
            time_dependent = !exprtk::collect_variables(text, vars->table, used) ||
                             std::find(used.begin(), used.end(), "t") != used.end();
        }
 
        bool depends_on_time() const { return time_dependent; }
 
        double operator()(double x, double y, double t = 0.0) const {
            return (*this)(x, y, 0.0, t);
        }
 
        double operator()(double x, double y, double z, double t) const {
            vars->x = x;
            vars->y = y;
            vars->z = z;
            vars->t = t;
            return vars->expr.value();
        }
 
        template <typename T>
        double at(const T & x, const int n_dim, double t = 0.0) const {
            return (*this)(double(x(0)), double(x(1)), n_dim > 2 ? double(x(2)) : 0.0, t);
        }
 
    private:
        // Heap-allocated so the symbol table's variable addresses survive moves
        struct Vars {
            double x = 0.0, y = 0.0, z = 0.0, t = 0.0;
            exprtk::symbol_table<double> table;
            exprtk::expression<double> expr;
        };
        std::unique_ptr<Vars> vars;
        bool time_dependent = true;
};
 
#endif // EXPRESSION_H