#pragma once

#include "api.hpp"

// x1, x2, x3
struct State
{
    double x1;
    double x2;
    double x3;
};

inline State add_scaled(const State& left, const State& right, double scale){
    return {
        left.x1 + scale * right.x1,
        left.x2 + scale * right.x2,
        left.x3 + scale * right.x3,
    };
}

// СОДУ
inline State derivative(const State& state,
                        const SolverParameters& parameters){
    return {
        -parameters.k1 * state.x1,
        parameters.k1 * state.x1 - parameters.k2 * state.x2,
        parameters.k2 * state.x2
    };
}

inline State runge_kutta_step(const State& state, const SolverParameters& parameters){
    double h = parameters.step;

    State k1 = derivative(state, parameters);
    State k2 = derivative(add_scaled(state, k1, h / 2), parameters);
    State k3 = derivative(add_scaled(state, k2, h / 2), parameters);
    State k4 = derivative(add_scaled(state, k3, h), parameters);

    return {
        state.x1 + h / 6.0 * (k1.x1 + 2 * k2.x1 + 2 * k3.x1 + k4.x1),
        state.x2 + h / 6.0 * (k1.x2 + 2 * k2.x2 + 2 * k3.x2 + k4.x2),
        state.x3 + h / 6.0 * (k1.x3 + 2 * k2.x3 + 2 * k3.x3 + k4.x3)
    };
}

inline State state_from_point(const SolutionPoint& point){
    return { point.x1, point.x2, point.x3 };
}

inline SolutionPoint make_step(double t, const State& state){
    return { t, state.x1, state.x2, state.x3 };
}