53 lines
1.4 KiB
C++
53 lines
1.4 KiB
C++
#pragma once
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#include "api.hpp"
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// x1, x2, x3
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struct State
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{
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double x1;
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double x2;
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double x3;
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};
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inline State add_scaled(const State& left, const State& right, double scale){
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return {
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left.x1 + scale * right.x1,
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left.x2 + scale * right.x2,
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left.x3 + scale * right.x3,
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};
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}
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// СОДУ
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inline State derivative(const State& state,
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const SolverParameters& parameters){
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return {
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-parameters.k1 * state.x1,
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parameters.k1 * state.x1 - parameters.k2 * state.x2,
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parameters.k2 * state.x2
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};
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}
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inline State runge_kutta_step(const State& state, const SolverParameters& parameters){
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double h = parameters.step;
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State k1 = derivative(state, parameters);
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State k2 = derivative(add_scaled(state, k1, h / 2), parameters);
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State k3 = derivative(add_scaled(state, k2, h / 2), parameters);
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State k4 = derivative(add_scaled(state, k3, h), parameters);
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return {
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state.x1 + h / 6.0 * (k1.x1 + 2 * k2.x1 + 2 * k3.x1 + k4.x1),
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state.x2 + h / 6.0 * (k1.x2 + 2 * k2.x2 + 2 * k3.x2 + k4.x2),
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state.x3 + h / 6.0 * (k1.x3 + 2 * k2.x3 + 2 * k3.x3 + k4.x3)
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};
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}
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inline State state_from_point(const SolutionPoint& point){
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return { point.x1, point.x2, point.x3 };
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}
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inline SolutionPoint make_step(double t, const State& state){
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return { t, state.x1, state.x2, state.x3 };
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}
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