acrobot.hpp

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#ifndef MLPACK_METHODS_RL_ENVIRONMENT_ACROBOT_HPP
#define MLPACK_METHODS_RL_ENVIRONMENT_ACROBOT_HPP

#include <mlpack/core.hpp>

namespace mlpack{
namespace rl{

class Acrobot
{
 public:
  /*
   * Implementation of Acrobot State. Each State is a tuple vector
   * (theta1, thetha2, angular velocity 1, angular velocity 2).
   */
  class State
  {
   public:
    State(): data(dimension) { /* nothing to do here */ }

    State(const arma::colvec& data) : data(data)
    { /* nothing to do here */ }

    arma::colvec& Data() { return data; }

    double Theta1() const { return data[0]; }
    double& Theta1() { return data[0]; }

    double Theta2() const { return data[1]; }
    double& Theta2() { return data[1]; }

    double AngularVelocity1() const { return data[2]; }
    double& AngularVelocity1() { return data[2]; }

    double AngularVelocity2() const { return data[3]; }
    double& AngularVelocity2() { return data[3]; }

    const arma::colvec& Encode() const { return data; }

    static constexpr size_t dimension = 4;

   private:
    arma::colvec data;
  };

  /*
   * Implementation of action for Acrobot
   */
  enum Action
  {
    negativeTorque,
    zeroTorque,
    positiveTorque,

    // Track the size of the action space.
    size
  };

  Acrobot(const double gravity = 9.81,
          const double linkLength1 = 1.0,
          const double linkLength2 = 1.0,
          const double linkMass1 = 1.0,
          const double linkMass2 = 1.0,
          const double linkCom1 = 0.5,
          const double linkCom2 = 0.5,
          const double linkMoi = 1.0,
          const double maxVel1 = 4 * M_PI,
          const double maxVel2 = 9 * M_PI,
          const double dt = 0.2,
          const double doneReward = 0,
          const size_t maxSteps = 0) :
      gravity(gravity),
      linkLength1(linkLength1),
      linkLength2(linkLength2),
      linkMass1(linkMass1),
      linkMass2(linkMass2),
      linkCom1(linkCom1),
      linkCom2(linkCom2),
      linkMoi(linkMoi),
      maxVel1(maxVel1),
      maxVel2(maxVel2),
      dt(dt),
      doneReward(doneReward),
      maxSteps(maxSteps),
      stepsPerformed(0)
  { /* Nothing to do here */ }

  double Sample(const State& state,
                const Action& action,
                State& nextState)
  {
    // Update the number of steps performed.
    stepsPerformed++;

    // Make a vector to estimate nextstate.
    arma::colvec currentState = {state.Theta1(), state.Theta2(),
        state.AngularVelocity1(), state.AngularVelocity2()};

    arma::colvec currentNextState = Rk4(currentState, Torque(action));

    nextState.Theta1() = Wrap(currentNextState[0], -M_PI, M_PI);

    nextState.Theta2() = Wrap(currentNextState[1], -M_PI, M_PI);

    nextState.AngularVelocity1() = math::ClampRange(currentNextState[2],
        -maxVel1, maxVel1);
    nextState.AngularVelocity2() = math::ClampRange(currentNextState[3],
        -maxVel2, maxVel2);

    // Check if the episode has terminated.
    bool done = IsTerminal(nextState);

    // Do not reward the agent if time ran out.
    if (done && maxSteps != 0 && stepsPerformed >= maxSteps)
      return 0;
    else if (done)
      return doneReward;

    return -1;
  };

  double Sample(const State& state, const Action& action)
  {
    State nextState;
    return Sample(state, action, nextState);
  }

  State InitialSample()
  {
    stepsPerformed = 0;
    return State((arma::randu<arma::colvec>(4) - 0.5) / 5.0);
  }

  bool IsTerminal(const State& state) const
  {
    if (maxSteps != 0 && stepsPerformed >= maxSteps)
    {
      Log::Info << "Episode terminated due to the maximum number of steps"
          "being taken.";
      return true;
    }
    else if (-std::cos(state.Theta1()) - std::cos(state.Theta1() +
        state.Theta2()) > 1.0)
    {
      Log::Info << "Episode terminated due to agent succeeding.";
      return true;
    }
    return false;
  }

  arma::colvec Dsdt(arma::colvec state, const double torque) const
  {
    const double m1 = linkMass1;
    const double m2 = linkMass2;
    const double l1 = linkLength1;
    const double lc1 = linkCom1;
    const double lc2 = linkCom2;
    const double I1 = linkMoi;
    const double I2 = linkMoi;
    const double g = gravity;
    const double a = torque;
    const double theta1 = state[0];
    const double theta2 = state[1];

    arma::colvec values(4);
    values[0] = state[2];
    values[1] = state[3];

    const double d1 = m1 * std::pow(lc1, 2) + m2 * (std::pow(l1, 2) +
        std::pow(lc2, 2) + 2 * l1 * lc2 * std::cos(theta2)) + I1 + I2;

    const double d2 = m2 * (std::pow(lc2, 2) + l1 * lc2 * std::cos(theta2)) +
        I2;

    const double phi2 = m2 * lc2 * g * std::cos(theta1 + theta2 - M_PI / 2.);

    const double phi1 = - m2 * l1 * lc2 * std::pow(values[1], 2) *
        std::sin(theta2) - 2 * m2 * l1 * lc2 * values[1] * values[0] *
        std::sin(theta2) + (m1 * lc1 +  m2 * l1) * g *
        std::cos(theta1 - M_PI / 2) + phi2;

    values[3] = (a + d2 / d1 * phi1 - m2 * l1 * lc2 * std::pow(values[0], 2) *
        std::sin(theta2) - phi2) / (m2 * std::pow(lc2, 2) + I2 -
        std::pow(d2, 2) / d1);

    values[2] = -(d2 * values[3] + phi1) / d1;

    return values;
  };

  double Wrap(double value,
              const double minimum,
              const double maximum) const
  {
    const double diff = maximum - minimum;

    if (value > maximum)
    {
      value = value - diff;
    }
    else if (value < minimum)
    {
      value = value + diff;
    }

    return value;
  };

  double Torque(const Action& action) const
  {
    // Add noise to the Torque Torque is action number - 1. {0,1,2} -> {-1,0,1}.
    return double(action - 1) + mlpack::math::Random(-0.1, 0.1);
  }

  arma::colvec Rk4(const arma::colvec state, const double torque) const
  {
    arma::colvec k1 = Dsdt(state, torque);
    arma::colvec k2 = Dsdt(state + dt * k1 / 2, torque);
    arma::colvec k3 = Dsdt(state + dt * k2 / 2, torque);
    arma::colvec k4 = Dsdt(state + dt * k3, torque);
    arma::colvec nextState = state + dt * (k1 + 2 * k2 + 2 * k3 + k4) / 6;

    return nextState;
  };

  size_t StepsPerformed() const { return stepsPerformed; }

  size_t MaxSteps() const { return maxSteps; }
  size_t& MaxSteps() { return maxSteps; }

 private:
  double gravity;

  double linkLength1;

  double linkLength2;

  double linkMass1;

  double linkMass2;

  double linkCom1;

  double linkCom2;

  double linkMoi;

  double maxVel1;

  double maxVel2;

  double dt;

  double doneReward;

  size_t maxSteps;

  size_t stepsPerformed;
};

} // namespace rl
} // namespace mlpack

#endif