// Copyright (c) FIRST and other WPILib contributors.
// Open Source Software; you can modify and/or share it under the terms of
// the WPILib BSD license file in the root directory of this project.

package org.wpilib.math.trajectory;

import java.util.Objects;
import org.wpilib.math.trajectory.struct.TrapezoidProfileStateStruct;
import org.wpilib.math.util.MathSharedStore;
import org.wpilib.util.struct.StructSerializable;

/**
 * A trapezoid-shaped velocity profile.
 *
 * <p>While this class can be used for a profiled movement from start to finish, the intended usage
 * is to filter a reference's dynamics based on trapezoidal velocity constraints. To compute the
 * reference obeying this constraint, do the following.
 *
 * <p>Initialization:
 *
 * <pre><code>
 * TrapezoidProfile.Constraints constraints =
 *   new TrapezoidProfile.Constraints(kMaxV, kMaxA);
 * TrapezoidProfile.State previousProfiledReference =
 *   new TrapezoidProfile.State(initialReference, 0.0);
 * TrapezoidProfile profile = new TrapezoidProfile(constraints);
 * </code></pre>
 *
 * <p>Run on update:
 *
 * <pre><code>
 * previousProfiledReference =
 * profile.calculate(timeSincePreviousUpdate, previousProfiledReference, unprofiledReference);
 * </code></pre>
 *
 * <p>where `unprofiledReference` is free to change between calls. Note that when the unprofiled
 * reference is within the constraints, `calculate()` returns the unprofiled reference unchanged.
 *
 * <p>Otherwise, a timer can be started to provide monotonic values for `calculate()` and to
 * determine when the profile has completed via `isFinished()`.
 */
public class TrapezoidProfile {
  // The direction of the profile, either 1 for forwards or -1 for inverted
  private int m_direction;

  private final Constraints m_constraints;
  private State m_current = new State();

  private double m_endAccel;
  private double m_endFullVelocity;
  private double m_endDecel;

  /** Profile constraints. */
  public static class Constraints {
    /** Maximum velocity. */
    public final double maxVelocity;

    /** Maximum acceleration. */
    public final double maxAcceleration;

    /**
     * Constructs constraints for a TrapezoidProfile.
     *
     * @param maxVelocity Maximum velocity, must be non-negative.
     * @param maxAcceleration Maximum acceleration, must be non-negative.
     */
    public Constraints(double maxVelocity, double maxAcceleration) {
      if (maxVelocity < 0.0 || maxAcceleration < 0.0) {
        throw new IllegalArgumentException("Constraints must be non-negative");
      }

      this.maxVelocity = maxVelocity;
      this.maxAcceleration = maxAcceleration;
      MathSharedStore.reportUsage("TrapezoidProfile", "");
    }
  }

  /** Profile state. */
  public static class State implements StructSerializable {
    /** The struct used to serialize this class. */
    public static final TrapezoidProfileStateStruct struct = new TrapezoidProfileStateStruct();

    /** The position at this state. */
    public double position;

    /** The velocity at this state. */
    public double velocity;

    /** Default constructor. */
    public State() {}

    /**
     * Constructs constraints for a Trapezoid Profile.
     *
     * @param position The position at this state.
     * @param velocity The velocity at this state.
     */
    public State(double position, double velocity) {
      this.position = position;
      this.velocity = velocity;
    }

    @Override
    public boolean equals(Object other) {
      return other instanceof State rhs
          && this.position == rhs.position
          && this.velocity == rhs.velocity;
    }

    @Override
    public int hashCode() {
      return Objects.hash(position, velocity);
    }
  }

  /**
   * Constructs a TrapezoidProfile.
   *
   * @param constraints The constraints on the profile, like maximum velocity.
   */
  public TrapezoidProfile(Constraints constraints) {
    m_constraints = constraints;
  }

  /**
   * Calculates the position and velocity for the profile at a time t where the current state is at
   * time t = 0.
   *
   * @param t How long to advance from the current state toward the desired state.
   * @param current The current state.
   * @param goal The desired state when the profile is complete.
   * @return The position and velocity of the profile at time t.
   */
  public State calculate(double t, State current, State goal) {
    m_direction = shouldFlipAcceleration(current, goal) ? -1 : 1;
    m_current = direct(current);
    goal = direct(goal);

    if (Math.abs(m_current.velocity) > m_constraints.maxVelocity) {
      m_current.velocity = Math.copySign(m_constraints.maxVelocity, m_current.velocity);
    }

    // Deal with a possibly truncated motion profile (with nonzero initial or
    // final velocity) by calculating the parameters as if the profile began and
    // ended at zero velocity
    double cutoffBegin = m_current.velocity / m_constraints.maxAcceleration;
    double cutoffDistBegin = cutoffBegin * cutoffBegin * m_constraints.maxAcceleration / 2.0;

    double cutoffEnd = goal.velocity / m_constraints.maxAcceleration;
    double cutoffDistEnd = cutoffEnd * cutoffEnd * m_constraints.maxAcceleration / 2.0;

    // Now we can calculate the parameters as if it was a full trapezoid instead
    // of a truncated one

    double fullTrapezoidDist =
        cutoffDistBegin + (goal.position - m_current.position) + cutoffDistEnd;
    double accelerationTime = m_constraints.maxVelocity / m_constraints.maxAcceleration;

    double fullVelocityDist =
        fullTrapezoidDist - accelerationTime * accelerationTime * m_constraints.maxAcceleration;

    // Handle the case where the profile never reaches full velocity
    if (fullVelocityDist < 0) {
      accelerationTime = Math.sqrt(fullTrapezoidDist / m_constraints.maxAcceleration);
      fullVelocityDist = 0;
    }

    m_endAccel = accelerationTime - cutoffBegin;
    m_endFullVelocity = m_endAccel + fullVelocityDist / m_constraints.maxVelocity;
    m_endDecel = m_endFullVelocity + accelerationTime - cutoffEnd;
    State result = new State(m_current.position, m_current.velocity);

    if (t < m_endAccel) {
      result.velocity += t * m_constraints.maxAcceleration;
      result.position += (m_current.velocity + t * m_constraints.maxAcceleration / 2.0) * t;
    } else if (t < m_endFullVelocity) {
      result.velocity = m_constraints.maxVelocity;
      result.position +=
          (m_current.velocity + m_endAccel * m_constraints.maxAcceleration / 2.0) * m_endAccel
              + m_constraints.maxVelocity * (t - m_endAccel);
    } else if (t <= m_endDecel) {
      result.velocity = goal.velocity + (m_endDecel - t) * m_constraints.maxAcceleration;
      double timeLeft = m_endDecel - t;
      result.position =
          goal.position
              - (goal.velocity + timeLeft * m_constraints.maxAcceleration / 2.0) * timeLeft;
    } else {
      result = goal;
    }

    return direct(result);
  }

  /**
   * Returns the time left until a target distance in the profile is reached.
   *
   * @param target The target distance.
   * @return The time left until a target distance in the profile is reached, or zero if no goal was
   *     set.
   */
  public double timeLeftUntil(double target) {
    double position = m_current.position * m_direction;
    double velocity = m_current.velocity * m_direction;

    double endAccel = m_endAccel * m_direction;
    double endFullVelocity = m_endFullVelocity * m_direction - endAccel;

    if (target < position) {
      endAccel = -endAccel;
      endFullVelocity = -endFullVelocity;
      velocity = -velocity;
    }

    endAccel = Math.max(endAccel, 0);
    endFullVelocity = Math.max(endFullVelocity, 0);

    final double acceleration = m_constraints.maxAcceleration;
    final double deceleration = -m_constraints.maxAcceleration;

    double distToTarget = Math.abs(target - position);
    if (distToTarget < 1e-6) {
      return 0;
    }

    double accelDist = velocity * endAccel + 0.5 * acceleration * endAccel * endAccel;

    double decelVelocity;
    if (endAccel > 0) {
      decelVelocity = Math.sqrt(Math.abs(velocity * velocity + 2 * acceleration * accelDist));
    } else {
      decelVelocity = velocity;
    }

    double fullVelocityDist = m_constraints.maxVelocity * endFullVelocity;
    double decelDist;

    if (accelDist > distToTarget) {
      accelDist = distToTarget;
      fullVelocityDist = 0;
      decelDist = 0;
    } else if (accelDist + fullVelocityDist > distToTarget) {
      fullVelocityDist = distToTarget - accelDist;
      decelDist = 0;
    } else {
      decelDist = distToTarget - fullVelocityDist - accelDist;
    }

    double accelTime =
        (-velocity + Math.sqrt(Math.abs(velocity * velocity + 2 * acceleration * accelDist)))
            / acceleration;

    double decelTime =
        (-decelVelocity
                + Math.sqrt(Math.abs(decelVelocity * decelVelocity + 2 * deceleration * decelDist)))
            / deceleration;

    double fullVelocityTime = fullVelocityDist / m_constraints.maxVelocity;

    return accelTime + fullVelocityTime + decelTime;
  }

  /**
   * Returns the total time the profile takes to reach the goal.
   *
   * @return The total time the profile takes to reach the goal, or zero if no goal was set.
   */
  public double totalTime() {
    return m_endDecel;
  }

  /**
   * Returns true if the profile has reached the goal.
   *
   * <p>The profile has reached the goal if the time since the profile started has exceeded the
   * profile's total time.
   *
   * @param t The time since the beginning of the profile.
   * @return True if the profile has reached the goal.
   */
  public boolean isFinished(double t) {
    return t >= totalTime();
  }

  /**
   * Returns true if the profile inverted.
   *
   * <p>The profile is inverted if goal position is less than the initial position.
   *
   * @param initial The initial state (usually the current state).
   * @param goal The desired state when the profile is complete.
   */
  private static boolean shouldFlipAcceleration(State initial, State goal) {
    return initial.position > goal.position;
  }

  // Flip the sign of the velocity and position if the profile is inverted
  private State direct(State in) {
    State result = new State(in.position, in.velocity);
    result.position = result.position * m_direction;
    result.velocity = result.velocity * m_direction;
    return result;
  }
}