using System;
using System.Collections.Generic;
using System.Text;
using RungeKutta.Integrators;

namespace RungeKutta.SimStep
{
    class Verlet : SimStepper
    {
        public State Integrate(State oldState, float timeStep, ODEs.ODE ode)
        {
            return Integrate(oldState, timeStep, ode.EvaluateForceAtState(oldState));
        }

        public State Integrate(State oldState, float timeStep, float acceleration)
        {
            float oldPos = oldState.position;

            if (oldState.prevPos == float.PositiveInfinity)
            {
                State copy = oldState;
                oldState = new SUVAT().Integrate(oldState, timeStep, acceleration);
                oldState.prevPos = copy.position;
                oldState.prevVel = copy.velocity;
                oldState.prevAccel = copy.acceleration;
                oldState.acceleration = acceleration;
            }
            else
            {
                //warm start
                oldState.prevVel = oldState.velocity;
                oldState.prevAccel = oldState.acceleration;
                oldState.acceleration = acceleration;
                oldState.position = VerletStep.Integrate(oldState.position, oldState.prevPos, timeStep, acceleration);
                oldState.velocity = Derivator.MeanValueTheoremStep.Derivate(oldState.prevPos, -timeStep, oldState.position, timeStep);
                oldState.prevPos = oldPos;                
            }

            return oldState;
        }

        public override string ToString()
        {
            return "Verlet";
        }

        public int ForceEvaluations
        {
            get
            {
                return 1;
            }
        }
    }
}