using System;
using System.Collections.Generic;
using System.Text;
using FRB;
using FRB.Text;

namespace RocketBrawl.Screens
{
    class TestScreen : Screen
    {
        Sprite s;
        State current;
        State previous;

        float t = 0.0f;
        float dt = 0.1f;

        float currentTime = 0.0f;
        float accumulator = 0.0f;

        Text time;


        public TestScreen(FrbEngineReferences refs)
            : base(string.Empty, string.Empty, refs)
        {
            s = sprMan.AddSprite("redball.bmp");

            current.x = 10;
            current.v = 3f;

            previous = current;

            time = textMan.AddText(string.Empty);
            time.x = -5;
            time.y = -5;
            TimeManager.StartFPSRecording();
        }

        public override void Activity()
        {
            base.Activity();


            float newTime = TimeManager.secondDifference;
            float deltaTime = newTime - currentTime;
            currentTime = newTime;

            if (deltaTime > 0.25f)
                deltaTime = 0.25f;

            accumulator += deltaTime;

            while (accumulator >= dt)
            {
                accumulator -= dt;
                previous = current;
                integrate(ref current, t, dt);
                t += dt;
            }

            State state = interpolate(ref previous, ref current, accumulator / dt);
            
            s.x = state.x;
            time.text = accumulator.ToString();
        }

        public override void Destroy()
        {
            base.Destroy();
            sprMan.RemoveSprite(s);
        }


        struct State
        {
            public float x;
            public float v;
        }

        struct Derivative
        {
            public float dx;
            public float dv;
        }

        State interpolate(ref State previous, ref State current, float alpha)
        {
            State state;
            state.x = current.x * alpha + previous.x * (1 - alpha);
            state.v = current.v * alpha + previous.v * (1 - alpha);
            return state;
        }

        float acceleration(ref State state, float t)
        {
            const float k = 10;
            const float b = 1;
            return -k * state.x - b * state.v;
        }

        Derivative evaluate(ref State initial, float t)
        {
            Derivative output;
            output.dx = initial.v;
            output.dv = acceleration(ref initial, t);
            return output;
        }

        Derivative evaluate(ref State initial, float t, float dt, ref Derivative d)
        {
            State state;
            state.x = initial.x + d.dx * dt;
            state.v = initial.v + d.dv * dt;
            Derivative output;
            output.dx = state.v;
            output.dv = acceleration(ref state, t + dt);
            return output;
        }

        void integrate(ref State state, float t, float dt)
        {
            Derivative a = evaluate(ref state, t);
            Derivative b = evaluate(ref state, t, dt * 0.5f, ref a);
            Derivative c = evaluate(ref state, t, dt * 0.5f, ref b);
            Derivative d = evaluate(ref state, t, dt, ref c);

            float dxdt = 1.0f / 6.0f * (a.dx + 2.0f * (b.dx + c.dx) + d.dx);
            float dvdt = 1.0f / 6.0f * (a.dv + 2.0f * (b.dv + c.dv) + d.dv);

            state.x = state.x + dxdt * dt;
            state.v = state.v + dvdt * dt;
        }
    }
}