using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;

namespace GeneratorTest
{
    public class Entity
    {
        public Vector2 Position;
    }

    /// <summary>
    /// This is the main type for your game
    /// </summary>
    public class Game1 : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        float currentDelta;
        List<Entity> entities = new List<Entity>();
        Texture2D tex;
        Stack<IEnumerator<object>> stack = new Stack<IEnumerator<object>>();

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
        }

        /// <summary>
        /// Allows the game to perform any initialization it needs to before starting to run.
        /// This is where it can query for any required services and load any non-graphic
        /// related content.  Calling base.Initialize will enumerate through any components
        /// and initialize them as well.
        /// </summary>
        protected override void Initialize()
        {
            base.Initialize();

            entities.Add(new Entity() { Position = new Vector2(100,100) });
            entities.Add(new Entity() { Position = new Vector2(300,100) });

            // Begin the cinematic.
            stack.Push(Cinematic().GetEnumerator());
        }

        /// <summary>
        /// LoadContent will be called once per game and is the place to load
        /// all of your content.
        /// </summary>
        protected override void LoadContent()
        {
            // Create a new SpriteBatch, which can be used to draw textures.
            spriteBatch = new SpriteBatch(GraphicsDevice);

            tex = Content.Load<Texture2D>("Sprite");
        }

        /// <summary>
        /// Allows the game to run logic such as updating the world,
        /// checking for collisions, gathering input, and playing audio.
        /// </summary>
        /// <param name="gameTime">Provides a snapshot of timing values.</param>
        protected override void Update(GameTime gameTime)
        {
            // Allows the game to exit
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
                this.Exit();

            currentDelta = (float)gameTime.ElapsedGameTime.Ticks / (float)TimeSpan.TicksPerSecond;

            // This is the complicated part.
            // When you run a generator you get an IEnumerable<>, then you have to call GetEnumerator()
            // to get an enumerator, then for an enumerator you can do the following to step through it:
            //
            // while(enumerator.MoveNext())
            // {
            //     do something with enumerator.Current
            // }
            //
            // In our case, we actually wait a frame before calling MoveNext again.
            // Also, as a nicety, if the generator returns another IEnumerable<>, treat it like a "subroutine"
            // and begin stepping it until it's over, and then return to stepping the caller.
            // This is why this resembles a programming language stack behavior so much.
            if (stack.Count > 0)
            {
                IEnumerator<object> e = stack.Peek();
                if (e.MoveNext())
                {
                    if (e.Current is IEnumerable<object>)
                    {
                        var newFrame = (IEnumerable<object>)e.Current;
                        stack.Push(newFrame.GetEnumerator());
                    }
                    // otherwise it's probably null, ignore it
                }
                else
                {
                    stack.Pop();
                }
            }

            base.Update(gameTime);
        }

        /// <summary>
        /// This is called when the game should draw itself.
        /// </summary>
        /// <param name="gameTime">Provides a snapshot of timing values.</param>
        protected override void Draw(GameTime gameTime)
        {
            GraphicsDevice.Clear(Color.CornflowerBlue);

            // Draw stuff
            spriteBatch.Begin();
            foreach (Entity ent in entities)
            {
                spriteBatch.Draw(tex, new Rectangle((int)ent.Position.X, (int)ent.Position.Y, 50, 50), Color.White);
            }
            spriteBatch.End();

            base.Draw(gameTime);
        }

        // Function that lets you wait for N seconds
        public IEnumerable<object> Wait(float seconds)
        {
            float t = 0;
            while (t < seconds)
            {
                t += currentDelta;
                yield return null;
            }
        }

        // Lerps an entity to the target position over N seconds
        public IEnumerable<object> MoveEntity(Entity ent, Vector2 targetPosition, float seconds)
        {
            Vector2 startPos = ent.Position;
            float t = 0;

            while (t < seconds)
            {
                ent.Position = Vector2.Lerp(startPos, targetPosition, t / seconds);
                t += currentDelta;
                yield return null;
            }

            ent.Position = targetPosition;
        }

        // This is the "cinematic" script
        // Notice how it almost looks like it just blocks, but it actually runs over several frames
        // Easier than dealing with an FSM
        public IEnumerable<object> Cinematic()
        {
            Entity a = entities[0];
            Entity b = entities[1];

            while (true)
            {
                yield return MoveEntity(a, new Vector2(100, 200), 1.0f);
                yield return Wait(0.25f);
                yield return MoveEntity(b, new Vector2(200, 200), 1.0f);
                yield return Wait(0.5f);
                yield return MoveEntity(b, new Vector2(100, 100), 0.5f);
                yield return MoveEntity(a, new Vector2(400, 200), 0.5f);
                yield return Wait(1.0f);
                yield return MoveEntity(a, new Vector2(100, 100), 0.2f);
                yield return MoveEntity(b, new Vector2(300, 100), 0.2f);
            }
        }
    }
}