using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.Xna.Framework;

namespace CarFire
{
    /// <summary>
    /// A class to manage the motion of objects on a grid of cells.
    /// Each update you can pass a direction and the manager will move
    /// the position to that point while also enforcing a speed limit.
    /// This class does not detect collisions, so care must be taken
    /// to only pass directions to a walkable cell during an update.
    /// </summary>
    public class MovementManager
    {
        #region Public Properties

        /// <summary>
        /// Get the current position in map coordinates.  This is the
        /// smooth, interpolated set of coordinates.
        /// </summary>
        public Vector2 Position { get { return mPosition; } }

        /// <summary>
        /// Get the grid coordinates where the object is at or
        /// is moving to.
        /// </summary>
        public Point Coordinates { get { return mCoordinates; } }

        /// <summary>
        /// Get and set the speed of movement in grid cells / second.
        /// </summary>
        public float Speed;

        #endregion


        #region Public Methods

        /// <summary>
        /// Construct a movement manager with the initial position of
        /// the thing you want to track.
        /// </summary>
        /// <param name="position">Grid coordinates.</param>
        public MovementManager(Point position)
        {
            mPosition = new Vector2((float)position.X, (float)position.Y);
            mCoordinates = position;
            mLastCoordinates = position;
            Speed = 1.0f;
        }

        /// <summary>
        /// Construct a movement manager with the initial position of
        /// the thing you want to track and its speed.
        /// </summary>
        /// <param name="position">Grid coordinates.</param>
        /// <param name="speed">Speed: Grid cells per second.</param>
        public MovementManager(Point position, float speed)
        {
            mPosition = new Vector2((float)position.X, (float)position.Y);
            mCoordinates = position;
            mLastCoordinates = position;
            Speed = speed;
        }


        /// <summary>
        /// Update the movement manager with the timeslice and no directions.
        /// </summary>
        /// <param name="timeSpan">The timeslice.</param>
        public void Update(TimeSpan timeSpan)
        {
            Update(timeSpan, false, false, false, false);
        }

        /// <summary>
        /// Update the movement manager with the timeslice and the directions
        /// the object is supposed to go.  The directions will be ignored if the
        /// object is currently in transit from one cell to another.
        /// </summary>
        /// <param name="timeSpan">The timeslice.</param>
        /// <param name="moveLeft">Want to move left.</param>
        /// <param name="moveRight">Want to move right.</param>
        /// <param name="moveUp">Want to move up.</param>
        /// <param name="moveDown">Want to move down.</param>
        public void Update(TimeSpan timeSpan, bool moveLeft, bool moveRight, bool moveUp, bool moveDown)
        {
            float passedTime = (float)timeSpan.TotalSeconds;

            bool requestMove = (moveLeft ^ moveRight) || (moveUp ^ moveDown);
            if (!mIsMoving && requestMove)
            {
                UpdateCoordinates(moveLeft, moveRight, moveUp, moveDown);

                mIsMoving = true;
                mTimeAccumulator = passedTime;

                RecalculatePosition(mTimeAccumulator / mInverseSpeed);
            }
            else if (mIsMoving)
            {
                mTimeAccumulator += passedTime;

                float alpha = mTimeAccumulator / mInverseSpeed;
                if (alpha >= 1.0f)
                {
                    if (requestMove)
                    {
                        mTimeAccumulator = mTimeAccumulator - mInverseSpeed;
                        alpha = mTimeAccumulator / mInverseSpeed;

                        UpdateCoordinates(moveLeft, moveRight, moveUp, moveDown);
                    }
                    else
                    {
                        mIsMoving = false;
                        alpha = 1.0f;
                    }
                }

                RecalculatePosition(alpha);
            }
        }


        /// <summary>
        /// Helper method to get neighbor cells from a point and directions.
        /// </summary>
        /// <param name="point">The point.</param>
        /// <param name="left">To the left.</param>
        /// <param name="right">To the right.</param>
        /// <param name="up">Above.</param>
        /// <param name="down">Below.</param>
        /// <returns>The neighbor cell coordinates.</returns>
        public static Point GetNeighborCell(Point point, bool left, bool right, bool up, bool down)
        {
            if (left) point.X--;
            if (right) point.X++;
            if (up) point.Y--;
            if (down) point.Y++;
            return point;
        }

        #endregion


        #region Private Methods

        void RecalculatePosition(float alpha)
        {
            Console.WriteLine("last: " + mLastCoordinates + ", now: " + mCoordinates + ", alpha: " + alpha);
            mPosition.X = (float)mLastCoordinates.X + alpha * ((float)mCoordinates.X - (float)mLastCoordinates.X);
            mPosition.Y = (float)mLastCoordinates.Y + alpha * ((float)mCoordinates.Y - (float)mLastCoordinates.Y);
        }

        void UpdateCoordinates(bool moveLeft, bool moveRight, bool moveUp, bool moveDown)
        {
            mLastCoordinates = mCoordinates;
            mCoordinates = GetNeighborCell(mCoordinates, moveLeft, moveRight, moveUp, moveDown);

            if ((moveLeft && moveUp) || (moveUp && moveRight) || (moveRight && moveDown) || (moveDown && moveLeft))
            {
                mInverseSpeed = 1.4f / Speed;
            }
            else
            {
                mInverseSpeed = 1.0f / Speed;
            }
        }

        #endregion


        #region Private Variables

        Vector2 mPosition;              // Position on the viewable map.
        Point mCoordinates;             // Position on the grid.
        Point mLastCoordinates;         // Last position on the grid.
        float mInverseSpeed;            // The time it takes to move from one cell to another.
        float mTimeAccumulator;         // Amount of time passed since last move.
        bool mIsMoving;                 // Whether or not it is currently in the process of moving.

        #endregion
    }
}