Physics for Goons

Mastiff.cs

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

namespace Mastiff
{
    class Physics
    {
        public Vector2 Position { get; set; }
        public Vector2 PrevPosition { get; set; }
        public Vector2 Velocity { get; set; }
        public float Acceleration { get; set; }
        public float MaxSpeed { get; set; }
        public float DeltaTime { get; set; }

        public Physics()
        {
            Acceleration = 0.2f;  // our rate of acceleration
            Velocity = Vector2.Zero;  // our starting velocity
            MaxSpeed = 8.0f;  // can't go faster than this
            Position = new Vector2(100, 100);  // let's start here
        }

        public void ApplyPhysics(GameTime gameTime, KeyboardState keyboardState)
        {
            DeltaTime = (float)gameTime.ElapsedGameTime.TotalSeconds;

            if (keyboardState.IsKeyDown(Keys.Right))
                Accelerate(1);
            else if (keyboardState.IsKeyDown(Keys.Left))
                Accelerate(-1);
            else
                Decelerate();

            Move();
        }

        
        // Increases the ship's velocity by it's acceleration value up to it's max speed.
        public void Accelerate(int direction)
        {
            // get our acceleration vector that we'll add to our velocity vector
            Vector2 accel = new Vector2(Acceleration * direction, 0);
            // adjust velocity now
            Velocity += accel;
            // make sure we aren't going over max speed in either direction.
            // if so, set velocity to capped max speed in either direction
            if (Velocity.Length() >= MaxSpeed || Velocity.Length() <= -MaxSpeed)
            {
                Velocity.Normalize();
                Velocity = new Vector2(Velocity.X * MaxSpeed * direction, 0);
            }
        }

        // Reduces the ship's velocity by it's acceleration value to a minimum of 0.
        public void Decelerate()
        {
            // if we're really close to stopping, just go ahead and stop
            if (this.Velocity.Length() <= 0.1f || this.Velocity.Length() >= -0.1f)
            {
                this.Velocity = Vector2.Zero;
                return;
            }

            // get our acceleration vector in proper direction
            Vector2 accel = Velocity;
            accel.Normalize();
            accel *= -Acceleration;
            // adjust velocity
            Velocity += accel;
        }

        // adjust position based on velocity
        public void Move()
        {
            Position += Velocity;
        }
    }
}