Windows Phone: Are you Game? Part 1

/*
* Farseer Physics Engine based on Box2D.XNA port:
* Copyright (c) 2010 Ian Qvist
* 
* Box2D.XNA port of Box2D:
* Copyright (c) 2009 Brandon Furtwangler, Nathan Furtwangler
*
* Original source Box2D:
* Copyright (c) 2006-2009 Erin Catto http://www.gphysics.com 
* 
* This software is provided 'as-is', without any express or implied 
* warranty.  In no event will the authors be held liable for any damages 
* arising from the use of this software. 
* Permission is granted to anyone to use this software for any purpose, 
* including commercial applications, and to alter it and redistribute it 
* freely, subject to the following restrictions: 
* 1. The origin of this software must not be misrepresented; you must not 
* claim that you wrote the original software. If you use this software 
* in a product, an acknowledgment in the product documentation would be 
* appreciated but is not required. 
* 2. Altered source versions must be plainly marked as such, and must not be 
* misrepresented as being the original software. 
* 3. This notice may not be removed or altered from any source distribution. 
*/

using System;
using System.Diagnostics;
using FarseerPhysics.Common;
using FarseerPhysics.Dynamics.Contacts;
using FarseerPhysics.Dynamics.Joints;
using Microsoft.Xna.Framework;

namespace FarseerPhysics.Dynamics
{
    /// <summary>
    /// This is an internal class.
    /// </summary>
    public class Island
    {
        public Body[] Bodies;
        public int BodyCount;
        public int ContactCount;
        public int JointCount;
        private int _bodyCapacity;
        private int _contactCapacity;
        private ContactManager _contactManager;
        private ContactSolver _contactSolver = new ContactSolver();
        private Contact[] _contacts;
        private int _jointCapacity;
        private Joint[] _joints;
        public float JointUpdateTime;

        private const float LinTolSqr = Settings.LinearSleepTolerance * Settings.LinearSleepTolerance;
        private const float AngTolSqr = Settings.AngularSleepTolerance * Settings.AngularSleepTolerance;

#if (!SILVERLIGHT)
        private Stopwatch _watch = new Stopwatch();
#endif

        public void Reset(int bodyCapacity, int contactCapacity, int jointCapacity, ContactManager contactManager)
        {
            _bodyCapacity = bodyCapacity;
            _contactCapacity = contactCapacity;
            _jointCapacity = jointCapacity;

            BodyCount = 0;
            ContactCount = 0;
            JointCount = 0;

            _contactManager = contactManager;

            if (Bodies == null || Bodies.Length < bodyCapacity)
            {
                Bodies = new Body[bodyCapacity];
            }

            if (_contacts == null || _contacts.Length < contactCapacity)
            {
                _contacts = new Contact[contactCapacity * 2];
            }

            if (_joints == null || _joints.Length < jointCapacity)
            {
                _joints = new Joint[jointCapacity * 2];
            }
        }

        public void Clear()
        {
            BodyCount = 0;
            ContactCount = 0;
            JointCount = 0;
        }

        private float _tmpTime;

        public void Solve(ref TimeStep step, ref Vector2 gravity)
        {
            // Integrate velocities and apply damping.
            for (int i = 0; i < BodyCount; ++i)
            {
                Body b = Bodies[i];

                if (b.BodyType != BodyType.Dynamic)
                {
                    continue;
                }

                // Integrate velocities.
                // FPE 3 only - Only apply gravity if the body wants it.
                if (b.IgnoreGravity)
                {
                    b.LinearVelocityInternal.X += step.dt * (b.InvMass * b.Force.X);
                    b.LinearVelocityInternal.Y += step.dt * (b.InvMass * b.Force.Y);
                    b.AngularVelocityInternal += step.dt * b.InvI * b.Torque;
                }
                else
                {
                    b.LinearVelocityInternal.X += step.dt * (gravity.X + b.InvMass * b.Force.X);
                    b.LinearVelocityInternal.Y += step.dt * (gravity.Y + b.InvMass * b.Force.Y);
                    b.AngularVelocityInternal += step.dt * b.InvI * b.Torque;
                }

                // Apply damping.
                // ODE: dv/dt + c * v = 0
                // Solution: v(t) = v0 * exp(-c * t)
                // Time step: v(t + dt) = v0 * exp(-c * (t + dt)) = v0 * exp(-c * t) * exp(-c * dt) = v * exp(-c * dt)
                // v2 = exp(-c * dt) * v1
                // Taylor expansion:
                // v2 = (1.0f - c * dt) * v1
                b.LinearVelocityInternal *= MathUtils.Clamp(1.0f - step.dt * b.LinearDamping, 0.0f, 1.0f);
                b.AngularVelocityInternal *= MathUtils.Clamp(1.0f - step.dt * b.AngularDamping, 0.0f, 1.0f);
            }

            // Partition contacts so that contacts with static bodies are solved last.
            int i1 = -1;
            for (int i2 = 0; i2 < ContactCount; ++i2)
            {
                Fixture fixtureA = _contacts[i2].FixtureA;
                Fixture fixtureB = _contacts[i2].FixtureB;
                Body bodyA = fixtureA.Body;
                Body bodyB = fixtureB.Body;
                bool nonStatic = bodyA.BodyType != BodyType.Static && bodyB.BodyType != BodyType.Static;
                if (nonStatic)
                {
                    ++i1;

                    //TODO: Only swap if they are not the same? see http://code.google.com/p/box2d/issues/detail?id=162
                    Contact tmp = _contacts[i1];
                    _contacts[i1] = _contacts[i2];
                    _contacts[i2] = tmp;
                }
            }

            // Initialize velocity constraints.
            _contactSolver.Reset(_contacts, ContactCount, step.dtRatio, Settings.EnableWarmstarting);
            _contactSolver.InitializeVelocityConstraints();

            if (Settings.EnableWarmstarting)
            {
                _contactSolver.WarmStart();
            }

#if (!SILVERLIGHT)
            if (Settings.EnableDiagnostics)
            {
                _watch.Start();
                _tmpTime = 0;
            }
#endif

            for (int i = 0; i < JointCount; ++i)
            {
                if (_joints[i].Enabled)
                    _joints[i].InitVelocityConstraints(ref step);
            }

#if (!SILVERLIGHT)
            if (Settings.EnableDiagnostics)
            {
                _tmpTime += _watch.ElapsedTicks;
            }
#endif

            // Solve velocity constraints.
            for (int i = 0; i < Settings.VelocityIterations; ++i)
            {
#if (!SILVERLIGHT)
                if (Settings.EnableDiagnostics)
                    _watch.Start();
#endif
                for (int j = 0; j < JointCount; ++j)
                {
                    Joint joint = _joints[j];

                    if (!joint.Enabled)
                        continue;

                    joint.SolveVelocityConstraints(ref step);
                    joint.Validate(step.inv_dt);
                }

#if (!SILVERLIGHT)
                if (Settings.EnableDiagnostics)
                {
                    _watch.Stop();
                    _tmpTime += _watch.ElapsedTicks;
                    _watch.Reset();
                }
#endif

                _contactSolver.SolveVelocityConstraints();
            }

            // Post-solve (store impulses for warm starting).
            _contactSolver.StoreImpulses();

            // Integrate positions.
            for (int i = 0; i < BodyCount; ++i)
            {
                Body b = Bodies[i];

                if (b.BodyType == BodyType.Static)
                {
                    continue;
                }

                // Check for large velocities.
                float translationX = step.dt * b.LinearVelocityInternal.X;
                float translationY = step.dt * b.LinearVelocityInternal.Y;
                float result = translationX * translationX + translationY * translationY;

                if (result > Settings.MaxTranslationSquared)
                {
                    float sq = (float)Math.Sqrt(result);

                    float ratio = Settings.MaxTranslation / sq;
                    b.LinearVelocityInternal.X *= ratio;
                    b.LinearVelocityInternal.Y *= ratio;
                }

                float rotation = step.dt * b.AngularVelocityInternal;
                if (rotation * rotation > Settings.MaxRotationSquared)
                {
                    float ratio = Settings.MaxRotation / Math.Abs(rotation);
                    b.AngularVelocityInternal *= ratio;
                }

                // Store positions for continuous collision.
                b.Sweep.C0.X = b.Sweep.C.X;
                b.Sweep.C0.Y = b.Sweep.C.Y;
                b.Sweep.A0 = b.Sweep.A;

                // Integrate
                b.Sweep.C.X += step.dt * b.LinearVelocityInternal.X;
                b.Sweep.C.Y += step.dt * b.LinearVelocityInternal.Y;
                b.Sweep.A += step.dt * b.AngularVelocityInternal;

                // Compute new transform
                b.SynchronizeTransform();

                // Note: shapes are synchronized later.
            }

            // Iterate over constraints.
            for (int i = 0; i < Settings.PositionIterations; ++i)
            {
                bool contactsOkay = _contactSolver.SolvePositionConstraints(Settings.ContactBaumgarte);
                bool jointsOkay = true;

#if (!SILVERLIGHT)
                if (Settings.EnableDiagnostics)
                    _watch.Start();
#endif
                for (int j = 0; j < JointCount; ++j)
                {
                    Joint joint = _joints[j];
                    if (!joint.Enabled)
                        continue;

                    bool jointOkay = joint.SolvePositionConstraints();
                    jointsOkay = jointsOkay && jointOkay;
                }

#if (!SILVERLIGHT)
                if (Settings.EnableDiagnostics)
                {
                    _watch.Stop();
                    _tmpTime += _watch.ElapsedTicks;
                    _watch.Reset();
                }
#endif
                if (contactsOkay && jointsOkay)
                {
                    // Exit early if the position errors are small.
                    break;
                }
            }

#if (!SILVERLIGHT)
            if (Settings.EnableDiagnostics)
            {
                JointUpdateTime = _tmpTime;
            }
#endif

            Report(_contactSolver.Constraints);

            if (Settings.AllowSleep)
            {
                float minSleepTime = Settings.MaxFloat;

                for (int i = 0; i < BodyCount; ++i)
                {
                    Body b = Bodies[i];
                    if (b.BodyType == BodyType.Static)
                    {
                        continue;
                    }

                    if ((b.Flags & BodyFlags.AutoSleep) == 0)
                    {
                        b.SleepTime = 0.0f;
                        minSleepTime = 0.0f;
                    }

                    if ((b.Flags & BodyFlags.AutoSleep) == 0 ||
                        b.AngularVelocityInternal * b.AngularVelocityInternal > AngTolSqr ||
                        Vector2.Dot(b.LinearVelocityInternal, b.LinearVelocityInternal) > LinTolSqr)
                    {
                        b.SleepTime = 0.0f;
                        minSleepTime = 0.0f;
                    }
                    else
                    {
                        b.SleepTime += step.dt;
                        minSleepTime = Math.Min(minSleepTime, b.SleepTime);
                    }
                }

                if (minSleepTime >= Settings.TimeToSleep)
                {
                    for (int i = 0; i < BodyCount; ++i)
                    {
                        Body b = Bodies[i];
                        b.Awake = false;
                    }
                }
            }
        }

        internal void SolveTOI(ref TimeStep subStep)
        {
            _contactSolver.Reset(_contacts, ContactCount, subStep.dtRatio, false);

            // Solve position constraints.
            const float kTOIBaumgarte = 0.75f;
            for (int i = 0; i < Settings.TOIPositionIterations; ++i)
            {
                bool contactsOkay = _contactSolver.SolvePositionConstraints(kTOIBaumgarte);
                if (contactsOkay)
                {
                    break;
                }

                if (i == Settings.TOIPositionIterations - 1)
                {
                    i += 0;
                }
            }

            // Leap of faith to new safe state.
            for (int i = 0; i < BodyCount; ++i)
            {
                Body body = Bodies[i];
                body.Sweep.A0 = body.Sweep.A;
                body.Sweep.C0 = body.Sweep.C;
            }

            // No warm starting is needed for TOI events because warm
            // starting impulses were applied in the discrete solver.
            _contactSolver.InitializeVelocityConstraints();

            // Solve velocity constraints.
            for (int i = 0; i < Settings.TOIVelocityIterations; ++i)
            {
                _contactSolver.SolveVelocityConstraints();
            }

            // Don't store the TOI contact forces for warm starting
            // because they can be quite large.

            // Integrate positions.
            for (int i = 0; i < BodyCount; ++i)
            {
                Body b = Bodies[i];

                if (b.BodyType == BodyType.Static)
                {
                    continue;
                }

                // Check for large velocities.
                float translationx = subStep.dt * b.LinearVelocityInternal.X;
                float translationy = subStep.dt * b.LinearVelocityInternal.Y;
                float dot = translationx * translationx + translationy * translationy;
                if (dot > Settings.MaxTranslationSquared)
                {
                    float norm = 1f / (float)Math.Sqrt(dot);
                    float value = Settings.MaxTranslation * subStep.inv_dt;
                    b.LinearVelocityInternal.X = value * (translationx * norm);
                    b.LinearVelocityInternal.Y = value * (translationy * norm);
                }

                float rotation = subStep.dt * b.AngularVelocity;
                if (rotation * rotation > Settings.MaxRotationSquared)
                {
                    if (rotation < 0.0)
                    {
                        b.AngularVelocityInternal = -subStep.inv_dt * Settings.MaxRotation;
                    }
                    else
                    {
                        b.AngularVelocityInternal = subStep.inv_dt * Settings.MaxRotation;
                    }
                }

                // Integrate
                b.Sweep.C.X += subStep.dt * b.LinearVelocityInternal.X;
                b.Sweep.C.Y += subStep.dt * b.LinearVelocityInternal.Y;
                b.Sweep.A += subStep.dt * b.AngularVelocityInternal;

                // Compute new transform
                b.SynchronizeTransform();

                // Note: shapes are synchronized later.
            }

            Report(_contactSolver.Constraints);
        }

        public void Add(Body body)
        {
            Debug.Assert(BodyCount < _bodyCapacity);
            Bodies[BodyCount++] = body;
        }

        public void Add(Contact contact)
        {
            Debug.Assert(ContactCount < _contactCapacity);
            _contacts[ContactCount++] = contact;
        }

        public void Add(Joint joint)
        {
            Debug.Assert(JointCount < _jointCapacity);
            _joints[JointCount++] = joint;
        }

        private void Report(ContactConstraint[] constraints)
        {
            if (_contactManager == null)
                return;

            for (int i = 0; i < ContactCount; ++i)
            {
                Contact c = _contacts[i];

                if (c.FixtureA.AfterCollision != null)
                    c.FixtureA.AfterCollision(c.FixtureA, c.FixtureB, c);

                if (c.FixtureB.AfterCollision != null)
                    c.FixtureB.AfterCollision(c.FixtureB, c.FixtureA, c);

                if (_contactManager.PostSolve != null)
                {
                    ContactConstraint cc = constraints[i];

                    _contactManager.PostSolve(c, cc);
                }
            }
        }
    }
}