ContactConstraintPart.h

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// Jolt Physics Library (https://github.com/jrouwe/JoltPhysics)
// SPDX-FileCopyrightText: 2026 Jorrit Rouwe
// SPDX-License-Identifier: MIT
 
#pragma once
 
#include <Jolt/Physics/Body/Body.h>
 
JPH_NAMESPACE_BEGIN
 
template <EMotionType Type1>
class ContactConstraintPart1
{
public:
    inline void                 Deactivate()
    {
        mEffectiveMass = 0.0f;
        mTotalLambda = 0.0f;
    }
 
    inline bool                 IsActive() const
    {
        return mEffectiveMass != 0.0f;
    }
 
    inline void                 SetTotalLambda(float inLambda)
    {
        mTotalLambda = inLambda;
    }
 
    inline float                GetTotalLambda() const
    {
        return mTotalLambda;
    }
 
protected:
    // Note: Constructor will not be called
    float                       mEffectiveMass;
    float                       mTotalLambda;
    float                       mBias;
};
 
template <>
class ContactConstraintPart1<EMotionType::Kinematic> : public ContactConstraintPart1<EMotionType::Static>
{
protected:
    // Note: Constructor will not be called
    Float3                      mR1PlusUxAxis;
};
 
template <>
class ContactConstraintPart1<EMotionType::Dynamic> : public ContactConstraintPart1<EMotionType::Kinematic>
{
protected:
    // Note: Constructor will not be called
    Float3                      mInvI1_R1PlusUxAxis;
};
 
template <EMotionType Type2>
class ContactConstraintPart2
{
};
 
template <>
class ContactConstraintPart2<EMotionType::Kinematic> : public ContactConstraintPart2<EMotionType::Static>
{
protected:
    // Note: Constructor will not be called
    Float3                      mR2xAxis;
};
 
template <>
class ContactConstraintPart2<EMotionType::Dynamic> : public ContactConstraintPart2<EMotionType::Kinematic>
{
protected:
    // Note: Constructor will not be called
    Float3                      mInvI2_R2xAxis;
};
 
static_assert(sizeof(ContactConstraintPart1<EMotionType::Kinematic>) == 3 * sizeof(float) + sizeof(Float3));
static_assert(sizeof(ContactConstraintPart1<EMotionType::Dynamic>) == 3 * sizeof(float) + 2 * sizeof(Float3));
static_assert(sizeof(ContactConstraintPart2<EMotionType::Kinematic>) == sizeof(Float3));
static_assert(sizeof(ContactConstraintPart2<EMotionType::Dynamic>) == 2 * sizeof(Float3));
 
template <EMotionType Type1, EMotionType Type2>
class ContactConstraintPart : public ContactConstraintPart1<Type1>, public ContactConstraintPart2<Type2>
{
private:
    JPH_INLINE bool             ApplyVelocityStep(MotionProperties *ioMotionProperties1, float inInvMass1, MotionProperties *ioMotionProperties2, float inInvMass2, Vec3Arg inWorldSpaceAxis, float inLambda) const
    {
        if (inLambda != 0.0f)
        {
            if constexpr (Type1 == EMotionType::Dynamic)
            {
                ioMotionProperties1->SubLinearVelocityStep((inLambda * inInvMass1) * inWorldSpaceAxis);
                ioMotionProperties1->SubAngularVelocityStep(inLambda * Vec3::sLoadFloat3Unsafe(this->mInvI1_R1PlusUxAxis));
            }
            if constexpr (Type2 == EMotionType::Dynamic)
            {
                ioMotionProperties2->AddLinearVelocityStep((inLambda * inInvMass2) * inWorldSpaceAxis);
                ioMotionProperties2->AddAngularVelocityStep(inLambda * Vec3::sLoadFloat3Unsafe(this->mInvI2_R2xAxis));
            }
            return true;
        }
 
        return false;
    }
 
public:
    JPH_INLINE void             CalculateConstraintProperties(float inInvMass1, Mat44Arg inInvI1, Vec3Arg inR1PlusU, float inInvMass2, Mat44Arg inInvI2, Vec3Arg inR2, Vec3Arg inWorldSpaceAxis, float inBias = 0.0f)
    {
        JPH_ASSERT(inWorldSpaceAxis.IsNormalized(1.0e-5f));
 
        // Calculate inverse effective mass: K = J M^-1 J^T
        float inv_effective_mass;
 
        if constexpr (Type1 != EMotionType::Static)
        {
            Vec3 r1_plus_u_x_axis = inR1PlusU.Cross(inWorldSpaceAxis);
            r1_plus_u_x_axis.StoreFloat3(&this->mR1PlusUxAxis);
 
            if constexpr (Type1 == EMotionType::Dynamic)
            {
                Vec3 invi1_r1_plus_u_x_axis = inInvI1.Multiply3x3(r1_plus_u_x_axis);
                invi1_r1_plus_u_x_axis.StoreFloat3(&this->mInvI1_R1PlusUxAxis);
 
                inv_effective_mass = inInvMass1 + invi1_r1_plus_u_x_axis.Dot(r1_plus_u_x_axis);
            }
            else
                inv_effective_mass = 0.0f;
        }
        else
            inv_effective_mass = 0.0f;
 
        if constexpr (Type2 != EMotionType::Static)
        {
            Vec3 r2_x_axis = inR2.Cross(inWorldSpaceAxis);
            r2_x_axis.StoreFloat3(&this->mR2xAxis);
 
            if constexpr (Type2 == EMotionType::Dynamic)
            {
                Vec3 invi2_r2_x_axis = inInvI2.Multiply3x3(r2_x_axis);
                invi2_r2_x_axis.StoreFloat3(&this->mInvI2_R2xAxis);
 
                inv_effective_mass += inInvMass2 + invi2_r2_x_axis.Dot(r2_x_axis);
            }
        }
 
        if (inv_effective_mass == 0.0f)
            this->Deactivate();
        else
        {
            this->mEffectiveMass = 1.0f / inv_effective_mass;
            this->mBias = inBias;
        }
    }
 
    inline void                 WarmStart(MotionProperties *ioMotionProperties1, float inInvMass1, MotionProperties *ioMotionProperties2, float inInvMass2, Vec3Arg inWorldSpaceAxis, float inWarmStartImpulseRatio)
    {
        this->mTotalLambda *= inWarmStartImpulseRatio;
 
        ApplyVelocityStep(ioMotionProperties1, inInvMass1, ioMotionProperties2, inInvMass2, inWorldSpaceAxis, this->mTotalLambda);
    }
 
    JPH_INLINE float            SolveVelocityConstraintGetTotalLambda(const MotionProperties *ioMotionProperties1, const MotionProperties *ioMotionProperties2, Vec3Arg inWorldSpaceAxis) const
    {
        // Calculate jacobian multiplied by linear velocity
        float jv;
        if constexpr (Type1 != EMotionType::Static && Type2 != EMotionType::Static)
            jv = inWorldSpaceAxis.Dot(ioMotionProperties1->GetLinearVelocity() - ioMotionProperties2->GetLinearVelocity());
        else if constexpr (Type1 != EMotionType::Static)
            jv = inWorldSpaceAxis.Dot(ioMotionProperties1->GetLinearVelocity());
        else if constexpr (Type2 != EMotionType::Static)
            jv = inWorldSpaceAxis.Dot(-ioMotionProperties2->GetLinearVelocity());
        else
            JPH_ASSERT(false); // Static vs static is nonsensical!
 
        // Calculate jacobian multiplied by angular velocity
        if constexpr (Type1 != EMotionType::Static)
            jv += Vec3::sLoadFloat3Unsafe(this->mR1PlusUxAxis).Dot(ioMotionProperties1->GetAngularVelocity());
        if constexpr (Type2 != EMotionType::Static)
            jv -= Vec3::sLoadFloat3Unsafe(this->mR2xAxis).Dot(ioMotionProperties2->GetAngularVelocity());
 
        // Lagrange multiplier is:
        //
        // lambda = -K^-1 (J v + b)
        float lambda = this->mEffectiveMass * (jv - this->mBias);
 
        // Return the total accumulated lambda
        return this->mTotalLambda + lambda;
    }
 
    JPH_INLINE bool             SolveVelocityConstraintApplyLambda(MotionProperties *ioMotionProperties1, float inInvMass1, MotionProperties *ioMotionProperties2, float inInvMass2, Vec3Arg inWorldSpaceAxis, float inTotalLambda)
    {
        float delta_lambda = inTotalLambda - this->mTotalLambda; // Calculate change in lambda
        this->mTotalLambda = inTotalLambda; // Store accumulated impulse
 
        return ApplyVelocityStep(ioMotionProperties1, inInvMass1, ioMotionProperties2, inInvMass2, inWorldSpaceAxis, delta_lambda);
    }
 
    JPH_INLINE bool             SolveVelocityConstraint(MotionProperties *ioMotionProperties1, float inInvMass1, MotionProperties *ioMotionProperties2, float inInvMass2, Vec3Arg inWorldSpaceAxis, float inMinLambda, float inMaxLambda)
    {
        float total_lambda = SolveVelocityConstraintGetTotalLambda(ioMotionProperties1, ioMotionProperties2, inWorldSpaceAxis);
 
        // Clamp impulse to specified range
        total_lambda = Clamp(total_lambda, inMinLambda, inMaxLambda);
 
        return SolveVelocityConstraintApplyLambda(ioMotionProperties1, inInvMass1, ioMotionProperties2, inInvMass2, inWorldSpaceAxis, total_lambda);
    }
 
    inline bool                 SolvePositionConstraint(Body &ioBody1, float inInvMass1, Body &ioBody2, float inInvMass2, Vec3Arg inWorldSpaceAxis, float inC, float inBaumgarte) const
    {
        if (inC != 0.0f)
        {
            float lambda = -this->mEffectiveMass * inBaumgarte * inC;
            if constexpr (Type1 == EMotionType::Dynamic)
            {
                ioBody1.SubPositionStep((lambda * inInvMass1) * inWorldSpaceAxis);
                ioBody1.SubRotationStep(lambda * Vec3::sLoadFloat3Unsafe(this->mInvI1_R1PlusUxAxis));
            }
            if constexpr (Type2 == EMotionType::Dynamic)
            {
                ioBody2.AddPositionStep((lambda * inInvMass2) * inWorldSpaceAxis);
                ioBody2.AddRotationStep(lambda * Vec3::sLoadFloat3Unsafe(this->mInvI2_R2xAxis));
            }
            return true;
        }
 
        return false;
    }
};
 
static_assert(sizeof(ContactConstraintPart<EMotionType::Dynamic, EMotionType::Dynamic>) == 3 * sizeof(float) + 4 * sizeof(Float3));
static_assert(sizeof(ContactConstraintPart<EMotionType::Dynamic, EMotionType::Kinematic>) == 3 * sizeof(float) + 3 * sizeof(Float3));
static_assert(sizeof(ContactConstraintPart<EMotionType::Dynamic, EMotionType::Static>) == 3 * sizeof(float) + 2 * sizeof(Float3));
static_assert(sizeof(ContactConstraintPart<EMotionType::Kinematic, EMotionType::Kinematic>) == 3 * sizeof(float) + 2 * sizeof(Float3));
static_assert(sizeof(ContactConstraintPart<EMotionType::Kinematic, EMotionType::Static>) == 3 * sizeof(float) + sizeof(Float3));
static_assert(sizeof(ContactConstraintPart<EMotionType::Static, EMotionType::Dynamic>) == 3 * sizeof(float) + 2 * sizeof(Float3));
static_assert(sizeof(ContactConstraintPart<EMotionType::Static, EMotionType::Kinematic>) == 3 * sizeof(float) + sizeof(Float3));
static_assert(sizeof(ContactConstraintPart<EMotionType::Static, EMotionType::Static>) == 3 * sizeof(float));
 
class ConcreteContactConstraintPart
{
public:
                                ConcreteContactConstraintPart() : mDD() { }
                                ~ConcreteContactConstraintPart() = default;
 
    inline bool                 SolveVelocityConstraint(Body &inBody1, float inInvMass1, float inInvInertiaScale1, Vec3Arg inR1PlusU, Body &inBody2, float inInvMass2, float inInvInertiaScale2, Vec3Arg inR2, Vec3Arg inWorldSpaceAxis, float inBias, float inMinLambda, float inMaxLambda)
    {
        // Dispatch to the correct templated form
        switch (inBody1.GetMotionType())
        {
        case EMotionType::Dynamic:
            {
                Mat44 inv_i1 = inInvInertiaScale1 * inBody1.GetInverseInertia();
                switch (inBody2.GetMotionType())
                {
                case EMotionType::Dynamic:
                    mDD.CalculateConstraintProperties(inInvMass1, inv_i1, inR1PlusU, inInvMass2, inInvInertiaScale2 * inBody2.GetInverseInertia(), inR2, inWorldSpaceAxis, inBias);
                    return mDD.SolveVelocityConstraint(inBody1.GetMotionPropertiesUnchecked(), inInvMass1, inBody2.GetMotionPropertiesUnchecked(), inInvMass2, inWorldSpaceAxis, inMinLambda, inMaxLambda);
 
                case EMotionType::Kinematic:
                    mDK.CalculateConstraintProperties(inInvMass1, inv_i1, inR1PlusU, 0 /* Will not be used */, Mat44() /* Will not be used */, inR2, inWorldSpaceAxis, inBias);
                    return mDK.SolveVelocityConstraint(inBody1.GetMotionPropertiesUnchecked(), inInvMass1, inBody2.GetMotionPropertiesUnchecked(), inInvMass2, inWorldSpaceAxis, inMinLambda, inMaxLambda);
 
                case EMotionType::Static:
                    mDS.CalculateConstraintProperties(inInvMass1, inv_i1, inR1PlusU, 0 /* Will not be used */, Mat44() /* Will not be used */, inR2, inWorldSpaceAxis, inBias);
                    return mDS.SolveVelocityConstraint(inBody1.GetMotionPropertiesUnchecked(), inInvMass1, nullptr /* Unused */, 0.0f /* Unused */, inWorldSpaceAxis, inMinLambda, inMaxLambda);
 
                default:
                    JPH_ASSERT(false);
                    break;
                }
                break;
            }
 
        case EMotionType::Kinematic:
            JPH_ASSERT(inBody2.IsDynamic());
            mKD.CalculateConstraintProperties(0 /* Will not be used */, Mat44() /* Will not be used */, inR1PlusU, inInvMass2, inInvInertiaScale2 * inBody2.GetInverseInertia(), inR2, inWorldSpaceAxis, inBias);
            return mKD.SolveVelocityConstraint(inBody1.GetMotionPropertiesUnchecked(), inInvMass1, inBody2.GetMotionPropertiesUnchecked(), inInvMass2, inWorldSpaceAxis, inMinLambda, inMaxLambda);
 
        case EMotionType::Static:
            JPH_ASSERT(inBody2.IsDynamic());
            mSD.CalculateConstraintProperties(0 /* Will not be used */, Mat44() /* Will not be used */, inR1PlusU, inInvMass2, inInvInertiaScale2 * inBody2.GetInverseInertia(), inR2, inWorldSpaceAxis, inBias);
            return mSD.SolveVelocityConstraint(nullptr /* Unused */, 0.0f /* Unused */, inBody2.GetMotionPropertiesUnchecked(), inInvMass2, inWorldSpaceAxis, inMinLambda, inMaxLambda);
 
        default:
            JPH_ASSERT(false);
            break;
        }
 
        return false;
    }
 
    inline float                GetTotalLambda() const
    {
        return mDD.GetTotalLambda();
    }
 
private:
    union
    {
        ContactConstraintPart<EMotionType::Dynamic, EMotionType::Dynamic>   mDD;
        ContactConstraintPart<EMotionType::Dynamic, EMotionType::Kinematic> mDK;
        ContactConstraintPart<EMotionType::Dynamic, EMotionType::Static>    mDS;
        ContactConstraintPart<EMotionType::Kinematic, EMotionType::Dynamic> mKD;
        ContactConstraintPart<EMotionType::Static, EMotionType::Dynamic>    mSD;
    };
    [[maybe_unused]] float      mPadding; // Pad an extra float so that we can use Vec3::sLoadFloat3Unsafe on the last Float3 member without worrying about reading past the end of the struct
};
 
JPH_NAMESPACE_END