void b2ContactSolver::InitializeVelocityConstraints()

in Box2D/Dynamics/Contacts/b2ContactSolver.cpp [138:247]
81 lines of code
8 McCabe index (conditional complexity)

void b2ContactSolver::InitializeVelocityConstraints()
{
	for (int32 i = 0; i < m_count; ++i)
	{
		b2ContactVelocityConstraint* vc = m_velocityConstraints + i;
		b2ContactPositionConstraint* pc = m_positionConstraints + i;

		float32 radiusA = pc->radiusA;
		float32 radiusB = pc->radiusB;
		b2Manifold* manifold = m_contacts[vc->contactIndex]->GetManifold();

		int32 indexA = vc->indexA;
		int32 indexB = vc->indexB;

		float32 mA = vc->invMassA;
		float32 mB = vc->invMassB;
		float32 iA = vc->invIA;
		float32 iB = vc->invIB;
		b2Vec2 localCenterA = pc->localCenterA;
		b2Vec2 localCenterB = pc->localCenterB;

		b2Vec2 cA = m_positions[indexA].c;
		float32 aA = m_positions[indexA].a;
		b2Vec2 vA = m_velocities[indexA].v;
		float32 wA = m_velocities[indexA].w;

		b2Vec2 cB = m_positions[indexB].c;
		float32 aB = m_positions[indexB].a;
		b2Vec2 vB = m_velocities[indexB].v;
		float32 wB = m_velocities[indexB].w;

		b2Assert(manifold->pointCount > 0);

		b2Transform xfA, xfB;
		xfA.q.Set(aA);
		xfB.q.Set(aB);
		xfA.p = cA - b2Mul(xfA.q, localCenterA);
		xfB.p = cB - b2Mul(xfB.q, localCenterB);

		b2WorldManifold worldManifold;
		worldManifold.Initialize(manifold, xfA, radiusA, xfB, radiusB);

		vc->normal = worldManifold.normal;

		int32 pointCount = vc->pointCount;
		for (int32 j = 0; j < pointCount; ++j)
		{
			b2VelocityConstraintPoint* vcp = vc->points + j;

			vcp->rA = worldManifold.points[j] - cA;
			vcp->rB = worldManifold.points[j] - cB;

			float32 rnA = b2Cross(vcp->rA, vc->normal);
			float32 rnB = b2Cross(vcp->rB, vc->normal);

			float32 kNormal = mA + mB + iA * rnA * rnA + iB * rnB * rnB;

			vcp->normalMass = kNormal > 0.0f ? 1.0f / kNormal : 0.0f;

			b2Vec2 tangent = b2Cross(vc->normal, 1.0f);

			float32 rtA = b2Cross(vcp->rA, tangent);
			float32 rtB = b2Cross(vcp->rB, tangent);

			float32 kTangent = mA + mB + iA * rtA * rtA + iB * rtB * rtB;

			vcp->tangentMass = kTangent > 0.0f ? 1.0f /  kTangent : 0.0f;

			// Setup a velocity bias for restitution.
			vcp->velocityBias = 0.0f;
			float32 vRel = b2Dot(vc->normal, vB + b2Cross(wB, vcp->rB) - vA - b2Cross(wA, vcp->rA));
			if (vRel < -b2_velocityThreshold)
			{
				vcp->velocityBias = -vc->restitution * vRel;
			}
		}

		// If we have two points, then prepare the block solver.
		if (vc->pointCount == 2)
		{
			b2VelocityConstraintPoint* vcp1 = vc->points + 0;
			b2VelocityConstraintPoint* vcp2 = vc->points + 1;

			float32 rn1A = b2Cross(vcp1->rA, vc->normal);
			float32 rn1B = b2Cross(vcp1->rB, vc->normal);
			float32 rn2A = b2Cross(vcp2->rA, vc->normal);
			float32 rn2B = b2Cross(vcp2->rB, vc->normal);

			float32 k11 = mA + mB + iA * rn1A * rn1A + iB * rn1B * rn1B;
			float32 k22 = mA + mB + iA * rn2A * rn2A + iB * rn2B * rn2B;
			float32 k12 = mA + mB + iA * rn1A * rn2A + iB * rn1B * rn2B;

			// Ensure a reasonable condition number.
			const float32 k_maxConditionNumber = 1000.0f;
			if (k11 * k11 < k_maxConditionNumber * (k11 * k22 - k12 * k12))
			{
				// K is safe to invert.
				vc->K.ex.Set(k11, k12);
				vc->K.ey.Set(k12, k22);
				vc->normalMass = vc->K.GetInverse();
			}
			else
			{
				// The constraints are redundant, just use one.
				// TODO_ERIN use deepest?
				vc->pointCount = 1;
			}
		}
	}
}