void b2WeldJoint::InitVelocityConstraints()

in Box2D/Dynamics/Joints/b2WeldJoint.cpp [58:161]
71 lines of code
6 McCabe index (conditional complexity)

void b2WeldJoint::InitVelocityConstraints(const b2SolverData& data)
{
	m_indexA = m_bodyA->m_islandIndex;
	m_indexB = m_bodyB->m_islandIndex;
	m_localCenterA = m_bodyA->m_sweep.localCenter;
	m_localCenterB = m_bodyB->m_sweep.localCenter;
	m_invMassA = m_bodyA->m_invMass;
	m_invMassB = m_bodyB->m_invMass;
	m_invIA = m_bodyA->m_invI;
	m_invIB = m_bodyB->m_invI;

	float32 aA = data.positions[m_indexA].a;
	b2Vec2 vA = data.velocities[m_indexA].v;
	float32 wA = data.velocities[m_indexA].w;

	float32 aB = data.positions[m_indexB].a;
	b2Vec2 vB = data.velocities[m_indexB].v;
	float32 wB = data.velocities[m_indexB].w;

	b2Rot qA(aA), qB(aB);

	m_rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
	m_rB = b2Mul(qB, m_localAnchorB - m_localCenterB);

	// J = [-I -r1_skew I r2_skew]
	//     [ 0       -1 0       1]
	// r_skew = [-ry; rx]

	// Matlab
	// K = [ mA+r1y^2*iA+mB+r2y^2*iB,  -r1y*iA*r1x-r2y*iB*r2x,          -r1y*iA-r2y*iB]
	//     [  -r1y*iA*r1x-r2y*iB*r2x, mA+r1x^2*iA+mB+r2x^2*iB,           r1x*iA+r2x*iB]
	//     [          -r1y*iA-r2y*iB,           r1x*iA+r2x*iB,                   iA+iB]

	float32 mA = m_invMassA, mB = m_invMassB;
	float32 iA = m_invIA, iB = m_invIB;

	b2Mat33 K;
	K.ex.x = mA + mB + m_rA.y * m_rA.y * iA + m_rB.y * m_rB.y * iB;
	K.ey.x = -m_rA.y * m_rA.x * iA - m_rB.y * m_rB.x * iB;
	K.ez.x = -m_rA.y * iA - m_rB.y * iB;
	K.ex.y = K.ey.x;
	K.ey.y = mA + mB + m_rA.x * m_rA.x * iA + m_rB.x * m_rB.x * iB;
	K.ez.y = m_rA.x * iA + m_rB.x * iB;
	K.ex.z = K.ez.x;
	K.ey.z = K.ez.y;
	K.ez.z = iA + iB;

	if (m_frequencyHz > 0.0f)
	{
		K.GetInverse22(&m_mass);

		float32 invM = iA + iB;
		float32 m = invM > 0.0f ? 1.0f / invM : 0.0f;

		float32 C = aB - aA - m_referenceAngle;

		// Frequency
		float32 omega = 2.0f * b2_pi * m_frequencyHz;

		// Damping coefficient
		float32 d = 2.0f * m * m_dampingRatio * omega;

		// Spring stiffness
		float32 k = m * omega * omega;

		// magic formulas
		float32 h = data.step.dt;
		m_gamma = h * (d + h * k);
		m_gamma = m_gamma != 0.0f ? 1.0f / m_gamma : 0.0f;
		m_bias = C * h * k * m_gamma;

		invM += m_gamma;
		m_mass.ez.z = invM != 0.0f ? 1.0f / invM : 0.0f;
	}
	else
	{
		K.GetSymInverse33(&m_mass);
		m_gamma = 0.0f;
		m_bias = 0.0f;
	}

	if (data.step.warmStarting)
	{
		// Scale impulses to support a variable time step.
		m_impulse *= data.step.dtRatio;

		b2Vec2 P(m_impulse.x, m_impulse.y);

		vA -= mA * P;
		wA -= iA * (b2Cross(m_rA, P) + m_impulse.z);

		vB += mB * P;
		wB += iB * (b2Cross(m_rB, P) + m_impulse.z);
	}
	else
	{
		m_impulse.SetZero();
	}

	data.velocities[m_indexA].v = vA;
	data.velocities[m_indexA].w = wA;
	data.velocities[m_indexB].v = vB;
	data.velocities[m_indexB].w = wB;
}