void b2FrictionJoint::InitVelocityConstraints()

in Box2D/Dynamics/Joints/b2FrictionJoint.cpp [56:129]
52 lines of code
3 McCabe index (conditional complexity)

void b2FrictionJoint::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);

	// Compute the effective mass matrix.
	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;

	b2Mat22 K;
	K.ex.x = mA + mB + iA * m_rA.y * m_rA.y + iB * m_rB.y * m_rB.y;
	K.ex.y = -iA * m_rA.x * m_rA.y - iB * m_rB.x * m_rB.y;
	K.ey.x = K.ex.y;
	K.ey.y = mA + mB + iA * m_rA.x * m_rA.x + iB * m_rB.x * m_rB.x;

	m_linearMass = K.GetInverse();

	m_angularMass = iA + iB;
	if (m_angularMass > 0.0f)
	{
		m_angularMass = 1.0f / m_angularMass;
	}

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

		b2Vec2 P(m_linearImpulse.x, m_linearImpulse.y);
		vA -= mA * P;
		wA -= iA * (b2Cross(m_rA, P) + m_angularImpulse);
		vB += mB * P;
		wB += iB * (b2Cross(m_rB, P) + m_angularImpulse);
	}
	else
	{
		m_linearImpulse.SetZero();
		m_angularImpulse = 0.0f;
	}

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