Optimize TruncatedSphere ff

Sample/HardParticle/TruncatedSphere.cpp

@@ -43,13 +43,13 @@ complex_t TruncatedSphere::formfactor_at_bottom(C3 q) const
                * (m_height * m_height * (3. * m_radius - m_height)
                   - m_dh * m_dh * (3. * m_radius - m_dh));
 
+    const complex_t Q2 = std::sqrt(q.x() * q.x() + q.y() * q.y()); // NOT the modulus!
+    const double R2 = m_radius * m_radius;
+
     complex_t integral = ComplexIntegrator().integrate(
         [=](double Z) {
-            double Rz = std::sqrt(m_radius * m_radius - Z * Z);
-            complex_t qx = q.x();
-            complex_t qy = q.y();
-            complex_t q_p = std::sqrt(qx * qx + qy * qy); // NOT the modulus!
-            return Rz * Rz * Math::Bessel::J1c(q_p * Rz) * exp_I(q.z() * Z);
+            const double R2Z2 = R2 - Z * Z;
+            return R2Z2 * Math::Bessel::J1c(Q2 * sqrt(R2Z2)) * exp_I(q.z() * Z);
         },
         m_radius - m_height, m_radius - m_dh);
     return M_TWOPI * integral * exp_I(q.z() * (m_height - m_radius));