simplify summation

Resample/Particle/ReMesocrystal.cpp

@@ -86,25 +86,16 @@ complex_t ReMesocrystal::structureFactor(const WavevectorInfo& wavevectors) cons
     complex_t exp_c = exp_I(m_lattice.basisVectorC().dot(q));
 
     std::vector<complex_t> exponents_b(range.y() + 1);
-    std::vector<complex_t> exponents_c(range.z() + 1);
 
     for (int j = 0; j <= range.y(); j++)
         exponents_b[j] = pow(exp_b, j + min.y());
-    for (int k = 0; k <= range.z(); k++)
-        exponents_c[k] = pow(exp_c, k + min.z());
-
-    complex_t denom_factor = Numeric::almostEqual(exp_c, 1., 1) ? 1. : 1. / (exp_c - 1.);
-
-    std::vector<complex_t> exp_c_sum_factor(range.z() + 1);
-    for (int k = 0; k <= range.z(); k++)
-        exp_c_sum_factor[k] = (pow(exp_c, k + 1) - 1.) * denom_factor;
 
     // main summation
 
     complex_t field_factor(0.0, 0.0);
 
     if (Numeric::almostEqual(exp_c, 1., 1)) {
-        // Special case if exp_c == 1
+        // if exp_c == 1, the sum is even simpler
 
         for (int i = 0; i <= range.x(); i++) {
             const complex_t e_a = pow(exp_a, i + min.x());
@@ -117,6 +108,14 @@ complex_t ReMesocrystal::structureFactor(const WavevectorInfo& wavevectors) cons
             }
         }
     } else {
+        std::vector<complex_t> exponents_c(range.z() + 1);
+        for (int k = 0; k <= range.z(); k++)
+            exponents_c[k] = pow(exp_c, k + min.z());
+
+        std::vector<complex_t> exp_c_sum_factor(range.z() + 1);
+        for (int k = 0; k <= range.z(); k++)
+            exp_c_sum_factor[k] = (pow(exp_c, k + 1) - 1.) / (exp_c - 1.);
+
         for (int i = 0; i <= range.x(); i++) {
             const complex_t e_a = pow(exp_a, i + min.x());