round to nearest integer for star polymer arms

sasmodels/models/star_polymer.c

@@ -2,16 +2,16 @@ double form_volume(void);
 
 double Iq(double q, double radius2, double arms);
 
-static double star_polymer_kernel(double q, double radius2, double arms)
+static double star_polymer_kernel(double q, double radius2, int n_arms)
 {
 
     double u_2 = radius2 * q * q;
-    double v = u_2 * arms / (3.0 * arms - 2.0);
+    double v = (u_2 * n_arms) / (3 * n_arms - 2);
 
     double term1 = v + expm1(-v);
-    double term2 = ((arms - 1.0)/2.0) * square(expm1(-v));
+    double term2 = ((n_arms - 1) * square(expm1(-v)))/2.0;
 
-    return (2.0 * (term1 + term2)) / (arms * v * v);
+    return (2.0 * (term1 + term2)) / (n_arms * v * v);
 
 }
 
@@ -22,5 +22,9 @@ double form_volume(void)
 
 double Iq(double q, double radius2, double arms)
 {
-    return star_polymer_kernel(q, radius2, arms);
+    // TODO: round or truncate?
+    // rounding: stacked_disk pearl_necklace multilayer_vesicle linear_pearls
+    //      lamellar_stack_caille lamellar_hg_stack_caille
+    // truncation: lamellar_stack_paracrystal
+    return star_polymer_kernel(q, radius2, int(arms + 0.5));
 }

sasmodels/models/star_polymer.py

@@ -79,7 +79,7 @@
 # pylint: disable=bad-whitespace, line-too-long
 #             ["name", "units", default, [lower, upper], "type","description"],
 parameters = [["rg_squared", "Ang^2", 100.0, [0.0, inf], "", "Ensemble radius of gyration SQUARED of the full polymer"],
-              ["arms",    "",      3,   [1.0, 6.0], "", "Number of arms in the model"],
+              ["arms",    "",      3,   [1.0, 6.0], "", "Number of arms in the model (integer)"],
              ]
 # pylint: enable=bad-whitespace, line-too-long