Changeset 4f2478e in sasmodels for sasmodels/models/lib

Timestamp:

Jan 28, 2016 1:19:03 PM (8 years ago)

Author:

Paul Kienzle <pkienzle@…>

Branches:

master, core_shell_microgels, costrafo411, magnetic_model, release_v0.94, release_v0.95, ticket-1257-vesicle-product, ticket_1156, ticket_1265_superball, ticket_822_more_unit_tests

Children:

a3e78c3

Parents:

bf6631a

Message:

simplify calculation for sph_j1c

File:

• sasmodels/models/lib/sph_j1c.c (modified) (3 diffs)

sasmodels/models/lib/sph_j1c.c

@@ -1 +1 @@
 /**
-* Spherical Bessel function j1(x)/x
+* Spherical Bessel function 3*j1(x)/x
 *
 * Used for low q to avoid cancellation error.
@@ -8 +8 @@
 * requires the first 3 terms.  Double precision requires the 4th term.
 * The fifth term is not needed, and is commented out.
+* Taylor expansion:
+*      1.0 + q2*(-3./30. + q2*(3./840.))+ q2*(-3./45360. + q2*(3./3991680.))))
+* Expression returned from Herbie (herbie.uwpise.org/demo):
+*      const double t = ((1. + 3.*q2*q2/5600.) - q2/20.);
+*      return t*t;
 */
 
@@ -18 +23 @@
     SINCOS(q, sin_q, cos_q);
 
-    const double bessel = (q < 1.e-1) ?
-        1.0 + q2*(-3./30. + q2*(3./840. + q2*(-3./45360.))) // + q2*(3./3991680.)))
+    const double bessel = (q < 0.384038453352533)
+        ? (1.0 + q2*(-3./30. + q2*(3./840.))
         : 3.0*(sin_q/q - cos_q)/q2;
 
     return bessel;
+
+ /*
+    // Code to test various expressions
+    if (sizeof(q2) > 4) {
+        return 3.0*(sin_q/q - cos_q)/q2;
+    } else if (q < 0.384038453352533) {
+        //const double t = ((1. + 3.*q2*q2/5600.) - q2/20.); return t*t;
+        return 1.0 + q2*q2*(3./840.) - q2*(3./30.);
+        //return 1.0 + q2*(-3./30. + q2*(3./840.));
+        //return 1.0 + q2*(-3./30. + q2*(3./840. + q2*(-3./45360.)));
+        //return 1.0 + q2*(-3./30. + q2*(3./840. + q2*(-3./45360. + q2*(3./3991680.))));
+    } else {
+        return 3.0*(sin_q/q - cos_q)/q2;
+    }
+*/
 }