extract() implementation for dihedral helix

doc/src/fix_adapt.rst

@@ -464,6 +464,8 @@ sub-style name. The dihedral styles that currently work with fix adapt are:
 +------------------------------------------------------------------------+----------------+----------------+
 | :doc:`cosine/squared/restricted <dihedral_cosine_squared_restricted>`  | k,phi0         | type dihedrals |
 +------------------------------------------------------------------------+----------------+----------------+
+| :doc:`helix <dihedral_helix>`                                          | a,b,c          | type dihedrals |
++------------------------------------------------------------------------+----------------+----------------+
 | :doc:`multi/harmonic <dihedral_multi_harmonic>`                        | a1,a2,a3,a4,a5 | type dihedrals |
 +------------------------------------------------------------------------+----------------+----------------+
 | :doc:`opls <dihedral_opls>`                                            | k1,k2,k3,k4    | type dihedrals |

src/EXTRA-MOLECULE/dihedral_helix.cpp

@@ -430,3 +430,16 @@ void DihedralHelix::born_matrix(int nd, int i1, int i2, int i3, int i4,
   du2 = -(9.0*bphi[type]*cos(3.0*phi) + cphi[type]*cos(phi + MY_PI4))*siinv*siinv +
           (3.0*bphi[type]*sin(3.0*phi) + cphi[type]*sin(phi + MY_PI4))*c*siinv*siinv*siinv;
 }
+
+/* ----------------------------------------------------------------------
+    return ptr to internal members upon request
+ ------------------------------------------------------------------------ */
+
+ void *DihedralHelix::extract(const char *str, int &dim)
+ {
+   dim = 1;
+   if (strcmp(str, "a") == 0) return (void *) aphi;
+   if (strcmp(str, "b") == 0) return (void *) bphi;
+   if (strcmp(str, "c") == 0) return (void *) cphi;
+   return nullptr;
+ }

src/EXTRA-MOLECULE/dihedral_helix.h

@@ -34,6 +34,7 @@ class DihedralHelix : public Dihedral {
   void read_restart(FILE *) override;
   void write_data(FILE *) override;
   void born_matrix(int, int, int, int, int, double &, double &) override;
+  void *extract(const char *, int &) override;
 
  protected:
   double *aphi, *bphi, *cphi;

unittest/force-styles/tests/dihedral-helix.yaml

@@ -16,7 +16,10 @@ dihedral_coeff: ! |
   3  56 10 32
   4  23 80 61
   5  19 90 13
-extract: ! ""
+extract: ! |
+  a 1
+  b 1
+  c 1
 natoms: 29
 init_energy: 4634.436545707672
 init_stress: ! |-