extract() implementation for dihedral cosine squared restricted

doc/src/fix_adapt.rst

@@ -457,11 +457,13 @@ all types from 1 to :math:`N`.  A leading asterisk means all types from
 If :doc:`dihedral_style hybrid <dihedral_hybrid>` is used, *dstyle* should be a
 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:`opls <dihedral_opls>`                                            | k1,k2,k3,k4    | type dihedrals |
-+---------------------------------------------------------+----------------+----------------+
++------------------------------------------------------------------------+----------------+----------------+
 | :doc:`quadratic <dihedral_quadratic>`                                  | k,phi0         | type dihedrals |
-+---------------------------------------------------------+----------------+----------------+
++------------------------------------------------------------------------+----------------+----------------+
 
 Note that internally, phi0 is stored in radians, so the variable
 this fix use to reset phi0 needs to generate values in radians.

src/EXTRA-MOLECULE/dihedral_cosine_squared_restricted.cpp

@@ -393,3 +393,15 @@ void DihedralCosineSquaredRestricted::born_matrix(int nd, int i1, int i2, int i3
 
   du2 = 2 * k[type] * numerator / denominator;
 }
+
+/* ----------------------------------------------------------------------
+    return ptr to internal members upon request
+ ------------------------------------------------------------------------ */
+ 
+ void *DihedralCosineSquaredRestricted::extract(const char *str, int &dim)
+ {
+   dim = 1;
+   if (strcmp(str, "k") == 0) return (void *) k;
+   if (strcmp(str, "phi0") == 0) return (void *) phi0;
+   return nullptr;
+ }

src/EXTRA-MOLECULE/dihedral_cosine_squared_restricted.h

@@ -34,6 +34,7 @@ class DihedralCosineSquaredRestricted : 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 *k, *phi0;

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

@@ -16,7 +16,9 @@ dihedral_coeff: ! |
   3  15.0 -10.0
   4  12.0 0.0
   5  11.0 45.0
-extract: ! ""
+extract: ! |
+  k 1
+  phi0 1
 natoms: 29
 init_energy: 10643.96352037142
 init_stress: ! |2-