add a lammps_eval() function to the C library interface and all derived wrappers

doc/src/Fortran.rst

@@ -321,6 +321,8 @@ of the contents of the :f:mod:`LIBLAMMPS` Fortran interface to LAMMPS.
    :ftype set_string_variable: subroutine
    :f set_internal_variable: :f:subr:`set_internal_variable`
    :ftype set_internal_variable: subroutine
+   :f eval: :f:func:`eval`
+   :ftype eval: function
    :f gather_atoms: :f:subr:`gather_atoms`
    :ftype gather_atoms: subroutine
    :f gather_atoms_concat: :f:subr:`gather_atoms_concat`

doc/src/Library_objects.rst

@@ -12,6 +12,7 @@ fixes, or variables in LAMMPS using the following functions:
 - :cpp:func:`lammps_set_string_variable`
 - :cpp:func:`lammps_set_internal_variable`
 - :cpp:func:`lammps_variable_info`
+- :cpp:func:`lammps_eval`
 
 -----------------------
 
@@ -55,6 +56,11 @@ fixes, or variables in LAMMPS using the following functions:
 
 -----------------------
 
+.. doxygenfunction:: lammps_eval
+   :project: progguide
+
+-----------------------
+
 .. doxygenenum:: _LMP_DATATYPE_CONST
 
 .. doxygenenum:: _LMP_STYLE_CONST

examples/COUPLE/plugin/liblammpsplugin.c

@@ -117,6 +117,7 @@ liblammpsplugin_t *liblammpsplugin_load(const char *lib)
   ADDSYM(set_string_variable);
   ADDSYM(set_internal_variable);
   ADDSYM(variable_info);
+  ADDSYM(eval);
 
   ADDSYM(gather_atoms);
   ADDSYM(gather_atoms_concat);

examples/COUPLE/plugin/liblammpsplugin.h

@@ -163,6 +163,7 @@ struct _liblammpsplugin {
   int (*set_string_variable)(void *, const char *, const char *);
   int (*set_internal_variable)(void *, const char *, double);
   int (*variable_info)(void *, int, char *, int);
+  double (*eval)(void *, const char *);
 
   void (*gather_atoms)(void *, const char *, int, int, void *);
   void (*gather_atoms_concat)(void *, const char *, int, int, void *);

fortran/lammps.f90

@@ -126,6 +126,7 @@ MODULE LIBLAMMPS
     PROCEDURE :: set_variable           => lmp_set_variable
     PROCEDURE :: set_string_variable    => lmp_set_string_variable
     PROCEDURE :: set_internal_variable  => lmp_set_internal_variable
+    PROCEDURE :: eval                   => lmp_eval
     PROCEDURE, PRIVATE :: lmp_gather_atoms_int
     PROCEDURE, PRIVATE :: lmp_gather_atoms_double
     GENERIC   :: gather_atoms           => lmp_gather_atoms_int, &
@@ -618,7 +619,14 @@ MODULE LIBLAMMPS
       INTEGER(c_int) :: lammps_set_internal_variable
     END FUNCTION lammps_set_internal_variable
 
-    SUBROUTINE lammps_gather_atoms(handle, name, type, count, data) BIND(C)
+    FUNCTION lammps_eval(handle, expr) BIND(C)
+      IMPORT :: c_ptr, c_double
+      IMPLICIT NONE
+      TYPE(c_ptr), VALUE :: handle, expr
+      REAL(c_double) :: lammps_eval
+    END FUNCTION lammps_eval
+
+    SUBROUTINE lammps_gather_atoms(handle, name, TYPE, count, DATA) BIND(C)
       IMPORT :: c_int, c_ptr
       IMPLICIT NONE
       TYPE(c_ptr), VALUE :: handle, name, data
@@ -1812,7 +1820,7 @@ CONTAINS
   SUBROUTINE lmp_set_internal_variable(self, name, val)
     CLASS(lammps), INTENT(IN) :: self
     CHARACTER(LEN=*), INTENT(IN) :: name
-    REAL(KIND=c_double), INTENT(IN) :: val
+    REAL(c_double), INTENT(IN) :: val
     INTEGER :: err
     TYPE(c_ptr) :: Cname
 
@@ -1826,6 +1834,18 @@ CONTAINS
     END IF
   END SUBROUTINE lmp_set_internal_variable
 
+  ! equivalent function to lammps_eval
+  FUNCTION lmp_eval(self, expr)
+    CLASS(lammps), INTENT(IN) :: self
+    CHARACTER(LEN=*), INTENT(IN) :: expr
+    REAL(c_double) :: lmp_eval
+    TYPE(c_ptr) :: Cexpr
+
+    Cexpr = f2c_string(expr)
+    lmp_eval = lammps_eval(self%handle, Cexpr)
+    CALL lammps_free(Cexpr)
+  END FUNCTION lmp_eval
+
   ! equivalent function to lammps_gather_atoms (for integers)
   SUBROUTINE lmp_gather_atoms_int(self, name, count, data)
     CLASS(lammps), INTENT(IN) :: self

python/lammps/core.py

@@ -339,6 +339,9 @@ class lammps(object):
     self.lib.lammps_extract_variable_datatype.argtypes = [c_void_p, c_char_p]
     self.lib.lammps_extract_variable_datatype.restype = c_int
 
+    self.lib.lammps_eval.argtypes = [c_void_p, c_char_p]
+    self.lib.lammps_eval.restype = c_double
+
     self.lib.lammps_fix_external_get_force.argtypes = [c_void_p, c_char_p]
     self.lib.lammps_fix_external_get_force.restype = POINTER(POINTER(c_double))
 
@@ -1533,6 +1536,30 @@ class lammps(object):
 
   # -------------------------------------------------------------------------
 
+  def eval(self, expr):
+    """ Evaluate a LAMMPS immediate variable expression
+
+    .. versionadded:: TBD
+
+    This function is a wrapper around the function :cpp:func:`lammps_eval`
+    of the C library interface.  It evaluates and expression like in
+    immediate variables and returns the value.
+
+    :param expr: immediate variable expression
+    :type name: string
+    :return: the result of the evaluation
+    :rtype: c_double
+    """
+
+    if expr: newexpr = expr.encode()
+    else: return None
+
+    with ExceptionCheck(self):
+      return self.lib.lammps_eval(self.lmp, newexpr)
+    return None
+
+  # -------------------------------------------------------------------------
+
   # return vector of atom properties gathered across procs
   # 3 variants to match src/library.cpp
   # name = atom property recognized by LAMMPS in atom->extract()

src/library.cpp

@@ -2904,6 +2904,36 @@ int lammps_variable_info(void *handle, int idx, char *buffer, int buf_size) {
   return 0;
 }
 
+/** Evaluate an immediate variable expression
+ *
+\verbatim embed:rst
+
+.. versionadded:: TBD
+
+This function takes a string with an expression, like what can be used
+for :doc:`equal style variables <variable>`, evaluates it and returns
+the resulting (scalar) value as a floating point number.
+
+\endverbatim
+
+ * \param handle   pointer to a previously created LAMMPS instance cast to ``void *``.
+ * \param expr     string with expression
+ * \return         result from expression */
+
+double lammps_eval(void *handle, const char *expr)
+{
+  auto lmp = (LAMMPS *) handle;
+  double result = 0.0;
+
+  BEGIN_CAPTURE
+  {
+    result = lmp->input->variable->compute_equal(expr);
+  }
+  END_CAPTURE
+
+  return result;
+}
+
 // ----------------------------------------------------------------------
 // Library functions for scatter/gather operations of data
 // ----------------------------------------------------------------------

src/library.h

@@ -189,6 +189,7 @@ int lammps_set_variable(void *handle, const char *name, const char *str);
 int lammps_set_string_variable(void *handle, const char *name, const char *str);
 int lammps_set_internal_variable(void *handle, const char *name, double value);
 int lammps_variable_info(void *handle, int idx, char *buf, int bufsize);
+double lammps_eval(void *handle, const char *expr);
 
 /* ----------------------------------------------------------------------
  * Library functions for scatter/gather operations of data

tools/swig/lammps.i

@@ -141,6 +141,8 @@ extern int    lammps_extract_variable_datatype(void *handle, const char *name);
 extern int    lammps_set_variable(void *, const char *, const char *);
 extern int    lammps_set_string_variable(void *, const char *, const char *);
 extern int    lammps_set_internal_variable(void *, const char *, double);
+extern int    lammps_variable_info(void *handle, int idx, char *buf, int bufsize);
+extern double lammps_eval(void *handle, const char *expr);
 
 extern void   lammps_gather_atoms(void *, char *, int, int, void *);
 extern void   lammps_gather_atoms_concat(void *, char *, int, int, void *);
@@ -332,6 +334,8 @@ extern int    lammps_extract_variable_datatype(void *handle, const char *name);
 extern int    lammps_set_variable(void *, const char *, const char *);
 extern int    lammps_set_string_variable(void *, const char *, const char *);
 extern int    lammps_set_internal_variable(void *, const char *, double);
+extern int    lammps_variable_info(void *handle, int idx, char *buf, int bufsize);
+extern double lammps_eval(void *handle, const char *expr);
 
 extern void   lammps_gather_atoms(void *, char *, int, int, void *);
 extern void   lammps_gather_atoms_concat(void *, char *, int, int, void *);