Documentation/UserGuide/Problem Generators

# What is a Problem Generator?

The purposes of the problem generator are to

• set initial conditions for all variables, in a function that must be called problem and has the following prototype

 void problem(DomainS *pDomain)

• enroll any problem-specific boundary conditions, if required. See Boundary Conditions.
• enroll any problem-specific user-defined history outputs, if required. See User-defined Output Variables.
• enroll any problem-specific physics controlled by function pointers, like forces due to a static gravitational potential, or optically-thin cooling. It will probably be necessary to read the Programmer Guide in order to understand the data structures and Mesh in Athena well enough to write a new problem generator. The existing files in the /src/prob directory can be used as starting templates.

In addition, the file containing the problem() function must also contain a number of other required functions

    * problem_write_restart() - writes problem-specific user data to restart files
* get_usr_expr()          - sets pointer to expression for special output data
* get_usr_out_fun()       - returns a user defined output function pointer
* Userwork_in_loop        - problem specific work IN     main loop
* Userwork_after_loop     - problem specific work AFTER  main loop


In particular, see User-defined Output Variables for a description of how to use the function get_usr_expr() to add new user-defined output variables using one of the existing file formats, and see User-defined Output Formats for a description of how to use the function get_usr_out_fun() to add new user-defined output formats.

It may also be necessary to include special user-defined functions in the same file that contains the problem generator if new output variables, or new output formats, are used.

A large number of problem generators are included in the ./athena/src/prob directory.

# Parsing the Input File

As metioned in the section on Input Files, data in the input file must be read using functions defined in a parser written for Athena located in the file src/par.c. It is quite likely that every problem generator will require the input of at least a few parameters from the <problem> block in the input file. The following functions can be used in the problem generator to read data from the input file.

    char  *par_gets(char *block, char *name);  /* reads a string called "name" from input block "block" */
int    par_geti(char *block, char *name);  /* reads an integer called "name" from input block "block" */
double par_getd(char *block, char *name);  /* reads a double called "name" from input block "block" */


The following three functions do the same thing, except set the value to that given in the def, if the name cannot be found in the input block. This is useful for setting default values to parameters without having to always include them in the input file.

    char  *par_gets_def(char *block, char *name, char   *def);
int    par_geti_def(char *block, char *name, int    def);
double par_getd_def(char *block, char *name, double def);


Most of the problem generators in the src/prob have examples of the usage of the above functions. Below are some examples.

    /* Read problem parameters.  Note Omega_0 set to 10^{-3} by default */
Omega_0 = par_getd_def("problem","Omega",1.0e-3);
qshear  = par_getd_def("problem","qshear",1.5);
amp = par_getd("problem","amp");
filename = par_gets("problem","fname");


Finally, sometimes it is useful to set parameters that are not already defined a specific <input block>. The following functions can be used for this purpose.

    void par_sets()       - sets/adds a string
void par_seti()       - sets/adds an integer
void par_setd()       - sets/adds a Real