= Adding a Filter Algorithm to PDAF =

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This text describes the implementation strategy and internal structure of PDAF valid for version 1.7.0 and later. If you use an earlier version of PDAF, we recommend to update to the most recent version. In this text, we assume that the reader is already familiar with PDAF to the extend of experience with the implementation of a model with PDAF that is described in the [ImplementationGuide implementation guide].

The internal structure of PDAF is organized into a generic part providing the infrastructure to perform ensemble forecasts and filter analysis steps. This generic part is independent of the particular filter algorithm and only distinguishes between ensemble based filters (all filters except SEEK) and mode based filters (currently only SEEK). The filter-specific routines are called through an internal interface.

Each filter algorithm consists of 6 routines that are described below. All routines are called trough the internal interface of PDAF, except for the "put state" routine (`PDAF_put_state_X` where X is the name of the selected filter). 

== PDAF's Internal Interface ==

Before explaining the filter-specific routines and the calling interface of each routine, we provide an overview of the internal interface routines of PDAF. The internal interface of PDAF is structured as follows ('X' is the name of the filter algorithm):

The routine `PDAF_init` calls
 * `PDAF_init_filters` - interface routine to `PDAF_X_init`[[BR]]
   * `PDAF_X_init` performs the filter-specific initialization of parameters and calls the user-supplied routine that initializes the initial ensemble of model states.
 * `PDAF_alloc_filters` - interface routine to `PDAF_X_alloc`[[BR]]
   * `PDAF_X_alloc` allocates the filter-specific arrays.
 * `PDAF_options_filters` - interface routine to `PDAF_X_options`
   * `PDAF_X_options` is an optional routine. Its purpose is to display an overview of available options for the filter algorithm.


The routine `PDAF_get_state` calls
 * `PDAF_initinfo_filters` - interface routine to `PDAF_X_initinfo`
   * `PDAF_X_initinfo` displays information on the current configuration of the filter algorithm.


The routine `PDAF_print_info` only includes the interface to `PDAF_X_memtime`
 * `PDAF_X_memtime` displays information on the execution duration of the different parts of the assimilation process as well as information on the amount of allocated memory. This functionality is optional.


The routine `PDAF_put_state_X` is called directly from the model code. There is a separate routine for each filter, mainly because of the fact that different user-supplied routines may be needed for the analysis step of the filter. However, the operations performed directly in `PDAF_put_state_X` are widely generic and the filter-specific analysis step is typically implemented as another subroutine.
The standard implementation calls
 * `PDAF_X_initinfo`
   * This is the same routine that is called by `PDAF_get_state`. Here, it is only called, if the offline mode of PDAF is used.
 * `PDAF_X_update`
   * This routine contains the actual assimilation or analysis step of the filter algorithm. 


When `PDAF_init` is called, the filter algorithm is chosen by its ID number. Internally to PDAF, each filter is identified by a string that is defined in `PDAF_init_filters`. The interface routines have a very simple structure. In general, they select the filter-specific routine based on the string identifying the filters. When a filter algorithm is added a line for the corresponding filter-specific routine has to be inserted to each of the interface routines. 

== Filter-specific routines ==

When a filter algorithm is added, the following filter routines have to be implemented and inserted to each interface routines described above.

 * `PDAF_X_init`
 * `PDAF_X_alloc`
 * `PDAF_X_options`
 * `PDAF_X_initinfo`
 * `PDAF_X_memtime`

In addition, the routine
 * `PDAF_put_state_X`
has to be implemented that is called directly in the model code.

We recommend to base on the routines of an existing filter, as most of the routines can be easily adapted to a new filter method.

=== `PDAF_X_init` ===

The routine `PDAF_X_init` performs the initialization of filter-specific parameters. In addition, it prints information about the configuration.

The interface is as follows:
{{{
  SUBROUTINE PDAF_X_init(subtype, param_int, dim_pint, param_real, dim_preal, &
                                 ensemblefilter, fixedbasis, verbose, outflag)
}}}
with the following arguments:
 * `subtype`: The subtype index of the filter algorithm [integer, input].
 * `param_int`: The array of integer parameters [integer(dim_pint), input].
 * `dim_pint`: The number of parameters in `param_int` [integer, input].
 * `param_real`: The array of real parameters [real(dim_preal), input].
 * `dim_preal`: The number of parameters in `param_real` [integer, input]
 * `ensemblefilter`: Flag, whether the filter is an ensemble filter or a mode-based filter [logical, output].
 * `fixedbasis`: Flag, whether the chosen `subtype` is a filter with fixed ensemble, such that only the ensemble mean is integrated by the model [logical, output].
 * `verbose`: Verbosity flag [integer, input]. Valid are the values provided to `PDAF_init`.
 * `outflag`: Error flag [integer, output]

The required operations are to initialize the PDAF-internal parameter variables from the provided values of `subtype`, `param_int`, and `param_real`. In the addition, the logical flags `ensemblefilter` and `fixedbasis` have to be set. The existing implementations also include some screen output about the configuration.

Please note:
 * The routine should check, whether the provided value of `subtype` is a valid choice. If this is not the case, the error flag should be set to 2.
 * Only parameters from `param_int` and `param_real` up to the value `dim_pint` and `dim_preal` should be considered in the initialization. The array may be bigger, but the user defined which parameters are to be used be setting the values of `dim_pint` and `dim_preal`.
 * The error flag `outflag` is initial set to 0. 
 * The internal parameters are declared and stored in the Fortran module `PDAF_mod_filter`. If a new filter algorithm requires additional parameters, their declaration should be added to the module.