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Template:NEW30

This How To describes the procedure for implementing a solver in GridLAB-D. The example used here illustrates a Newton's Method solver that solver N independent single variable equations.

Step 1 - Stub in the implementation file

Create the file core/solvers/newton_method.cpp and open it for editing, adding the following code:

// $Id$
#include <stdlib.h>
#include "gridlabd.h"
static unsigned int version = 1;
// TODO structure of solver data (see Step 3)
static unsigned int max_iterations = 100;
EXPORT int newton_method_init(CALLBACKS *fntable)
{
  callback=fntable;
  return 1;
}
EXPORT int newton_method_set(char *param,...)
{
  int n=0;
  va_list arg;
  va_start(arg,param);
  char *tag = param;
  while ( tag!=NULL )
  {
    // TODO handle set params (see Step 4)
    {
      gl_error("newton_method_config(char *param='%s',...): tag '%s' is not recognized");
      return n;
    }
    tag = va_arg(arg,char*);
    n++;
    }
  return n;
}
EXPORT int newton_method_get(char *param,...)
{
  int n=0;
  va_list arg;
  va_start(arg,param);
  char *tag = param;
  while ( tar!=NULL )
  {
    // TODO handle get params (see Step 5)
    {
      gl_error("newton_method_config(char *param='%s',...): tag '%s' is not recognized");
      return n;
    }
    tag = va_arg(arg,char*);
    n++;
    }
  return n;
}
EXPORT int newton_method_solve(NMDATA *data)
{
  // TODO implement solver (see Step 6)
  return 0;
}

Step 2 - Add to build files

  1. In MSVC, right-click on the "solvers" project and add the implementation file. In other environments, open core/solvers/solvers.vcproj and add the implementation file in the appropriate place in the XML code.
  2. Open core/solvers/Makefile.am and add newton_method.cpp to glsolvers_la_SOURCES

Step 3 - Define the data structures

Add the following code at the TODO comment

typedef struct {
  unsigned int n; // dimensions (default 1)
  double *x; // current values of x
  double (**f)(double); // functions
  double (**df)(double); // derivatives
  double *p; // precisions
  unsigned int *m; // multiplicities (default is 1)
  unsigned char *s; // status (0=failed, 1=converge, 2=non-converged)
} NMDATA;
static unsigned int dimension = 1; // default number of dimensions for new data

Step 4 - Implement newton_method_set

Add the following code at the TODO comment

    // set the maximum number of iterations
    if ( strcmp(param,"max_iterations")==0 )
      max_iterations = va_arg(arg,unsigned int);
    
    // set the number of dimensions to use for the next data allocation
    else if ( strcmp(param,"dimension")==0 )
      dimensions = va_arg(arg,unsigned int);
    else

Step 5 - Implement newton_method_get

Add the following code at the TODO comment

    // get the current version of the solver
    if ( strcmp(param,"version")==0 )
      *va_arg(arg,unsigned int*) = version;
    
    // get the maximum number of iterations currently in use
    else if ( strcmp(param,"max_iterations")==0 )
      *va_arg(arg,unsigned int*) = max_iterations;
    
    // get the number of dimension currently is use
    else if ( strcmp(param,"dimensions")==0 )
      *va_arg(arg,unsigned int*) = dimensions;
    
    // get a new solver data block using the current number of dimensions
    else if ( strcmp(param,"init_data")==0 )
    {
      NMDATA *data = (NMDATA*)va_arg(arg,NMDATA*);
      if ( data->n==0 ) data->n = dimensions;
      data->df = (double(**)(double))malloc(sizeof(void*)*dimensions);
      data->f = (double(**)(double))malloc(sizeof(void*)*dimensions);
      data->x = (double*)malloc(sizeof(double)*dimensions);
      data->m = (unsigned int*)malloc(sizeof(unsigned int)*dimensions);
      data->p = (double*)malloc(sizeof(double)*dimensions);
      data->s = (unsigned char*)malloc(sizeof(unsigned int)*dimensions);
      int i;
      for ( i=0 ; i<dimensions ; i++ )
      {
        data->x[i] = 0;
        data->df[i] = NULL;
        data->f[i] = NULL;
        data->m[i] = 1;
        data->p[i] = 1e-8;
        data->s[i] = 1;
      }
    }
    else

Step 6 - Implement newton_method_solve

Add the following code at the TODO comment

  // n is an index, s is the return status
  int n, s=1;
  
  // for each equation
  for ( n=0 ; n<data->n ; n++ )
  {
    // map the solution data to local variables
    double &p = data->p[n];
    double &x = data->x[n];
    double (*f)(double) = data->f[n];
    double (*df)(double) = data->df[n];
    unsigned int &m = data->m[n];
    
    // dx is the correction to the solution, i is the iteration counter
    double dx;
    unsigned int i=0;
    
    // iterate until the correction is less than the precision given (status=1)
    do {
    
      // slope at the current solution
      double dydx = (*df)(x);
      
      // if slope is not usable
      if ( isnan(dydx) || dydx==0 )
      {
        // no solution (status=0)
        x = NaN;
        s = data->s[n] = 0;
        break;
      }
      
      // if slope is infinite
      else if ( !isfinite(dydx) )
        // current solution is used (status=1)
        break;
      
      // compute new correction
      else
      {
        dx = m*(*f)(x)/dydx;
        x -= dx;
      }
      
      // if iteration limit reached current solution is used (status=2)
      if ( i++>max_iterations )
      {
         if ( s>0 ) s=2; // only flag if not already flagged for failure
         data->s[n] = 2;
         break;
      }
    } while ( fabs(dx)>p );
  }

Step 7 - Document the solver

  1. Add the solver to the Xref:Solvers See Also list by adding a new link of the form [[Tech:Newton's Method|Newton's Method]]
  2. Click on the new link and write the page. Document the Synopsis, Description, Version, See also sections at least. Don't forget to document the set/get parameters and the data structure used to access the solver.
  3. Add a brief description of the solver to the Dev:Solvers page with a link to the new Tech page.

Tips and Tricks

Check the version 
Each time the data structure is changed, the version number should be changed to prevent inconsistent data usage and mysterious crashes.
Do not use globals for anything other than defaults 
Global variables can cause problems if more than once instance of the solver is used, or if multithreading is used.
Use on throw(const char[]) or throw (const char *) 
No other catch is certain to be available.
Remember that the solve function has 3 return states
0 means at least one of the equations had no solution
1 means all the equations converged to a solution
2 means at least one of the equations did not converge before the maximum iteration limit was reached.

Version

The general solvers library was introduced in Hassayampa (Version 3.0).

See also