source: CIVL/examples/compare/petscBad/ex2a.c

/* ------------------------------------------------------------------------

    Solid Fuel Ignition (SFI) problem.  This problem is modeled by
    the partial differential equation
  
            -Laplacian u - lambda*exp(u) = 0,  0 < x,y < 1,
  
    with boundary conditions
   
             u = 0  for  x = 0, x = 1, y = 0, y = 1.
  
    A finite difference approximation with the usual 5-point stencil
    is used to discretize the boundary value problem to obtain a nonlinear 
    system of equations.
  ------------------------------------------------------------------------- */

#ifdef _CIVL
#include <civlc.cvh>
#endif

#include "petsc.h"

PetscErrorCode PetscLogFlops(double ){return 0;}
PetscErrorCode CHKERRQ(PetscErrorCode ){return 0;}


typedef struct {
   PassiveReal param;          /* test problem parameter */
} AppCtx;

/* ------------------------------------------------------------------- */
#undef __FUNCT__
#define __FUNCT__ "FormFunctionLocal"
/* 
   FormFunctionLocal - Evaluates nonlinear function, F(x) on local process patch
 */
PetscErrorCode FormFunctionLocal(DMDALocalInfo *info,PetscScalar **x,PetscScalar **f,AppCtx *user)
{
  PetscErrorCode ierr;
  PetscInt       i,j;
  PetscReal      lambda,hx,hy,hxdhy,hydhx,sc;
  PetscScalar    u,uxx,uyy;

  PetscFunctionBegin;

  lambda = user->param;
#ifdef _CIVL
  $assume(info->mx != 1);
  $assume(info->my != 1);
#endif
  hx     = 1.0/(PetscReal)(info->mx-1);
  hy     = 1.0/(PetscReal)(info->my-1);
  sc     = hx*hy*lambda;
  hxdhy  = hx/hy;
  hydhx  = hy/hx;
  /*
     Compute function over the locally owned part of the grid
  */
  for (j=info->ys; j<info->ys+info->ym; j++) {
    for (i=info->xs; i<info->xs+info->xm; i++) {
      if (i == 0 || j == 0 || i == info->mx-1 || j == info->my-1) {
        f[j][i] = 2.0*(hydhx+hxdhy)*x[j][i];
      } else {
        u       = x[j][i];
        uxx     = (2.0*u - x[j][i-1] - x[j][i+1])*hydhx;
        uyy     = (2.0*u - x[j-1][i] - x[j+1][i])*hxdhy;
        f[j][i] = uxx + uyy - sc*PetscExpScalar(u);
      }
    }
  }
  ierr = PetscLogFlops(11.0*info->ym*info->xm);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}