/*
       Code for Timestepping with explicit Euler.
*/
#include <private/tsimpl.h>                /*I   "petscts.h"   I*/

typedef struct {
  Vec update;     /* work vector where new solution is formed  */
} TS_Euler;

#undef __FUNCT__
#define __FUNCT__ "TSStep_Euler"
static PetscErrorCode TSStep_Euler(TS ts,PetscInt *steps,PetscReal *ptime)
{
  TS_Euler       *euler = (TS_Euler*)ts->data;
  Vec            sol = ts->vec_sol,update = euler->update;
  PetscInt       i;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  *steps = -ts->steps;
  *ptime  = ts->ptime;

  ierr = TSMonitor(ts,ts->steps,ts->ptime,sol);CHKERRQ(ierr);

  for (i=0; i<ts->max_steps; i++) {
    if (ts->ptime + ts->time_step > ts->max_time) break;
    ierr = TSPreStep(ts);CHKERRQ(ierr);

    ierr = TSComputeRHSFunction(ts,ts->ptime,sol,update);CHKERRQ(ierr);

    ierr = VecAXPY(sol,ts->time_step,update);CHKERRQ(ierr);
    ts->ptime += ts->time_step;
    ts->steps++;

    ierr = TSPostStep(ts);CHKERRQ(ierr);
    ierr = TSMonitor(ts,ts->steps,ts->ptime,sol);CHKERRQ(ierr);
  }

  *steps += ts->steps;
  *ptime  = ts->ptime;
  PetscFunctionReturn(0);
}
/*------------------------------------------------------------*/

#undef __FUNCT__
#define __FUNCT__ "TSSetUp_Euler"
static PetscErrorCode TSSetUp_Euler(TS ts)
{
  TS_Euler       *euler = (TS_Euler*)ts->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = VecDuplicate(ts->vec_sol,&euler->update);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSReset_Euler"
static PetscErrorCode TSReset_Euler(TS ts)
{
  TS_Euler       *euler = (TS_Euler*)ts->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = VecDestroy(&euler->update);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSDestroy_Euler"
static PetscErrorCode TSDestroy_Euler(TS ts)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = TSReset_Euler(ts);CHKERRQ(ierr);
  ierr = PetscFree(ts->data);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}
/*------------------------------------------------------------*/

#undef __FUNCT__
#define __FUNCT__ "TSSetFromOptions_Euler"
static PetscErrorCode TSSetFromOptions_Euler(TS ts)
{
  PetscFunctionBegin;
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSView_Euler"
static PetscErrorCode TSView_Euler(TS ts,PetscViewer viewer)
{
  PetscFunctionBegin;
  PetscFunctionReturn(0);
}

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

/*MC
      TSEULER - ODE solver using the explicit forward Euler method

  Level: beginner

.seealso:  TSCreate(), TS, TSSetType(), TSBEULER

M*/
EXTERN_C_BEGIN
#undef __FUNCT__
#define __FUNCT__ "TSCreate_Euler"
PetscErrorCode  TSCreate_Euler(TS ts)
{
  TS_Euler       *euler;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ts->ops->setup           = TSSetUp_Euler;
  ts->ops->step            = TSStep_Euler;
  ts->ops->reset           = TSReset_Euler;
  ts->ops->destroy         = TSDestroy_Euler;
  ts->ops->setfromoptions  = TSSetFromOptions_Euler;
  ts->ops->view            = TSView_Euler;

  ierr = PetscNewLog(ts,TS_Euler,&euler);CHKERRQ(ierr);
  ts->data = (void*)euler;

  PetscFunctionReturn(0);
}
EXTERN_C_END