ts/event/tsevent.c

#include <petsc/private/tsimpl.h> /*I  "petscts.h" I*/

/*
  TSEventInitialize - Initializes TSEvent for TSSolve
*/
PetscErrorCode TSEventInitialize(TSEvent event, TS ts, PetscReal t, Vec U)
{
  PetscFunctionBegin;
  if (!event) PetscFunctionReturn(PETSC_SUCCESS);
  PetscValidPointer(event, 1);
  PetscValidHeaderSpecific(ts, TS_CLASSID, 2);
  PetscValidHeaderSpecific(U, VEC_CLASSID, 4);
  event->ptime_prev = t;
  event->iterctr    = 0;
  PetscCall((*event->eventhandler)(ts, t, U, event->fvalue_prev, event->ctx));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode TSEventDestroy(TSEvent *event)
{
  PetscInt i;

  PetscFunctionBegin;
  PetscValidPointer(event, 1);
  if (!*event) PetscFunctionReturn(PETSC_SUCCESS);
  if (--(*event)->refct > 0) {
    *event = NULL;
    PetscFunctionReturn(PETSC_SUCCESS);
  }

  PetscCall(PetscFree((*event)->fvalue));
  PetscCall(PetscFree((*event)->fvalue_prev));
  PetscCall(PetscFree((*event)->fvalue_right));
  PetscCall(PetscFree((*event)->zerocrossing));
  PetscCall(PetscFree((*event)->side));
  PetscCall(PetscFree((*event)->direction));
  PetscCall(PetscFree((*event)->terminate));
  PetscCall(PetscFree((*event)->events_zero));
  PetscCall(PetscFree((*event)->vtol));

  for (i = 0; i < (*event)->recsize; i++) PetscCall(PetscFree((*event)->recorder.eventidx[i]));
  PetscCall(PetscFree((*event)->recorder.eventidx));
  PetscCall(PetscFree((*event)->recorder.nevents));
  PetscCall(PetscFree((*event)->recorder.stepnum));
  PetscCall(PetscFree((*event)->recorder.time));

  PetscCall(PetscViewerDestroy(&(*event)->monitor));
  PetscCall(PetscFree(*event));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  TSSetPostEventIntervalStep - Set the time-step used immediately following an event interval

  Logically Collective

  Input Parameters:
+ ts - time integration context
- dt - post event interval step

  Options Database Key:
. -ts_event_post_eventinterval_step <dt> time-step after event interval

  Level: advanced

  Notes:
 `TSSetPostEventIntervalStep()` allows one to set a time-step that is used immediately following an event interval.

  This function should be called from the postevent function set with `TSSetEventHandler()`.

  The post event interval time-step should be selected based on the dynamics following the event.
  If the dynamics are stiff, a conservative (small) step should be used.
  If not, then a larger time-step can be used.

.seealso: [](chapter_ts), `TS`, `TSEvent`, `TSSetEventHandler()`
@*/
PetscErrorCode TSSetPostEventIntervalStep(TS ts, PetscReal dt)
{
  PetscFunctionBegin;
  ts->event->timestep_posteventinterval = dt;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   TSSetEventTolerances - Set tolerances for event zero crossings

   Logically Collective

   Input Parameters:
+  ts - time integration context
.  tol - scalar tolerance, `PETSC_DECIDE` to leave current value
-  vtol - array of tolerances or `NULL`, used in preference to tol if present

   Options Database Key:
.  -ts_event_tol <tol> - tolerance for event zero crossing

   Level: beginner

   Notes:
   Must call `TSSetEventHandler(`) before setting the tolerances.

   The size of `vtol` is equal to the number of events.

   The tolerance is some measure of how close the event function is to zero for the event detector to stop
   and declare the time of the event has been detected.

.seealso: [](chapter_ts), `TS`, `TSEvent`, `TSSetEventHandler()`
@*/
PetscErrorCode TSSetEventTolerances(TS ts, PetscReal tol, PetscReal vtol[])
{
  TSEvent  event;
  PetscInt i;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(ts, TS_CLASSID, 1);
  if (vtol) PetscValidRealPointer(vtol, 3);
  PetscCheck(ts->event, PetscObjectComm((PetscObject)ts), PETSC_ERR_USER, "Must set the events first by calling TSSetEventHandler()");

  event = ts->event;
  if (vtol) {
    for (i = 0; i < event->nevents; i++) event->vtol[i] = vtol[i];
  } else {
    if ((tol != (PetscReal)PETSC_DECIDE) || (tol != (PetscReal)PETSC_DEFAULT)) {
      for (i = 0; i < event->nevents; i++) event->vtol[i] = tol;
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   TSSetEventHandler - Sets a function used for detecting events

   Logically Collective

   Input Parameters:
+  ts - the `TS` context obtained from `TSCreate()`
.  nevents - number of local events
.  direction - direction of zero crossing to be detected. -1 => Zero crossing in negative direction,
               +1 => Zero crossing in positive direction, 0 => both ways (one for each event)
.  terminate - flag to indicate whether time stepping should be terminated after
               event is detected (one for each event)
.  eventhandler - a change in sign of this function (see `direction`) is used to determine an even has occurred
.  postevent - [optional] post-event function, this function can change properties of the solution, ODE etc at the time of the event
-  ctx       - [optional] user-defined context for private data for the
               event detector and post event routine (use `NULL` if no
               context is desired)

   Calling sequence of `eventhandler`:
$   PetscErrorCode eventhandler(TS ts, PetscReal t, Vec U, PetscScalar fvalue[], void* ctx)
+  ts  - the `TS` context
.  t   - current time
.  U   - current iterate
.  ctx - [optional] context passed with eventhandler
-  fvalue    - function value of events at time t

   Calling sequence of `postevent`:
$   PetscErrorCode postevent(TS ts, PetscInt nevents_zero, PetscInt events_zero[], PetscReal t, Vec U, PetscBool forwardsolve, void *ctx)
+  ts - the `TS` context
.  nevents_zero - number of local events whose event function is zero
.  events_zero  - indices of local events which have reached zero
.  t            - current time
.  U            - current solution
.  forwardsolve - Flag to indicate whether `TS` is doing a forward solve (1) or adjoint solve (0)
-  ctx          - the context passed with eventhandler

   Level: intermediate

   Note:
   The `eventhandler` is actually the event detector function and the `postevent` function actually handles the desired changes that
   should take place at the time of the event

.seealso: [](chapter_ts), `TSEvent`, `TSCreate()`, `TSSetTimeStep()`, `TSSetConvergedReason()`
@*/
PetscErrorCode TSSetEventHandler(TS ts, PetscInt nevents, PetscInt direction[], PetscBool terminate[], PetscErrorCode (*eventhandler)(TS, PetscReal, Vec, PetscScalar[], void *), PetscErrorCode (*postevent)(TS, PetscInt, PetscInt[], PetscReal, Vec, PetscBool, void *), void *ctx)
{
  TSAdapt   adapt;
  PetscReal hmin;
  TSEvent   event;
  PetscInt  i;
  PetscBool flg;
#if defined PETSC_USE_REAL_SINGLE
  PetscReal tol = 1e-4;
#else
  PetscReal tol = 1e-6;
#endif

  PetscFunctionBegin;
  PetscValidHeaderSpecific(ts, TS_CLASSID, 1);
  if (nevents) {
    PetscValidIntPointer(direction, 3);
    PetscValidBoolPointer(terminate, 4);
  }

  PetscCall(PetscNew(&event));
  PetscCall(PetscMalloc1(nevents, &event->fvalue));
  PetscCall(PetscMalloc1(nevents, &event->fvalue_prev));
  PetscCall(PetscMalloc1(nevents, &event->fvalue_right));
  PetscCall(PetscMalloc1(nevents, &event->zerocrossing));
  PetscCall(PetscMalloc1(nevents, &event->side));
  PetscCall(PetscMalloc1(nevents, &event->direction));
  PetscCall(PetscMalloc1(nevents, &event->terminate));
  PetscCall(PetscMalloc1(nevents, &event->vtol));
  for (i = 0; i < nevents; i++) {
    event->direction[i]    = direction[i];
    event->terminate[i]    = terminate[i];
    event->zerocrossing[i] = PETSC_FALSE;
    event->side[i]         = 0;
  }
  PetscCall(PetscMalloc1(nevents, &event->events_zero));
  event->nevents                    = nevents;
  event->eventhandler               = eventhandler;
  event->postevent                  = postevent;
  event->ctx                        = ctx;
  event->timestep_posteventinterval = ts->time_step;
  PetscCall(TSGetAdapt(ts, &adapt));
  PetscCall(TSAdaptGetStepLimits(adapt, &hmin, NULL));
  event->timestep_min = hmin;

  event->recsize = 8; /* Initial size of the recorder */
  PetscOptionsBegin(((PetscObject)ts)->comm, ((PetscObject)ts)->prefix, "TS Event options", "TS");
  {
    PetscCall(PetscOptionsReal("-ts_event_tol", "Scalar event tolerance for zero crossing check", "TSSetEventTolerances", tol, &tol, NULL));
    PetscCall(PetscOptionsName("-ts_event_monitor", "Print choices made by event handler", "", &flg));
    PetscCall(PetscOptionsInt("-ts_event_recorder_initial_size", "Initial size of event recorder", "", event->recsize, &event->recsize, NULL));
    PetscCall(PetscOptionsReal("-ts_event_post_eventinterval_step", "Time step after event interval", "", event->timestep_posteventinterval, &event->timestep_posteventinterval, NULL));
    PetscCall(PetscOptionsReal("-ts_event_post_event_step", "Time step after event", "", event->timestep_postevent, &event->timestep_postevent, NULL));
    PetscCall(PetscOptionsReal("-ts_event_dt_min", "Minimum time step considered for TSEvent", "", event->timestep_min, &event->timestep_min, NULL));
  }
  PetscOptionsEnd();

  PetscCall(PetscMalloc1(event->recsize, &event->recorder.time));
  PetscCall(PetscMalloc1(event->recsize, &event->recorder.stepnum));
  PetscCall(PetscMalloc1(event->recsize, &event->recorder.nevents));
  PetscCall(PetscMalloc1(event->recsize, &event->recorder.eventidx));
  for (i = 0; i < event->recsize; i++) PetscCall(PetscMalloc1(event->nevents, &event->recorder.eventidx[i]));
  /* Initialize the event recorder */
  event->recorder.ctr = 0;

  for (i = 0; i < event->nevents; i++) event->vtol[i] = tol;
  if (flg) PetscCall(PetscViewerASCIIOpen(PETSC_COMM_SELF, "stdout", &event->monitor));

  PetscCall(TSEventDestroy(&ts->event));
  ts->event        = event;
  ts->event->refct = 1;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
  TSEventRecorderResize - Resizes (2X) the event recorder arrays whenever the recording limit (event->recsize)
                          is reached.
*/
static PetscErrorCode TSEventRecorderResize(TSEvent event)
{
  PetscReal *time;
  PetscInt  *stepnum;
  PetscInt  *nevents;
  PetscInt **eventidx;
  PetscInt   i, fact = 2;

  PetscFunctionBegin;

  /* Create large arrays */
  PetscCall(PetscMalloc1(fact * event->recsize, &time));
  PetscCall(PetscMalloc1(fact * event->recsize, &stepnum));
  PetscCall(PetscMalloc1(fact * event->recsize, &nevents));
  PetscCall(PetscMalloc1(fact * event->recsize, &eventidx));
  for (i = 0; i < fact * event->recsize; i++) PetscCall(PetscMalloc1(event->nevents, &eventidx[i]));

  /* Copy over data */
  PetscCall(PetscArraycpy(time, event->recorder.time, event->recsize));
  PetscCall(PetscArraycpy(stepnum, event->recorder.stepnum, event->recsize));
  PetscCall(PetscArraycpy(nevents, event->recorder.nevents, event->recsize));
  for (i = 0; i < event->recsize; i++) PetscCall(PetscArraycpy(eventidx[i], event->recorder.eventidx[i], event->recorder.nevents[i]));

  /* Destroy old arrays */
  for (i = 0; i < event->recsize; i++) PetscCall(PetscFree(event->recorder.eventidx[i]));
  PetscCall(PetscFree(event->recorder.eventidx));
  PetscCall(PetscFree(event->recorder.nevents));
  PetscCall(PetscFree(event->recorder.stepnum));
  PetscCall(PetscFree(event->recorder.time));

  /* Set pointers */
  event->recorder.time     = time;
  event->recorder.stepnum  = stepnum;
  event->recorder.nevents  = nevents;
  event->recorder.eventidx = eventidx;

  /* Double size */
  event->recsize *= fact;

  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
   Helper routine to handle user postevents and recording
*/
static PetscErrorCode TSPostEvent(TS ts, PetscReal t, Vec U)
{
  TSEvent   event     = ts->event;
  PetscBool terminate = PETSC_FALSE;
  PetscBool restart   = PETSC_FALSE;
  PetscInt  i, ctr, stepnum;
  PetscBool inflag[2], outflag[2];
  PetscBool forwardsolve = PETSC_TRUE; /* Flag indicating that TS is doing a forward solve */

  PetscFunctionBegin;
  if (event->postevent) {
    PetscObjectState state_prev, state_post;
    PetscCall(PetscObjectStateGet((PetscObject)U, &state_prev));
    PetscCall((*event->postevent)(ts, event->nevents_zero, event->events_zero, t, U, forwardsolve, event->ctx));
    PetscCall(PetscObjectStateGet((PetscObject)U, &state_post));
    if (state_prev != state_post) restart = PETSC_TRUE;
  }

  /* Handle termination events and step restart */
  for (i = 0; i < event->nevents_zero; i++)
    if (event->terminate[event->events_zero[i]]) terminate = PETSC_TRUE;
  inflag[0] = restart;
  inflag[1] = terminate;
  PetscCall(MPIU_Allreduce(inflag, outflag, 2, MPIU_BOOL, MPI_LOR, ((PetscObject)ts)->comm));
  restart   = outflag[0];
  terminate = outflag[1];
  if (restart) PetscCall(TSRestartStep(ts));
  if (terminate) PetscCall(TSSetConvergedReason(ts, TS_CONVERGED_EVENT));
  event->status = terminate ? TSEVENT_NONE : TSEVENT_RESET_NEXTSTEP;

  /* Reset event residual functions as states might get changed by the postevent callback */
  if (event->postevent) {
    PetscCall(VecLockReadPush(U));
    PetscCall((*event->eventhandler)(ts, t, U, event->fvalue, event->ctx));
    PetscCall(VecLockReadPop(U));
  }

  /* Cache current time and event residual functions */
  event->ptime_prev = t;
  for (i = 0; i < event->nevents; i++) event->fvalue_prev[i] = event->fvalue[i];

  /* Record the event in the event recorder */
  PetscCall(TSGetStepNumber(ts, &stepnum));
  ctr = event->recorder.ctr;
  if (ctr == event->recsize) PetscCall(TSEventRecorderResize(event));
  event->recorder.time[ctr]    = t;
  event->recorder.stepnum[ctr] = stepnum;
  event->recorder.nevents[ctr] = event->nevents_zero;
  for (i = 0; i < event->nevents_zero; i++) event->recorder.eventidx[ctr][i] = event->events_zero[i];
  event->recorder.ctr++;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/* Uses Anderson-Bjorck variant of regula falsi method */
static inline PetscReal TSEventComputeStepSize(PetscReal tleft, PetscReal t, PetscReal tright, PetscScalar fleft, PetscScalar f, PetscScalar fright, PetscInt side, PetscReal dt)
{
  PetscReal new_dt, scal = 1.0;
  if (PetscRealPart(fleft) * PetscRealPart(f) < 0) {
    if (side == 1) {
      scal = (PetscRealPart(fright) - PetscRealPart(f)) / PetscRealPart(fright);
      if (scal < PETSC_SMALL) scal = 0.5;
    }
    new_dt = (scal * PetscRealPart(fleft) * t - PetscRealPart(f) * tleft) / (scal * PetscRealPart(fleft) - PetscRealPart(f)) - tleft;
  } else {
    if (side == -1) {
      scal = (PetscRealPart(fleft) - PetscRealPart(f)) / PetscRealPart(fleft);
      if (scal < PETSC_SMALL) scal = 0.5;
    }
    new_dt = (PetscRealPart(f) * tright - scal * PetscRealPart(fright) * t) / (PetscRealPart(f) - scal * PetscRealPart(fright)) - t;
  }
  return PetscMin(dt, new_dt);
}

static PetscErrorCode TSEventDetection(TS ts)
{
  TSEvent   event = ts->event;
  PetscReal t;
  PetscInt  i;
  PetscInt  fvalue_sign, fvalueprev_sign;
  PetscInt  in, out;

  PetscFunctionBegin;
  PetscCall(TSGetTime(ts, &t));
  for (i = 0; i < event->nevents; i++) {
    if (PetscAbsScalar(event->fvalue[i]) < event->vtol[i]) {
      if (!event->iterctr) event->zerocrossing[i] = PETSC_TRUE;
      event->status = TSEVENT_LOCATED_INTERVAL;
      if (event->monitor) {
        PetscCall(PetscViewerASCIIPrintf(event->monitor, "TSEvent: iter %" PetscInt_FMT " - Event %" PetscInt_FMT " interval detected due to zero value (tol=%g) [%g - %g]\n", event->iterctr, i, (double)event->vtol[i], (double)event->ptime_prev, (double)t));
      }
      continue;
    }
    if (PetscAbsScalar(event->fvalue_prev[i]) < event->vtol[i]) continue; /* avoid duplicative detection if the previous endpoint is an event location */
    fvalue_sign     = PetscSign(PetscRealPart(event->fvalue[i]));
    fvalueprev_sign = PetscSign(PetscRealPart(event->fvalue_prev[i]));
    if (fvalueprev_sign != 0 && (fvalue_sign != fvalueprev_sign)) {
      if (!event->iterctr) event->zerocrossing[i] = PETSC_TRUE;
      event->status = TSEVENT_LOCATED_INTERVAL;
      if (event->monitor) PetscCall(PetscViewerASCIIPrintf(event->monitor, "TSEvent: iter %" PetscInt_FMT " - Event %" PetscInt_FMT " interval detected due to sign change [%g - %g]\n", event->iterctr, i, (double)event->ptime_prev, (double)t));
    }
  }
  in = (PetscInt)event->status;
  PetscCall(MPIU_Allreduce(&in, &out, 1, MPIU_INT, MPI_MAX, PetscObjectComm((PetscObject)ts)));
  event->status = (TSEventStatus)out;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode TSEventLocation(TS ts, PetscReal *dt)
{
  TSEvent   event = ts->event;
  PetscInt  i;
  PetscReal t;
  PetscInt  fvalue_sign, fvalueprev_sign;
  PetscInt  rollback = 0, in[2], out[2];

  PetscFunctionBegin;
  PetscCall(TSGetTime(ts, &t));
  event->nevents_zero = 0;
  for (i = 0; i < event->nevents; i++) {
    if (event->zerocrossing[i]) {
      if (PetscAbsScalar(event->fvalue[i]) < event->vtol[i] || *dt < event->timestep_min || PetscAbsReal((*dt) / ((event->ptime_right - event->ptime_prev) / 2)) < event->vtol[i]) { /* stopping criteria */
        event->status          = TSEVENT_ZERO;
        event->fvalue_right[i] = event->fvalue[i];
        continue;
      }
      /* Compute new time step */
      *dt             = TSEventComputeStepSize(event->ptime_prev, t, event->ptime_right, event->fvalue_prev[i], event->fvalue[i], event->fvalue_right[i], event->side[i], *dt);
      fvalue_sign     = PetscSign(PetscRealPart(event->fvalue[i]));
      fvalueprev_sign = PetscSign(PetscRealPart(event->fvalue_prev[i]));
      switch (event->direction[i]) {
      case -1:
        if (fvalue_sign < 0) {
          rollback               = 1;
          event->fvalue_right[i] = event->fvalue[i];
          event->side[i]         = 1;
        }
        break;
      case 1:
        if (fvalue_sign > 0) {
          rollback               = 1;
          event->fvalue_right[i] = event->fvalue[i];
          event->side[i]         = 1;
        }
        break;
      case 0:
        if (fvalue_sign != fvalueprev_sign) { /* trigger rollback only when there is a sign change */
          rollback               = 1;
          event->fvalue_right[i] = event->fvalue[i];
          event->side[i]         = 1;
        }
        break;
      }
      if (event->status == TSEVENT_PROCESSING) event->side[i] = -1;
    }
  }
  in[0] = (PetscInt)event->status;
  in[1] = rollback;
  PetscCall(MPIU_Allreduce(in, out, 2, MPIU_INT, MPI_MAX, PetscObjectComm((PetscObject)ts)));
  event->status = (TSEventStatus)out[0];
  rollback      = out[1];
  /* If rollback is true, the status will be overwritten so that an event at the endtime of current time step will be postponed to guarantee correct order */
  if (rollback) event->status = TSEVENT_LOCATED_INTERVAL;
  if (event->status == TSEVENT_ZERO) {
    for (i = 0; i < event->nevents; i++) {
      if (event->zerocrossing[i]) {
        if (PetscAbsScalar(event->fvalue[i]) < event->vtol[i] || *dt < event->timestep_min || PetscAbsReal((*dt) / ((event->ptime_right - event->ptime_prev) / 2)) < event->vtol[i]) { /* stopping criteria */
          event->events_zero[event->nevents_zero++] = i;
          if (event->monitor) PetscCall(PetscViewerASCIIPrintf(event->monitor, "TSEvent: iter %" PetscInt_FMT " - Event %" PetscInt_FMT " zero crossing located at time %g\n", event->iterctr, i, (double)t));
          event->zerocrossing[i] = PETSC_FALSE;
        }
      }
      event->side[i] = 0;
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode TSEventHandler(TS ts)
{
  TSEvent   event;
  PetscReal t;
  Vec       U;
  PetscInt  i;
  PetscReal dt, dt_min, dt_reset = 0.0;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(ts, TS_CLASSID, 1);
  if (!ts->event) PetscFunctionReturn(PETSC_SUCCESS);
  event = ts->event;

  PetscCall(TSGetTime(ts, &t));
  PetscCall(TSGetTimeStep(ts, &dt));
  PetscCall(TSGetSolution(ts, &U));

  if (event->status == TSEVENT_NONE) {
    event->timestep_prev = dt;
    event->ptime_end     = t;
  }
  if (event->status == TSEVENT_RESET_NEXTSTEP) {
    /* user has specified a PostEventInterval dt */
    dt = event->timestep_posteventinterval;
    if (ts->exact_final_time == TS_EXACTFINALTIME_MATCHSTEP) {
      PetscReal maxdt = ts->max_time - t;
      dt              = dt > maxdt ? maxdt : (PetscIsCloseAtTol(dt, maxdt, 10 * PETSC_MACHINE_EPSILON, 0) ? maxdt : dt);
    }
    PetscCall(TSSetTimeStep(ts, dt));
    event->status = TSEVENT_NONE;
  }

  PetscCall(VecLockReadPush(U));
  PetscCall((*event->eventhandler)(ts, t, U, event->fvalue, event->ctx));
  PetscCall(VecLockReadPop(U));

  /* Detect the events */
  PetscCall(TSEventDetection(ts));

  /* Locate the events */
  if (event->status == TSEVENT_LOCATED_INTERVAL || event->status == TSEVENT_PROCESSING) {
    /* Approach the zero crosing by setting a new step size */
    PetscCall(TSEventLocation(ts, &dt));
    /* Roll back when new events are detected */
    if (event->status == TSEVENT_LOCATED_INTERVAL) {
      PetscCall(TSRollBack(ts));
      PetscCall(TSSetConvergedReason(ts, TS_CONVERGED_ITERATING));
      event->iterctr++;
    }
    PetscCall(MPIU_Allreduce(&dt, &dt_min, 1, MPIU_REAL, MPIU_MIN, PetscObjectComm((PetscObject)ts)));
    if (dt_reset > 0.0 && dt_reset < dt_min) dt_min = dt_reset;
    PetscCall(TSSetTimeStep(ts, dt_min));
    /* Found the zero crossing */
    if (event->status == TSEVENT_ZERO) {
      PetscCall(TSPostEvent(ts, t, U));

      dt = event->ptime_end - t;
      if (PetscAbsReal(dt) < PETSC_SMALL) { /* we hit the event, continue with the candidate time step */
        dt            = event->timestep_prev;
        event->status = TSEVENT_NONE;
      }
      if (event->timestep_postevent) { /* user has specified a PostEvent dt*/
        dt = event->timestep_postevent;
      }
      if (ts->exact_final_time == TS_EXACTFINALTIME_MATCHSTEP) {
        PetscReal maxdt = ts->max_time - t;
        dt              = dt > maxdt ? maxdt : (PetscIsCloseAtTol(dt, maxdt, 10 * PETSC_MACHINE_EPSILON, 0) ? maxdt : dt);
      }
      PetscCall(TSSetTimeStep(ts, dt));
      event->iterctr = 0;
    }
    /* Have not found the zero crosing yet */
    if (event->status == TSEVENT_PROCESSING) {
      if (event->monitor) PetscCall(PetscViewerASCIIPrintf(event->monitor, "TSEvent: iter %" PetscInt_FMT " - Stepping forward as no event detected in interval [%g - %g]\n", event->iterctr, (double)event->ptime_prev, (double)t));
      event->iterctr++;
    }
  }
  if (event->status == TSEVENT_LOCATED_INTERVAL) { /* The step has been rolled back */
    event->status      = TSEVENT_PROCESSING;
    event->ptime_right = t;
  } else {
    for (i = 0; i < event->nevents; i++) event->fvalue_prev[i] = event->fvalue[i];
    event->ptime_prev = t;
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode TSAdjointEventHandler(TS ts)
{
  TSEvent   event;
  PetscReal t;
  Vec       U;
  PetscInt  ctr;
  PetscBool forwardsolve = PETSC_FALSE; /* Flag indicating that TS is doing an adjoint solve */

  PetscFunctionBegin;
  PetscValidHeaderSpecific(ts, TS_CLASSID, 1);
  if (!ts->event) PetscFunctionReturn(PETSC_SUCCESS);
  event = ts->event;

  PetscCall(TSGetTime(ts, &t));
  PetscCall(TSGetSolution(ts, &U));

  ctr = event->recorder.ctr - 1;
  if (ctr >= 0 && PetscAbsReal(t - event->recorder.time[ctr]) < PETSC_SMALL) {
    /* Call the user postevent function */
    if (event->postevent) {
      PetscCall((*event->postevent)(ts, event->recorder.nevents[ctr], event->recorder.eventidx[ctr], t, U, forwardsolve, event->ctx));
      event->recorder.ctr--;
    }
  }

  PetscCall(PetscBarrier((PetscObject)ts));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  TSGetNumEvents - Get the numbers of events currently set to be detected

  Logically Collective

  Input Parameter:
. ts - the `TS` context

  Output Parameter:
. nevents - the number of events

  Level: intermediate

.seealso: [](chapter_ts), `TSEvent`, `TSSetEventHandler()`
@*/
PetscErrorCode TSGetNumEvents(TS ts, PetscInt *nevents)
{
  PetscFunctionBegin;
  *nevents = ts->event->nevents;
  PetscFunctionReturn(PETSC_SUCCESS);
}