runner.c

/*******************************************************************************
 * This file is part of SWIFT.
 * Copyright (c) 2012 Pedro Gonnet (pedro.gonnet@durham.ac.uk)
 *                    Matthieu Schaller (matthieu.schaller@durham.ac.uk)
 *               2015 Peter W. Draper (p.w.draper@durham.ac.uk)
 *               2016 John A. Regan (john.a.regan@durham.ac.uk)
 *                    Tom Theuns (tom.theuns@durham.ac.uk)
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published
 * by the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 ******************************************************************************/

/* Config parameters. */
#include "../config.h"

/* Some standard headers. */
#include <float.h>
#include <limits.h>
#include <stdlib.h>

/* MPI headers. */
#ifdef WITH_MPI
#include <mpi.h>
#endif

/* This object's header. */
#include "runner.h"

/* Local headers. */
#include "active.h"
#include "approx_math.h"
#include "atomic.h"
#include "cell.h"
#include "const.h"
#include "cooling.h"
#include "debug.h"
#include "drift.h"
#include "engine.h"
#include "error.h"
#include "gravity.h"
#include "hydro.h"
#include "hydro_properties.h"
#include "kick.h"
#include "minmax.h"
#include "runner_doiact_vec.h"
#include "scheduler.h"
#include "sort_part.h"
#include "sourceterms.h"
#include "space.h"
#include "stars.h"
#include "task.h"
#include "timers.h"
#include "timestep.h"

/* Import the density loop functions. */
#define FUNCTION density
#include "runner_doiact.h"

/* Import the gradient loop functions (if required). */
#ifdef EXTRA_HYDRO_LOOP
#undef FUNCTION
#define FUNCTION gradient
#include "runner_doiact.h"
#endif

/* Import the force loop functions. */
#undef FUNCTION
#define FUNCTION force
#include "runner_doiact.h"

/* Import the gravity loop functions. */
#include "runner_doiact_fft.h"
#include "runner_doiact_grav.h"

/**
 * @brief Perform source terms
 *
 * @param r runner task
 * @param c cell
 * @param timer 1 if the time is to be recorded.
 */
void runner_do_sourceterms(struct runner *r, struct cell *c, int timer) {
  const int count = c->count;
  const double cell_min[3] = {c->loc[0], c->loc[1], c->loc[2]};
  const double cell_width[3] = {c->width[0], c->width[1], c->width[2]};
  struct sourceterms *sourceterms = r->e->sourceterms;
  const int dimen = 3;

  TIMER_TIC;

  /* Recurse? */
  if (c->split) {
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL) runner_do_sourceterms(r, c->progeny[k], 0);
  } else {

    if (count > 0) {

      /* do sourceterms in this cell? */
      const int incell =
          sourceterms_test_cell(cell_min, cell_width, sourceterms, dimen);
      if (incell == 1) {
        sourceterms_apply(r, sourceterms, c);
      }
    }
  }

  if (timer) TIMER_TOC(timer_dosource);
}

/**
 * @brief Calculate gravity acceleration from external potential
 *
 * @param r runner task
 * @param c cell
 * @param timer 1 if the time is to be recorded.
 */
void runner_do_grav_external(struct runner *r, struct cell *c, int timer) {

  struct gpart *restrict gparts = c->gparts;
  const int gcount = c->gcount;
  const struct engine *e = r->e;
  const struct external_potential *potential = e->external_potential;
  const struct phys_const *constants = e->physical_constants;
  const double time = r->e->time;

  TIMER_TIC;

  /* Anything to do here? */
  if (!cell_is_active(c, e)) return;

  /* Recurse? */
  if (c->split) {
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL) runner_do_grav_external(r, c->progeny[k], 0);
  } else {

    /* Loop over the gparts in this cell. */
    for (int i = 0; i < gcount; i++) {

      /* Get a direct pointer on the part. */
      struct gpart *restrict gp = &gparts[i];

      /* Is this part within the time step? */
      if (gpart_is_active(gp, e)) {
        external_gravity_acceleration(time, potential, constants, gp);
      }
    }
  }

  if (timer) TIMER_TOC(timer_dograv_external);
}

/**
 * @brief Calculate change in thermal state of particles induced
 * by radiative cooling and heating.
 *
 * @param r runner task
 * @param c cell
 * @param timer 1 if the time is to be recorded.
 */
void runner_do_cooling(struct runner *r, struct cell *c, int timer) {

  struct part *restrict parts = c->parts;
  struct xpart *restrict xparts = c->xparts;
  const int count = c->count;
  const struct engine *e = r->e;
  const struct cooling_function_data *cooling_func = e->cooling_func;
  const struct phys_const *constants = e->physical_constants;
  const struct unit_system *us = e->internal_units;
  const double timeBase = e->timeBase;

  TIMER_TIC;

  /* Anything to do here? */
  if (!cell_is_active(c, e)) return;

  /* Recurse? */
  if (c->split) {
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL) runner_do_cooling(r, c->progeny[k], 0);
  } else {

    /* Loop over the parts in this cell. */
    for (int i = 0; i < count; i++) {

      /* Get a direct pointer on the part. */
      struct part *restrict p = &parts[i];
      struct xpart *restrict xp = &xparts[i];

      if (part_is_active(p, e)) {

        /* Let's cool ! */
        const double dt = get_timestep(p->time_bin, timeBase);
        cooling_cool_part(constants, us, cooling_func, p, xp, dt);
      }
    }
  }

  if (timer) TIMER_TOC(timer_do_cooling);
}

/**
 * @brief Sort the entries in ascending order using QuickSort.
 *
 * @param sort The entries
 * @param N The number of entries.
 */
void runner_do_sort_ascending(struct entry *sort, int N) {

  struct {
    short int lo, hi;
  } qstack[10];
  int qpos, i, j, lo, hi, imin;
  struct entry temp;
  float pivot;

  /* Sort parts in cell_i in decreasing order with quicksort */
  qstack[0].lo = 0;
  qstack[0].hi = N - 1;
  qpos = 0;
  while (qpos >= 0) {
    lo = qstack[qpos].lo;
    hi = qstack[qpos].hi;
    qpos -= 1;
    if (hi - lo < 15) {
      for (i = lo; i < hi; i++) {
        imin = i;
        for (j = i + 1; j <= hi; j++)
          if (sort[j].d < sort[imin].d) imin = j;
        if (imin != i) {
          temp = sort[imin];
          sort[imin] = sort[i];
          sort[i] = temp;
        }
      }
    } else {
      pivot = sort[(lo + hi) / 2].d;
      i = lo;
      j = hi;
      while (i <= j) {
        while (sort[i].d < pivot) i++;
        while (sort[j].d > pivot) j--;
        if (i <= j) {
          if (i < j) {
            temp = sort[i];
            sort[i] = sort[j];
            sort[j] = temp;
          }
          i += 1;
          j -= 1;
        }
      }
      if (j > (lo + hi) / 2) {
        if (lo < j) {
          qpos += 1;
          qstack[qpos].lo = lo;
          qstack[qpos].hi = j;
        }
        if (i < hi) {
          qpos += 1;
          qstack[qpos].lo = i;
          qstack[qpos].hi = hi;
        }
      } else {
        if (i < hi) {
          qpos += 1;
          qstack[qpos].lo = i;
          qstack[qpos].hi = hi;
        }
        if (lo < j) {
          qpos += 1;
          qstack[qpos].lo = lo;
          qstack[qpos].hi = j;
        }
      }
    }
  }
}

/**
 * @brief Sort the particles in the given cell along all cardinal directions.
 *
 * @param r The #runner.
 * @param c The #cell.
 * @param flags Cell flag.
 * @param clock Flag indicating whether to record the timing or not, needed
 *      for recursive calls.
 */
void runner_do_sort(struct runner *r, struct cell *c, int flags, int clock) {

  struct entry *finger;
  struct entry *fingers[8];
  struct part *parts = c->parts;
  struct xpart *xparts = c->xparts;
  struct entry *sort;
  const int count = c->count;
  float buff[8];

  TIMER_TIC;

  /* Check that the particles have been moved to the current time */
  if (!cell_is_drifted(c, r->e)) error("Sorting un-drifted cell");

  /* Clean-up the flags, i.e. filter out what's already been sorted, but
     only if the sorts are recent. */
  if (c->ti_sort == r->e->ti_current) {
    /* Ignore dimensions that have been sorted. */
    flags &= ~c->sorted;
  } else {
    /* Clean old (stale) sorts. */
    c->sorted = 0;
  }
  if (flags == 0) return;

  /* Sorting an un-drifted cell? */
  if (!cell_is_drifted(c, r->e)) error("Sorting undrifted cell.");

  /* start by allocating the entry arrays. */
  if (c->sort == NULL || c->sortsize < count) {
    if (c->sort != NULL) free(c->sort);
    c->sortsize = count * 1.1;
    if ((c->sort = (struct entry *)malloc(sizeof(struct entry) *
                                          (c->sortsize + 1) * 13)) == NULL)
      error("Failed to allocate sort memory.");
  }
  sort = c->sort;

  /* Does this cell have any progeny? */
  if (c->split) {

    /* Fill in the gaps within the progeny. */
    float dx_max_sort = 0.0f;
    for (int k = 0; k < 8; k++) {
      if (c->progeny[k] != NULL) {
        if (flags & ~c->progeny[k]->sorted ||
            c->progeny[k]->dx_max_sort > c->dmin * space_maxreldx)
          runner_do_sort(r, c->progeny[k], flags, 0);
        dx_max_sort = max(dx_max_sort, c->progeny[k]->dx_max_sort);
      }
    }
    c->dx_max_sort = dx_max_sort;

    /* Loop over the 13 different sort arrays. */
    for (int j = 0; j < 13; j++) {

      /* Has this sort array been flagged? */
      if (!(flags & (1 << j))) continue;

      /* Init the particle index offsets. */
      int off[8];
      off[0] = 0;
      for (int k = 1; k < 8; k++)
        if (c->progeny[k - 1] != NULL)
          off[k] = off[k - 1] + c->progeny[k - 1]->count;
        else
          off[k] = off[k - 1];

      /* Init the entries and indices. */
      int inds[8];
      for (int k = 0; k < 8; k++) {
        inds[k] = k;
        if (c->progeny[k] != NULL && c->progeny[k]->count > 0) {
          fingers[k] = &c->progeny[k]->sort[j * (c->progeny[k]->count + 1)];
          buff[k] = fingers[k]->d;
          off[k] = off[k];
        } else
          buff[k] = FLT_MAX;
      }

      /* Sort the buffer. */
      for (int i = 0; i < 7; i++)
        for (int k = i + 1; k < 8; k++)
          if (buff[inds[k]] < buff[inds[i]]) {
            int temp_i = inds[i];
            inds[i] = inds[k];
            inds[k] = temp_i;
          }

      /* For each entry in the new sort list. */
      finger = &sort[j * (count + 1)];
      for (int ind = 0; ind < count; ind++) {

        /* Copy the minimum into the new sort array. */
        finger[ind].d = buff[inds[0]];
        finger[ind].i = fingers[inds[0]]->i + off[inds[0]];

        /* Update the buffer. */
        fingers[inds[0]] += 1;
        buff[inds[0]] = fingers[inds[0]]->d;

        /* Find the smallest entry. */
        for (int k = 1; k < 8 && buff[inds[k]] < buff[inds[k - 1]]; k++) {
          int temp_i = inds[k - 1];
          inds[k - 1] = inds[k];
          inds[k] = temp_i;
        }

      } /* Merge. */

      /* Add a sentinel. */
      sort[j * (count + 1) + count].d = FLT_MAX;
      sort[j * (count + 1) + count].i = 0;

      /* Mark as sorted. */
      c->sorted |= (1 << j);

    } /* loop over sort arrays. */

  } /* progeny? */

  /* Otherwise, just sort. */
  else {

    /* Fill the sort array. */
    for (int k = 0; k < count; k++) {
      if (xparts != NULL) {
        xparts[k].x_diff_sort[0] = 0.0f;
        xparts[k].x_diff_sort[1] = 0.0f;
        xparts[k].x_diff_sort[2] = 0.0f;
      }
      const double px[3] = {parts[k].x[0], parts[k].x[1], parts[k].x[2]};
      for (int j = 0; j < 13; j++)
        if (flags & (1 << j)) {
          sort[j * (count + 1) + k].i = k;
          sort[j * (count + 1) + k].d = px[0] * runner_shift[j][0] +
                                        px[1] * runner_shift[j][1] +
                                        px[2] * runner_shift[j][2];
        }
    }

    /* Add the sentinel and sort. */
    for (int j = 0; j < 13; j++)
      if (flags & (1 << j)) {
        sort[j * (count + 1) + count].d = FLT_MAX;
        sort[j * (count + 1) + count].i = 0;
        runner_do_sort_ascending(&sort[j * (count + 1)], count);
        c->sorted |= (1 << j);
      }

    /* Finally, clear the dx_max_sort field of this cell. */
    c->dx_max_sort = 0.f;

    /* If this was not just an update, invalidate the sorts above this one. */
    if (c->ti_sort < r->e->ti_current)
      for (struct cell *finger = c->parent; finger != NULL;
           finger = finger->parent)
        finger->sorted = 0;
  }

  /* Update the sort timer. */
  c->ti_sort = r->e->ti_current;

#ifdef SWIFT_DEBUG_CHECKS
  /* Verify the sorting. */
  for (int j = 0; j < 13; j++) {
    if (!(flags & (1 << j))) continue;
    finger = &sort[j * (count + 1)];
    for (int k = 1; k < count; k++) {
      if (finger[k].d < finger[k - 1].d)
        error("Sorting failed, ascending array.");
      if (finger[k].i >= count) error("Sorting failed, indices borked.");
    }
  }
#endif

  if (clock) TIMER_TOC(timer_dosort);
}

/**
 * @brief Initialize the multipoles before the gravity calculation.
 *
 * @param r The runner thread.
 * @param c The cell.
 * @param timer 1 if the time is to be recorded.
 */
void runner_do_init_grav(struct runner *r, struct cell *c, int timer) {

  const struct engine *e = r->e;

  TIMER_TIC;

#ifdef SWIFT_DEBUG_CHECKS
  if (!(e->policy & engine_policy_self_gravity))
    error("Grav-init task called outside of self-gravity calculation");
#endif

  /* Anything to do here? */
  if (!cell_is_active(c, e)) return;

  /* Drift the multipole */
  cell_drift_multipole(c, e);

  /* Reset the gravity acceleration tensors */
  gravity_field_tensors_init(&c->multipole->pot);

  /* Recurse? */
  if (c->split) {
    for (int k = 0; k < 8; k++) {
      if (c->progeny[k] != NULL) runner_do_init_grav(r, c->progeny[k], 0);
    }
  }

  if (timer) TIMER_TOC(timer_init_grav);
}

/**
 * @brief Intermediate task after the gradient loop that does final operations
 * on the gradient quantities and optionally slope limits the gradients
 *
 * @param r The runner thread.
 * @param c The cell.
 * @param timer Are we timing this ?
 */
void runner_do_extra_ghost(struct runner *r, struct cell *c, int timer) {

#ifdef EXTRA_HYDRO_LOOP

  struct part *restrict parts = c->parts;
  const int count = c->count;
  const struct engine *e = r->e;

  TIMER_TIC;

  /* Anything to do here? */
  if (!cell_is_active(c, e)) return;

  /* Recurse? */
  if (c->split) {
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL) runner_do_extra_ghost(r, c->progeny[k], 0);
  } else {

    /* Loop over the parts in this cell. */
    for (int i = 0; i < count; i++) {

      /* Get a direct pointer on the part. */
      struct part *restrict p = &parts[i];

      if (part_is_active(p, e)) {

        /* Get ready for a force calculation */
        hydro_end_gradient(p);
      }
    }
  }

  if (timer) TIMER_TOC(timer_do_extra_ghost);

#else
  error("SWIFT was not compiled with the extra hydro loop activated.");
#endif
}

/**
 * @brief Intermediate task after the density to check that the smoothing
 * lengths are correct.
 *
 * @param r The runner thread.
 * @param c The cell.
 * @param timer Are we timing this ?
 */
void runner_do_ghost(struct runner *r, struct cell *c, int timer) {

  struct part *restrict parts = c->parts;
  struct xpart *restrict xparts = c->xparts;
  const struct engine *e = r->e;
  const struct space *s = e->s;
  const float hydro_h_max = e->hydro_properties->h_max;
  const float target_wcount = e->hydro_properties->target_neighbours;
  const float max_wcount =
      target_wcount + e->hydro_properties->delta_neighbours;
  const float min_wcount =
      target_wcount - e->hydro_properties->delta_neighbours;
  const int max_smoothing_iter = e->hydro_properties->max_smoothing_iterations;
  int redo = 0, count = 0;

  TIMER_TIC;

  /* Anything to do here? */
  if (!cell_is_active(c, e)) return;

  /* Recurse? */
  if (c->split) {
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL) runner_do_ghost(r, c->progeny[k], 0);
  } else {

    /* Init the list of active particles that have to be updated. */
    int *pid = NULL;
    if ((pid = malloc(sizeof(int) * c->count)) == NULL)
      error("Can't allocate memory for pid.");
    for (int k = 0; k < c->count; k++)
      if (part_is_active(&parts[k], e)) {
        pid[count] = k;
        ++count;
      }

    /* While there are particles that need to be updated... */
    for (int num_reruns = 0; count > 0 && num_reruns < max_smoothing_iter;
         num_reruns++) {

      /* Reset the redo-count. */
      redo = 0;

      /* Loop over the remaining active parts in this cell. */
      for (int i = 0; i < count; i++) {

        /* Get a direct pointer on the part. */
        struct part *restrict p = &parts[pid[i]];
        struct xpart *restrict xp = &xparts[pid[i]];

#ifdef SWIFT_DEBUG_CHECKS
        /* Is this part within the timestep? */
        if (!part_is_active(p, e)) error("Ghost applied to inactive particle");
#endif

        /* Finish the density calculation */
        hydro_end_density(p);

        /* Did we get the right number of neighbours? */
        if (p->density.wcount > max_wcount || p->density.wcount < min_wcount) {

          float h_corr = 0.f;

          /* If no derivative, double the smoothing length. */
          if (p->density.wcount_dh == 0.0f) h_corr = p->h;

          /* Otherwise, compute the smoothing length update (Newton step). */
          else {
            h_corr = (target_wcount - p->density.wcount) / p->density.wcount_dh;

            /* Truncate to the range [ -p->h/2 , p->h ]. */
            h_corr = (h_corr < p->h) ? h_corr : p->h;
            h_corr = (h_corr > -0.5f * p->h) ? h_corr : -0.5f * p->h;
          }

          /* Ok, correct then */
          p->h += h_corr;

          /* If below the absolute maximum, try again */
          if (p->h < hydro_h_max) {

            /* Flag for another round of fun */
            pid[redo] = pid[i];
            redo += 1;

            /* Re-initialise everything */
            hydro_init_part(p, &s->hs);

            /* Off we go ! */
            continue;
          } else {

            /* Ok, this particle is a lost cause... */
            p->h = hydro_h_max;
          }
        }

        /* We now have a particle whose smoothing length has converged */

        /* As of here, particle force variables will be set. */

        /* Compute variables required for the force loop */
        hydro_prepare_force(p, xp);

        /* The particle force values are now set.  Do _NOT_
           try to read any particle density variables! */

        /* Prepare the particle for the force loop over neighbours */
        hydro_reset_acceleration(p);
      }

      /* We now need to treat the particles whose smoothing length had not
       * converged again */

      /* Re-set the counter for the next loop (potentially). */
      count = redo;
      if (count > 0) {

        /* Climb up the cell hierarchy. */
        for (struct cell *finger = c; finger != NULL; finger = finger->parent) {

          /* Run through this cell's density interactions. */
          for (struct link *l = finger->density; l != NULL; l = l->next) {

            /* Self-interaction? */
            if (l->t->type == task_type_self)
              runner_doself_subset_density(r, finger, parts, pid, count);

            /* Otherwise, pair interaction? */
            else if (l->t->type == task_type_pair) {

              /* Left or right? */
              if (l->t->ci == finger)
                runner_dopair_subset_density(r, finger, parts, pid, count,
                                             l->t->cj);
              else
                runner_dopair_subset_density(r, finger, parts, pid, count,
                                             l->t->ci);

            }

            /* Otherwise, sub-self interaction? */
            else if (l->t->type == task_type_sub_self)
              runner_dosub_subset_density(r, finger, parts, pid, count, NULL,
                                          -1, 1);

            /* Otherwise, sub-pair interaction? */
            else if (l->t->type == task_type_sub_pair) {

              /* Left or right? */
              if (l->t->ci == finger)
                runner_dosub_subset_density(r, finger, parts, pid, count,
                                            l->t->cj, -1, 1);
              else
                runner_dosub_subset_density(r, finger, parts, pid, count,
                                            l->t->ci, -1, 1);
            }
          }
        }
      }
    }

#ifdef SWIFT_DEBUG_CHECKS
    if (count) {
      error("Smoothing length failed to converge on %i particles.", count);
    }
#else
    if (count)
      message("Smoothing length failed to converge on %i particles.", count);
#endif

    /* Be clean */
    free(pid);
  }

  if (timer) TIMER_TOC(timer_do_ghost);
}

/**
 * @brief Unskip any tasks associated with active cells.
 *
 * @param c The cell.
 * @param e The engine.
 */
static void runner_do_unskip(struct cell *c, struct engine *e) {

  /* Ignore empty cells. */
  if (c->count == 0 && c->gcount == 0) return;

  /* Skip inactive cells. */
  if (!cell_is_active(c, e)) return;

  /* Recurse */
  if (c->split) {
    for (int k = 0; k < 8; k++) {
      if (c->progeny[k] != NULL) {
        struct cell *cp = c->progeny[k];
        runner_do_unskip(cp, e);
      }
    }
  }

  /* Unskip any active tasks. */
  const int forcerebuild = cell_unskip_tasks(c, &e->sched);
  if (forcerebuild) atomic_inc(&e->forcerebuild);
}

/**
 * @brief Mapper function to unskip active tasks.
 *
 * @param map_data An array of #cell%s.
 * @param num_elements Chunk size.
 * @param extra_data Pointer to an #engine.
 */
void runner_do_unskip_mapper(void *map_data, int num_elements,
                             void *extra_data) {

  struct engine *e = (struct engine *)extra_data;
  struct cell *cells = (struct cell *)map_data;

  for (int ind = 0; ind < num_elements; ind++) {
    struct cell *c = &cells[ind];
    if (c != NULL) runner_do_unskip(c, e);
  }
}
/**
 * @brief Drift particles in real space.
 *
 * @param r The runner thread.
 * @param c The cell.
 * @param timer Are we timing this ?
 */
void runner_do_drift_particles(struct runner *r, struct cell *c, int timer) {

  TIMER_TIC;

  cell_drift_particles(c, r->e);

  if (timer) TIMER_TOC(timer_drift);
}

/**
 * @brief Perform the first half-kick on all the active particles in a cell.
 *
 * @param r The runner thread.
 * @param c The cell.
 * @param timer Are we timing this ?
 */
void runner_do_kick1(struct runner *r, struct cell *c, int timer) {

  const struct engine *e = r->e;
  struct part *restrict parts = c->parts;
  struct xpart *restrict xparts = c->xparts;
  struct gpart *restrict gparts = c->gparts;
  struct spart *restrict sparts = c->sparts;
  const int count = c->count;
  const int gcount = c->gcount;
  const int scount = c->scount;
  const integertime_t ti_current = e->ti_current;
  const double timeBase = e->timeBase;

  TIMER_TIC;

  /* Anything to do here? */
  if (!cell_is_starting(c, e)) return;

  /* Recurse? */
  if (c->split) {
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL) runner_do_kick1(r, c->progeny[k], 0);
  } else {

    /* Loop over the parts in this cell. */
    for (int k = 0; k < count; k++) {

      /* Get a handle on the part. */
      struct part *restrict p = &parts[k];
      struct xpart *restrict xp = &xparts[k];

      /* If particle needs to be kicked */
      if (part_is_starting(p, e)) {

        const integertime_t ti_step = get_integer_timestep(p->time_bin);
        const integertime_t ti_begin =
            get_integer_time_begin(ti_current + 1, p->time_bin);

#ifdef SWIFT_DEBUG_CHECKS
        const integertime_t ti_end =
            get_integer_time_end(ti_current + 1, p->time_bin);

        if (ti_begin != ti_current)
          error(
              "Particle in wrong time-bin, ti_end=%lld, ti_begin=%lld, "
              "ti_step=%lld time_bin=%d ti_current=%lld",
              ti_end, ti_begin, ti_step, p->time_bin, ti_current);
#endif

        /* do the kick */
        kick_part(p, xp, ti_begin, ti_begin + ti_step / 2, timeBase);
      }
    }

    /* Loop over the gparts in this cell. */
    for (int k = 0; k < gcount; k++) {

      /* Get a handle on the part. */
      struct gpart *restrict gp = &gparts[k];

      /* If the g-particle has no counterpart and needs to be kicked */
      if (gp->type == swift_type_dark_matter && gpart_is_starting(gp, e)) {

        const integertime_t ti_step = get_integer_timestep(gp->time_bin);
        const integertime_t ti_begin =
            get_integer_time_begin(ti_current + 1, gp->time_bin);

#ifdef SWIFT_DEBUG_CHECKS
        const integertime_t ti_end =
            get_integer_time_end(ti_current + 1, gp->time_bin);

        if (ti_begin != ti_current)
          error(
              "Particle in wrong time-bin, ti_end=%lld, ti_begin=%lld, "
              "ti_step=%lld time_bin=%d ti_current=%lld",
              ti_end, ti_begin, ti_step, gp->time_bin, ti_current);
#endif

        /* do the kick */
        kick_gpart(gp, ti_begin, ti_begin + ti_step / 2, timeBase);
      }
    }

    /* Loop over the star particles in this cell. */
    for (int k = 0; k < scount; k++) {

      /* Get a handle on the s-part. */
      struct spart *restrict sp = &sparts[k];

      /* If particle needs to be kicked */
      if (spart_is_starting(sp, e)) {

        const integertime_t ti_step = get_integer_timestep(sp->time_bin);
        const integertime_t ti_begin =
            get_integer_time_begin(ti_current + 1, sp->time_bin);

#ifdef SWIFT_DEBUG_CHECKS
        const integertime_t ti_end =
            get_integer_time_end(ti_current + 1, sp->time_bin);