runner_doiact.h

/*******************************************************************************
 * This file is part of SWIFT.
 * Copyright (c) 2012 Pedro Gonnet (pedro.gonnet@durham.ac.uk)
 *               2016 Matthieu Schaller (matthieu.schaller@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/>.
 *
 ******************************************************************************/

/* Before including this file, define FUNCTION, which is the
   name of the interaction function. This creates the interaction functions
   runner_dopair_FUNCTION, runner_dopair_FUNCTION_naive, runner_doself_FUNCTION,
   and runner_dosub_FUNCTION calling the pairwise interaction function
   runner_iact_FUNCTION. */

#define PASTE(x, y) x##_##y

#define _DOPAIR1_BRANCH(f) PASTE(runner_dopair1_branch, f)
#define DOPAIR1_BRANCH _DOPAIR1_BRANCH(FUNCTION)

#define _DOPAIR1(f) PASTE(runner_dopair1, f)
#define DOPAIR1 _DOPAIR1(FUNCTION)

#define _DOPAIR2(f) PASTE(runner_dopair2, f)
#define DOPAIR2 _DOPAIR2(FUNCTION)

#define _DOPAIR_SUBSET(f) PASTE(runner_dopair_subset, f)
#define DOPAIR_SUBSET _DOPAIR_SUBSET(FUNCTION)

#define _DOPAIR_SUBSET_NOSORT(f) PASTE(runner_dopair_subset_nosort, f)
#define DOPAIR_SUBSET_NOSORT _DOPAIR_SUBSET_NOSORT(FUNCTION)

#define _DOPAIR_SUBSET_NAIVE(f) PASTE(runner_dopair_subset_naive, f)
#define DOPAIR_SUBSET_NAIVE _DOPAIR_SUBSET_NAIVE(FUNCTION)

#define _DOPAIR1_NAIVE(f) PASTE(runner_dopair1_naive, f)
#define DOPAIR1_NAIVE _DOPAIR1_NAIVE(FUNCTION)

#define _DOPAIR2_NAIVE(f) PASTE(runner_dopair2_naive, f)
#define DOPAIR2_NAIVE _DOPAIR2_NAIVE(FUNCTION)

#define _DOSELF2_NAIVE(f) PASTE(runner_doself2_naive, f)
#define DOSELF2_NAIVE _DOSELF2_NAIVE(FUNCTION)

#define _DOSELF1(f) PASTE(runner_doself1, f)
#define DOSELF1 _DOSELF1(FUNCTION)

#define _DOSELF2(f) PASTE(runner_doself2, f)
#define DOSELF2 _DOSELF2(FUNCTION)

#define _DOSELF_SUBSET(f) PASTE(runner_doself_subset, f)
#define DOSELF_SUBSET _DOSELF_SUBSET(FUNCTION)

#define _DOSUB_SELF1(f) PASTE(runner_dosub_self1, f)
#define DOSUB_SELF1 _DOSUB_SELF1(FUNCTION)

#define _DOSUB_PAIR1(f) PASTE(runner_dosub_pair1, f)
#define DOSUB_PAIR1 _DOSUB_PAIR1(FUNCTION)

#define _DOSUB_SELF2(f) PASTE(runner_dosub_self2, f)
#define DOSUB_SELF2 _DOSUB_SELF2(FUNCTION)

#define _DOSUB_PAIR2(f) PASTE(runner_dosub_pair2, f)
#define DOSUB_PAIR2 _DOSUB_PAIR2(FUNCTION)

#define _DOSUB_SUBSET(f) PASTE(runner_dosub_subset, f)
#define DOSUB_SUBSET _DOSUB_SUBSET(FUNCTION)

#define _IACT_NONSYM(f) PASTE(runner_iact_nonsym, f)
#define IACT_NONSYM _IACT_NONSYM(FUNCTION)

#define _IACT(f) PASTE(runner_iact, f)
#define IACT _IACT(FUNCTION)

#define _IACT_NONSYM_VEC(f) PASTE(runner_iact_nonsym_vec, f)
#define IACT_NONSYM_VEC _IACT_NONSYM_VEC(FUNCTION)

#define _IACT_VEC(f) PASTE(runner_iact_vec, f)
#define IACT_VEC _IACT_VEC(FUNCTION)

#define _TIMER_DOSELF(f) PASTE(timer_doself, f)
#define TIMER_DOSELF _TIMER_DOSELF(FUNCTION)

#define _TIMER_DOPAIR(f) PASTE(timer_dopair, f)
#define TIMER_DOPAIR _TIMER_DOPAIR(FUNCTION)

#define _TIMER_DOSUB_SELF(f) PASTE(timer_dosub_self, f)
#define TIMER_DOSUB_SELF _TIMER_DOSUB_SELF(FUNCTION)

#define _TIMER_DOSUB_PAIR(f) PASTE(timer_dosub_pair, f)
#define TIMER_DOSUB_PAIR _TIMER_DOSUB_PAIR(FUNCTION)

#define _TIMER_DOSELF_SUBSET(f) PASTE(timer_doself_subset, f)
#define TIMER_DOSELF_SUBSET _TIMER_DOSELF_SUBSET(FUNCTION)

#define _TIMER_DOPAIR_SUBSET(f) PASTE(timer_dopair_subset, f)
#define TIMER_DOPAIR_SUBSET _TIMER_DOPAIR_SUBSET(FUNCTION)

/**
 * @brief Compute the interactions between a cell pair (non-symmetric case).
 *
 * Inefficient version using a brute-force algorithm.
 *
 * @param r The #runner.
 * @param ci The first #cell.
 * @param cj The second #cell.
 */
void DOPAIR1_NAIVE(struct runner *r, struct cell *restrict ci,
                   struct cell *restrict cj) {

  const struct engine *e = r->e;

#ifndef SWIFT_DEBUG_CHECKS
  error("Don't use in actual runs ! Slow code !");
#endif

  TIMER_TIC;

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

  const int count_i = ci->count;
  const int count_j = cj->count;
  struct part *restrict parts_i = ci->parts;
  struct part *restrict parts_j = cj->parts;

  /* Get the relative distance between the pairs, wrapping. */
  double shift[3] = {0.0, 0.0, 0.0};
  for (int k = 0; k < 3; k++) {
    if (cj->loc[k] - ci->loc[k] < -e->s->dim[k] / 2)
      shift[k] = e->s->dim[k];
    else if (cj->loc[k] - ci->loc[k] > e->s->dim[k] / 2)
      shift[k] = -e->s->dim[k];
  }

  /* Loop over the parts in ci. */
  for (int pid = 0; pid < count_i; pid++) {

    /* Get a hold of the ith part in ci. */
    struct part *restrict pi = &parts_i[pid];
    const int pi_active = part_is_active(pi, e);
    const float hi = pi->h;
    const float hig2 = hi * hi * kernel_gamma2;
    const float pix[3] = {pi->x[0] - (cj->loc[0] + shift[0]),
                          pi->x[1] - (cj->loc[1] + shift[1]),
                          pi->x[2] - (cj->loc[2] + shift[2])};

    /* Loop over the parts in cj. */
    for (int pjd = 0; pjd < count_j; pjd++) {

      /* Get a pointer to the jth particle. */
      struct part *restrict pj = &parts_j[pjd];
      const float hj = pj->h;
      const float hjg2 = hj * hj * kernel_gamma2;
      const int pj_active = part_is_active(pj, e);

      /* Compute the pairwise distance. */
      const float pjx[3] = {pj->x[0] - cj->loc[0], pj->x[1] - cj->loc[1],
                            pj->x[2] - cj->loc[2]};
      float dx[3] = {pix[0] - pjx[0], pix[1] - pjx[1], pix[2] - pjx[2]};
      const float r2 = dx[0] * dx[0] + dx[1] * dx[1] + dx[2] * dx[2];

      /* Hit or miss? */
      if (r2 < hig2 && pi_active) {

        IACT_NONSYM(r2, dx, hi, hj, pi, pj);
      }
      if (r2 < hjg2 && pj_active) {

        dx[0] = -dx[0];
        dx[1] = -dx[1];
        dx[2] = -dx[2];

        IACT_NONSYM(r2, dx, hj, hi, pj, pi);
      }

    } /* loop over the parts in cj. */
  }   /* loop over the parts in ci. */

  TIMER_TOC(TIMER_DOPAIR);
}

/**
 * @brief Compute the interactions between a cell pair (symmetric case).
 *
 * Inefficient version using a brute-force algorithm.
 *
 * @param r The #runner.
 * @param ci The first #cell.
 * @param cj The second #cell.
 */
void DOPAIR2_NAIVE(struct runner *r, struct cell *restrict ci,
                   struct cell *restrict cj) {

  const struct engine *e = r->e;

#ifndef SWIFT_DEBUG_CHECKS
  error("Don't use in actual runs ! Slow code !");
#endif

  TIMER_TIC;

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

  const int count_i = ci->count;
  const int count_j = cj->count;
  struct part *restrict parts_i = ci->parts;
  struct part *restrict parts_j = cj->parts;

  /* Get the relative distance between the pairs, wrapping. */
  double shift[3] = {0.0, 0.0, 0.0};
  for (int k = 0; k < 3; k++) {
    if (cj->loc[k] - ci->loc[k] < -e->s->dim[k] / 2)
      shift[k] = e->s->dim[k];
    else if (cj->loc[k] - ci->loc[k] > e->s->dim[k] / 2)
      shift[k] = -e->s->dim[k];
  }

  /* Loop over the parts in ci. */
  for (int pid = 0; pid < count_i; pid++) {

    /* Get a hold of the ith part in ci. */
    struct part *restrict pi = &parts_i[pid];
    const int pi_active = part_is_active(pi, e);
    const float hi = pi->h;
    const float hig2 = hi * hi * kernel_gamma2;
    const float pix[3] = {pi->x[0] - (cj->loc[0] + shift[0]),
                          pi->x[1] - (cj->loc[1] + shift[1]),
                          pi->x[2] - (cj->loc[2] + shift[2])};

    /* Loop over the parts in cj. */
    for (int pjd = 0; pjd < count_j; pjd++) {

      /* Get a pointer to the jth particle. */
      struct part *restrict pj = &parts_j[pjd];
      const int pj_active = part_is_active(pj, e);
      const float hj = pj->h;
      const float hjg2 = hj * hj * kernel_gamma2;

      /* Compute the pairwise distance. */
      const float pjx[3] = {pj->x[0] - cj->loc[0], pj->x[1] - cj->loc[1],
                            pj->x[2] - cj->loc[2]};
      float dx[3] = {pix[0] - pjx[0], pix[1] - pjx[1], pix[2] - pjx[2]};
      const float r2 = dx[0] * dx[0] + dx[1] * dx[1] + dx[2] * dx[2];

      /* Hit or miss? */
      if (r2 < hig2 || r2 < hjg2) {

        if (pi_active && pj_active) {

          IACT(r2, dx, hi, hj, pi, pj);
        } else if (pi_active) {

          IACT_NONSYM(r2, dx, hi, hj, pi, pj);
        } else if (pj_active) {

          dx[0] = -dx[0];
          dx[1] = -dx[1];
          dx[2] = -dx[2];

          IACT_NONSYM(r2, dx, hj, hi, pj, pi);
        }
      }
    } /* loop over the parts in cj. */
  }   /* loop over the parts in ci. */

  TIMER_TOC(TIMER_DOPAIR);
}

/**
 * @brief Compute the interactions within a cell (symmetric case).
 *
 * Inefficient version using a brute-force algorithm.
 *
 * @param r The #runner.
 * @param c The #cell.
 */
void DOSELF2_NAIVE(struct runner *r, struct cell *restrict c) {

  const struct engine *e = r->e;

#ifndef SWIFT_DEBUG_CHECKS
  error("Don't use in actual runs ! Slow code !");
#endif

  TIMER_TIC;

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

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

  /* Loop over the parts in ci. */
  for (int pid = 0; pid < count; pid++) {

    /* Get a hold of the ith part in ci. */
    struct part *restrict pi = &parts[pid];
    const double pix[3] = {pi->x[0], pi->x[1], pi->x[2]};
    const float hi = pi->h;
    const float hig2 = hi * hi * kernel_gamma2;
    const int pi_active = part_is_active(pi, e);

    /* Loop over the parts in cj. */
    for (int pjd = pid + 1; pjd < count; pjd++) {

      /* Get a pointer to the jth particle. */
      struct part *restrict pj = &parts[pjd];
      const float hj = pj->h;
      const int pj_active = part_is_active(pj, e);

      /* Compute the pairwise distance. */
      float r2 = 0.0f;
      float dx[3];
      for (int k = 0; k < 3; k++) {
        dx[k] = pix[k] - pj->x[k];
        r2 += dx[k] * dx[k];
      }
      const float hjg2 = hj * hj * kernel_gamma2;

      /* Hit or miss? */
      if (r2 < hig2 || r2 < hjg2) {

        if (pi_active && pj_active) {

          IACT(r2, dx, hi, hj, pi, pj);
        } else if (pi_active) {

          IACT_NONSYM(r2, dx, hi, hj, pi, pj);
        } else if (pj_active) {

          dx[0] = -dx[0];
          dx[1] = -dx[1];
          dx[2] = -dx[2];

          IACT_NONSYM(r2, dx, hj, hi, pj, pi);
        }
      }

    } /* loop over the parts in cj. */

  } /* loop over the parts in ci. */

  TIMER_TOC(TIMER_DOSELF);
}

/**
 * @brief Compute the interactions between a cell pair, but only for the
 *      given indices in ci.
 *
 * Version using a brute-force algorithm.
 *
 * @param r The #runner.
 * @param ci The first #cell.
 * @param parts_i The #part to interact with @c cj.
 * @param ind The list of indices of particles in @c ci to interact with.
 * @param count The number of particles in @c ind.
 * @param cj The second #cell.
 */
void DOPAIR_SUBSET_NAIVE(struct runner *r, struct cell *restrict ci,
                         struct part *restrict parts_i, int *restrict ind,
                         int count, struct cell *restrict cj) {

  const struct engine *e = r->e;

#ifndef SWIFT_DEBUG_CHECKS
  error("Don't use in actual runs ! Slow code !");
#endif

  TIMER_TIC;

  const int count_j = cj->count;
  struct part *restrict parts_j = cj->parts;

  /* Get the relative distance between the pairs, wrapping. */
  double shift[3] = {0.0, 0.0, 0.0};
  for (int k = 0; k < 3; k++) {
    if (cj->loc[k] - ci->loc[k] < -e->s->dim[k] / 2)
      shift[k] = e->s->dim[k];
    else if (cj->loc[k] - ci->loc[k] > e->s->dim[k] / 2)
      shift[k] = -e->s->dim[k];
  }

  /* Loop over the parts_i. */
  for (int pid = 0; pid < count; pid++) {

    /* Get a hold of the ith part in ci. */
    struct part *restrict pi = &parts_i[ind[pid]];
    double pix[3];
    for (int k = 0; k < 3; k++) pix[k] = pi->x[k] - shift[k];
    const float hi = pi->h;
    const float hig2 = hi * hi * kernel_gamma2;

#ifdef SWIFT_DEBUG_CHECKS
    if (!part_is_active(pi, e))
      error("Trying to correct smoothing length of inactive particle !");

#endif

    /* Loop over the parts in cj. */
    for (int pjd = 0; pjd < count_j; pjd++) {

      /* Get a pointer to the jth particle. */
      struct part *restrict pj = &parts_j[pjd];

      /* Compute the pairwise distance. */
      float r2 = 0.0f;
      float dx[3];
      for (int k = 0; k < 3; k++) {
        dx[k] = pix[k] - pj->x[k];
        r2 += dx[k] * dx[k];
      }

#ifdef SWIFT_DEBUG_CHECKS
      /* Check that particles have been drifted to the current time */
      if (pi->ti_drift != e->ti_current)
        error("Particle pi not drifted to current time");
      if (pj->ti_drift != e->ti_current)
        error("Particle pj not drifted to current time");
#endif

      /* Hit or miss? */
      if (r2 < hig2) {

        IACT_NONSYM(r2, dx, hi, pj->h, pi, pj);
      }
    } /* loop over the parts in cj. */
  }   /* loop over the parts in ci. */

  TIMER_TOC(timer_dopair_subset_naive);
}

/**
 * @brief Compute the interactions between a cell pair, but only for the
 *      given indices in ci.
 *
 * @param r The #runner.
 * @param ci The first #cell.
 * @param parts_i The #part to interact with @c cj.
 * @param ind The list of indices of particles in @c ci to interact with.
 * @param count The number of particles in @c ind.
 * @param cj The second #cell.
 */
void DOPAIR_SUBSET(struct runner *r, struct cell *restrict ci,
                   struct part *restrict parts_i, int *restrict ind, int count,
                   struct cell *restrict cj) {

  const struct engine *e = r->e;

  TIMER_TIC;

  const int count_j = cj->count;
  struct part *restrict parts_j = cj->parts;

  /* Get the relative distance between the pairs, wrapping. */
  double shift[3] = {0.0, 0.0, 0.0};
  for (int k = 0; k < 3; k++) {
    if (cj->loc[k] - ci->loc[k] < -e->s->dim[k] / 2)
      shift[k] = e->s->dim[k];
    else if (cj->loc[k] - ci->loc[k] > e->s->dim[k] / 2)
      shift[k] = -e->s->dim[k];
  }

  /* Get the sorting index. */
  int sid = 0;
  for (int k = 0; k < 3; k++)
    sid = 3 * sid + ((cj->loc[k] - ci->loc[k] + shift[k] < 0)
                         ? 0
                         : (cj->loc[k] - ci->loc[k] + shift[k] > 0) ? 2 : 1);

  /* Switch the cells around? */
  const int flipped = runner_flip[sid];
  sid = sortlistID[sid];

  /* Has the cell cj been sorted? */
  if (!(cj->sorted & (1 << sid)) ||
      cj->dx_max_sort_old > space_maxreldx * cj->dmin)
    error("Interacting unsorted cells.");

  /* Pick-out the sorted lists. */
  const struct entry *restrict sort_j = cj->sort[sid];
  const float dxj = cj->dx_max_sort;

  /* Parts are on the left? */
  if (!flipped) {

    /* Loop over the parts_i. */
    for (int pid = 0; pid < count; pid++) {

      /* Get a hold of the ith part in ci. */
      struct part *restrict pi = &parts_i[ind[pid]];
      const double pix = pi->x[0] - (shift[0]);
      const double piy = pi->x[1] - (shift[1]);
      const double piz = pi->x[2] - (shift[2]);
      const float hi = pi->h;
      const float hig2 = hi * hi * kernel_gamma2;
      const double di = hi * kernel_gamma + dxj + pix * runner_shift[sid][0] +
                        piy * runner_shift[sid][1] + piz * runner_shift[sid][2];

      /* Loop over the parts in cj. */
      for (int pjd = 0; pjd < count_j && sort_j[pjd].d < di; pjd++) {

        /* Get a pointer to the jth particle. */
        struct part *restrict pj = &parts_j[sort_j[pjd].i];
        const float hj = pj->h;
        const double pjx = pj->x[0];
        const double pjy = pj->x[1];
        const double pjz = pj->x[2];

        /* Compute the pairwise distance. */
        float dx[3] = {pix - pjx, piy - pjy, piz - pjz};
        const float r2 = dx[0] * dx[0] + dx[1] * dx[1] + dx[2] * dx[2];

        /* Hit or miss? */
        if (r2 < hig2) {

          IACT_NONSYM(r2, dx, hi, hj, pi, pj);
        }
      } /* loop over the parts in cj. */
    }   /* loop over the parts in ci. */
  }

  /* Parts are on the right. */
  else {

    /* Loop over the parts_i. */
    for (int pid = 0; pid < count; pid++) {

      /* Get a hold of the ith part in ci. */
      struct part *restrict pi = &parts_i[ind[pid]];
      const double pix = pi->x[0] - (shift[0]);
      const double piy = pi->x[1] - (shift[1]);
      const double piz = pi->x[2] - (shift[2]);
      const float hi = pi->h;
      const float hig2 = hi * hi * kernel_gamma2;
      const double di = -hi * kernel_gamma - dxj + pix * runner_shift[sid][0] +
                        piy * runner_shift[sid][1] + piz * runner_shift[sid][2];

      /* Loop over the parts in cj. */
      for (int pjd = count_j - 1; pjd >= 0 && di < sort_j[pjd].d; pjd--) {

        /* Get a pointer to the jth particle. */
        struct part *restrict pj = &parts_j[sort_j[pjd].i];
        const float hj = pj->h;
        const double pjx = pj->x[0];
        const double pjy = pj->x[1];
        const double pjz = pj->x[2];

        /* Compute the pairwise distance. */
        float dx[3] = {pix - pjx, piy - pjy, piz - pjz};
        const float r2 = dx[0] * dx[0] + dx[1] * dx[1] + dx[2] * dx[2];

        /* Hit or miss? */
        if (r2 < hig2) {

          IACT_NONSYM(r2, dx, hi, hj, pi, pj);
        }
      } /* loop over the parts in cj. */
    }   /* loop over the parts in ci. */
  }

  TIMER_TOC(timer_dopair_subset);
}

/**
 * @brief Compute the interactions between a cell pair, but only for the
 *      given indices in ci.
 *
 * @param r The #runner.
 * @param ci The first #cell.
 * @param parts The #part to interact.
 * @param ind The list of indices of particles in @c ci to interact with.
 * @param count The number of particles in @c ind.
 */
void DOSELF_SUBSET(struct runner *r, struct cell *restrict ci,
                   struct part *restrict parts, int *restrict ind, int count) {

#ifdef SWIFT_DEBUG_CHECKS
  const struct engine *e = r->e;
#endif

  TIMER_TIC;

  const int count_i = ci->count;
  struct part *restrict parts_j = ci->parts;

  /* Loop over the parts in ci. */
  for (int pid = 0; pid < count; pid++) {

    /* Get a hold of the ith part in ci. */
    struct part *pi = &parts[ind[pid]];
    const float pix[3] = {pi->x[0] - ci->loc[0], pi->x[1] - ci->loc[1],
                          pi->x[2] - ci->loc[2]};
    const float hi = pi->h;
    const float hig2 = hi * hi * kernel_gamma2;

#ifdef SWIFT_DEBUG_CHECKS
    if (!part_is_active(pi, e)) error("Inactive particle in subset function!");
#endif

    /* Loop over the parts in cj. */
    for (int pjd = 0; pjd < count_i; pjd++) {

      /* Get a pointer to the jth particle. */
      struct part *restrict pj = &parts_j[pjd];

      /* Compute the pairwise distance. */
      const float pjx[3] = {pj->x[0] - ci->loc[0], pj->x[1] - ci->loc[1],
                            pj->x[2] - ci->loc[2]};
      float dx[3] = {pix[0] - pjx[0], pix[1] - pjx[1], pix[2] - pjx[2]};
      const float r2 = dx[0] * dx[0] + dx[1] * dx[1] + dx[2] * dx[2];

      /* Hit or miss? */
      if (r2 > 0.f && r2 < hig2) {

        IACT_NONSYM(r2, dx, hi, pj->h, pi, pj);
      }
    } /* loop over the parts in cj. */
  }   /* loop over the parts in ci. */

  TIMER_TOC(timer_doself_subset);
}

/**
 * @brief Compute the interactions between a cell pair (non-symmetric).
 *
 * @param r The #runner.
 * @param ci The first #cell.
 * @param cj The second #cell.
 * @param sid The direction of the pair
 * @param shift The shift vector to apply to the particles in ci.
 */
void DOPAIR1(struct runner *r, struct cell *ci, struct cell *cj, const int sid,
             const double *shift) {

  const struct engine *restrict e = r->e;

  // DOPAIR1_NAIVE(r, ci, cj);
  // return;

  TIMER_TIC;

  /* Get the cutoff shift. */
  double rshift = 0.0;
  for (int k = 0; k < 3; k++) rshift += shift[k] * runner_shift[sid][k];

  /* Pick-out the sorted lists. */
  const struct entry *restrict sort_i = ci->sort[sid];
  const struct entry *restrict sort_j = cj->sort[sid];

#ifdef SWIFT_DEBUG_CHECKS
  /* Check that the dx_max_sort values in the cell are indeed an upper
     bound on particle movement. */
  for (int pid = 0; pid < ci->count; pid++) {
    const struct part *p = &ci->parts[sort_i[pid].i];
    const float d = p->x[0] * runner_shift[sid][0] +
                    p->x[1] * runner_shift[sid][1] +
                    p->x[2] * runner_shift[sid][2];
    if (fabsf(d - sort_i[pid].d) - ci->dx_max_sort >
        1.0e-4 * max(fabsf(d), ci->dx_max_sort_old))
      error(
          "particle shift diff exceeds dx_max_sort in cell ci. ci->nodeID=%d "
          "cj->nodeID=%d d=%e sort_i[pid].d=%e ci->dx_max_sort=%e "
          "ci->dx_max_sort_old=%e",
          ci->nodeID, cj->nodeID, d, sort_i[pid].d, ci->dx_max_sort,
          ci->dx_max_sort_old);
  }
  for (int pjd = 0; pjd < cj->count; pjd++) {
    const struct part *p = &cj->parts[sort_j[pjd].i];
    const float d = p->x[0] * runner_shift[sid][0] +
                    p->x[1] * runner_shift[sid][1] +
                    p->x[2] * runner_shift[sid][2];
    if (fabsf(d - sort_j[pjd].d) - cj->dx_max_sort >
        1.0e-4 * max(fabsf(d), cj->dx_max_sort_old))
      error(
          "particle shift diff exceeds dx_max_sort in cell cj. cj->nodeID=%d "
          "ci->nodeID=%d d=%e sort_j[pjd].d=%e cj->dx_max_sort=%e "
          "cj->dx_max_sort_old=%e",
          cj->nodeID, ci->nodeID, d, sort_j[pjd].d, cj->dx_max_sort,
          cj->dx_max_sort_old);
  }
#endif /* SWIFT_DEBUG_CHECKS */

  /* Get some other useful values. */
  const double hi_max = ci->h_max * kernel_gamma - rshift;
  const double hj_max = cj->h_max * kernel_gamma;
  const int count_i = ci->count;
  const int count_j = cj->count;
  struct part *restrict parts_i = ci->parts;
  struct part *restrict parts_j = cj->parts;
  const double di_max = sort_i[count_i - 1].d - rshift;
  const double dj_min = sort_j[0].d;
  const float dx_max = (ci->dx_max_sort + cj->dx_max_sort);

  if (cell_is_active(ci, e)) {

    /* Loop over the parts in ci. */
    for (int pid = count_i - 1;
         pid >= 0 && sort_i[pid].d + hi_max + dx_max > dj_min; pid--) {

      /* Get a hold of the ith part in ci. */
      struct part *restrict pi = &parts_i[sort_i[pid].i];
      const float hi = pi->h;

      /* Skip inactive particles */
      if (!part_is_active(pi, e)) continue;

      /* Is there anything we need to interact with ? */
      const double di = sort_i[pid].d + hi * kernel_gamma + dx_max - rshift;
      if (di < dj_min) continue;

      /* Get some additional information about pi */
      const float hig2 = hi * hi * kernel_gamma2;
      const float pix = pi->x[0] - (cj->loc[0] + shift[0]);
      const float piy = pi->x[1] - (cj->loc[1] + shift[1]);
      const float piz = pi->x[2] - (cj->loc[2] + shift[2]);

      /* Loop over the parts in cj. */
      for (int pjd = 0; pjd < count_j && sort_j[pjd].d < di; pjd++) {

        /* Recover pj */
        struct part *pj = &parts_j[sort_j[pjd].i];
        const float hj = pj->h;
        const float pjx = pj->x[0] - cj->loc[0];
        const float pjy = pj->x[1] - cj->loc[1];
        const float pjz = pj->x[2] - cj->loc[2];

        /* Compute the pairwise distance. */
        float dx[3] = {pix - pjx, piy - pjy, piz - pjz};
        const float r2 = dx[0] * dx[0] + dx[1] * dx[1] + dx[2] * dx[2];

#ifdef SWIFT_DEBUG_CHECKS
        /* Check that particles have been drifted to the current time */
        if (pi->ti_drift != e->ti_current)
          error("Particle pi not drifted to current time");
        if (pj->ti_drift != e->ti_current)
          error("Particle pj not drifted to current time");
#endif

        /* Hit or miss? */
        if (r2 < hig2) {

          IACT_NONSYM(r2, dx, hi, hj, pi, pj);
        }
      } /* loop over the parts in cj. */
    }   /* loop over the parts in ci. */
  }     /* Cell ci is active */

  if (cell_is_active(cj, e)) {

    /* Loop over the parts in cj. */
    for (int pjd = 0; pjd < count_j && sort_j[pjd].d - hj_max - dx_max < di_max;
         pjd++) {

      /* Get a hold of the jth part in cj. */
      struct part *pj = &parts_j[sort_j[pjd].i];
      const float hj = pj->h;

      /* Skip inactive particles */
      if (!part_is_active(pj, e)) continue;

      /* Is there anything we need to interact with ? */
      const double dj = sort_j[pjd].d - hj * kernel_gamma - dx_max + rshift;
      if (dj - rshift > di_max) continue;

      /* Get some additional information about pj */
      const float hjg2 = hj * hj * kernel_gamma2;
      const float pjx = pj->x[0] - cj->loc[0];
      const float pjy = pj->x[1] - cj->loc[1];
      const float pjz = pj->x[2] - cj->loc[2];

      /* Loop over the parts in ci. */
      for (int pid = count_i - 1; pid >= 0 && sort_i[pid].d > dj; pid--) {

        /* Recover pi */
        struct part *pi = &parts_i[sort_i[pid].i];
        const float hi = pi->h;
        const float pix = pi->x[0] - (cj->loc[0] + shift[0]);
        const float piy = pi->x[1] - (cj->loc[1] + shift[1]);
        const float piz = pi->x[2] - (cj->loc[2] + shift[2]);

        /* Compute the pairwise distance. */
        float dx[3] = {pjx - pix, pjy - piy, pjz - piz};
        const float r2 = dx[0] * dx[0] + dx[1] * dx[1] + dx[2] * dx[2];

#ifdef SWIFT_DEBUG_CHECKS
        /* Check that particles have been drifted to the current time */
        if (pi->ti_drift != e->ti_current)
          error("Particle pi not drifted to current time");
        if (pj->ti_drift != e->ti_current)
          error("Particle pj not drifted to current time");
#endif

        /* Hit or miss? */
        if (r2 < hjg2) {

          IACT_NONSYM(r2, dx, hj, hi, pj, pi);
        }
      } /* loop over the parts in ci. */
    }   /* loop over the parts in cj. */
  }     /* Cell cj is active */

  TIMER_TOC(TIMER_DOPAIR);
}

/**
 * @brief Determine which version of DOPAIR1 needs to be called depending on the
 * orientation of the cells or whether DOPAIR1 needs to be called at all.
 *
 * @param r #runner
 * @param ci #cell ci
 * @param cj #cell cj
 *
 */
void DOPAIR1_BRANCH(struct runner *r, struct cell *ci, struct cell *cj) {

  const struct engine *restrict e = r->e;

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

  /* Check that cells are drifted. */
  if (!cell_are_part_drifted(ci, e) || !cell_are_part_drifted(cj, e))
    error("Interacting undrifted cells.");

  /* Get the sort ID. */
  double shift[3] = {0.0, 0.0, 0.0};
  const int sid = space_getsid(e->s, &ci, &cj, shift);

  /* Have the cells been sorted? */
  if (!(ci->sorted & (1 << sid)) ||
      ci->dx_max_sort_old > space_maxreldx * ci->dmin)
    error("Interacting unsorted cells.");
  if (!(cj->sorted & (1 << sid)) ||
      cj->dx_max_sort_old > space_maxreldx * cj->dmin)
    error("Interacting unsorted cells.");

#if defined(WITH_VECTORIZATION) && defined(GADGET2_SPH) && \
    (DOPAIR1_BRANCH == runner_dopair1_density_branch)
  if (!sort_is_corner(sid))
    runner_dopair1_density_vec(r, ci, cj, sid, shift);
  else
    DOPAIR1(r, ci, cj, sid, shift);
#else
  DOPAIR1(r, ci, cj, sid, shift);
#endif
}

/**
 * @brief Compute the interactions between a cell pair (symmetric)
 *
 * @param r The #runner.
 * @param ci The first #cell.
 * @param cj The second #cell.
 */
void DOPAIR2(struct runner *r, struct cell *ci, struct cell *cj) {

  struct engine *restrict e = r->e;

  // DOPAIR2_NAIVE(r, ci, cj);
  // return;

  TIMER_TIC;

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

  if (!cell_are_part_drifted(ci, e) || !cell_are_part_drifted(cj, e))
    error("Interacting undrifted cells.");

  /* Get the shift ID. */
  double shift[3] = {0.0, 0.0, 0.0};
  const int sid = space_getsid(e->s, &ci, &cj, shift);

  /* Have the cells been sorted? */
  if (!(ci->sorted & (1 << sid)) ||