cell.c 172 KB
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/*******************************************************************************
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 * This file is part of SWIFT.
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 * Copyright (c) 2012 Pedro Gonnet (pedro.gonnet@durham.ac.uk)
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 *                    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)
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 *
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 * 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.
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 *
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 * 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.
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 *
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 * 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/>.
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 *
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 ******************************************************************************/

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

/* Some standard headers. */
#include <float.h>
#include <limits.h>
#include <math.h>
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#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
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/* MPI headers. */
#ifdef WITH_MPI
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#include <mpi.h>
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#endif

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/* Switch off timers. */
#ifdef TIMER
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#undef TIMER
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#endif

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

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/* Local headers. */
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#include "active.h"
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#include "atomic.h"
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#include "chemistry.h"
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#include "drift.h"
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#include "engine.h"
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#include "error.h"
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#include "gravity.h"
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#include "hydro.h"
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#include "hydro_properties.h"
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#include "memswap.h"
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#include "minmax.h"
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#include "scheduler.h"
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#include "space.h"
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#include "space_getsid.h"
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#include "stars.h"
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#include "timers.h"
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#include "tools.h"
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#include "tracers.h"
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/* Global variables. */
int cell_next_tag = 0;

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/**
 * @brief Get the size of the cell subtree.
 *
 * @param c The #cell.
 */
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int cell_getsize(struct cell *c) {
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  /* Number of cells in this subtree. */
  int count = 1;
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  /* Sum up the progeny if split. */
  if (c->split)
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    for (int k = 0; k < 8; k++)
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      if (c->progeny[k] != NULL) count += cell_getsize(c->progeny[k]);

  /* Return the final count. */
  return count;
}

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/**
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 * @brief Link the cells recursively to the given #part array.
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 *
 * @param c The #cell.
 * @param parts The #part array.
 *
 * @return The number of particles linked.
 */
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int cell_link_parts(struct cell *c, struct part *parts) {

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#ifdef SWIFT_DEBUG_CHECKS
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  if (c->nodeID == engine_rank)
    error("Linking foreign particles in a local cell!");

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  if (c->hydro.parts != NULL)
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    error("Linking parts into a cell that was already linked");
#endif

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  c->hydro.parts = parts;

  /* Fill the progeny recursively, depth-first. */
  if (c->split) {
    int offset = 0;
    for (int k = 0; k < 8; k++) {
      if (c->progeny[k] != NULL)
        offset += cell_link_parts(c->progeny[k], &parts[offset]);
    }
  }

  /* Return the total number of linked particles. */
  return c->hydro.count;
}

/**
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 * @brief Link the cells recursively to the given #gpart array.
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 *
 * @param c The #cell.
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 * @param gparts The #gpart array.
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 *
 * @return The number of particles linked.
 */
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int cell_link_gparts(struct cell *c, struct gpart *gparts) {
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#ifdef SWIFT_DEBUG_CHECKS
  if (c->nodeID == engine_rank)
    error("Linking foreign particles in a local cell!");

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  if (c->grav.parts != NULL)
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    error("Linking gparts into a cell that was already linked");
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#endif
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  c->grav.parts = gparts;
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  /* Fill the progeny recursively, depth-first. */
  if (c->split) {
    int offset = 0;
    for (int k = 0; k < 8; k++) {
      if (c->progeny[k] != NULL)
        offset += cell_link_gparts(c->progeny[k], &gparts[offset]);
    }
  }

  /* Return the total number of linked particles. */
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  return c->grav.count;
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}

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/**
 * @brief Link the cells recursively to the given #spart array.
 *
 * @param c The #cell.
 * @param sparts The #spart array.
 *
 * @return The number of particles linked.
 */
int cell_link_sparts(struct cell *c, struct spart *sparts) {

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#ifdef SWIFT_DEBUG_CHECKS
  if (c->nodeID == engine_rank)
    error("Linking foreign particles in a local cell!");

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  if (c->stars.parts != NULL)
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    error("Linking sparts into a cell that was already linked");
#endif

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  c->stars.parts = sparts;
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  /* Fill the progeny recursively, depth-first. */
  if (c->split) {
    int offset = 0;
    for (int k = 0; k < 8; k++) {
      if (c->progeny[k] != NULL)
        offset += cell_link_sparts(c->progeny[k], &sparts[offset]);
    }
  }

  /* Return the total number of linked particles. */
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  return c->stars.count;
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}

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/**
 * @brief Recurse down foreign cells until reaching one with hydro
 * tasks; then trigger the linking of the #part array from that
 * level.
 *
 * @param c The #cell.
 * @param parts The #part array.
 *
 * @return The number of particles linked.
 */
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int cell_link_foreign_parts(struct cell *c, struct part *parts) {

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#ifdef WITH_MPI

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#ifdef SWIFT_DEBUG_CHECKS
  if (c->nodeID == engine_rank)
    error("Linking foreign particles in a local cell!");
#endif

  /* Do we have a hydro task at this level? */
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  if (cell_get_recv(c, task_subtype_xv) != NULL) {
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    /* Recursively attach the parts */
    const int counts = cell_link_parts(c, parts);
#ifdef SWIFT_DEBUG_CHECKS
    if (counts != c->hydro.count)
      error("Something is wrong with the foreign counts");
#endif
    return counts;
  }

  /* Go deeper to find the level where the tasks are */
  if (c->split) {
    int count = 0;
    for (int k = 0; k < 8; k++) {
      if (c->progeny[k] != NULL) {
        count += cell_link_foreign_parts(c->progeny[k], &parts[count]);
      }
    }
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    return count;
  } else {
    return 0;
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  }
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#else
  error("Calling linking of foregin particles in non-MPI mode.");
#endif
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}

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/**
 * @brief Recurse down foreign cells until reaching one with gravity
 * tasks; then trigger the linking of the #gpart array from that
 * level.
 *
 * @param c The #cell.
 * @param gparts The #gpart array.
 *
 * @return The number of particles linked.
 */
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int cell_link_foreign_gparts(struct cell *c, struct gpart *gparts) {

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#ifdef WITH_MPI

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#ifdef SWIFT_DEBUG_CHECKS
  if (c->nodeID == engine_rank)
    error("Linking foreign particles in a local cell!");
#endif

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  /* Do we have a gravity task at this level? */
  if (cell_get_recv(c, task_subtype_gpart) != NULL) {
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    /* Recursively attach the gparts */
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    const int counts = cell_link_gparts(c, gparts);
#ifdef SWIFT_DEBUG_CHECKS
    if (counts != c->grav.count)
      error("Something is wrong with the foreign counts");
#endif
    return counts;
  }

  /* Go deeper to find the level where the tasks are */
  if (c->split) {
    int count = 0;
    for (int k = 0; k < 8; k++) {
      if (c->progeny[k] != NULL) {
        count += cell_link_foreign_gparts(c->progeny[k], &gparts[count]);
      }
    }
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    return count;
  } else {
    return 0;
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  }
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#else
  error("Calling linking of foregin particles in non-MPI mode.");
#endif
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}

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/**
 * @brief Recursively count the number of #part in foreign cells that
 * are in cells with hydro-related tasks.
 *
 * @param c The #cell.
 *
 * @return The number of particles linked.
 */
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int cell_count_parts_for_tasks(const struct cell *c) {

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#ifdef WITH_MPI

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#ifdef SWIFT_DEBUG_CHECKS
  if (c->nodeID == engine_rank)
    error("Counting foreign particles in a local cell!");
#endif

  /* Do we have a hydro task at this level? */
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  if (cell_get_recv(c, task_subtype_xv) != NULL) {
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    return c->hydro.count;
  }

  if (c->split) {
    int count = 0;
    for (int k = 0; k < 8; ++k) {
      if (c->progeny[k] != NULL) {
        count += cell_count_parts_for_tasks(c->progeny[k]);
      }
    }
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    return count;
  } else {
    return 0;
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  }
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#else
  error("Calling linking of foregin particles in non-MPI mode.");
#endif
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}

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/**
 * @brief Recursively count the number of #gpart in foreign cells that
 * are in cells with gravity-related tasks.
 *
 * @param c The #cell.
 *
 * @return The number of particles linked.
 */
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int cell_count_gparts_for_tasks(const struct cell *c) {

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#ifdef WITH_MPI

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#ifdef SWIFT_DEBUG_CHECKS
  if (c->nodeID == engine_rank)
    error("Counting foreign particles in a local cell!");
#endif

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  /* Do we have a gravity task at this level? */
  if (cell_get_recv(c, task_subtype_gpart) != NULL) {
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    return c->grav.count;
  }

  if (c->split) {
    int count = 0;
    for (int k = 0; k < 8; ++k) {
      if (c->progeny[k] != NULL) {
        count += cell_count_gparts_for_tasks(c->progeny[k]);
      }
    }
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    return count;
  } else {
    return 0;
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  }
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#else
  error("Calling linking of foregin particles in non-MPI mode.");
#endif
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}

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/**
 * @brief Pack the data of the given cell and all it's sub-cells.
 *
 * @param c The #cell.
 * @param pc Pointer to an array of packed cells in which the
 *      cells will be packed.
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 * @param with_gravity Are we running with gravity and hence need
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 *
 * @return The number of packed cells.
 */
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int cell_pack(struct cell *restrict c, struct pcell *restrict pc,
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              const int with_gravity) {
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#ifdef WITH_MPI

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  /* Start by packing the data of the current cell. */
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  pc->hydro.h_max = c->hydro.h_max;
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  pc->stars.h_max = c->stars.h_max;
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  pc->hydro.ti_end_min = c->hydro.ti_end_min;
  pc->hydro.ti_end_max = c->hydro.ti_end_max;
  pc->grav.ti_end_min = c->grav.ti_end_min;
  pc->grav.ti_end_max = c->grav.ti_end_max;
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  pc->stars.ti_end_min = c->stars.ti_end_min;
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  pc->stars.ti_end_max = c->stars.ti_end_max;
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  pc->hydro.ti_old_part = c->hydro.ti_old_part;
  pc->grav.ti_old_part = c->grav.ti_old_part;
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  pc->grav.ti_old_multipole = c->grav.ti_old_multipole;
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  pc->stars.ti_old_part = c->stars.ti_old_part;
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  pc->hydro.count = c->hydro.count;
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  pc->grav.count = c->grav.count;
  pc->stars.count = c->stars.count;
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  pc->maxdepth = c->maxdepth;
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  /* Copy the Multipole related information */
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  if (with_gravity) {
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    const struct gravity_tensors *mp = c->grav.multipole;
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    pc->grav.m_pole = mp->m_pole;
    pc->grav.CoM[0] = mp->CoM[0];
    pc->grav.CoM[1] = mp->CoM[1];
    pc->grav.CoM[2] = mp->CoM[2];
    pc->grav.CoM_rebuild[0] = mp->CoM_rebuild[0];
    pc->grav.CoM_rebuild[1] = mp->CoM_rebuild[1];
    pc->grav.CoM_rebuild[2] = mp->CoM_rebuild[2];
    pc->grav.r_max = mp->r_max;
    pc->grav.r_max_rebuild = mp->r_max_rebuild;
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  }

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#ifdef SWIFT_DEBUG_CHECKS
  pc->cellID = c->cellID;
#endif
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  /* Fill in the progeny, depth-first recursion. */
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  int count = 1;
  for (int k = 0; k < 8; k++)
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    if (c->progeny[k] != NULL) {
      pc->progeny[k] = count;
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      count += cell_pack(c->progeny[k], &pc[count], with_gravity);
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    } else {
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      pc->progeny[k] = -1;
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    }
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  /* Return the number of packed cells used. */
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  c->mpi.pcell_size = count;
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  return count;
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#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
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}

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/**
 * @brief Pack the tag of the given cell and all it's sub-cells.
 *
 * @param c The #cell.
 * @param tags Pointer to an array of packed tags.
 *
 * @return The number of packed tags.
 */
int cell_pack_tags(const struct cell *c, int *tags) {

#ifdef WITH_MPI

  /* Start by packing the data of the current cell. */
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  tags[0] = c->mpi.tag;
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  /* Fill in the progeny, depth-first recursion. */
  int count = 1;
  for (int k = 0; k < 8; k++)
    if (c->progeny[k] != NULL)
      count += cell_pack_tags(c->progeny[k], &tags[count]);

#ifdef SWIFT_DEBUG_CHECKS
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  if (c->mpi.pcell_size != count) error("Inconsistent tag and pcell count!");
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#endif  // SWIFT_DEBUG_CHECKS

  /* Return the number of packed tags used. */
  return count;

#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
}

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/**
 * @brief Unpack the data of a given cell and its sub-cells.
 *
 * @param pc An array of packed #pcell.
 * @param c The #cell in which to unpack the #pcell.
 * @param s The #space in which the cells are created.
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 * @param with_gravity Are we running with gravity and hence need
 *      to exchange multipoles?
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 *
 * @return The number of cells created.
 */
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int cell_unpack(struct pcell *restrict pc, struct cell *restrict c,
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                struct space *restrict s, const int with_gravity) {
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#ifdef WITH_MPI

  /* Unpack the current pcell. */
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  c->hydro.h_max = pc->hydro.h_max;
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  c->stars.h_max = pc->stars.h_max;
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  c->hydro.ti_end_min = pc->hydro.ti_end_min;
  c->hydro.ti_end_max = pc->hydro.ti_end_max;
  c->grav.ti_end_min = pc->grav.ti_end_min;
  c->grav.ti_end_max = pc->grav.ti_end_max;
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  c->stars.ti_end_min = pc->stars.ti_end_min;
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  c->stars.ti_end_max = pc->stars.ti_end_max;
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  c->hydro.ti_old_part = pc->hydro.ti_old_part;
  c->grav.ti_old_part = pc->grav.ti_old_part;
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  c->grav.ti_old_multipole = pc->grav.ti_old_multipole;
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  c->stars.ti_old_part = pc->stars.ti_old_part;
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  c->hydro.count = pc->hydro.count;
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  c->grav.count = pc->grav.count;
  c->stars.count = pc->stars.count;
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  c->maxdepth = pc->maxdepth;

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#ifdef SWIFT_DEBUG_CHECKS
  c->cellID = pc->cellID;
#endif
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  /* Copy the Multipole related information */
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  if (with_gravity) {
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    struct gravity_tensors *mp = c->grav.multipole;
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    mp->m_pole = pc->grav.m_pole;
    mp->CoM[0] = pc->grav.CoM[0];
    mp->CoM[1] = pc->grav.CoM[1];
    mp->CoM[2] = pc->grav.CoM[2];
    mp->CoM_rebuild[0] = pc->grav.CoM_rebuild[0];
    mp->CoM_rebuild[1] = pc->grav.CoM_rebuild[1];
    mp->CoM_rebuild[2] = pc->grav.CoM_rebuild[2];
    mp->r_max = pc->grav.r_max;
    mp->r_max_rebuild = pc->grav.r_max_rebuild;
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  }
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  /* Number of new cells created. */
  int count = 1;

  /* Fill the progeny recursively, depth-first. */
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  c->split = 0;
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  for (int k = 0; k < 8; k++)
    if (pc->progeny[k] >= 0) {
      struct cell *temp;
      space_getcells(s, 1, &temp);
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      temp->hydro.count = 0;
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      temp->grav.count = 0;
      temp->stars.count = 0;
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      temp->loc[0] = c->loc[0];
      temp->loc[1] = c->loc[1];
      temp->loc[2] = c->loc[2];
      temp->width[0] = c->width[0] / 2;
      temp->width[1] = c->width[1] / 2;
      temp->width[2] = c->width[2] / 2;
      temp->dmin = c->dmin / 2;
      if (k & 4) temp->loc[0] += temp->width[0];
      if (k & 2) temp->loc[1] += temp->width[1];
      if (k & 1) temp->loc[2] += temp->width[2];
      temp->depth = c->depth + 1;
      temp->split = 0;
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      temp->hydro.dx_max_part = 0.f;
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      temp->hydro.dx_max_sort = 0.f;
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      temp->stars.dx_max_part = 0.f;
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      temp->stars.dx_max_sort = 0.f;
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      temp->nodeID = c->nodeID;
      temp->parent = c;
      c->progeny[k] = temp;
      c->split = 1;
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      count += cell_unpack(&pc[pc->progeny[k]], temp, s, with_gravity);
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    }

  /* Return the total number of unpacked cells. */
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  c->mpi.pcell_size = count;
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  return count;

#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
}

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/**
 * @brief Unpack the tags of a given cell and its sub-cells.
 *
 * @param tags An array of tags.
 * @param c The #cell in which to unpack the tags.
 *
 * @return The number of tags created.
 */
int cell_unpack_tags(const int *tags, struct cell *restrict c) {

#ifdef WITH_MPI

  /* Unpack the current pcell. */
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  c->mpi.tag = tags[0];
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  /* Number of new cells created. */
  int count = 1;

  /* Fill the progeny recursively, depth-first. */
  for (int k = 0; k < 8; k++)
    if (c->progeny[k] != NULL) {
      count += cell_unpack_tags(&tags[count], c->progeny[k]);
    }

#ifdef SWIFT_DEBUG_CHECKS
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  if (c->mpi.pcell_size != count) error("Inconsistent tag and pcell count!");
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#endif  // SWIFT_DEBUG_CHECKS

  /* Return the total number of unpacked tags. */
  return count;

#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
}

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/**
 * @brief Pack the time information of the given cell and all it's sub-cells.
 *
 * @param c The #cell.
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 * @param pcells (output) The end-of-timestep information we pack into
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 *
 * @return The number of packed cells.
 */
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int cell_pack_end_step_hydro(struct cell *restrict c,
                             struct pcell_step_hydro *restrict pcells) {
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#ifdef WITH_MPI

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  /* Pack this cell's data. */
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  pcells[0].ti_end_min = c->hydro.ti_end_min;
  pcells[0].ti_end_max = c->hydro.ti_end_max;
  pcells[0].dx_max_part = c->hydro.dx_max_part;
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  /* Fill in the progeny, depth-first recursion. */
  int count = 1;
  for (int k = 0; k < 8; k++)
    if (c->progeny[k] != NULL) {
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      count += cell_pack_end_step_hydro(c->progeny[k], &pcells[count]);
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    }

  /* Return the number of packed values. */
  return count;
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#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
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}

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/**
 * @brief Unpack the time information of a given cell and its sub-cells.
 *
 * @param c The #cell
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 * @param pcells The end-of-timestep information to unpack
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 *
 * @return The number of cells created.
 */
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int cell_unpack_end_step_hydro(struct cell *restrict c,
                               struct pcell_step_hydro *restrict pcells) {
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#ifdef WITH_MPI

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  /* Unpack this cell's data. */
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  c->hydro.ti_end_min = pcells[0].ti_end_min;
  c->hydro.ti_end_max = pcells[0].ti_end_max;
  c->hydro.dx_max_part = pcells[0].dx_max_part;
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  /* Fill in the progeny, depth-first recursion. */
  int count = 1;
  for (int k = 0; k < 8; k++)
    if (c->progeny[k] != NULL) {
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      count += cell_unpack_end_step_hydro(c->progeny[k], &pcells[count]);
    }

  /* Return the number of packed values. */
  return count;

#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
}

/**
 * @brief Pack the time information of the given cell and all it's sub-cells.
 *
 * @param c The #cell.
 * @param pcells (output) The end-of-timestep information we pack into
 *
 * @return The number of packed cells.
 */
int cell_pack_end_step_grav(struct cell *restrict c,
                            struct pcell_step_grav *restrict pcells) {

#ifdef WITH_MPI

  /* Pack this cell's data. */
  pcells[0].ti_end_min = c->grav.ti_end_min;
  pcells[0].ti_end_max = c->grav.ti_end_max;

  /* Fill in the progeny, depth-first recursion. */
  int count = 1;
  for (int k = 0; k < 8; k++)
    if (c->progeny[k] != NULL) {
      count += cell_pack_end_step_grav(c->progeny[k], &pcells[count]);
    }

  /* Return the number of packed values. */
  return count;

#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
}

/**
 * @brief Unpack the time information of a given cell and its sub-cells.
 *
 * @param c The #cell
 * @param pcells The end-of-timestep information to unpack
 *
 * @return The number of cells created.
 */
int cell_unpack_end_step_grav(struct cell *restrict c,
                              struct pcell_step_grav *restrict pcells) {

#ifdef WITH_MPI

  /* Unpack this cell's data. */
  c->grav.ti_end_min = pcells[0].ti_end_min;
  c->grav.ti_end_max = pcells[0].ti_end_max;

  /* Fill in the progeny, depth-first recursion. */
  int count = 1;
  for (int k = 0; k < 8; k++)
    if (c->progeny[k] != NULL) {
      count += cell_unpack_end_step_grav(c->progeny[k], &pcells[count]);
    }

  /* Return the number of packed values. */
  return count;

#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
}

/**
 * @brief Pack the time information of the given cell and all it's sub-cells.
 *
 * @param c The #cell.
 * @param pcells (output) The end-of-timestep information we pack into
 *
 * @return The number of packed cells.
 */
int cell_pack_end_step_stars(struct cell *restrict c,
                             struct pcell_step_stars *restrict pcells) {

#ifdef WITH_MPI

  /* Pack this cell's data. */
  pcells[0].ti_end_min = c->stars.ti_end_min;
  pcells[0].ti_end_max = c->stars.ti_end_max;
  pcells[0].dx_max_part = c->stars.dx_max_part;

  /* Fill in the progeny, depth-first recursion. */
  int count = 1;
  for (int k = 0; k < 8; k++)
    if (c->progeny[k] != NULL) {
      count += cell_pack_end_step_stars(c->progeny[k], &pcells[count]);
    }

  /* Return the number of packed values. */
  return count;

#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
}

/**
 * @brief Unpack the time information of a given cell and its sub-cells.
 *
 * @param c The #cell
 * @param pcells The end-of-timestep information to unpack
 *
 * @return The number of cells created.
 */
int cell_unpack_end_step_stars(struct cell *restrict c,
                               struct pcell_step_stars *restrict pcells) {

#ifdef WITH_MPI

  /* Unpack this cell's data. */
  c->stars.ti_end_min = pcells[0].ti_end_min;
  c->stars.ti_end_max = pcells[0].ti_end_max;
  c->stars.dx_max_part = pcells[0].dx_max_part;

  /* Fill in the progeny, depth-first recursion. */
  int count = 1;
  for (int k = 0; k < 8; k++)
    if (c->progeny[k] != NULL) {
      count += cell_unpack_end_step_stars(c->progeny[k], &pcells[count]);
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    }

  /* Return the number of packed values. */
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  return count;
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#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
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}
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/**
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 * @brief Pack the multipole information of the given cell and all it's
 * sub-cells.
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 *
 * @param c The #cell.
 * @param pcells (output) The multipole information we pack into
 *
 * @return The number of packed cells.
 */
int cell_pack_multipoles(struct cell *restrict c,
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                         struct gravity_tensors *restrict pcells) {
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#ifdef WITH_MPI

  /* Pack this cell's data. */
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  pcells[0] = *c->grav.multipole;
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  /* Fill in the progeny, depth-first recursion. */
  int count = 1;
  for (int k = 0; k < 8; k++)
    if (c->progeny[k] != NULL) {
      count += cell_pack_multipoles(c->progeny[k], &pcells[count]);
    }

  /* Return the number of packed values. */
  return count;

#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
}

/**
 * @brief Unpack the multipole information of a given cell and its sub-cells.
 *
 * @param c The #cell
 * @param pcells The multipole information to unpack
 *
 * @return The number of cells created.
 */
int cell_unpack_multipoles(struct cell *restrict c,
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                           struct gravity_tensors *restrict pcells) {
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#ifdef WITH_MPI

  /* Unpack this cell's data. */
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  *c->grav.multipole = pcells[0];
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  /* Fill in the progeny, depth-first recursion. */
  int count = 1;
  for (int k = 0; k < 8; k++)
    if (c->progeny[k] != NULL) {
      count += cell_unpack_multipoles(c->progeny[k], &pcells[count]);
    }

  /* Return the number of packed values. */
  return count;

#else
  error("SWIFT was not compiled with MPI support.");
  return 0;
#endif
}

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/**
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 * @brief Lock a cell for access to its array of #part and hold its parents.
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 *
 * @param c The #cell.
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 * @return 0 on success, 1 on failure
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 */
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int cell_locktree(struct cell *c) {

  TIMER_TIC

  /* First of all, try to lock this cell. */
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  if (c->hydro.hold || lock_trylock(&c->hydro.lock) != 0) {
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    TIMER_TOC(timer_locktree);
    return 1;
  }

  /* Did somebody hold this cell in the meantime? */
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  if (c->hydro.hold) {
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    /* Unlock this cell. */
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    if (lock_unlock(&c->hydro.lock) != 0) error("Failed to unlock cell.");
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    /* Admit defeat. */
    TIMER_TOC(timer_locktree);
    return 1;
  }

  /* Climb up the tree and lock/hold/unlock. */
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  struct cell *finger;
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  for (finger = c->parent; finger != NULL; finger = finger->parent) {

    /* Lock this cell. */
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    if (lock_trylock(&finger->hydro.lock) != 0) break;
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    /* Increment the hold. */
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    atomic_inc(&finger->hydro.hold);
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    /* Unlock the cell. */
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    if (lock_unlock(&finger->hydro.lock) != 0) error("Failed to unlock cell.");
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  }

  /* If we reached the top of the tree, we're done. */
  if (finger == NULL) {
    TIMER_TOC(timer_locktree);
    return 0;
  }

  /* Otherwise, we hit a snag. */
  else {

    /* Undo the holds up to finger. */
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    for (struct cell *finger2 = c->parent; finger2 != finger;
         finger2 = finger2->parent)
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      atomic_dec(&finger2->hydro.hold);
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    /* Unlock this cell. */
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    if (lock_unlock(&c->hydro.lock) != 0) error("Failed to unlock cell.");
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    /* Admit defeat. */
    TIMER_TOC(timer_locktree);
    return 1;
  }
}

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/**
 * @brief Lock a cell for access to its array of #gpart and hold its parents.
 *
 * @param c The #cell.
 * @return 0 on success, 1 on failure
 */
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int cell_glocktree(struct cell *c) {

  TIMER_TIC

  /* First of all, try to lock this cell. */
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  if (c->grav.phold || lock_trylock(&c->grav.plock) != 0) {
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    TIMER_TOC(timer_locktree);
    return 1;
  }

  /* Did somebody hold this cell in the meantime? */
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  if (c->grav.phold) {
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    /* Unlock this cell. */
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    if (lock_unlock(&c->grav.plock) != 0) error("Failed to unlock cell.");
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    /* Admit defeat. */
    TIMER_TOC(timer_locktree);
    return 1;
  }

  /* Climb up the tree and lock/hold/unlock. */
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  struct cell *finger;
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  for (finger = c->parent; finger != NULL; finger = finger->parent) {

    /* Lock this cell. */
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    if (lock_trylock(&finger->grav.plock) != 0) break;
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    /* Increment the hold. */
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    atomic_inc(&finger->grav.phold);
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    /* Unlock the cell. */
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    if (lock_unlock(&finger->grav.plock) != 0) error("Failed to unlock cell.");
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  }

  /* If we reached the top of the tree, we're done. */
  if (finger == NULL) {
    TIMER_TOC(timer_locktree);
    return 0;
  }

  /* Otherwise, we hit a snag. */
  else {

    /* Undo the holds up to finger. */
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    for (struct cell *finger2 = c->parent; finger2 != finger;
         finger2 = finger2->parent)
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      atomic_dec(&finger2->grav.phold);
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    /* Unlock this cell. */
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    if (lock_unlock(&c->grav.plock) != 0) error("Failed to unlock cell.");
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    /* Admit defeat. */
    TIMER_TOC(timer_locktree);
    return 1;