runner.c 48 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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 ******************************************************************************/
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/* Config parameters. */
#include "../config.h"
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/* Some standard headers. */
#include <float.h>
#include <limits.h>
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#include <stdlib.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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/* This object's header. */
#include "runner.h"

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/* Local headers. */
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#include "approx_math.h"
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#include "atomic.h"
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#include "const.h"
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#include "debug.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_properties.h"
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#include "hydro.h"
#include "minmax.h"
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#include "scheduler.h"
#include "space.h"
#include "task.h"
#include "timers.h"
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/* Orientation of the cell pairs */
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const float runner_shift[13 * 3] = {
    5.773502691896258e-01, 5.773502691896258e-01,  5.773502691896258e-01,
    7.071067811865475e-01, 7.071067811865475e-01,  0.0,
    5.773502691896258e-01, 5.773502691896258e-01,  -5.773502691896258e-01,
    7.071067811865475e-01, 0.0,                    7.071067811865475e-01,
    1.0,                   0.0,                    0.0,
    7.071067811865475e-01, 0.0,                    -7.071067811865475e-01,
    5.773502691896258e-01, -5.773502691896258e-01, 5.773502691896258e-01,
    7.071067811865475e-01, -7.071067811865475e-01, 0.0,
    5.773502691896258e-01, -5.773502691896258e-01, -5.773502691896258e-01,
    0.0,                   7.071067811865475e-01,  7.071067811865475e-01,
    0.0,                   1.0,                    0.0,
    0.0,                   7.071067811865475e-01,  -7.071067811865475e-01,
    0.0,                   0.0,                    1.0, };
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/* Does the axis need flipping ? */
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const char runner_flip[27] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0,
                              0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
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/* #define MY_CELL 428428428 */
/* #define DX 0.1 */
/* #define NX 1000 */
/* #define CELL_ID                                                  \ */
/*   ((int)(c->loc[0] / DX) * NX *NX + (int)(c->loc[1] / DX) * NX + \ */
/*    (int)(c->loc[2] / DX)) */
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/* #define OUT \ */
/*   if (CELL_ID == MY_CELL) { \ */
/*     message(" cell= %d gcount=%d time=%f \n", CELL_ID, c->gcount,
 * r->e->time); \ */
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/*   } */
/* #define OUT  message(" cell %d %d %f \n",CELL_ID, c->count, r->e->time); */
/* #define OUT  if(CELL_ID == MY_CELL) message("\n cell %f %f %f %d %d */
/* %f\n",c->loc[0],c->loc[1],c->loc[2], CELL_ID, c->count, r->e->time); */
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/* Import the density loop functions. */
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#define FUNCTION density
#include "runner_doiact.h"

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

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

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/**
 * @brief Calculate gravity acceleration from external potential
 *
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 * @param r runner task
 * @param c cell
 * @param timer 1 if the time is to be recorded.
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 */
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void runner_do_grav_external(struct runner *r, struct cell *c, int timer) {
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  struct gpart *g, *gparts = c->gparts;
  int i, k, gcount = c->gcount;
  const int ti_current = r->e->ti_current;
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  const struct external_potential *potential = r->e->external_potential;
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  const struct phys_const *constants = r->e->physical_constants;
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  TIMER_TIC;
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  /* Recurse? */
  if (c->split) {
    for (k = 0; k < 8; k++)
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      if (c->progeny[k] != NULL) runner_do_grav_external(r, c->progeny[k], 0);
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    return;
  }

#ifdef TASK_VERBOSE
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  OUT;
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#endif
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  /* Loop over the parts in this cell. */
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  for (i = 0; i < gcount; i++) {
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    /* Get a direct pointer on the part. */
    g = &gparts[i];

    /* Is this part within the time step? */
    if (g->ti_end <= ti_current) {
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      external_gravity(potential, constants, g);
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      /* /\* check for energy and angular momentum conservation - begin by */
      /*  * synchronizing velocity*\/ */
      /* const float dx = g->x[0] - r->e->potential->point_mass.x; */
      /* const float dy = g->x[1] - r->e->potential->point_mass.y; */
      /* const float dz = g->x[2] - r->e->potential->point_mass.z; */
      /* const float dr = sqrtf((dx * dx) + (dy * dy) + (dz * dz)); */
      /* const float rinv = 1.f / sqrtf(dx * dx + dy * dy + dz * dz); */

      /* const int current_dti = g->ti_end - g->ti_begin; */
      /* const float dt = 0.5f * current_dti * r->e->timeBase; */
      /* const float vx = g->v_full[0] + dt * g->a_grav[0]; */
      /* const float vy = g->v_full[1] + dt * g->a_grav[1]; */
      /* const float vz = g->v_full[2] + dt * g->a_grav[2]; */

      /* /\* E/L *\/ */
      /* float L[3], E; */
      /* E = 0.5 * ((vx * vx) + (vy * vy) + (vz * vz)) - */
      /*     r->e->physical_constants->newton_gravity * */
      /*         r->e->potential->point_mass.mass * rinv; */
      /* L[0] = dy * vz - dz * vy; */
      /* L[1] = dz * vx - dx * vz; */
      /* L[2] = dx * vy - dy * vx; */
      /* if (abs(g->id) == 1) { */
      /*   float v2 = vx * vx + vy * vy + vz * vz; */
      /*   float fg = r->e->physical_constants->newton_gravity * */
      /*              r->e->potential->point_mass.mass * rinv; */
      /*   float fga = sqrtf((g->a_grav[0] * g->a_grav[0]) + */
      /*                     (g->a_grav[1] * g->a_grav[1]) + */
      /*                     (g->a_grav[2] * g->a_grav[2])) * */
      /*               dr; */
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      /*   // message("grav_external time= %f\t V_c^2= %f GM/r= %f E= %f L[2]=
       * %f */
      /*   // x= %f y= %f vx= %f vy= %f\n", r->e->time, v2, fg, E, L[2],
       * g->x[0], */
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      /*   // g->x[1], vx, vy); */
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      /*   message("%f\t %f %f %f %f %f %f %f %f %f %f %f %f %f\n", r->e->time,
       */
      /*           g->tx, g->tv, dt, v2, fg, fga, dr, E, L[2], g->x[0], g->x[1],
       */
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      /*           vx, vy); */
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      /*   /\* message(" G=%e M=%e\n", r->e->physical_constants->newton_gravity,
       */
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      /*    * r->e->potential->point_mass.mass); *\/ */
      /*   /\* exit(-1); *\/ */
      /* } */
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    }
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  }
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  if (timer) TIMER_TOC(timer_dograv_external);
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}

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

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/**
 * @brief Sort the particles in the given cell along all cardinal directions.
 *
 * @param r The #runner.
 * @param c The #cell.
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 * @param flags Cell flag.
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 * @param clock Flag indicating whether to record the timing or not, needed
 *      for recursive calls.
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 */
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void runner_do_sort(struct runner *r, struct cell *c, int flags, int clock) {
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  struct entry *finger;
  struct entry *fingers[8];
  struct part *parts = c->parts;
  struct entry *sort;
  int j, k, count = c->count;
  int i, ind, off[8], inds[8], temp_i, missing;
  // float shift[3];
  float buff[8], px[3];

  TIMER_TIC

  /* Clean-up the flags, i.e. filter out what's already been sorted. */
  flags &= ~c->sorted;
  if (flags == 0) return;

  /* 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. */
    for (k = 0; k < 8; k++) {
      if (c->progeny[k] == NULL) continue;
      missing = flags & ~c->progeny[k]->sorted;
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      if (missing) runner_do_sort(r, c->progeny[k], missing, 0);
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    }

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

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

      /* Init the particle index offsets. */
      for (off[0] = 0, 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. */
      for (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 (i = 0; i < 7; i++)
        for (k = i + 1; k < 8; k++)
          if (buff[inds[k]] < buff[inds[i]]) {
            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 (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 (k = 1; k < 8 && buff[inds[k]] < buff[inds[k - 1]]; k++) {
          temp_i = inds[k - 1];
          inds[k - 1] = inds[k];
          inds[k] = temp_i;
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        }
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      } /* 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 (k = 0; k < count; k++) {
      px[0] = parts[k].x[0];
      px[1] = parts[k].x[1];
      px[2] = parts[k].x[2];
      for (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[3 * j + 0] +
                                        px[1] * runner_shift[3 * j + 1] +
                                        px[2] * runner_shift[3 * j + 2];
        }
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    }
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    /* Add the sentinel and sort. */
    for (j = 0; j < 13; j++)
      if (flags & (1 << j)) {
        sort[j * (count + 1) + count].d = FLT_MAX;
        sort[j * (count + 1) + count].i = 0;
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        runner_do_sort_ascending(&sort[j * (count + 1)], count);
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        c->sorted |= (1 << j);
      }
  }

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  /* Verify the sorting. */
  /* for ( j = 0 ; j < 13 ; j++ ) {
      if ( !( flags & (1 << j) ) )
          continue;
      finger = &sort[ j*(count + 1) ];
      for ( 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." );
          }
      } */
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  if (clock) TIMER_TOC(timer_dosort);
}

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void runner_do_gsort(struct runner *r, struct cell *c, int flags, int clock) {
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  struct entry *finger;
  struct entry *fingers[8];
  struct gpart *gparts = c->gparts;
  struct entry *gsort;
  int j, k, count = c->gcount;
  int i, ind, off[8], inds[8], temp_i, missing;
  // float shift[3];
  float buff[8], px[3];

  TIMER_TIC

  /* Clean-up the flags, i.e. filter out what's already been sorted. */
  flags &= ~c->gsorted;
  if (flags == 0) return;

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

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

    /* Fill in the gaps within the progeny. */
    for (k = 0; k < 8; k++) {
      if (c->progeny[k] == NULL) continue;
      missing = flags & ~c->progeny[k]->gsorted;
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      if (missing) runner_do_gsort(r, c->progeny[k], missing, 0);
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    }

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

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

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

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

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

      /* For each entry in the new sort list. */
      finger = &gsort[j * (count + 1)];
      for (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 (k = 1; k < 8 && buff[inds[k]] < buff[inds[k - 1]]; k++) {
          temp_i = inds[k - 1];
          inds[k - 1] = inds[k];
          inds[k] = temp_i;
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        }
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      } /* Merge. */

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

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

    } /* loop over sort arrays. */

  } /* progeny? */

  /* Otherwise, just sort. */
  else {

    /* Fill the sort array. */
    for (k = 0; k < count; k++) {
      px[0] = gparts[k].x[0];
      px[1] = gparts[k].x[1];
      px[2] = gparts[k].x[2];
      for (j = 0; j < 13; j++)
        if (flags & (1 << j)) {
          gsort[j * (count + 1) + k].i = k;
          gsort[j * (count + 1) + k].d = px[0] * runner_shift[3 * j + 0] +
                                         px[1] * runner_shift[3 * j + 1] +
                                         px[2] * runner_shift[3 * j + 2];
        }
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    }
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    /* Add the sentinel and sort. */
    for (j = 0; j < 13; j++)
      if (flags & (1 << j)) {
        gsort[j * (count + 1) + count].d = FLT_MAX;
        gsort[j * (count + 1) + count].i = 0;
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        runner_do_sort_ascending(&gsort[j * (count + 1)], count);
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        c->gsorted |= (1 << j);
      }
  }

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  /* Verify the sorting. */
  /* for ( j = 0 ; j < 13 ; j++ ) {
      if ( !( flags & (1 << j) ) )
          continue;
      finger = &c->gsort[ j*(count + 1) ];
      for ( k = 1 ; k < count ; k++ ) {
          if ( finger[k].d < finger[k-1].d )
              error( "Sorting failed, ascending array." );
          if ( finger[k].i < 0 || finger[k].i >= count )
              error( "Sorting failed, indices borked." );
          }
      } */
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  if (clock) TIMER_TOC(timer_dosort);
}

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/**
 * @brief Initialize the particles before the density calculation
 *
 * @param r The runner thread.
 * @param c The cell.
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 * @param timer 1 if the time is to be recorded.
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 */
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void runner_do_init(struct runner *r, struct cell *c, int timer) {
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  struct part *const parts = c->parts;
  struct gpart *const gparts = c->gparts;
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  const int count = c->count;
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  const int gcount = c->gcount;
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  const int ti_current = r->e->ti_current;
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  TIMER_TIC;
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  /* Recurse? */
  if (c->split) {
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    for (int k = 0; k < 8; k++)
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      if (c->progeny[k] != NULL) runner_do_init(r, c->progeny[k], 0);
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    return;
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  } else {

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    /* Loop over the parts in this cell. */
    for (int i = 0; i < count; i++) {
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      /* Get a direct pointer on the part. */
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      struct part *const p = &parts[i];
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      if (p->ti_end <= ti_current) {
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        /* Get ready for a density calculation */
        hydro_init_part(p);
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      }
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    }
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    /* Loop over the gparts in this cell. */
    for (int i = 0; i < gcount; i++) {

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

      if (gp->ti_end <= ti_current) {

        /* Get ready for a density calculation */
        gravity_init_part(gp);
      }
    }
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  }
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  if (timer) TIMER_TOC(timer_init);
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}

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/**
 * @brief Intermediate task between density and force
 *
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 * @param r The runner thread.
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 * @param c The cell.
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 */
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void runner_do_ghost(struct runner *r, struct cell *c) {
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  struct part *p, *parts = c->parts;
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  struct xpart *xp, *xparts = c->xparts;
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  struct cell *finger;
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  int redo, count = c->count;
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  int *pid;
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  float h_corr;
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  const int ti_current = r->e->ti_current;
  const double timeBase = r->e->timeBase;
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  const float target_wcount = r->e->hydro_properties->target_neighbours;
  const float max_wcount =
      target_wcount + r->e->hydro_properties->delta_neighbours;
  const float min_wcount =
      target_wcount - r->e->hydro_properties->delta_neighbours;
  const int max_smoothing_iter =
      r->e->hydro_properties->max_smoothing_iterations;
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  TIMER_TIC;

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

  /* Init the IDs that have to be updated. */
  if ((pid = (int *)alloca(sizeof(int) * count)) == NULL)
    error("Call to alloca failed.");
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  for (int k = 0; k < count; k++) pid[k] = k;
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  /* While there are particles that need to be updated... */
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  for (int num_reruns = 0; count > 0 && num_reruns < max_smoothing_iter;
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       num_reruns++) {
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    /* Reset the redo-count. */
    redo = 0;
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    /* Loop over the parts in this cell. */
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    for (int i = 0; i < count; i++) {
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      /* Get a direct pointer on the part. */
      p = &parts[pid[i]];
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      xp = &xparts[pid[i]];
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      /* Is this part within the timestep? */
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      if (p->ti_end <= ti_current) {
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        /* Finish the density calculation */
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        hydro_end_density(p, ti_current);
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        /* If no derivative, double the smoothing length. */
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        if (p->density.wcount_dh == 0.0f) h_corr = p->h;
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        /* Otherwise, compute the smoothing length update (Newton step). */
        else {
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          h_corr = (target_wcount - p->density.wcount) / p->density.wcount_dh;
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          /* Truncate to the range [ -p->h/2 , p->h ]. */
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          h_corr = fminf(h_corr, p->h);
          h_corr = fmaxf(h_corr, -p->h * 0.5f);
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        }
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        /* Did we get the right number density? */
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        if (p->density.wcount > max_wcount || p->density.wcount < min_wcount) {
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          /* Ok, correct then */
          p->h += h_corr;

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          /* Flag for another round of fun */
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          pid[redo] = pid[i];
          redo += 1;
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          /* Re-initialise everything */
          hydro_init_part(p);
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          /* Off we go ! */
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          continue;
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        }

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        /* We now have a particle whose smoothing length has converged */
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        /* As of here, particle force variables will be set. */
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        /* Compute variables required for the force loop */
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        hydro_prepare_force(p, xp, ti_current, timeBase);
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        /* The particle force values are now set.  Do _NOT_
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           try to read any particle density variables! */
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        /* Prepare the particle for the force loop over neighbours */
        hydro_reset_acceleration(p);
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      }
    }

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

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

      /* Climb up the cell hierarchy. */
      for (finger = c; finger != NULL; finger = finger->parent) {
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        /* Run through this cell's density interactions. */
        for (struct link *l = finger->density; l != NULL; l = l->next) {
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          /* 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 interaction? */
          else if (l->t->type == task_type_sub) {

            /* 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);
          }
        }
      }
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    }
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  }
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  if (count)
    message("Smoothing length failed to converge on %i particles.", count);
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  TIMER_TOC(timer_do_ghost);
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}

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/**
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 * @brief Drift particles and g-particles forward in time
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 *
 * @param r The runner thread.
 * @param c The cell.
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 * @param timer Are we timing this ?
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 */
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void runner_do_drift(struct runner *r, struct cell *c, int timer) {
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  const double timeBase = r->e->timeBase;
  const double dt = (r->e->ti_current - r->e->ti_old) * timeBase;
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  const int ti_old = r->e->ti_old;
  const int ti_current = r->e->ti_current;
  struct part *const parts = c->parts;
  struct xpart *const xparts = c->xparts;
  struct gpart *const gparts = c->gparts;
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  float dx_max = 0.f, dx2_max = 0.f, h_max = 0.f;
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  TIMER_TIC
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#ifdef TASK_VERBOSE
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  OUT;
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#endif
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  /* No children? */
  if (!c->split) {
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    /* Loop over all the g-particles in the cell */
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    const int nr_gparts = c->gcount;
    for (size_t k = 0; k < nr_gparts; k++) {

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

      /* Drift... */
      gp->x[0] += gp->v_full[0] * dt;
      gp->x[1] += gp->v_full[1] * dt;
      gp->x[2] += gp->v_full[2] * dt;
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      /* Compute offset since last cell construction */
      gp->x_diff[0] -= gp->v_full[0] * dt;
      gp->x_diff[1] -= gp->v_full[1] * dt;
      gp->x_diff[2] -= gp->v_full[2] * dt;

      /* Compute (square of) motion since last cell construction */
      const float dx2 = gp->x_diff[0] * gp->x_diff[0] +
                        gp->x_diff[1] * gp->x_diff[1] +
                        gp->x_diff[2] * gp->x_diff[2];
      dx2_max = fmaxf(dx2_max, dx2);
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    }

    /* Loop over all the particles in the cell (more work for these !) */
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    const size_t nr_parts = c->count;
    for (size_t k = 0; k < nr_parts; k++) {
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      /* Get a handle on the part. */
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      struct part *const p = &parts[k];
      struct xpart *const xp = &xparts[k];
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      /* Useful quantity */
      const float h_inv = 1.0f / p->h;

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      /* Drift... */
      p->x[0] += xp->v_full[0] * dt;
      p->x[1] += xp->v_full[1] * dt;
      p->x[2] += xp->v_full[2] * dt;
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      /* Predict velocities (for hydro terms) */
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      p->v[0] += p->a_hydro[0] * dt;
      p->v[1] += p->a_hydro[1] * dt;
      p->v[2] += p->a_hydro[2] * dt;
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      /* Predict smoothing length */
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      const float w1 = p->h_dt * h_inv * dt;
      if (fabsf(w1) < 0.2f)
        p->h *= approx_expf(w1); /* 4th order expansion of exp(w) */
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      else
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        p->h *= expf(w1);
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      /* Predict density */
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      const float w2 = -3.0f * p->h_dt * h_inv * dt;
      if (fabsf(w2) < 0.2f)
        p->rho *= approx_expf(w2); /* 4th order expansion of exp(w) */
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      else
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        p->rho *= expf(w2);
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      /* Predict the values of the extra fields */
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      hydro_predict_extra(p, xp, ti_old, ti_current, timeBase);
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      /* Compute offset since last cell construction */
      xp->x_diff[0] -= xp->v_full[0] * dt;
      xp->x_diff[1] -= xp->v_full[1] * dt;
      xp->x_diff[2] -= xp->v_full[2] * dt;

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      /* Compute (square of) motion since last cell construction */
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      const float dx2 = xp->x_diff[0] * xp->x_diff[0] +
                        xp->x_diff[1] * xp->x_diff[1] +
                        xp->x_diff[2] * xp->x_diff[2];
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      dx2_max = fmaxf(dx2_max, dx2);
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      /* Maximal smoothing length */
      h_max = fmaxf(p->h, h_max);
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    }
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    /* Now, get the maximal particle motion from its square */
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    dx_max = sqrtf(dx2_max);
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  }
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  /* Otherwise, aggregate data from children. */
  else {

    /* Loop over the progeny. */
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL) {
        struct cell *cp = c->progeny[k];
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        runner_do_drift(r, cp, 0);
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        dx_max = fmaxf(dx_max, cp->dx_max);
        h_max = fmaxf(h_max, cp->h_max);
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      }
  }

  /* Store the values */
  c->h_max = h_max;
  c->dx_max = dx_max;
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  if (timer) TIMER_TOC(timer_drift);
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}
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/**
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 * @brief Kick particles in momentum space and collect statistics (fixed
 * time-step case)
 *
 * @param r The runner thread.
 * @param c The cell.
 * @param timer Are we timing this ?
 */
void runner_do_kick_fixdt(struct runner *r, struct cell *c, int timer) {

  const double global_dt = r->e->dt_max;
  const double timeBase = r->e->timeBase;
  const int count = c->count;
  const int gcount = c->gcount;
  struct part *const parts = c->parts;
  struct xpart *const xparts = c->xparts;
  struct gpart *const gparts = c->gparts;

  int updated = 0, g_updated = 0;
  int ti_end_min = max_nr_timesteps, ti_end_max = 0;
  double e_kin = 0.0, e_int = 0.0, e_pot = 0.0, mass = 0.0;
  float mom[3] = {0.0f, 0.0f, 0.0f};
  float ang[3] = {0.0f, 0.0f, 0.0f};

  TIMER_TIC

#ifdef TASK_VERBOSE
  OUT;
#endif

  /* The new time-step */
  const int new_dti = global_dt / timeBase;

  /* No children? */
  if (!c->split) {

    /* Loop over the g-particles and kick the active ones. */
    for (int k = 0; k < gcount; k++) {

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

      /* If the g-particle has no counterpart and needs to be kicked */
      if (gp->id < 0) {

        /* First, finish the force calculation */
        gravity_end_force(gp);

        /* Compute the time step for this kick */
        const int ti_start = (gp->ti_begin + gp->ti_end) / 2;
        const int ti_end = gp->ti_end + new_dti / 2;
        const double dt = (ti_end - ti_start) * timeBase;
        const double half_dt = (ti_end - gp->ti_end) * timeBase;

        /* Move particle forward in time */
        gp->ti_begin = gp->ti_end;
        gp->ti_end = gp->ti_begin + new_dti;

        /* Kick particles in momentum space */
        gp->v_full[0] += gp->a_grav[0] * dt;
        gp->v_full[1] += gp->a_grav[1] * dt;
        gp->v_full[2] += gp->a_grav[2] * dt;

        /* Extra kick work */
        gravity_kick_extra(gp, dt, half_dt);

        /* Number of updated g-particles */
        g_updated++;

        /* Minimal time for next end of time-step */
        ti_end_min = min(gp->ti_end, ti_end_min);
        ti_end_max = max(gp->ti_end, ti_end_max);
      }
    }

    /* Now do the hydro ones... */

    /* Loop over the particles and kick the active ones. */
    for (int k = 0; k < count; k++) {

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

      /* First, finish the force loop */
      p->h_dt *= p->h * 0.333333333f;

      /* And do the same of the extra variable */
      hydro_end_force(p);
      if (p->gpart != NULL) gravity_end_force(p->gpart);

      /* Compute the time step for this kick */
      const int ti_start = (p->ti_begin + p->ti_end) / 2;