diff --git a/tests/testActivePair.c b/tests/testActivePair.c
new file mode 100644
index 0000000000000000000000000000000000000000..990370d44d124a8065b95870b17e939b3618913c
--- /dev/null
+++ b/tests/testActivePair.c
@@ -0,0 +1,328 @@
+/*******************************************************************************
+ * This file is part of SWIFT.
+ * Copyright (C) 2015 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/>.
+ *
+ ******************************************************************************/
+#include "../config.h"
+
+/* Some standard headers. */
+#include <fenv.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+/* Local headers. */
+#include "swift.h"
+
+/* n is both particles per axis and box size:
+ * particles are generated on a mesh with unit spacing
+ */
+struct cell *make_cell(size_t n, double *offset, double size, double h,
+ double density, unsigned long long *partId,
+ double pert) {
+ const size_t count = n * n * n;
+ const double volume = size * size * size;
+ struct cell *cell = malloc(sizeof(struct cell));
+ bzero(cell, sizeof(struct cell));
+
+ if (posix_memalign((void **)&cell->parts, part_align,
+ count * sizeof(struct part)) != 0) {
+ error("couldn't allocate particles, no. of particles: %d", (int)count);
+ }
+ bzero(cell->parts, count * sizeof(struct part));
+
+ /* Construct the parts */
+ struct part *part = cell->parts;
+ for (size_t x = 0; x < n; ++x) {
+ for (size_t y = 0; y < n; ++y) {
+ for (size_t z = 0; z < n; ++z) {
+ part->x[0] =
+ offset[0] +
+ size * (x + 0.5 + random_uniform(-0.5, 0.5) * pert) / (float)n;
+ part->x[1] =
+ offset[1] +
+ size * (y + 0.5 + random_uniform(-0.5, 0.5) * pert) / (float)n;
+ part->x[2] =
+ offset[2] +
+ size * (z + 0.5 + random_uniform(-0.5, 0.5) * pert) / (float)n;
+ part->v[0] = random_uniform(-0.05, 0.05);
+ part->v[1] = random_uniform(-0.05, 0.05);
+ part->v[2] = random_uniform(-0.05, 0.05);
+ part->h = size * h / (float)n;
+ part->id = ++(*partId);
+#if defined(GIZMO_SPH) || defined(SHADOWFAX_SPH)
+ part->conserved.mass = density * volume / count;
+#else
+ part->mass = density * volume / count;
+#endif
+ part->time_bin = 1;
+ //part->time_bin = num_time_bins + 1;
+
+#ifdef SWIFT_DEBUG_CHECKS
+ part->ti_drift = 8;
+ part->ti_kick = 8;
+#endif
+
+ ++part;
+ }
+ }
+ }
+
+ /* Cell properties */
+ cell->split = 0;
+ cell->h_max = h;
+ cell->count = count;
+ cell->dx_max_part = 0.;
+ cell->dx_max_sort = 0.;
+ cell->width[0] = n;
+ cell->width[1] = n;
+ cell->width[2] = n;
+ cell->loc[0] = offset[0];
+ cell->loc[1] = offset[1];
+ cell->loc[2] = offset[2];
+
+ cell->ti_old_part = 8;
+ cell->ti_end_min = 8;
+ cell->ti_end_max = 8;
+
+ shuffle_particles(cell->parts, cell->count);
+
+ cell->sorted = 0;
+ cell->sort = NULL;
+
+ return cell;
+}
+
+void clean_up(struct cell *ci) {
+ free(ci->parts);
+ free(ci->sort);
+ free(ci);
+}
+
+/**
+ * @brief Initializes all particles field to be ready for a density calculation
+ */
+void zero_particle_fields(struct cell *c) {
+ for (int pid = 0; pid < c->count; pid++) {
+ hydro_init_part(&c->parts[pid], NULL);
+ }
+}
+
+/**
+ * @brief Dump all the particles to a file
+ */
+void dump_particle_fields(char *fileName, struct cell *ci, struct cell *cj) {
+ FILE *file = fopen(fileName, "w");
+
+ /* Write header */
+ fprintf(file,
+ "# %4s %10s %10s %10s %10s %10s %10s %13s %13s %13s %13s %13s "
+ "%13s %13s %13s\n",
+ "ID", "pos_x", "pos_y", "pos_z", "v_x", "v_y", "v_z", "rho", "rho_dh",
+ "wcount", "wcount_dh", "div_v", "curl_vx", "curl_vy", "curl_vz");
+
+ fprintf(file, "# ci --------------------------------------------\n");
+
+ for (int pid = 0; pid < ci->count; pid++) {
+ fprintf(file,
+ "%6llu %10f %10f %10f %10f %10f %10f %13e %13e %13e %13e %13e "
+ "%13e %13e %13e\n",
+ ci->parts[pid].id, ci->parts[pid].x[0], ci->parts[pid].x[1],
+ ci->parts[pid].x[2], ci->parts[pid].v[0], ci->parts[pid].v[1],
+ ci->parts[pid].v[2], hydro_get_density(&ci->parts[pid]),
+#if defined(GIZMO_SPH) || defined(SHADOWFAX_SPH)
+ 0.f,
+#else
+ ci->parts[pid].density.rho_dh,
+#endif
+ ci->parts[pid].density.wcount, ci->parts[pid].density.wcount_dh,
+#if defined(GADGET2_SPH) || defined(DEFAULT_SPH) || defined(HOPKINS_PE_SPH)
+ ci->parts[pid].density.div_v, ci->parts[pid].density.rot_v[0],
+ ci->parts[pid].density.rot_v[1], ci->parts[pid].density.rot_v[2]
+#else
+ 0., 0., 0., 0.
+#endif
+ );
+ }
+
+ fprintf(file, "# cj --------------------------------------------\n");
+
+ for (int pjd = 0; pjd < cj->count; pjd++) {
+ fprintf(file,
+ "%6llu %10f %10f %10f %10f %10f %10f %13e %13e %13e %13e %13e "
+ "%13e %13e %13e\n",
+ cj->parts[pjd].id, cj->parts[pjd].x[0], cj->parts[pjd].x[1],
+ cj->parts[pjd].x[2], cj->parts[pjd].v[0], cj->parts[pjd].v[1],
+ cj->parts[pjd].v[2], hydro_get_density(&cj->parts[pjd]),
+#if defined(GIZMO_SPH) || defined(SHADOWFAX_SPH)
+ 0.f,
+#else
+ cj->parts[pjd].density.rho_dh,
+#endif
+ cj->parts[pjd].density.wcount, cj->parts[pjd].density.wcount_dh,
+#if defined(GADGET2_SPH) || defined(DEFAULT_SPH) || defined(HOPKINS_PE_SPH)
+ cj->parts[pjd].density.div_v, cj->parts[pjd].density.rot_v[0],
+ cj->parts[pjd].density.rot_v[1], cj->parts[pjd].density.rot_v[2]
+#else
+ 0., 0., 0., 0.
+#endif
+ );
+ }
+
+ fclose(file);
+}
+
+/* Just a forward declaration... */
+void runner_dopair1_density(struct runner *r, struct cell *ci, struct cell *cj);
+void runner_doself1_density_vec(struct runner *r, struct cell *ci);
+void runner_dopair1_branch_density(struct runner *r, struct cell *ci,
+ struct cell *cj);
+
+int main(int argc, char *argv[]) {
+ size_t particles = 0, runs = 0, volume, type = 0;
+ double offset[3] = {0, 0, 0}, h = 1.1255, size = 1., rho = 1.;
+ double perturbation = 0.1;
+ struct cell *ci, *cj;
+ struct space space;
+ struct engine engine;
+ struct runner runner;
+ char c;
+ static unsigned long long partId = 0;
+ char outputFileNameExtension[200] = "";
+ char outputFileName[200] = "";
+ ticks tic, toc, time;
+
+ /* Initialize CPU frequency, this also starts time. */
+ unsigned long long cpufreq = 0;
+ clocks_set_cpufreq(cpufreq);
+
+ /* Choke on FP-exceptions */
+ feenableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);
+
+ srand(0);
+
+ while ((c = getopt(argc, argv, "h:p:r:t:d:f:")) != -1) {
+ switch (c) {
+ case 'h':
+ sscanf(optarg, "%lf", &h);
+ break;
+ case 'p':
+ sscanf(optarg, "%zu", &particles);
+ break;
+ case 'r':
+ sscanf(optarg, "%zu", &runs);
+ break;
+ case 't':
+ sscanf(optarg, "%zu", &type);
+ break;
+ case 'd':
+ sscanf(optarg, "%lf", &perturbation);
+ break;
+ case 'f':
+ strcpy(outputFileNameExtension, optarg);
+ break;
+ case '?':
+ error("Unknown option.");
+ break;
+ }
+ }
+
+ if (h < 0 || particles == 0 || runs == 0 || type > 2) {
+ printf(
+ "\nUsage: %s -p PARTICLES_PER_AXIS -r NUMBER_OF_RUNS [OPTIONS...]\n"
+ "\nGenerates a cell pair, filled with particles on a Cartesian grid."
+ "\nThese are then interacted using runner_dopair1_density."
+ "\n\nOptions:"
+ "\n-t TYPE=0 - cells share face (0), edge (1) or corner (2)"
+ "\n-h DISTANCE=1.1255 - smoothing length"
+ "\n-d pert - perturbation to apply to the particles [0,1["
+ "\n-f fileName - part of the file name used to save the dumps\n",
+ argv[0]);
+ exit(1);
+ }
+
+ space.periodic = 0;
+
+ engine.s = &space;
+ engine.time = 0.1f;
+ engine.ti_current = 8;
+ engine.max_active_bin = num_time_bins;
+ runner.e = &engine;
+
+ volume = particles * particles * particles;
+ message("particles: %zu B\npositions: 0 B", 2 * volume * sizeof(struct part));
+
+ ci = make_cell(particles, offset, size, h, rho, &partId, perturbation);
+ for (size_t i = 0; i < type + 1; ++i) offset[i] = 1.;
+ cj = make_cell(particles, offset, size, h, rho, &partId, perturbation);
+
+ runner_do_sort(&runner, ci, 0x1FFF, 0, 0);
+ runner_do_sort(&runner, cj, 0x1FFF, 0, 0);
+
+ time = 0;
+ /* Zero the fields */
+ zero_particle_fields(ci);
+ zero_particle_fields(cj);
+
+#ifdef WITH_VECTORIZATION
+ runner.ci_cache.count = 0;
+ cache_init(&runner.ci_cache, 512);
+ runner.cj_cache.count = 0;
+ cache_init(&runner.cj_cache, 512);
+#endif
+
+ tic = getticks();
+
+ /* Run the test */
+ runner_dopair1_branch_density(&runner, ci, cj);
+
+ toc = getticks();
+ time += toc - tic;
+
+ /* Dump if necessary */
+ sprintf(outputFileName, "swift_dopair_%s.dat", outputFileNameExtension);
+ dump_particle_fields(outputFileName, ci, cj);
+
+ /* Output timing */
+ message("SWIFT calculation took %lli ticks.", time / runs);
+
+ /* Now perform a brute-force version for accuracy tests */
+
+ /* Zero the fields */
+ zero_particle_fields(ci);
+ zero_particle_fields(cj);
+
+ tic = getticks();
+
+ /* Run the brute-force test */
+ pairs_all_density(&runner, ci, cj);
+
+ toc = getticks();
+
+ /* Dump */
+ sprintf(outputFileName, "brute_force_%s.dat", outputFileNameExtension);
+ dump_particle_fields(outputFileName, ci, cj);
+
+ /* Output timing */
+ message("Brute force calculation took %lli ticks.", toc - tic);
+
+ /* Clean things to make the sanitizer happy ... */
+ clean_up(ci);
+ clean_up(cj);
+
+ return 0;
+}
diff --git a/tests/testActivePair.sh.in b/tests/testActivePair.sh.in
new file mode 100755
index 0000000000000000000000000000000000000000..eb3cf5da3e9f456f2bb7b12f2e884e778fa86bca
--- /dev/null
+++ b/tests/testActivePair.sh.in
@@ -0,0 +1,11 @@
+#!/bin/bash
+
+echo ""
+
+rm -f brute_force_pair_active.dat swift_dopair_active.dat
+
+./testActivePair -p 6 -r 1 -d 0 -f active
+
+python @srcdir@/difffloat.py brute_force_active.dat swift_dopair_active.dat @srcdir@/tolerance_pair_active.dat
+
+exit $?