When merging BHs, accumulate the number of seeds

src/black_holes/EAGLE/black_holes.h

@@ -59,6 +59,8 @@ __attribute__((always_inline)) INLINE static void black_holes_first_init_bpart(
   bp->total_accreted_mass = 0.f;
   bp->accretion_rate = 0.f;
   bp->formation_time = -1.f;
+  bp->cumulative_number_seeds = 1;
+  bp->number_of_mergers = 0;
 }
 
 /**
@@ -244,6 +246,36 @@ __attribute__((always_inline)) INLINE static void black_holes_swallow_part(
  */
 __attribute__((always_inline)) INLINE static void black_holes_swallow_bpart(
     struct bpart* bpi, const struct bpart* bpj, const struct cosmology* cosmo) {
+
+  /* Get the current dynamical masses */
+  const float bpi_dyn_mass = bpi->mass;
+  const float bpj_dyn_mass = bpj->mass;
+
+  /* Increase the masses of the BH. */
+  bpi->mass += bpj->mass;
+  bpi->gpart->mass += bpj->mass;
+  bpi->subgrid_mass += bpj->subgrid_mass;
+
+  /* Update the BH momentum */
+  const float BH_mom[3] = {bpi_dyn_mass * bpi->v[0] + bpj_dyn_mass * bpj->v[0],
+                           bpi_dyn_mass * bpi->v[1] + bpj_dyn_mass * bpj->v[1],
+                           bpi_dyn_mass * bpi->v[2] + bpj_dyn_mass * bpj->v[2]};
+
+  bpi->v[0] = BH_mom[0] / bpi->mass;
+  bpi->v[1] = BH_mom[1] / bpi->mass;
+  bpi->v[2] = BH_mom[2] / bpi->mass;
+  bpi->gpart->v_full[0] = bpi->v[0];
+  bpi->gpart->v_full[1] = bpi->v[1];
+  bpi->gpart->v_full[2] = bpi->v[2];
+
+  /* Update the energy reservoir */
+  bpi->energy_reservoir += bpj->energy_reservoir;
+
+  /* Add up all the BH seeds */
+  bpi->cumulative_number_seeds += bpj->cumulative_number_seeds;
+
+  /* We had another merger */
+  bpi->number_of_mergers++;
 }
 
 /**
@@ -446,6 +478,8 @@ INLINE static void black_holes_create_from_gas(
 
   /* We haven't accreted anything yet */
   bp->total_accreted_mass = 0.f;
+  bp->cumulative_number_seeds = 1;
+  bp->number_of_mergers = 0;
 
   /* Initial metal masses */
   const float gas_mass = hydro_get_mass(p);

src/black_holes/EAGLE/black_holes_io.h

@@ -77,7 +77,7 @@ INLINE static void black_holes_write_particles(const struct bpart *bparts,
                                                int *num_fields) {
 
   /* Say how much we want to write */
-  *num_fields = 12;
+  *num_fields = 13;
 
   /* List what we want to write */
   list[0] = io_make_output_field_convert_bpart(
@@ -106,6 +106,9 @@ INLINE static void black_holes_write_particles(const struct bpart *bparts,
   list[11] = io_make_output_field("TotalAccretedMass", FLOAT, 1,
                                   UNIT_CONV_MASS_PER_UNIT_TIME, bparts,
                                   total_accreted_mass);
+  list[12] =
+      io_make_output_field("CumulativeNumberSeeds", INT, 1, UNIT_CONV_NO_UNITS,
+                           bparts, cumulative_number_seeds);
 
 #ifdef DEBUG_INTERACTIONS_BLACK_HOLES
 

src/black_holes/EAGLE/black_holes_part.h

@@ -103,6 +103,12 @@ struct bpart {
   /*! Integer number of neighbours */
   int num_ngbs;
 
+  /*! Number of seeds in this BH (i.e. itself + the merged ones) */
+  int cumulative_number_seeds;
+
+  /*! Total number of BH merger events (i.e. not including all progenies) */
+  int number_of_mergers;
+
   /*! Properties used in the feedback loop to distribute to gas neighbours. */
   struct {