Have a default unit system for the snapshots when none is provided

src/engine.c

@@ -2672,7 +2672,7 @@ void engine_dump_snapshot(struct engine *e) {
   struct clocks_time time1, time2;
   clocks_gettime(&time1);
 
-  if (e->verbose) message("writing snapshot at t=%f.", e->time);
+  if (e->verbose) message("writing snapshot at t=%e.", e->time);
 
 /* Dump... */
 #if defined(WITH_MPI)
@@ -2808,7 +2808,7 @@ void engine_init(struct engine *e, struct space *s,
   e->ti_nextSnapshot = 0;
   parser_get_param_string(params, "Snapshots:basename", e->snapshotBaseName);
   e->snapshotUnits = malloc(sizeof(struct UnitSystem));
-  units_init(e->snapshotUnits, params, "Snapshots");
+  units_init_default(e->snapshotUnits, params, "Snapshots", internal_units);
   e->dt_min = parser_get_param_double(params, "TimeIntegration:dt_min");
   e->dt_max = parser_get_param_double(params, "TimeIntegration:dt_max");
   e->file_stats = NULL;
@@ -3228,6 +3228,6 @@ void engine_compute_next_snapshot_time(struct engine *e) {
     const float next_snapshot_time =
         e->ti_nextSnapshot * e->timeBase + e->timeBegin;
     if (e->verbose)
-      message("Next output time set to t=%f.", next_snapshot_time);
+      message("Next output time set to t=%e.", next_snapshot_time);
   }
 }

src/units.c

@@ -66,6 +66,41 @@ void units_init(struct UnitSystem* us, const struct swift_params* params,
   us->UnitTemperature_in_cgs = parser_get_param_double(params, buffer);
 }
 
+/**
+ * @brief Initialises the UnitSystem structure with the constants given in
+ * rhe parameter file. Uses a default if the values are not present in the file.
+ *
+ * @param us The UnitSystem to initialize.
+ * @param params The parsed parameter file.
+ * @param category The section of the parameter file to read from.
+ * @param def The default unit system to copy from if required.
+ */
+void units_init_default(struct UnitSystem* us,
+                        const struct swift_params* params, const char* category,
+                        const struct UnitSystem* def) {
+
+  if (!def) error("Default UnitSystem not allocated");
+
+  char buffer[200];
+  sprintf(buffer, "%s:UnitMass_in_cgs", category);
+  us->UnitMass_in_cgs =
+      parser_get_opt_param_double(params, buffer, def->UnitMass_in_cgs);
+  sprintf(buffer, "%s:UnitLength_in_cgs", category);
+  us->UnitLength_in_cgs =
+      parser_get_opt_param_double(params, buffer, def->UnitLength_in_cgs);
+  sprintf(buffer, "%s:UnitVelocity_in_cgs", category);
+  const double defaultVelocity = def->UnitLength_in_cgs / def->UnitTime_in_cgs;
+  const double unitVelocity =
+      parser_get_opt_param_double(params, buffer, defaultVelocity);
+  us->UnitTime_in_cgs = us->UnitLength_in_cgs / unitVelocity;
+  sprintf(buffer, "%s:UnitCurrent_in_cgs", category);
+  us->UnitCurrent_in_cgs =
+      parser_get_opt_param_double(params, buffer, def->UnitCurrent_in_cgs);
+  sprintf(buffer, "%s:UnitTemp_in_cgs", category);
+  us->UnitTemperature_in_cgs =
+      parser_get_opt_param_double(params, buffer, def->UnitTemperature_in_cgs);
+}
+
 /**
  * @brief Returns the base unit conversion factor for a given unit system
  * @param us The UnitSystem used

src/units.h

@@ -94,6 +94,10 @@ enum UnitConversionFactor {
 
 void units_init(struct UnitSystem*, const struct swift_params*,
                 const char* category);
+void units_init_default(struct UnitSystem* us,
+                        const struct swift_params* params, const char* category,
+                        const struct UnitSystem* def);
+
 int units_are_equal(const struct UnitSystem* a, const struct UnitSystem* b);
 
 /* Base units */