Define MAX_NUM_OF_NEIGHBOURS at configure time.

configure.ac

@@ -689,14 +689,21 @@ esac
 # Check if debugging interactions is switched on.
 AC_ARG_ENABLE([debug-interactions],
    [AS_HELP_STRING([--enable-debug-interactions],
-     [Activate interaction debugging @<:@yes/no@:>@]
+     [Activate interaction debugging, logging a maximum of @<:@N@:>@ neighbours. Defaults to 256 if no value set.]
    )],
    [enable_debug_interactions="$enableval"],
    [enable_debug_interactions="no"]
 )
-if test "$enable_debug_interactions" = "yes"; then
+if test "$enable_debug_interactions" != "no"; then
   if test "$with_hydro" = "gadget2"; then
-    AC_DEFINE([DEBUG_INTERACTIONS_SPH],1,[Enable interaction debugging])
+      AC_DEFINE([DEBUG_INTERACTIONS_SPH],1,[Enable interaction debugging])
+    if test "$enable_debug_interactions" == "yes"; then
+      AC_DEFINE([MAX_NUM_OF_NEIGHBOURS],256,[The maximum number of particle neighbours to be logged])
+      [enable_debug_interactions="yes (Logging up to 256 neighbours)"]
+    else
+      AC_DEFINE_UNQUOTED([MAX_NUM_OF_NEIGHBOURS], [$enableval] ,[The maximum number of particle neighbours to be logged])
+      [enable_debug_interactions="yes (Logging up to $enableval neighbours)"]
+    fi
   else 
     [enable_debug_interactions="no (only available for gadget2 hydro scheme)"]
   fi

src/hydro/Gadget2/hydro_part.h

@@ -33,10 +33,6 @@
 
 #include "cooling_struct.h"
 
-#ifdef DEBUG_INTERACTIONS_SPH
-#define NUM_OF_NEIGHBOURS 256
-#endif
-
 /* Extra particle data not needed during the SPH loops over neighbours. */
 struct xpart {
 
@@ -149,10 +145,10 @@ struct part {
 
 #ifdef DEBUG_INTERACTIONS_SPH
   /*! List of interacting particles in the density SELF and PAIR */
-  long long ids_ngbs_density[NUM_OF_NEIGHBOURS];
+  long long ids_ngbs_density[MAX_NUM_OF_NEIGHBOURS];
 
   /*! List of interacting particles in the force SELF and PAIR */
-  long long ids_ngbs_force[NUM_OF_NEIGHBOURS];
+  long long ids_ngbs_force[MAX_NUM_OF_NEIGHBOURS];
 
   /*! Number of interactions in the density SELF and PAIR */
   int num_ngb_density;