Add some comments to the random number generator to make sure we don't change...

Add some comments to the random number generator to make sure we don't change the numbers by mistake.

src/random.h

@@ -27,6 +27,10 @@
 
 /**
  * @brief The categories of random number generated.
+ *
+ * The values of the fields are carefully chose prime
+ * numbers. Only change them if you know what you are
+ * doing!
  */
 enum random_number_type {
   random_number_star_formation = 7,
@@ -39,8 +43,9 @@ enum random_number_type {
  * @brief Returns a pseudo-random number in the range [0, 1[.
  *
  * We generate numbers that are always reproducible for a given particle ID and
- * simulation time. If more than one number per time-step per particle is
- * needed, additional randomness can be obtained by using the type argument.
+ * simulation time (on the integer time-line). If more than one number per
+ * time-step per particle is needed, additional randomness can be obtained by
+ * using the type argument.
  *
  * @param id The ID of the particle for which to generate a number.
  * @param ti_current The time (on the time-line) for which to generate a number.
@@ -56,6 +61,10 @@ INLINE static double random_unit_interval(const long long int id,
   static const double RAND_MAX_inv = 1. / ((double)RAND_MAX);
 
   /* Calculate the seed */
+  /* WARNING: Only change the math if you really know what you are doing!
+     The numbers are carefully chosen prime numbers that prevent correlation
+     with either the current integer time or the particle IDs.
+     The calculation overflows on purpose.  */
   unsigned int seed = ((937LL * id + 1109LL) % 2147987LL +
                        (ti_current - 1) % 1514917LL + (long long)type) %
                       seed_range;