improved prng

README.md

@@ -85,7 +85,7 @@ Name|Args|Implemented?|UTF-8 safe?|Meaning
 - `sf`: store (raw) file. 3 arguments: `#(sf,fname,form)`. Stores the raw value from `form` into a named file. The value is always written in its entirety (the form pointer is ignored) and segment gap bytes, if there are any, are written "as is". Returns a null value. The form doesn't get deleted from the internal form storage. The file is fully overwritten if it already exists.
 - `ff`: fetch (raw) file. 3 arguments: `#(ff,fname,form)`. Reads a raw string from the named file into `form`. Returns null value (check the target form afterwards).
 - `tm`: local/UTC/Epoch time. 2 or 3 arguments: `#(tl,fmt[,U])`. Returns the local (or UTC, if the third parameter `U` is specified) time formatted according to the strftime-compatible `fmt` string or `E` format. If the format string is just `E` (Epoch), returns the amount of seconds since 00:00:00 UTC, January 1, 1970.
-- `rn`: random number. 3 arguments: `#(rn,n1,n2)`. Returns a (pseudo-)random integer number in the range `n1` (included) to `n2` (not included). Implemented using the xorshift64 algorithm.
+- `rn`: random number. 3 arguments: `#(rn,n1,n2)`. Returns a (pseudo-)random integer number in the range `n1` (included) to `n2` (not included). Implemented using the double-pass xorshift64* algorithm.
 - `os`: run an OS command. 2 arguments: `#(os,cmd)`. Runs a command in the external OS shell (the one determined by the `system()` C call) and returns the command exit code. The output is not captured.
 
 For self-contained nntrac environments that have no external shell (or the shell is nntrac itself), you can disable the `os` primitive by building nntrac with the `-DNNT_NO_EXTSHELL` flag.

nntrac.c

@@ -1061,9 +1061,11 @@ char *prim_tm(char *arglist, char *res, int *reslen) {
 }
 
 static unsigned long long nnt_rngs; /* PRNG state*/    
-unsigned long long xorshift64() {
-  nnt_rngs^=(nnt_rngs<<13);nnt_rngs^=(nnt_rngs>>7);nnt_rngs^=(nnt_rngs<<17);
-  return nnt_rngs;
+unsigned long long xorshift64s() {
+  nnt_rngs^=(nnt_rngs>>12);nnt_rngs^=(nnt_rngs<<25);nnt_rngs^=(nnt_rngs>>27);
+  unsigned long long inter = (nnt_rngs * 0x2545F4914F6CDD1DULL) >> 32;
+  nnt_rngs^=(nnt_rngs>>12);nnt_rngs^=(nnt_rngs<<25);nnt_rngs^=(nnt_rngs>>27);
+  return (inter << 32) | ((nnt_rngs * 0x2545F4914F6CDD1DULL) & 0xFFFFFFFF);
 }
 
 /* [new] rn primitive: (pseudo)random number */
@@ -1073,7 +1075,7 @@ char *prim_rn(char *arglist, char *res, int *reslen) {
        *tos = strtok(NULL, NNT_ADEL_S);
   long long a = (froms == NULL) ? 0 : snum(froms),
             b = (tos == NULL) ? ARITH_LIMIT : snum(tos);
-  res = n2s(a == b ? a : (a + xorshift64() % (b - a)), res, reslen);
+  res = n2s(a == b ? a : (a + xorshift64s() % (b - a)), res, reslen);
   return res;
 }
 
@@ -1081,7 +1083,7 @@ char *prim_rn(char *arglist, char *res, int *reslen) {
 void nnt_init() {
   nnt_forms = malloc(0); /* init the forms table */
   nnt_primitives = malloc(0); /* init the primitives table */
-  nnt_rngs = time(NULL); xorshift64(); /* init the PRNG */
+  nnt_rngs = time(NULL); xorshift64s(); /* init the PRNG */
   nnt_regprimitive("ps", &prim_ps);
   nnt_regprimitive("rc", &prim_rc);
   nnt_regprimitive("rs", &prim_rs);