COMMON: rand() replacment using PCG

Author: chrisws

samples/distro-examples/tests/all.bas

@@ -198,7 +198,7 @@ print "RIGHT:" + RIGHT (s,2)
 print "RIGHTOF:" + RIGHTOF (s1, s2)
 print "RIGHTOFLAST:" + RIGHTOFLAST (s1, s2)
 print "RINSTR:" + RINSTR (2, s1, s2)
-print "RND:" + RND
+print "RND:" + iff(RND>=0 && RND<=1.0,1,0)
 print "ROUND:" + ROUND (x,22)
 print "RTRIM:" + RTRIM (s)
 print "RUN:" '+ RUN cmdstr

samples/distro-examples/tests/output/all.out

@@ -183,7 +183,7 @@ RIGHT:gs
 RIGHTOF:
 RIGHTOFLAST:
 RINSTR:0
-RND:0.75898406142369
+RND:1
 ROUND:12.3
 RTRIM:catsanddogs
 RUN:

src/common/Makefile.am

@@ -54,6 +54,7 @@ libsb_common_a_SOURCES =                  \
     device.c device.h                     \
     screen.c                              \
     system.c                              \
+    random.c                              \
     eval.c                                \
     extlib.c extlib.h                     \
     file.c                                \

src/common/blib.c

@@ -2051,18 +2051,18 @@ void cmd_restore() {
  * RANDOMIZE [num]
  */
 void cmd_randomize() {
-  long seed;
+  var_int_t seed;
 
   byte code = code_peek();
   switch (code) {
   case kwTYPE_LINE:
   case kwTYPE_EOC:
-    srand(clock());
+    pcg32_srand(clock());
     break;
   default:
     seed = par_getint();
     if (!prog_error) {
-      srand(seed);
+      pcg32_srand(seed);
     }
   };
 }

src/common/blib_func.c

@@ -480,7 +480,7 @@ var_num_t cmd_math0(long funcCode) {
     }
     break;
   case kwRND:
-    r = ((var_num_t) rand()) / (RAND_MAX + 1.0);
+    r = pcg32_rand();
     break;
   default:
     rt_raise("Unsupported built-in function call %ld", funcCode);

src/common/random.c

@@ -0,0 +1,40 @@
+// This file is part of SmallBASIC
+//
+// lowlevel device (OS) I/O
+//
+// This program is distributed under the terms of the GPL v2.0 or later
+// Download the GNU Public License (GPL) from www.gnu.org
+//
+// Copyright(C) 2000 Nicholas Christopoulos
+
+#include "config.h"
+
+#include "common/sys.h"
+#include <stdint.h>
+#include <limits.h>
+
+// see:
+// https://en.wikipedia.org/wiki/Permuted_congruential_generator
+// https://www.pcg-random.org/download.html
+
+static uint64_t state      = 0x4d595df4d0f33173;
+static uint64_t multiplier = 6364136223846793005u;
+static uint64_t increment  = 1442695040888963407u;
+
+static uint32_t rotr32(uint32_t x, unsigned r) {
+  return x >> r | x << (-r & 31);
+}
+
+var_num_t pcg32_rand() {
+  uint64_t x = state;
+  unsigned count = (unsigned)(x >> 59);
+  state = x * multiplier + increment;
+  x ^= x >> 18;
+  int32_t r = rotr32((uint32_t)(x >> 27), count);
+  return ((var_num_t)abs(r)) / (INT_MAX + 1.0);
+}
+
+void pcg32_srand(var_int_t seed) {
+  state = seed + increment;
+  pcg32_rand();
+}

src/common/sys.h

@@ -120,6 +120,9 @@ typedef uint32_t bcip_t;
 #include "include/var.h"
 #include "common/str.h"
 
+var_num_t pcg32_rand(void);
+void pcg32_srand(var_int_t seed);
+
 #if !defined(O_BINARY)
 #define O_BINARY 0
 #endif