Use a repeatable cross-platform PRNG.

1 files changed, 33 insertions, 0 deletions

diff --git a/util.c b/util.c
index 3c61cde..a2573a4 100644
--- a/util.c
+++ b/util.c
@@ -31,3 +31,36 @@ xrealloc(void *ptr, size_t size)
   }
   return ret;
 }
+
+// Based on sample rand() implementation from POSIX.1-2001
+
+static unsigned long xrand_next = 0;
+
+void xsrand(unsigned int seed) {
+  xrand_next = seed;
+}
+
+static unsigned int xrand_simple(void) {
+  xrand_next = xrand_next*1103515245 + 12345;
+  return (unsigned int)(xrand_next/65536)%32768;
+}
+
+static unsigned int xrand_full(void) {
+  unsigned int low = xrand_simple();
+  unsigned int high = xrand_simple();
+  return low | (high << 15);
+}
+
+#define XRAND_RANGE 1073741824U
+
+unsigned int
+xrand(unsigned int range)
+{
+  // Compensate for bias if XRAND_RANGE isn't a multiple of range
+  unsigned int limit = XRAND_RANGE - XRAND_RANGE%range;
+  unsigned int res;
+  do {
+    res = xrand_full();
+  } while (res >= limit);
+  return res%range;
+}