(struct irand_state, irand_init, irand32, irand_mod): Moved back here,

from rand-isaac.c.

diff --git a/src/shred.c b/src/shred.c
index 54bc49c..3e47aad 100644 (file)
--- a/src/shred.c
+++ b/src/shred.c
@@ -227,6 +227,66 @@ to be recovered later.\n\
   exit (status);
 }
 
+/* Single-word RNG built on top of ISAAC */
+struct irand_state
+{
+  uint32_t r[ISAAC_WORDS];
+  unsigned int numleft;
+  struct isaac_state *s;
+};
+
+static void
+irand_init (struct irand_state *r, struct isaac_state *s)
+{
+  r->numleft = 0;
+  r->s = s;
+}
+
+/*
+ * We take from the end of the block deliberately, so if we need
+ * only a small number of values, we choose the final ones which are
+ * marginally better mixed than the initial ones.
+ */
+static uint32_t
+irand32 (struct irand_state *r)
+{
+  if (!r->numleft)
+    {
+      isaac_refill (r->s, r->r);
+      r->numleft = ISAAC_WORDS;
+    }
+  return r->r[--r->numleft];
+}
+
+/*
+ * Return a uniformly distributed random number between 0 and n,
+ * inclusive.  Thus, the result is modulo n+1.
+ *
+ * Theory of operation: as x steps through every possible 32-bit number,
+ * x % n takes each value at least 2^32 / n times (rounded down), but
+ * the values less than 2^32 % n are taken one additional time.  Thus,
+ * x % n is not perfectly uniform.  To fix this, the values of x less
+ * than 2^32 % n are disallowed, and if the RNG produces one, we ask
+ * for a new value.
+ */
+static uint32_t
+irand_mod (struct irand_state *r, uint32_t n)
+{
+  uint32_t x;
+  uint32_t lim;
+
+  if (!++n)
+    return irand32 (r);
+
+  lim = -n % n;                        /* == (2**32-n) % n == 2**32 % n */
+  do
+    {
+      x = irand32 (r);
+    }
+  while (x < lim);
+  return x % n;
+}
+
 /*
  * Fill a buffer with a fixed pattern.
  *