nobu	2016-05-10 15:46:09 +0900 (Tue, 10 May 2016)

  New Revision: 54969

  https://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=revision&revision=54969

  Log:
    random.c: reuse bits
    
    * random.c (random_ulong_limited): reduce calling bytes methods by
      reusing dropped bits.

  Modified files:
    trunk/random.c
Index: random.c
===================================================================
--- random.c	(revision 54968)
+++ random.c	(revision 54969)
@@ -991,9 +991,21 @@ random_ulong_limited(VALUE obj, rb_rando https://github.com/ruby/ruby/blob/trunk/random.c#L991
 {
     if (!limit) return 0;
     if (!rnd) {
-	unsigned long val, mask = make_mask(limit);
+	const int w = sizeof(limit) * CHAR_BIT - nlz_long(limit);
+	const int n = w > 32 ? sizeof(unsigned long) : sizeof(uint32_t);
+	const unsigned long mask = ~(~0UL << w);
+	const unsigned long full = ~(~0UL << n * CHAR_BIT);
+	unsigned long val, bits = 0, rest = 0;
 	do {
-	    obj_random_bytes(obj, &val, sizeof(unsigned long));
+	    if (mask & ~rest) {
+		union {uint32_t u32; unsigned long ul;} buf;
+		obj_random_bytes(obj, &buf, n);
+		rest = full;
+		bits = (n == sizeof(uint32_t)) ? buf.u32 : buf.ul;
+	    }
+	    val = bits;
+	    bits >>= w;
+	    rest >>= w;
 	    val &= mask;
 	} while (limit < val);
 	return val;

--
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