diff libtommath/bn_mp_cnt_lsb.c @ 330:5488db2e9e4e

merge of 332f709a4cb39cde4cedab7c3be89e05f3023067 and ca4ca78b82c5d430c69ce01bf794e8886ce81431
author Matt Johnston <matt@ucc.asn.au>
date Sat, 10 Jun 2006 16:39:40 +0000
parents eed26cff980b
children 5ff8218bcee9
line wrap: on
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--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_mp_cnt_lsb.c	Sat Jun 10 16:39:40 2006 +0000
@@ -0,0 +1,49 @@
+#include <tommath.h>
+#ifdef BN_MP_CNT_LSB_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis
+ *
+ * LibTomMath is a library that provides multiple-precision
+ * integer arithmetic as well as number theoretic functionality.
+ *
+ * The library was designed directly after the MPI library by
+ * Michael Fromberger but has been written from scratch with
+ * additional optimizations in place.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, [email protected], http://math.libtomcrypt.org
+ */
+
+static const int lnz[16] = { 
+   4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
+};
+
+/* Counts the number of lsbs which are zero before the first zero bit */
+int mp_cnt_lsb(mp_int *a)
+{
+   int x;
+   mp_digit q, qq;
+
+   /* easy out */
+   if (mp_iszero(a) == 1) {
+      return 0;
+   }
+
+   /* scan lower digits until non-zero */
+   for (x = 0; x < a->used && a->dp[x] == 0; x++);
+   q = a->dp[x];
+   x *= DIGIT_BIT;
+
+   /* now scan this digit until a 1 is found */
+   if ((q & 1) == 0) {
+      do {
+         qq  = q & 15;
+         x  += lnz[qq];
+         q >>= 4;
+      } while (qq == 0);
+   }
+   return x;
+}
+
+#endif