Gather up another utility.

[u/mdw/catacomb] / mp-gcd.c

diff --git a/mp-gcd.c b/mp-gcd.c
index a17a502..5e663a1 100644 (file)
--- a/mp-gcd.c
+++ b/mp-gcd.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: mp-gcd.c,v 1.5 2000/10/08 12:02:41 mdw Exp $
+ * $Id: mp-gcd.c,v 1.7 2004/04/08 01:36:15 mdw Exp $
  *
  * Extended GCD calculation
  *
@@ -27,27 +27,6 @@
  * MA 02111-1307, USA.
  */
 
-/*----- Revision history --------------------------------------------------* 
- *
- * $Log: mp-gcd.c,v $
- * Revision 1.5  2000/10/08 12:02:41  mdw
- * Use Euclid's algorithm rather than the binary one.
- *
- * Revision 1.4  2000/06/17 11:34:46  mdw
- * More hacking for the signs of the outputs.
- *
- * Revision 1.3  1999/12/10 23:18:39  mdw
- * Change interface for suggested destinations.
- *
- * Revision 1.2  1999/11/22 20:49:56  mdw
- * Fix bug which failed to favour `x' when `y' wasn't wanted and the two
- * arguments needed swapping.
- *
- * Revision 1.1  1999/11/17 18:02:16  mdw
- * New multiprecision integer arithmetic suite.
- *
- */
-
 /*----- Header files ------------------------------------------------------*/
 
 #include "mp.h"
@@ -74,12 +53,10 @@ void mp_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
   mp *q = MP_NEW;
   unsigned f = 0;
 
-  enum {
-    f_swap = 1u,
-    f_aneg = 2u,
-    f_bneg = 4u,
-    f_ext = 8u
-  };
+#define f_swap 1u
+#define f_aneg 2u
+#define f_bneg 4u
+#define f_ext 8u
 
   /* --- Sort out some initial flags --- */
 
@@ -252,6 +229,27 @@ void mp_gcd(mp **gcd, mp **xx, mp **yy, mp *a, mp *b)
   MP_DROP(a); MP_DROP(b);
 }
 
+/* -- @mp_modinv@ --- *
+ *
+ * Arguments:  @mp *d@ = destination
+ *             @mp *x@ = argument
+ *             @mp *p@ = modulus
+ *
+ * Returns:    The inverse %$x^{-1} \bmod p$%.
+ *
+ * Use:                Computes a modular inverse.    An assertion fails if %$p$%
+ *             has no inverse.
+ */
+
+mp *mp_modinv(mp *d, mp *x, mp *p)
+{
+  mp *g = MP_NEW;
+  mp_gcd(&g, 0, &d, p, x);
+  assert(MP_EQ(g, MP_ONE));
+  mp_drop(g);
+  return (d);
+}
+
 /*----- Test rig ----------------------------------------------------------*/
 
 #ifdef TEST_RIG