X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/ceb3f0c0a3b7bb3fa3250d31b04c382894095e52..cf76bcbb2097c36912fae1c8a6cc77f0ce4bfafd:/calc/gfx.cal

diff --git a/calc/gfx.cal b/calc/gfx.cal
index 5b19cb3..e38f4b7 100644
--- a/calc/gfx.cal
+++ b/calc/gfx.cal
@@ -1,6 +1,6 @@
 /* -*-apcalc-*-
  *
- * $Id: gfx.cal,v 1.1.4.1 2004/03/21 22:39:46 mdw Exp $
+ * $Id: gfx.cal,v 1.3 2004/04/08 01:36:15 mdw Exp $
  *
  * Testbed for %$\gf{2}$% poltnomial arithmetic
  *
@@ -27,17 +27,6 @@
  * MA 02111-1307, USA.
  */
 
-/*----- Revision history --------------------------------------------------* 
- *
- * $Log: gfx.cal,v $
- * Revision 1.1.4.1  2004/03/21 22:39:46  mdw
- * Elliptic curves on binary fields work.
- *
- * Revision 1.1  2000/10/08 16:01:37  mdw
- * Prototypes of various bits of code.
- *
- */
-
 /*----- Object types ------------------------------------------------------*/
 
 obj gf { x };
@@ -108,24 +97,36 @@ define gf_mod(x, y)
   return gf(l[[1]]);
 }
 
-define gf_inv(a, b)
+define gf_gcd(a, b)
 {
-  local g, x, y, X, Y, u, v, t, q, r;
-  x = gf(1); X = gf(0);
-  y = gf(0); Y = gf(1);
-
-  if (b == gf(0)) { g = a; } else if (a == gf(0)) { g = b; }
+  local swap = 0;
+  local g, x = 1, X = 0, y = 0, Y = 1, q, r, t;
+  if (a.x < b.x) {
+    t = a; a = b; b = t;
+    swap = 1;
+  }
+  if (b == gf(0))
+    g = a;
   else {
     while (b != gf(0)) {
-      q = gf_div(b, a); r = gf_mod(b, a);
+      q = gf_div(a, b); r = gf_mod(a, b);
       t = X * q + x; x = X; X = t;
       t = Y * q + y; y = Y; Y = t;
-      b = a; a = r;
+      a = b; b = r;
     }
     g = a;
   }
-  if (g != gf(1)) quit "not coprime in gf_inv";
-  return Y;
+  if (swap) {
+    t = x; x = y; y = t;
+  }
+  return list(g, x, y);
+}
+
+define gf_inv(a, b)
+{
+  local l = gf_gcd(b, a);
+  if (l[[0]] != gf(1)) quit "not coprime in gf_inv";
+  return l[[2]];
 }
 
 /*----- That's all, folks -------------------------------------------------*/