Field exponentiation. Add field size to field structure. Make check

[u/mdw/catacomb] / field-exp.c

diff --git a/field-exp.c b/field-exp.c
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+++ b/field-exp.c
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+/* -*-c-*-
+ *
+ * $Id$
+ *
+ * Exponentiation in finite fields
+ *
+ * (c) 2004 Straylight/Edgeware
+ */
+
+/*----- Licensing notice --------------------------------------------------* 
+ *
+ * This file is part of Catacomb.
+ *
+ * Catacomb is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Library General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ * 
+ * Catacomb is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Library General Public License for more details.
+ * 
+ * You should have received a copy of the GNU Library General Public
+ * License along with Catacomb; if not, write to the Free
+ * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+ * MA 02111-1307, USA.
+ */
+
+/*----- Header files ------------------------------------------------------*/
+
+#include "field.h"
+#include "field-exp.h"
+
+/*----- Main code ---------------------------------------------------------*/
+
+/* --- @field_exp@ --- *
+ *
+ * Arguments:  @field *f@ = pointer to field
+ *             @mp *d@ = fake destination
+ *             @mp *a@ = base
+ *             @mp *e@ = exponent
+ *
+ * Returns:    Result, %$a^e$%.
+ *
+ * Use:                Exponentiation in a finite field.  Note that all quantities
+ *             are in internal format.
+ */
+
+mp *field_exp(field *f, mp *d, mp *a, mp *e)
+{
+  mp *x = MP_COPY(f->one);
+  mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
+
+  MP_COPY(a);
+  MP_SHRINK(e);
+  if (MP_ZEROP(e))
+    ;
+  else {
+    if (MP_NEGP(e))
+      a = F_INV(f, a, a);
+    if (MP_LEN(e) < EXP_THRESH)
+      EXP_SIMPLE(x, a, e);
+    else
+      EXP_WINDOW(x, a, e);
+  }
+  mp_drop(d);
+  mp_drop(spare);
+  mp_drop(a);
+  return (x);
+}
+
+/*----- That's all, folks -------------------------------------------------*/