raisemod.c

/* raisemod.c: bcmath library file. */
/*
    Copyright (C) 1991, 1992, 1993, 1994, 1997 Free Software Foundation, Inc.
    Copyright (C) 2000 Philip A. Nelson

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2 of the License, or (at your option) any later version.

    This library 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
    Lesser General Public License for more details.  (LICENSE)

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to:

      The Free Software Foundation, Inc.
      59 Temple Place, Suite 330
      Boston, MA 02111-1307 USA.

    You may contact the author by:
       e-mail:  philnelson@acm.org
      us-mail:  Philip A. Nelson
                Computer Science Department, 9062
                Western Washington University
                Bellingham, WA 98226-9062

*************************************************************************/

#include "bcmath.h"
#include "private.h"
#include <stddef.h>

/* Raise BASE to the EXPO power, reduced modulo MOD.  The result is placed in RESULT. */
raise_mod_status bc_raisemod(bc_num base, bc_num expo, bc_num mod, bc_num *result, size_t scale)
{
	bc_num power, exponent, modulus, parity, temp;

	/* Check the base for scale digits. */
	if (base->n_scale != 0) {
		return BASE_HAS_FRACTIONAL;
	}
	/* Check the exponent for scale digits. */
	if (expo->n_scale != 0) {
		return EXPO_HAS_FRACTIONAL;
	}
	if (bc_is_neg(expo)) {
		return EXPO_IS_NEGATIVE;
	}
	/* Check the modulus for scale digits. */
	if (mod->n_scale != 0) {
		return MOD_HAS_FRACTIONAL;
	}
	/* Modulus cannot be 0 */
	if (bc_is_zero(mod)) {
		return MOD_IS_ZERO;
	}

	/* Any integer number mod 1 (or -1) must be equal to 0 */
	if (_bc_do_compare(mod, BCG(_one_), mod->n_scale, false) == BCMATH_EQUAL) {
		bc_free_num (result);
		*result = bc_new_num(1, scale);
		return OK;
	}

	/* Set initial values. */
	power = bc_copy_num(base);
	exponent = bc_copy_num(expo);
	modulus = bc_copy_num(mod);
	temp = bc_copy_num(BCG(_one_));
	bc_init_num(&parity);

	/* Do the calculation. */
	while (!bc_is_zero(exponent)) {
		(void) bc_divmod(exponent, BCG(_two_), &exponent, &parity, 0);
		if (!bc_is_zero(parity)) {
			bc_multiply_ex(temp, power, &temp, scale);
			(void) bc_modulo(temp, modulus, &temp, scale);
		}
		bc_multiply_ex(power, power, &power, scale);
		(void) bc_modulo(power, modulus, &power, scale);
	}

	/* Assign the value. */
	bc_free_num (&power);
	bc_free_num (&exponent);
	bc_free_num (&modulus);
	bc_free_num (result);
	bc_free_num (&parity);
	*result = temp;
	return OK;
}