raisemod.c

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/* 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;
}