#include <tommath.h>
#ifdef BN_MP_INVMOD_SLOW_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
 *
 * LibTomMath is a library that provides multiple-precision
 * integer arithmetic as well as number theoretic functionality.
 *
 * The library was designed directly after the MPI library by
 * Michael Fromberger but has been written from scratch with
 * additional optimizations in place.
 *
 * The library is free for all purposes without any express
 * guarantee it works.
 *
 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
 */

/* hac 14.61, pp608 */
int mp_invmod_slow(mp_int * a, mp_int * b, mp_int * c)
{
	mp_int x, y, u, v, A, B, C, D;
	int res;

	/* b cannot be negative */
	if (b->sign == MP_NEG || mp_iszero(b) == 1) {
		return MP_VAL;
	}

	/* init temps */
	if ((res = mp_init_multi(&x, &y, &u, &v,
				 &A, &B, &C, &D, NULL)) != MP_OKAY) {
		return res;
	}

	/* x = a, y = b */
	if ((res = mp_mod(a, b, &x)) != MP_OKAY) {
		goto LBL_ERR;
	}
	if ((res = mp_copy(b, &y)) != MP_OKAY) {
		goto LBL_ERR;
	}

	/* 2. [modified] if x,y are both even then return an error! */
	if (mp_iseven(&x) == 1 && mp_iseven(&y) == 1) {
		res = MP_VAL;
		goto LBL_ERR;
	}

	/* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
	if ((res = mp_copy(&x, &u)) != MP_OKAY) {
		goto LBL_ERR;
	}
	if ((res = mp_copy(&y, &v)) != MP_OKAY) {
		goto LBL_ERR;
	}
	mp_set(&A, 1);
	mp_set(&D, 1);

top:
	/* 4.  while u is even do */
	while (mp_iseven(&u) == 1) {
		/* 4.1 u = u/2 */
		if ((res = mp_div_2(&u, &u)) != MP_OKAY) {
			goto LBL_ERR;
		}
		/* 4.2 if A or B is odd then */
		if (mp_isodd(&A) == 1 || mp_isodd(&B) == 1) {
			/* A = (A+y)/2, B = (B-x)/2 */
			if ((res = mp_add(&A, &y, &A)) != MP_OKAY) {
				goto LBL_ERR;
			}
			if ((res = mp_sub(&B, &x, &B)) != MP_OKAY) {
				goto LBL_ERR;
			}
		}
		/* A = A/2, B = B/2 */
		if ((res = mp_div_2(&A, &A)) != MP_OKAY) {
			goto LBL_ERR;
		}
		if ((res = mp_div_2(&B, &B)) != MP_OKAY) {
			goto LBL_ERR;
		}
	}

	/* 5.  while v is even do */
	while (mp_iseven(&v) == 1) {
		/* 5.1 v = v/2 */
		if ((res = mp_div_2(&v, &v)) != MP_OKAY) {
			goto LBL_ERR;
		}
		/* 5.2 if C or D is odd then */
		if (mp_isodd(&C) == 1 || mp_isodd(&D) == 1) {
			/* C = (C+y)/2, D = (D-x)/2 */
			if ((res = mp_add(&C, &y, &C)) != MP_OKAY) {
				goto LBL_ERR;
			}
			if ((res = mp_sub(&D, &x, &D)) != MP_OKAY) {
				goto LBL_ERR;
			}
		}
		/* C = C/2, D = D/2 */
		if ((res = mp_div_2(&C, &C)) != MP_OKAY) {
			goto LBL_ERR;
		}
		if ((res = mp_div_2(&D, &D)) != MP_OKAY) {
			goto LBL_ERR;
		}
	}

	/* 6.  if u >= v then */
	if (mp_cmp(&u, &v) != MP_LT) {
		/* u = u - v, A = A - C, B = B - D */
		if ((res = mp_sub(&u, &v, &u)) != MP_OKAY) {
			goto LBL_ERR;
		}

		if ((res = mp_sub(&A, &C, &A)) != MP_OKAY) {
			goto LBL_ERR;
		}

		if ((res = mp_sub(&B, &D, &B)) != MP_OKAY) {
			goto LBL_ERR;
		}
	} else {
		/* v - v - u, C = C - A, D = D - B */
		if ((res = mp_sub(&v, &u, &v)) != MP_OKAY) {
			goto LBL_ERR;
		}

		if ((res = mp_sub(&C, &A, &C)) != MP_OKAY) {
			goto LBL_ERR;
		}

		if ((res = mp_sub(&D, &B, &D)) != MP_OKAY) {
			goto LBL_ERR;
		}
	}

	/* if not zero goto step 4 */
	if (mp_iszero(&u) == 0)
		goto top;

	/* now a = C, b = D, gcd == g*v */

	/* if v != 1 then there is no inverse */
	if (mp_cmp_d(&v, 1) != MP_EQ) {
		res = MP_VAL;
		goto LBL_ERR;
	}

	/* if its too low */
	while (mp_cmp_d(&C, 0) == MP_LT) {
		if ((res = mp_add(&C, b, &C)) != MP_OKAY) {
			goto LBL_ERR;
		}
	}

	/* too big */
	while (mp_cmp_mag(&C, b) != MP_LT) {
		if ((res = mp_sub(&C, b, &C)) != MP_OKAY) {
			goto LBL_ERR;
		}
	}

	/* C is now the inverse */
	mp_exch(&C, c);
	res = MP_OKAY;
LBL_ERR:mp_clear_multi(&x, &y, &u, &v, &A, &B, &C, &D, NULL);
	return res;
}
#endif

/* $Source: /cvs/libtom/libtommath/bn_mp_invmod_slow.c,v $ */
/* $Revision: 1.4 $ */
/* $Date: 2006/12/28 01:25:13 $ */