bn_mp_div.c Go to the documentation of this file. #include <tommath.h> #ifdef BN_MP_DIV_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://math.libtomcrypt.com */ #ifdef BN_MP_DIV_SMALL /* slower bit-bang division... also smaller */ int mp_div(mp_int * a, mp_int * b, mp_int * c, mp_int * d) { mp_int ta, tb, tq, q; int res, n, n2; /* is divisor zero ? */ if (mp_iszero (b) == 1) { return MP_VAL; } /* if a < b then q=0, r = a */ if (mp_cmp_mag (a, b) == MP_LT) { if (d != NULL) { res = mp_copy (a, d); } else { res = MP_OKAY; } if (c != NULL) { mp_zero (c); } return res; } /* init our temps */ if ((res = mp_init_multi(&ta, &tb, &tq, &q, NULL) != MP_OKAY)) { return res; } mp_set(&tq, 1); n = mp_count_bits(a) - mp_count_bits(b); if (((res = mp_abs(a, &ta)) != MP_OKAY) || ((res = mp_abs(b, &tb)) != MP_OKAY) || ((res = mp_mul_2d(&tb, n, &tb)) != MP_OKAY) || ((res = mp_mul_2d(&tq, n, &tq)) != MP_OKAY)) { goto LBL_ERR; } while (n-- >= 0) { if (mp_cmp(&tb, &ta) != MP_GT) { if (((res = mp_sub(&ta, &tb, &ta)) != MP_OKAY) || ((res = mp_add(&q, &tq, &q)) != MP_OKAY)) { goto LBL_ERR; } } if (((res = mp_div_2d(&tb, 1, &tb, NULL)) != MP_OKAY) || ((res = mp_div_2d(&tq, 1, &tq, NULL)) != MP_OKAY)) { goto LBL_ERR; } } /* now q == quotient and ta == remainder */ n = a->sign; n2 = (a->sign == b->sign ? MP_ZPOS : MP_NEG); if (c != NULL) { mp_exch(c, &q); c->sign = (mp_iszero(c) == MP_YES) ? MP_ZPOS : n2; } if (d != NULL) { mp_exch(d, &ta); d->sign = (mp_iszero(d) == MP_YES) ? MP_ZPOS : n; } LBL_ERR: mp_clear_multi(&ta, &tb, &tq, &q, NULL); return res; } #else /* integer signed division. * c*b + d == a [e.g. a/b, c=quotient, d=remainder] * HAC pp.598 Algorithm 14.20 * * Note that the description in HAC is horribly * incomplete. For example, it doesn't consider * the case where digits are removed from 'x' in * the inner loop. It also doesn't consider the * case that y has fewer than three digits, etc.. * * The overall algorithm is as described as * 14.20 from HAC but fixed to treat these cases. */ int mp_div (mp_int * a, mp_int * b, mp_int * c, mp_int * d) { mp_int q, x, y, t1, t2; int res, n, t, i, norm, neg; /* is divisor zero ? */ if (mp_iszero (b) == 1) { return MP_VAL; } /* if a < b then q=0, r = a */ if (mp_cmp_mag (a, b) == MP_LT) { if (d != NULL) { res = mp_copy (a, d); } else { res = MP_OKAY; } if (c != NULL) { mp_zero (c); } return res; } if ((res = mp_init_size (&q, a->used + 2)) != MP_OKAY) { return res; } q.used = a->used + 2; if ((res = mp_init (&t1)) != MP_OKAY) { goto LBL_Q; } if ((res = mp_init (&t2)) != MP_OKAY) { goto LBL_T1; } if ((res = mp_init_copy (&x, a)) != MP_OKAY) { goto LBL_T2; } if ((res = mp_init_copy (&y, b)) != MP_OKAY) { goto LBL_X; } /* fix the sign */ neg = (a->sign == b->sign) ? MP_ZPOS : MP_NEG; x.sign = y.sign = MP_ZPOS; /* normalize both x and y, ensure that y >= b/2, [b == 2**DIGIT_BIT] */ norm = mp_count_bits(&y) % DIGIT_BIT; if (norm < (int)(DIGIT_BIT-1)) { norm = (DIGIT_BIT-1) - norm; if ((res = mp_mul_2d (&x, norm, &x)) != MP_OKAY) { goto LBL_Y; } if ((res = mp_mul_2d (&y, norm, &y)) != MP_OKAY) { goto LBL_Y; } } else { norm = 0; } /* note hac does 0 based, so if used==5 then its 0,1,2,3,4, e.g. use 4 */ n = x.used - 1; t = y.used - 1; /* while (x >= y*b**n-t) do { q[n-t] += 1; x -= y*b**{n-t} } */ if ((res = mp_lshd (&y, n - t)) != MP_OKAY) { /* y = y*b**{n-t} */ goto LBL_Y; } while (mp_cmp (&x, &y) != MP_LT) { ++(q.dp[n - t]); if ((res = mp_sub (&x, &y, &x)) != MP_OKAY) { goto LBL_Y; } } /* reset y by shifting it back down */ mp_rshd (&y, n - t); /* step 3. for i from n down to (t + 1) */ for (i = n; i >= (t + 1); i--) { if (i > x.used) { continue; } /* step 3.1 if xi == yt then set q{i-t-1} to b-1, * otherwise set q{i-t-1} to (xi*b + x{i-1})/yt */ if (x.dp[i] == y.dp[t]) { q.dp[i - t - 1] = ((((mp_digit)1) << DIGIT_BIT) - 1); } else { mp_word tmp; tmp = ((mp_word) x.dp[i]) << ((mp_word) DIGIT_BIT); tmp |= ((mp_word) x.dp[i - 1]); tmp /= ((mp_word) y.dp[t]); if (tmp > (mp_word) MP_MASK) tmp = MP_MASK; q.dp[i - t - 1] = (mp_digit) (tmp & (mp_word) (MP_MASK)); } /* while (q{i-t-1} * (yt * b + y{t-1})) > xi * b**2 + xi-1 * b + xi-2 do q{i-t-1} -= 1; */ q.dp[i - t - 1] = (q.dp[i - t - 1] + 1) & MP_MASK; do { q.dp[i - t - 1] = (q.dp[i - t - 1] - 1) & MP_MASK; /* find left hand */ mp_zero (&t1); t1.dp[0] = (t - 1 < 0) ? 0 : y.dp[t - 1]; t1.dp[1] = y.dp[t]; t1.used = 2; if ((res = mp_mul_d (&t1, q.dp[i - t - 1], &t1)) != MP_OKAY) { goto LBL_Y; } /* find right hand */ t2.dp[0] = (i - 2 < 0) ? 0 : x.dp[i - 2]; t2.dp[1] = (i - 1 < 0) ? 0 : x.dp[i - 1]; t2.dp[2] = x.dp[i]; t2.used = 3; } while (mp_cmp_mag(&t1, &t2) == MP_GT); /* step 3.3 x = x - q{i-t-1} * y * b**{i-t-1} */ if ((res = mp_mul_d (&y, q.dp[i - t - 1], &t1)) != MP_OKAY) { goto LBL_Y; } if ((res = mp_lshd (&t1, i - t - 1)) != MP_OKAY) { goto LBL_Y; } if ((res = mp_sub (&x, &t1, &x)) != MP_OKAY) { goto LBL_Y; } /* if x < 0 then { x = x + y*b**{i-t-1}; q{i-t-1} -= 1; } */ if (x.sign == MP_NEG) { if ((res = mp_copy (&y, &t1)) != MP_OKAY) { goto LBL_Y; } if ((res = mp_lshd (&t1, i - t - 1)) != MP_OKAY) { goto LBL_Y; } if ((res = mp_add (&x, &t1, &x)) != MP_OKAY) { goto LBL_Y; } q.dp[i - t - 1] = (q.dp[i - t - 1] - 1UL) & MP_MASK; } } /* now q is the quotient and x is the remainder * [which we have to normalize] */ /* get sign before writing to c */ x.sign = x.used == 0 ? MP_ZPOS : a->sign; if (c != NULL) { mp_clamp (&q); mp_exch (&q, c); c->sign = neg; } if (d != NULL) { mp_div_2d (&x, norm, &x, NULL); mp_exch (&x, d); } res = MP_OKAY; LBL_Y:mp_clear (&y); LBL_X:mp_clear (&x); LBL_T2:mp_clear (&t2); LBL_T1:mp_clear (&t1); LBL_Q:mp_clear (&q); return res; } #endif #endif /* $Source: /cvsroot/tcl/libtommath/bn_mp_div.c,v $ */ /* $Revision: 1.4 $ */ /* $Date: 2006/12/01 19:45:38 $ */

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