## File: bn_mp_n_root.c

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libtommath 0.42.0-1
 `123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132` ``````#include #ifdef BN_MP_N_ROOT_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 */ /* find the n'th root of an integer * * Result found such that (c)**b <= a and (c+1)**b > a * * This algorithm uses Newton's approximation * x[i+1] = x[i] - f(x[i])/f'(x[i]) * which will find the root in log(N) time where * each step involves a fair bit. This is not meant to * find huge roots [square and cube, etc]. */ int mp_n_root (mp_int * a, mp_digit b, mp_int * c) { mp_int t1, t2, t3; int res, neg; /* input must be positive if b is even */ if ((b & 1) == 0 && a->sign == MP_NEG) { return MP_VAL; } if ((res = mp_init (&t1)) != MP_OKAY) { return res; } if ((res = mp_init (&t2)) != MP_OKAY) { goto LBL_T1; } if ((res = mp_init (&t3)) != MP_OKAY) { goto LBL_T2; } /* if a is negative fudge the sign but keep track */ neg = a->sign; a->sign = MP_ZPOS; /* t2 = 2 */ mp_set (&t2, 2); do { /* t1 = t2 */ if ((res = mp_copy (&t2, &t1)) != MP_OKAY) { goto LBL_T3; } /* t2 = t1 - ((t1**b - a) / (b * t1**(b-1))) */ /* t3 = t1**(b-1) */ if ((res = mp_expt_d (&t1, b - 1, &t3)) != MP_OKAY) { goto LBL_T3; } /* numerator */ /* t2 = t1**b */ if ((res = mp_mul (&t3, &t1, &t2)) != MP_OKAY) { goto LBL_T3; } /* t2 = t1**b - a */ if ((res = mp_sub (&t2, a, &t2)) != MP_OKAY) { goto LBL_T3; } /* denominator */ /* t3 = t1**(b-1) * b */ if ((res = mp_mul_d (&t3, b, &t3)) != MP_OKAY) { goto LBL_T3; } /* t3 = (t1**b - a)/(b * t1**(b-1)) */ if ((res = mp_div (&t2, &t3, &t3, NULL)) != MP_OKAY) { goto LBL_T3; } if ((res = mp_sub (&t1, &t3, &t2)) != MP_OKAY) { goto LBL_T3; } } while (mp_cmp (&t1, &t2) != MP_EQ); /* result can be off by a few so check */ for (;;) { if ((res = mp_expt_d (&t1, b, &t2)) != MP_OKAY) { goto LBL_T3; } if (mp_cmp (&t2, a) == MP_GT) { if ((res = mp_sub_d (&t1, 1, &t1)) != MP_OKAY) { goto LBL_T3; } } else { break; } } /* reset the sign of a first */ a->sign = neg; /* set the result */ mp_exch (&t1, c); /* set the sign of the result */ c->sign = neg; res = MP_OKAY; LBL_T3:mp_clear (&t3); LBL_T2:mp_clear (&t2); LBL_T1:mp_clear (&t1); return res; } #endif /* \$Source\$ */ /* \$Revision: 0.41 \$ */ /* \$Date: 2007-04-18 09:58:18 +0000 \$ */ ``````