libtom/libtommath

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 #include #ifdef BN_MP_REDUCE_2K_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, tstdenis82@gmail.com, http://libtom.org */ /* reduces a modulo n where n is of the form 2**p - d */ int mp_reduce_2k(mp_int *a, mp_int *n, mp_digit d) { mp_int q; int p, res; if ((res = mp_init(&q)) != MP_OKAY) { return res; } p = mp_count_bits(n); top: /* q = a/2**p, a = a mod 2**p */ if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) { goto ERR; } if (d != 1) { /* q = q * d */ if ((res = mp_mul_d(&q, d, &q)) != MP_OKAY) { goto ERR; } } /* a = a + q */ if ((res = s_mp_add(a, &q, a)) != MP_OKAY) { goto ERR; } if (mp_cmp_mag(a, n) != MP_LT) { if ((res = s_mp_sub(a, n, a)) != MP_OKAY) { goto ERR; } goto top; } ERR: mp_clear(&q); return res; } #endif /* ref: \$Format:%D\$ */ /* git commit: \$Format:%H\$ */ /* commit time: \$Format:%ai\$ */