fuchsia / third_party / dropbear / 1c5fda515f96c27d4e3b732d887f418453f1cb14 / . / bn_fast_s_mp_mul_digs.c

#include <tommath.h> | |

#ifdef BN_FAST_S_MP_MUL_DIGS_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@iahu.ca, http://math.libtomcrypt.org | |

*/ | |

/* Fast (comba) multiplier | |

* | |

* This is the fast column-array [comba] multiplier. It is | |

* designed to compute the columns of the product first | |

* then handle the carries afterwards. This has the effect | |

* of making the nested loops that compute the columns very | |

* simple and schedulable on super-scalar processors. | |

* | |

* This has been modified to produce a variable number of | |

* digits of output so if say only a half-product is required | |

* you don't have to compute the upper half (a feature | |

* required for fast Barrett reduction). | |

* | |

* Based on Algorithm 14.12 on pp.595 of HAC. | |

* | |

*/ | |

int fast_s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs) | |

{ | |

int olduse, res, pa, ix, iz; | |

mp_digit W[MP_WARRAY]; | |

register mp_word _W; | |

/* grow the destination as required */ | |

if (c->alloc < digs) { | |

if ((res = mp_grow (c, digs)) != MP_OKAY) { | |

return res; | |

} | |

} | |

/* number of output digits to produce */ | |

pa = MIN(digs, a->used + b->used); | |

/* clear the carry */ | |

_W = 0; | |

for (ix = 0; ix < pa; ix++) { | |

int tx, ty; | |

int iy; | |

mp_digit *tmpx, *tmpy; | |

/* get offsets into the two bignums */ | |

ty = MIN(b->used-1, ix); | |

tx = ix - ty; | |

/* setup temp aliases */ | |

tmpx = a->dp + tx; | |

tmpy = b->dp + ty; | |

/* this is the number of times the loop will iterrate, essentially | |

while (tx++ < a->used && ty-- >= 0) { ... } | |

*/ | |

iy = MIN(a->used-tx, ty+1); | |

/* execute loop */ | |

for (iz = 0; iz < iy; ++iz) { | |

_W += ((mp_word)*tmpx++)*((mp_word)*tmpy--); | |

} | |

/* store term */ | |

W[ix] = ((mp_digit)_W) & MP_MASK; | |

/* make next carry */ | |

_W = _W >> ((mp_word)DIGIT_BIT); | |

} | |

/* store final carry */ | |

W[ix] = (mp_digit)(_W & MP_MASK); | |

/* setup dest */ | |

olduse = c->used; | |

c->used = pa; | |

{ | |

register mp_digit *tmpc; | |

tmpc = c->dp; | |

for (ix = 0; ix < pa+1; ix++) { | |

/* now extract the previous digit [below the carry] */ | |

*tmpc++ = W[ix]; | |

} | |

/* clear unused digits [that existed in the old copy of c] */ | |

for (; ix < olduse; ix++) { | |

*tmpc++ = 0; | |

} | |

} | |

mp_clamp (c); | |

return MP_OKAY; | |

} | |

#endif |