diff options
Diffstat (limited to 'libtommath/bn_mp_sub_d.c')
-rw-r--r-- | libtommath/bn_mp_sub_d.c | 89 |
1 files changed, 89 insertions, 0 deletions
diff --git a/libtommath/bn_mp_sub_d.c b/libtommath/bn_mp_sub_d.c new file mode 100644 index 0000000..d8cc473 --- /dev/null +++ b/libtommath/bn_mp_sub_d.c @@ -0,0 +1,89 @@ +#include <tommath.h> +#ifdef BN_MP_SUB_D_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 + */ + +/* single digit subtraction */ +int +mp_sub_d (mp_int * a, mp_digit b, mp_int * c) +{ + mp_digit *tmpa, *tmpc, mu; + int res, ix, oldused; + + /* grow c as required */ + if (c->alloc < a->used + 1) { + if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) { + return res; + } + } + + /* if a is negative just do an unsigned + * addition [with fudged signs] + */ + if (a->sign == MP_NEG) { + a->sign = MP_ZPOS; + res = mp_add_d(a, b, c); + a->sign = c->sign = MP_NEG; + return res; + } + + /* setup regs */ + oldused = c->used; + tmpa = a->dp; + tmpc = c->dp; + + /* if a <= b simply fix the single digit */ + if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) { + if (a->used == 1) { + *tmpc++ = b - *tmpa; + } else { + *tmpc++ = b; + } + ix = 1; + + /* negative/1digit */ + c->sign = MP_NEG; + c->used = 1; + } else { + /* positive/size */ + c->sign = MP_ZPOS; + c->used = a->used; + + /* subtract first digit */ + *tmpc = *tmpa++ - b; + mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1); + *tmpc++ &= MP_MASK; + + /* handle rest of the digits */ + for (ix = 1; ix < a->used; ix++) { + *tmpc = *tmpa++ - mu; + mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1); + *tmpc++ &= MP_MASK; + } + } + + /* zero excess digits */ + while (ix++ < oldused) { + *tmpc++ = 0; + } + mp_clamp(c); + return MP_OKAY; +} + +#endif + +/* $Source: /root/tcl/repos-to-convert/tcl/libtommath/bn_mp_sub_d.c,v $ */ +/* $Revision: 1.1.1.1.2.1 $ */ +/* $Date: 2005/09/26 20:16:53 $ */ |