# libtom/libtommath

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 /* 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 */ #include /* 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; }
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