# libtom/libtommath

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 /* LibTomMath, multiple-precision integer library -- Tom St Denis * * LibTomMath is library that provides for multiple-precision * integer arithmetic as well as number theoretic functionality. * * The library is 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 /* high level subtraction (handles signs) */ int mp_sub (mp_int * a, mp_int * b, mp_int * c) { int sa, sb, res; sa = a->sign; sb = b->sign; /* handle four cases */ if (sa == MP_ZPOS && sb == MP_ZPOS) { /* both positive, a - b, but if b>a then we do -(b - a) */ if (mp_cmp_mag (a, b) == MP_LT) { /* b>a */ res = s_mp_sub (b, a, c); c->sign = MP_NEG; } else { res = s_mp_sub (a, b, c); c->sign = MP_ZPOS; } } else if (sa == MP_ZPOS && sb == MP_NEG) { /* a - -b == a + b */ res = s_mp_add (a, b, c); c->sign = MP_ZPOS; } else if (sa == MP_NEG && sb == MP_ZPOS) { /* -a - b == -(a + b) */ res = s_mp_add (a, b, c); c->sign = MP_NEG; } else { /* -a - -b == b - a, but if a>b == -(a - b) */ if (mp_cmp_mag (a, b) == MP_GT) { res = s_mp_sub (a, b, c); c->sign = MP_NEG; } else { res = s_mp_sub (b, a, c); c->sign = MP_ZPOS; } } return res; }
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