## Mercurial > dropbear

### view libtommath/bn_mp_sub.c @ 709:abd99ecd7ec2

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use an empty writebuf rather than a NULL one

author | Matt Johnston <matt@ucc.asn.au> |
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date | Wed, 20 Mar 2013 22:31:07 +0800 |

parents | 5ff8218bcee9 |

children | 60fc6476e044 |

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#include <tommath.h> #ifdef BN_MP_SUB_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, [email protected], http://math.libtomcrypt.com */ /* 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; if (sa != sb) { /* subtract a negative from a positive, OR */ /* subtract a positive from a negative. */ /* In either case, ADD their magnitudes, */ /* and use the sign of the first number. */ c->sign = sa; res = s_mp_add (a, b, c); } else { /* subtract a positive from a positive, OR */ /* subtract a negative from a negative. */ /* First, take the difference between their */ /* magnitudes, then... */ if (mp_cmp_mag (a, b) != MP_LT) { /* Copy the sign from the first */ c->sign = sa; /* The first has a larger or equal magnitude */ res = s_mp_sub (a, b, c); } else { /* The result has the *opposite* sign from */ /* the first number. */ c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS; /* The second has a larger magnitude */ res = s_mp_sub (b, a, c); } } return res; } #endif /* $Source: /cvs/libtom/libtommath/bn_mp_sub.c,v $ */ /* $Revision: 1.3 $ */ /* $Date: 2006/03/31 14:18:44 $ */