pb_sub.c

   1 /* LibTomPoly, Polynomial Basis Math -- Tom St Denis
   2  *
   3  * LibTomPoly is a public domain library that provides
   4  * polynomial basis arithmetic support.  It relies on
   5  * LibTomMath for large integer support.
   6  *
   7  * This library is free for all purposes without any
   8  * express guarantee that it works.
   9  *
  10  * Tom St Denis, tomstdenis@iahu.ca, http://poly.libtomcrypt.org
  11  */
  12 #include <tompoly.h>
  13
  14 int pb_sub(pb_poly *a, pb_poly *b, pb_poly *c)
  15 {
  16    int neg, err, x, y, z, characteristic;
  17    pb_poly *tmp;
  18
  19    /* grow c to be the max size */
  20    y = PB_MAX(a->used, b->used);
  21    if (c->alloc < y) {
  22       if ((err = pb_grow(c, y)) != MP_OKAY) {
  23          return err;
  24       }
  25    }
  26
  27    /* do we need to concern char */
  28    characteristic = mp_iszero(&(c->characteristic));
  29
  30    /* sub the terms */
  31    z = PB_MIN(a->used, b->used);
  32    for (x = 0; x < z; x++) {
  33        if ((err = mp_sub(&(a->terms[x]), &(b->terms[x]), &(c->terms[x]))) != MP_OKAY) {
  34           return err;
  35        }
  36        if (characteristic == MP_NO) {
  37           if ((err = mp_mod(&(c->terms[x]), &(c->characteristic), &(c->terms[x]))) != MP_OKAY) {
  38              return err;
  39           }
  40        }
  41    }
  42
  43    /* excess digits? */
  44    if (y != z) {
  45        if (a->used == y) {
  46           tmp = a;
  47           neg = 0;
  48        } else {
  49           tmp = b;
  50           neg = 1;
  51        }
  52        for (x = z; x < y; x++) {
  53           if (characteristic == MP_NO) {
  54              if ((err = mp_mod(&(tmp->terms[x]), &(c->characteristic), &(c->terms[x]))) != MP_OKAY) {
  55                 return err;
  56              }
  57              if (neg) {
  58                 if ((err = mp_sub(&(c->characteristic), &(c->terms[x]), &(c->terms[x]))) != MP_OKAY) {
  59                    return err;
  60                 }
  61              }
  62           } else {
  63              if (neg) {
  64                 if ((err = mp_neg(&(tmp->terms[x]), &(c->terms[x]))) != MP_OKAY) {
  65                    return err;
  66                 }
  67              } else {
  68                 if ((err = mp_copy(&(tmp->terms[x]), &(c->terms[x]))) != MP_OKAY) {
  69                    return err;
  70                 }
  71              }
  72           }
  73        }
  74    }
  75
  76    /* zero excess */
  77    for (x = y; x < c->used; x++) {
  78        mp_zero(&(c->terms[x]));
  79    }
  80    c->used = y;
  81    pb_clamp(c);
  82
  83    return MP_OKAY;
  84 }
  85
  86