lngmath

lngmath.c

void lngcpy(unsigned char *dst, int dst_length, unsigned char *const src, int src_length) {
  if (dst_length <= src_length) {
    memcpy(dst, src, dst_length);
  } else {
    /* src_length < dst_length */
    memcpy(dst, src, src_length);
    memset(dst + src_length, 0, dst_length - src_length);
  }
}

void lngadd(unsigned char *dst, int dst_length, unsigned char *const src, int src_length) {
  int i = 0;
  int carry = 0;
  int tmp;
  while (i < dst_length && i < src_length) {
    tmp = carry + dst[i] + src[i];
    dst[i] = (unsigned char)(tmp & 0xff);
    carry = tmp >> 8;
  }
  while (carry && i < dst_length) {
    dst[i] = (unsigned char)(tmp & 0xff);
    carry = tmp >> 8;
  }
}

void lngsub(unsigned char *dst, int dst_length, unsigned char *const src, int src_length) {
  int i = 0;
  int carry = 0;
  int tmp;
  while (i < dst_length && i < src_length) {
    tmp = carry + dst[i] - src[i];
    dst[i] = (unsigned char)(tmp & 0xff);
    carry = tmp >> 8;
  }
  while (carry && i < dst_length) {
    dst[i] = (unsigned char)(tmp & 0xff);
    carry = tmp >> 8;
  }
}

void lngmul(unsigned char *dst, int dst_length, unsigned char *const src, int src_length) {
  int i, j, k;
  unsigned long long tmp;
  
  for (i = dst_length-1; i >= 0; i--) {

    tmp = 0;    
    for (j = 0; j < src_length; j++) {
      k = i - j;
      if (k < 0) break;
      tmp += src[j] * dst[k];
    }
    dst[i] = (tmp & 0xff); tmp >>= 8;

    k = i+1;
    while (k < dst_length && tmp) {
      tmp += dst[k];
      dst[k] = tmp & 0xff;
      tmp >>= 8;
      k++;
    }

  }
}