void JBIG2Bitmap::combine(JBIG2Bitmap *bitmap, int x, int y,
			  Guint combOp) {
  int x0, x1, y0, y1, xx, yy;
  Guchar *srcPtr, *destPtr;
  Guint src0, src1, src, dest, s1, s2, m1, m2, m3;
  GBool oneByte;

  if (y < 0) {
    y0 = -y;
  } else {
    y0 = 0;
  }
  if (y + bitmap->h > h) {
    y1 = h - y;
  } else {
    y1 = bitmap->h;
  }
  if (y0 >= y1) {
    return;
  }

  if (x >= 0) {
    x0 = x & ~7;
  } else {
    x0 = 0;
  }
  x1 = x + bitmap->w;
  if (x1 > w) {
    x1 = w;
  }
  if (x0 >= x1) {
    return;
  }

  s1 = x & 7;
  s2 = 8 - s1;
  m1 = 0xff >> (x1 & 7);
  m2 = 0xff << (((x1 & 7) == 0) ? 0 : 8 - (x1 & 7));
  m3 = (0xff >> s1) & m2;

  oneByte = x0 == ((x1 - 1) & ~7);

  for (yy = y0; yy < y1; ++yy) {

    if (oneByte) {
      if (x >= 0) {
	destPtr = data + (y + yy) * line + (x >> 3);
	srcPtr = bitmap->data + yy * bitmap->line;
	dest = *destPtr;
	src1 = *srcPtr;
	switch (combOp) {
	case 0: // or
	  dest |= (src1 >> s1) & m2;
	  break;
	case 1: // and
	  dest &= ((0xff00 | src1) >> s1) | m1;
	  break;
	case 2: // xor
	  dest ^= (src1 >> s1) & m2;
	  break;
	case 3: // xnor
	  dest ^= ((src1 ^ 0xff) >> s1) & m2;
	  break;
	case 4: // replace
	  dest = (dest & ~m3) | ((src1 >> s1) & m3);
	  break;
	}
	*destPtr = dest;
      } else {
	destPtr = data + (y + yy) * line;
	srcPtr = bitmap->data + yy * bitmap->line + (-x >> 3);
	dest = *destPtr;
	src1 = *srcPtr;
	switch (combOp) {
	case 0: // or
	  dest |= src1 & m2;
	  break;
	case 1: // and
	  dest &= src1 | m1;
	  break;
	case 2: // xor
	  dest ^= src1 & m2;
	  break;
	case 3: // xnor
	  dest ^= (src1 ^ 0xff) & m2;
	  break;
	case 4: // replace
	  dest = (src1 & m2) | (dest & m1);
	  break;
	}
	*destPtr = dest;
      }

    } else {

      if (x >= 0) {
	destPtr = data + (y + yy) * line + (x >> 3);
	srcPtr = bitmap->data + yy * bitmap->line;
	src1 = *srcPtr++;
	dest = *destPtr;
	switch (combOp) {
	case 0: // or
	  dest |= src1 >> s1;
	  break;
	case 1: // and
	  dest &= (0xff00 | src1) >> s1;
	  break;
	case 2: // xor
	  dest ^= src1 >> s1;
	  break;
	case 3: // xnor
	  dest ^= (src1 ^ 0xff) >> s1;
	  break;
	case 4: // replace
	  dest = (dest & (0xff << s2)) | (src1 >> s1);
	  break;
	}
	*destPtr++ = dest;
	xx = x0 + 8;
      } else {
	destPtr = data + (y + yy) * line;
	srcPtr = bitmap->data + yy * bitmap->line + (-x >> 3);
	src1 = *srcPtr++;
	xx = x0;
      }

      for (; xx < x1 - 8; xx += 8) {
	dest = *destPtr;
	src0 = src1;
	src1 = *srcPtr++;
	src = (((src0 << 8) | src1) >> s1) & 0xff;
	switch (combOp) {
	case 0: // or
	  dest |= src;
	  break;
	case 1: // and
	  dest &= src;
	  break;
	case 2: // xor
	  dest ^= src;
	  break;
	case 3: // xnor
	  dest ^= src ^ 0xff;
	  break;
	case 4: // replace
	  dest = src;
	  break;
	}
	*destPtr++ = dest;
      }

      dest = *destPtr;
      src0 = src1;
      src1 = *srcPtr++;
      src = (((src0 << 8) | src1) >> s1) & 0xff;
      switch (combOp) {
      case 0: // or
	dest |= src & m2;
	break;
      case 1: // and
	dest &= src | m1;
	break;
      case 2: // xor
	dest ^= src & m2;
	break;
      case 3: // xnor
	dest ^= (src ^ 0xff) & m2;
	break;
      case 4: // replace
	dest = (src & m2) | (dest & m1);
	break;
      }
      *destPtr = dest;
    }
  }
}

This weakness involves numeric computation errors, such as integer overflows, underflows, or precision losses, which can lead to miscalculations and exploitable behaviors in software.