sclass CountingInputStream extends FilterInputStream {
  long counter;
  
  *(InputStream in) { super(in); }
  
  @Override
  public int read() throws IOException {
    int c = in.read();
    if (c >= 0) ++counter;
    ret c;
  }

  @Override
  public int read(byte[] b) throws IOException {
    int n = in.read(b);
    counter += max(n, 0);
    ret n;
  }

  @Override
  public int read(byte[] b, int off, int len) throws IOException {
    int n = in.read(b, off, len);
    counter += max(n, 0);
    ret n;
  }
  
  long getFilePointer() { ret counter; }
}

Began life as a copy of #1029366