fn write(path: &str, data: String) -> Result<usize> {
    FILE_MAP.with(|cell| {
        let mut map = cell.borrow_mut();
        let path = Path::new(path);
        let file = match map.entry(filename(path)?) {
            Entry::Occupied(elem) => elem.into_mut(),
            Entry::Vacant(elem) => elem.insert(open(path)?),
        };

        Ok(file.write(&data.into_bytes())?)
    })
}