from voussoirkit import cacheclass

import etiquette

class FileCacheManager:
    '''
    The FileCacheManager serves ETag and Cache-Control headers for disk files.

    We consider the following cases:

    Client does not have the file (or has disabled their cache):
        Server sends file, provides ETag, tells client to save it for max-age.

    Client has the file, but it has been a long time, beyond the max-age:
        Client provides the old ETag. If it's still valid, Server responds with
        304 Not Modified and no data. Client keeps the old file.

    Client has the file, and it is within the max-age:
        Client does not make a request at all.

    We use the file's MD5 hash as the ETag, and will only recalculate it if the
    file's mtime has changed since the last request.
    '''
    def __init__(self, maxlen, max_age, max_filesize):
        self.cache = cacheclass.Cache(maxlen=maxlen)
        self.max_age = int(max_age)
        self.max_filesize = max(int(max_filesize), 0) or None

    def get(self, filepath):
        try:
            return self.cache[filepath]
        except KeyError:
            pass

        if (self.max_filesize is not None) and (filepath.size > self.max_filesize):
            return None

        cache_file = CacheFile(filepath, max_age=self.max_age)
        self.cache[filepath] = cache_file
        return cache_file

    def matches(self, request, filepath):
        client_etag = request.headers.get('If-None-Match', None)
        if client_etag is None:
            return False

        server_value = self.get(filepath)
        if server_value is None:
            return False

        server_etag = server_value.get_etag()
        if client_etag != server_etag:
            return False

        return server_value.get_headers()

class CacheFile:
    def __init__(self, filepath, max_age):
        self.filepath = filepath
        self.max_age = int(max_age)
        self._stored_hash_time = None
        self._stored_hash_value = None

    def get_etag(self):
        mtime = self.filepath.stat.st_mtime
        do_refresh = (self._stored_hash_value is None) or (mtime > self._stored_hash_time)

        if do_refresh:
            self._stored_hash_time = mtime
            self._stored_hash_value = etiquette.helpers.hash_file_md5(self.filepath)
        return self._stored_hash_value

    def get_headers(self):
        headers = {
            'ETag': self.get_etag(),
            'Cache-Control': f'max-age={self.max_age}',
        }
        return headers