Adds a strip index file

This should reduce the amount of time we spend on querying for comics that are already present

comic_download/comic_strip.py

@@ -263,6 +263,73 @@ class Comic(ImageRepo, ABC):
         """The title of the comic."""
         self.author = None
         """The author(s) of the comic. Can be a string for a name, a list of names, or a list of objects containing the names under "name" and role under "role"."""
+        
+        self.strip_index = {}
+        """
+        The strip index maps comic identifiers to their filenames (sans stems). If we
+        cache this data, we can find out where each strip lives without having to
+        load data from the website, thus using less requests for already-downloaded
+        strips.
+        """
+        self._index_loaded = False
+        """A flag for when the index has been successfully loaded."""
+        self._index_lock = Condition(Lock())
+        """A lock for enforcing thread safety when loading the comic index file."""
+
+    def read_index(self, base_path: Path):
+        """
+        Saves the strip index to the same base path we download comic strips to.
+        It is saved to "index.json."
+
+        :param base_path: The folder to save the index to.
+        """
+        with self._index_lock:
+            if not self._index_loaded:
+                index_path = Path(base_path, "index.json")
+                if index_path.exists():
+                    try:
+                        with open(Path(base_path, "index.json"), 'r') as index_fp:
+                            self.strip_index = json.load(index_fp)
+                    except json.JSONDecodeError as e:
+                        logging.exception(e)
+                        self.strip_index = {}
+                self._index_loaded = True
+            self._index_lock.notify_all()
+
+    def save_index(self, base_path: Path):
+        """
+        Saves the strip index to the same base path we download comic strips to.
+        It is saved to "index.json."
+
+        :param base_path: The folder to save the index to.
+        """
+        with self._index_lock:
+            with open(Path(base_path, "index.json"), 'w') as index_fp:
+                json.dump(self.strip_index, index_fp)
+
+    def index_strip(self, strip):
+        """
+        Registers a comic to the index. This does not save the comic index.
+
+        :param strip: The comic strip to index
+        """
+        with self._index_lock:
+            self.strip_index[strip.identifier] = strip.get_filename()
+
+    def is_strip_present(self, base_path: Path, suffix: str, identifier) -> bool:
+        """
+        Checks to see if a given strip has been saved to the file.
+
+        :param base_path: The folder we expect the strip to show up in.
+        :param suffix: The type suffix we want to check (i.e. ".cbz")
+        :param identifier: The identifier to check.
+        :returns: True if the strip file has been saved.
+        """
+        if not identifier in self.strip_index:
+            return False
+        path = Path(base_path, self.strip_index[identifier] + suffix)
+        return path.exists()
+            
 
     #@abstractmethod
     @classmethod
@@ -382,6 +449,7 @@ class ComicStrip(ImageRepo, ABC):
     def load_data(self):
         super().load_data()
         self.comic.load_data()
+        self.comic.index_strip(self)
 
     def download_data(self):
         super().download_data()