Introduce Chars

This improves the implementation by introducing a class to manage the characters

src/ilp_keyboard_layout_optimization/data_aquisition/chars.py

@@ -10,13 +10,28 @@ from ..type_aliases import Bigram, CharTuple
 
 
 class Chars:
+    """A unified interface to a collection of characters and corresponding bigrams
+
+    Parameters
+    ----------
+    chars : str or CharTupel, optional
+        A string of concatenated (special) characters or a CharTupel of single
+        characters, that are supposed to be considered. Defaults to the most common
+        letters, numbers and punctuation in German texts.
+    """
+
     _chars: str
+    _monos: CharTuple
+    _bis: Tuple[Bigram]
 
     def __init__(self, chars: Optional[Union[str, CharTuple]] = None):
         if chars is None:
             self._chars = (
-                """abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890,"""
-                r"""üöäÜÖÄß.:;-–…_[](){}^\/#!?+<>&$|~`*=%"'@"""
+                string.ascii_lowercase
+                + string.ascii_uppercase
+                + string.digits
+                + string.punctuation
+                + "üöäÜÖÄß–…"
             )
         else:
             self.chars = chars
@@ -31,11 +46,33 @@ class Chars:
             self._chars = chars
         else:  # isinstance(chars, CharTuple):
             self._chars = "".join(char for char in chars)
+        try:
+            del self._monos
+        except AttributeError:
+            pass
+        try:
+            del self._bis
+        except AttributeError:
+            pass
 
     @property
-    def char_tuple(self) -> CharTuple:
-        return self._str2char_tuple(self.chars)
+    def monos(self) -> CharTuple:
+        try:
+            return self._monos
+        except AttributeError:
+            self._monos = self._str2char_tuple(self.chars)
+            return self._monos
 
     @staticmethod
-    def _str2char_tuple(string: str) -> CharTuple:
-        return tuple(char for char in string)
+    def _str2char_tuple(char_str: str) -> CharTuple:
+        return tuple(char for char in char_str)
+
+    @property
+    def bis(self):
+        try:
+            return self._bis
+        except AttributeError:
+            self._bis = tuple(
+                "".join(bigram_tuple) for bigram_tuple in product(self.chars, repeat=2)
+            )
+            return self._bis