More clever username generation

authens/backends.py

@@ -70,21 +70,26 @@ class ENSCASBackend:
         worry about data races).
         """
 
+        # Ideally we can have username = cas_login.
+        if not UserModel.objects.filter(username=cas_login).exists():
+            return cas_login
+
+        # Otherwise, add a numerical suffix.
+
+        # If there was a numerical suffix already, remove it:
+        # https://git.eleves.ens.fr/klub-dev-ens/authens/issues/5
+        i = len(cas_login) - 1
+        while i >= 0 and cas_login[i] in "0123456789":
+            i -= 1
+        radical = cas_login[:i + 1]
+
+        # Find an integer i such that radical + str(i) is not taken.
         taken = UserModel.objects.values_list("username", flat=True)
-
-        # This should handle most cases and produce a nice username.
-        prefered = [cas_login, "cas_" + cas_login]
-        pref_taken = taken.filter(username__in=prefered)
-        for name in prefered:
-            if name not in pref_taken:
-                return name
-
-        # Worst case: generate a username of the form cas_login + int
-        taken = taken.filter(username__startswith=cas_login)
+        taken = taken.filter(username__startswith=radical)
         i = 2
-        while cas_login + str(i) in taken:
+        while radical + str(i) in taken:
             i += 1
-        return cas_login + str(i)
+        return radical + str(i)
 
     def _get_or_create(self, cas_login, entrance_year):
         """Handles account retrieval, creation and invalidation as described above.