Add function to calculate unique prefix of two strings

poezio/common.py

@@ -17,6 +17,7 @@ import subprocess
 import time
 import string
 import logging
+import itertools
 
 from slixmpp import JID, InvalidJID, Message
 from poezio.poezio_shlex import shlex
@@ -468,3 +469,22 @@ def safeJID(*args, **kwargs) -> JID:
             exc_info=True,
         )
         return JID('')
+
+
+def unique_prefix_of(a: str, b: str) -> str:
+    """
+    Return the unique prefix of `a` with `b`.
+
+    Corner cases:
+
+    * If `a` and `b` share no prefix, the first letter of `a` is returned.
+    * If `a` and `b` are equal, `a` is returned.
+    * If `a` is a prefix of `b`, `a` is returned.
+    * If `b` is a prefix of `a`, `b` plus the first letter of `a` after the
+      common prefix is returned.
+    """
+    for i, (ca, cb) in enumerate(itertools.zip_longest(a, b)):
+        if ca != cb:
+            return a[:i+1]
+    # both are equal, return a
+    return a

test/test_common.py

@@ -11,7 +11,7 @@ from poezio.common import (_datetime_tuple as datetime_tuple, get_utc_time,
                            get_local_time, shell_split, _find_argument_quoted
                            as find_argument_quoted, _find_argument_unquoted as
                            find_argument_unquoted, parse_str_to_secs,
-                           parse_secs_to_str, safeJID)
+                           parse_secs_to_str, safeJID, unique_prefix_of)
 
 def test_utc_time():
     delta = timedelta(seconds=-3600)
@@ -63,3 +63,22 @@ def test_parse_secs_to_str():
 def test_safeJID():
     assert safeJID('toto@titi/tata') == JID('toto@titi/tata')
     assert safeJID('toto@…') == JID('')
+
+def test_unique_prefix_of__no_shared_prefix():
+    assert unique_prefix_of("a", "b") == "a"
+    assert unique_prefix_of("foo", "bar") == "f"
+    assert unique_prefix_of("foo", "") == "f"
+
+def test_unique_prefix_of__equal():
+    assert unique_prefix_of("foo", "foo") == "foo"
+
+def test_unique_prefix_of__a_prefix():
+    assert unique_prefix_of("foo", "foobar") == "foo"
+
+def test_unique_prefix_of__b_prefix():
+    assert unique_prefix_of("foobar", "foo") == "foob"
+
+def test_unique_prefix_of__normal_shared_prefix():
+    assert unique_prefix_of("foobar", "foobaz") == "foobar"
+    assert unique_prefix_of("fnord", "funky") == "fn"
+    assert unique_prefix_of("asbestos", "aspergers") == "asb"