utils: move elide_filename to own function

Also increase the elide limit in TempDownloadManager to 50, since that's
probably still enough for all systems.

qutebrowser/browser/webkit/downloads.py

@@ -1334,8 +1334,7 @@ class TempDownloadManager(QObject):
         encoding = sys.getfilesystemencoding()
         suggested_name = utils.force_encoding(suggested_name, encoding)
         # Make sure that the filename is not too long
-        if len(suggested_name) > 50:
+        suggested_name = utils.elide_filename(suggested_name, 50)
-            suggested_name = suggested_name[:25] + '...' + suggested_name[-25:]
         fobj = tempfile.NamedTemporaryFile(dir=tmpdir.name, delete=False,
                                            suffix=suggested_name)
         self.files.append(fobj)

qutebrowser/utils/utils.py

@@ -58,6 +58,38 @@ def elide(text, length):
         return text[:length - 1] + '\u2026'
 
 
+def elide_filename(filename, length):
+    """Elide a filename to the given length.
+
+    The difference to the elide() is that the text is removed from
+    the middle instead of from the end. This preserves file name extensions.
+    Additionally, standard ASCII dots are used ("...") instead of the unicode
+    "…" (U+2026) so it works regardless of the filesystem encoding.
+
+    This function does not handle path separators.
+
+    Args:
+        filename: The filename to elide.
+        length: The maximum length of the filename, must be at least 3.
+
+    Return:
+        The elided filename.
+    """
+    elidestr = '...'
+    if length < len(elidestr):
+        raise ValueError('length must be greater or equal to 3')
+    if len(filename) <= length:
+        return filename
+    # Account for '...'
+    length -= len(elidestr)
+    left = length // 2
+    right = length - left
+    if right == 0:
+        return filename[:left] + elidestr
+    else:
+        return filename[:left] + elidestr + filename[-right:]
+
+
 def compact_text(text, elidelength=None):
     """Remove leading whitespace and newlines from a text and maybe elide it.
 

tests/unit/utils/test_utils.py

@@ -94,6 +94,24 @@ class TestEliding:
         assert utils.elide(text, length) == expected
 
 
+class TestElidingFilenames:
+
+    """Test elide_filename."""
+
+    def test_too_small(self):
+        """Test eliding to less than 3 characters which should fail."""
+        with pytest.raises(ValueError):
+            utils.elide_filename('foo', 1)
+
+    @pytest.mark.parametrize('filename, length, expected', [
+        ('foobar', 3, '...'),
+        ('foobar.txt', 50, 'foobar.txt'),
+        ('foobarbazqux.py', 10, 'foo...x.py'),
+    ])
+    def test_elided(self, filename, length, expected):
+        assert utils.elide_filename(filename, length) == expected
+
+
 @pytest.fixture(params=[True, False])
 def freezer(request, monkeypatch):
     if request.param and not getattr(sys, 'frozen', False):