boxes: Improve trigger sequence for generic_autocomplete.

This updates generic_autocomplete to trigger autocomplete functions even
when there are some characters before the prefix_string.

Tests amended.

Fixes #541.

Author: preetmishra

tests/ui_tools/test_boxes.py

@@ -81,6 +81,13 @@ def test_generic_autocomplete_no_prefix(self, mocker, write_box, text,
         ('@_', 3, None),  # Reached last match
         ('@_', 4, None),  # Beyond end
         ('@_', -1, '@_**Human 2**'),
+        # When some character is preceding the prefix_string (trigger).
+        ('(@H', 0, '(@**Human Myself**'),
+        ('(@H', 1, '(@**Human 1**'),
+        ('>@_H', 0, '>@_**Human Myself**'),
+        ('>@_H', 1, '>@_**Human 1**'),
+        ('-@G', 0, '-@*Group 1*'),
+        ('-@G', 1, '-@*Group 2*'),
     ])
     def test_generic_autocomplete_mentions(self, write_box, users_fixture,
                                            text, state, required_typeahead,
@@ -110,6 +117,8 @@ def test_generic_autocomplete_mentions(self, write_box, users_fixture,
         ('#St', 0, '#**Stream 1**'),
         ('#St', 1, '#**Stream 2**'),
         ('#Stream 1', 0, '#**Stream 1**'),
+        ('[#Stream', 0, '[#**Stream 1**'),
+        ('(#Stream', 1, '(#**Stream 2**'),
     ])
     def test_generic_autocomplete_streams(self, write_box, streams_fixture,
                                           text, state, required_typeahead):
@@ -139,6 +148,8 @@ def test_generic_autocomplete_streams(self, write_box, streams_fixture,
         (':', -1, ':smirk:'),
         (':nomatch', 0, None),
         (':nomatch', -1, None),
+        ('(:smi', 0, '(:smile:'),
+        ('&:smi', 1, '&:smiley:'),
         ])
     def test_generic_autocomplete_emojis(self, write_box, emojis_fixture, text,
                                          mocker, state, required_typeahead):

zulipterminal/ui_tools/boxes.py

@@ -101,20 +101,22 @@ def generic_autocomplete(self, text: str, state: int) -> Optional[str]:
             'mentions': self.autocomplete_mentions,
             'streams': self.autocomplete_streams,
         }
-        if text.startswith('@_'):
+        if '@_' in text:
             form, prefix_string = 'mentions', '@_'
-        elif text.startswith('@'):
+        elif '@' in text:
             form, prefix_string = 'mentions', '@'
-        elif text.startswith('#'):
+        elif '#' in text:
             form, prefix_string = 'streams', '#'
-        elif text.startswith(':'):
+        elif ':' in text:
             form, prefix_string = 'emojis', ':'
         else:
             return text
-        typeahead = autocomplete_forms[form](text, prefix_string)
+        prefix_index = max(text.find(prefix_string), 0)
+        typeahead = autocomplete_forms[form](text[prefix_index:],
+                                             prefix_string)
 
         try:
-            return typeahead[state]
+            return text[:prefix_index] + typeahead[state]
         except (IndexError, TypeError):
             return None