Fix performance issue with word completion

When pasting many words with <a-p> we can end-up with a huge
concatenated word and many selections, the previous code ended
up iterating from each selection cursor to that word start and
end to find the word under the cursor.

This could lead to performance issue as each selection would
trigger iteration on that huge word. This is unnecessary as
word completion has a word length limit, so we now take it into
account to avoid iterating to far from the cursor position.

src/insert_completer.cc

@@ -86,9 +86,12 @@ InsertCompletion complete_word(const SelectionList& sels,
     HashMap<StringView, int> sel_word_counts;
     for (int i = 0; i < sels.size(); ++i)
     {
+        int len = 0;
+        auto is_short_enough_word = [&] (Codepoint c) { return len++ < WordDB::max_word_len && is_word_pred(c); };
+
         Utf8It end{buffer.iterator_at(sels[i].cursor()), buffer};
         Utf8It begin = end-1;
-        if (not skip_while_reverse(begin, buffer.begin(), is_word_pred) and
+        if (not skip_while_reverse(begin, buffer.begin(), is_short_enough_word) and
             begin < end) // (begin might == end if end == buffer.begin())
             ++begin;
 
@@ -98,11 +101,14 @@ InsertCompletion complete_word(const SelectionList& sels,
             prefix = buffer.substr(word_begin, end.base().coord());
         }
 
-        skip_while(end, buffer.end(), is_word_pred);
+        skip_while(end, buffer.end(), is_short_enough_word);
 
+        if (len <= WordDB::max_word_len)
+        {
             StringView word = buffer.substr(begin.base().coord(), end.base().coord());
             ++sel_word_counts[word];
         }
+    }
 
     struct RankedMatchAndBuffer : RankedMatch
     {

src/word_db.cc

@@ -21,8 +21,6 @@ WordDB& get_word_db(const Buffer& buffer)
 
 struct WordSplitter
 {
-    static constexpr CharCount max_word_len = 50;
-
     struct Iterator
     {
         Iterator(const char* begin, const WordSplitter& splitter)
@@ -48,7 +46,7 @@ struct WordSplitter
                     utf8::to_next(m_word_end, end);
                     ++word_len;
                 }
-                if (m_word_begin == end or word_len < max_word_len)
+                if (m_word_begin == end or word_len < WordDB::max_word_len)
                     break;
             }
 

src/word_db.hh

@@ -18,6 +18,8 @@ class Buffer;
 class WordDB : public OptionManagerWatcher
 {
 public:
+    static constexpr CharCount max_word_len = 50;
+
     WordDB(const Buffer& buffer);
     ~WordDB();
     WordDB(const WordDB&) = delete;