Speed up WordSplitter

Only do utf8 decoding once per codepoint instead of twice, limit
the byte length instead of the codepoint length.

src/word_db.cc

@@ -34,30 +34,21 @@ struct WordSplitter
             const auto* end = m_splitter->m_content.end();
             auto extra_chars = m_splitter->m_extra_word_chars;
 
-            while (true)
+            do
             {
-                m_word_begin = m_word_end;
+                auto it = m_word_begin = m_word_end;
-                while (m_word_begin != end and not is_word(utf8::codepoint(m_word_begin, end), extra_chars))
+                while (it != end and not is_word(utf8::read_codepoint(it, end), extra_chars))
-                    utf8::to_next(m_word_begin, end);
+                    m_word_begin = it;
-                m_word_end = m_word_begin;
+
-                CharCount word_len = 0;
+                m_word_end = it;
-                while (m_word_end != end and is_word(utf8::codepoint(m_word_end, end), extra_chars))
+                while (it != end and is_word(utf8::read_codepoint(it, end), extra_chars))
-                {
+                    m_word_end = it;
-                    utf8::to_next(m_word_end, end);
+            } while (m_word_begin != end and (m_word_end - m_word_begin) > WordDB::max_word_len);
-                    ++word_len;
-                }
-                if (m_word_begin == end or word_len < WordDB::max_word_len)
-                    break;
-            }
 
             return *this;
         }
 
-        friend bool operator==(const Iterator& lhs, const Iterator& rhs)
+        friend bool operator==(const Iterator& lhs, const Iterator& rhs) = default;
-        { return lhs.m_word_begin == rhs.m_word_begin and lhs.m_word_end == rhs.m_word_end; }
-
-        friend bool operator!=(const Iterator& lhs, const Iterator& rhs)
-        { return not (lhs == rhs); }
 
         const char* m_word_begin;
         const char* m_word_end;

src/word_db.hh

@@ -18,7 +18,7 @@ class Buffer;
 class WordDB : public OptionManagerWatcher
 {
 public:
-    static constexpr CharCount max_word_len = 50;
+    static constexpr ByteCount max_word_len = 50;
 
     WordDB(const Buffer& buffer);
     ~WordDB();