Add utf8_utf16_length.

GitOrigin-RevId: d5e713df1f3e0cdf70004d0898c5b55246dd014e

tdutils/td/utils/port/wstring_convert.cpp

@@ -21,10 +21,7 @@ Result<std::wstring> to_wstring(CSlice slice) {
     return Status::Error("Wrong encoding");
   }
 
-  size_t wstring_len = 0;
-  for (auto c : slice) {
-    wstring_len += ((c & 0xc0) != 0x80) + ((c & 0xf8) == 0xf0);
-  }
+  size_t wstring_len = utf8_utf16_length(slice);
 
   std::wstring result(wstring_len, static_cast<wchar_t>(0));
   if (wstring_len) {

tdutils/td/utils/utf8.h

@@ -28,6 +28,15 @@ inline size_t utf8_length(Slice str) {
   return result;
 }
 
+/// returns length of UTF-8 string in UTF-16 code units
+inline size_t utf8_utf16_length(Slice str) {
+  size_t result = 0;
+  for (auto c : str) {
+    result += is_utf8_character_first_code_unit(c) + ((c & 0xf8) == 0xf0);
+  }
+  return result;
+}
+
 /// appends a Unicode character using UTF-8 encoding
 void append_utf8_character(string &str, uint32 ch);