expose multibyte-to-codepoint conversion

ostd/string.hh

@@ -82,15 +82,12 @@ using char_range = basic_char_range<char>;
 using string_range = basic_char_range<char const>;
 
 namespace utf {
-    /* @brief Get the number of Unicode code points in a valid UTF-8 string.
+    /* @brief Get the Unicode code point for a multibyte sequence.
      *
-     * If an invalid UTF-8 sequence is encountered, it returns the length
+     * The string is advanced past the UTF-8 character in the front.
-     * until that sequence.
+     * If the decoding fails, `false` is returned, otherwise it's `true`.
-     *
-     * If you need to get the continuation string, use the error-handling
-     * overload of the function.
      */
-    std::size_t length(string_range r);
+    bool codepoint(string_range &r, char32_t &ret);
 
     /* @brief Get the number of Unicode code points in a string.
      *
@@ -98,12 +95,22 @@ namespace utf {
      * once it can't it returns the number of valid ones with the rest
      * of the input string range being in `cont`. That means if the entire
      * string is a valid UTF-8 string, `cont` will be empty, otherwise it
-     * will begin at the first unvalid UTF-8 code point.
+     * will begin at the first invalid UTF-8 code point.
      *
      * If you're sure the string is valid or you don't need to handle the
-     * error, you can use the more convenient overload above.
+     * error, you can use the more convenient overload below.
      */
     std::size_t length(string_range r, string_range &cont);
+
+    /* @brief Get the number of Unicode code points in a valid UTF-8 string.
+     *
+     * If an invalid UTF-8 sequence is encountered, it returns the length
+     * until that sequence.
+     *
+     * If you need to get the continuation string, use the general
+     * error-handling overload of the function.
+     */
+    std::size_t length(string_range r);
 } /* namespace utf */
 
 /** @brief A string slice type.

src/string.cc

@@ -11,11 +11,7 @@ namespace utf {
 
 constexpr std::uint32_t MaxCodepoint = 0x10FFFF;
 
-static inline bool is_u8cont(std::uint32_t ch) {
+static inline bool codepoint_dec(string_range &r, char32_t &cret) {
-    return (ch & 0xC0) == 0x80;
-}
-
-static inline bool codepoint(string_range &r, char32_t &cret) {
     static const std::uint32_t ulim[] = { 0xFF, 0x7F, 0x7FF, 0xFFFF };
     if (r.empty()) {
         return false;
@@ -65,9 +61,13 @@ static inline bool codepoint(string_range &r, char32_t &cret) {
     return true;
 }
 
+bool codepoint(string_range &r, char32_t &ret) {
+    return codepoint_dec(r, ret);
+}
+
 std::size_t length(string_range r, string_range &cont) {
     std::size_t ret = 0;
-    for (char32_t ch = U'\0'; codepoint(r, ch); ++ret) {
+    for (char32_t ch = U'\0'; codepoint_dec(r, ch); ++ret) {
         continue;
     }
     cont = r;