Fix multibyte string processing

Signed-off-by: Pavel Artsishevsky <polter.rnd@gmail.com>

include/slimlog/pattern.h

@@ -333,35 +333,33 @@ class Pattern {
      */
     static auto get_string_specs(StringViewType value) -> Placeholder::StringSpecs;
 
+    /**
+     * @brief Writes the source string to the destination buffer.
+     *
+     * @tparam StringView String view type, convertible to `std::basic_string_view`.
+     * @param dst Destination buffer where the string will be written.
+     * @param src Source string view to be written.
+     * @param codepoints Number of codepoints the source string contains.
+     */
+    template<typename StringView>
+    constexpr static void write_string(auto& dst, StringView&& src, std::size_t codepoints);
+
     /**
      * @brief Writes the source string to the destination buffer with specific alignment.
      *
      * This function writes the source string to the destination buffer, applying the specified
      * alignment and fill character.
      *
-     * @tparam T Character type for the string view.
+     * @tparam StringView String view type, convertible to `std::basic_string_view`.
      * @param dst Destination buffer where the string will be written.
      * @param src Source string view to be written.
      * @param specs String specifications, including alignment and fill character.
      * @param codepoints Number of codepoints the source string contains.
      */
     template<typename StringView>
-    constexpr static void write_padded(
+    constexpr static void write_string_padded(
         auto& dst, StringView&& src, const Placeholder::StringSpecs& specs, std::size_t codepoints);
 
-    /**
-     * @brief Converts a multi-byte string to a single-byte string.
-     *
-     * This function converts a multi-byte string to a single-byte string and appends the result to
-     * the provided destination stream buffer.
-     *
-     * @tparam T Character type of the source string.
-     * @param out Destination stream buffer where the converted string will be appended.
-     * @param data Source multi-byte string to be converted.
-     * @param codepoints Number of codepoints the data string contains.
-     */
-    static void from_multibyte(auto& out, std::string_view data, std::size_t codepoints);
-
     std::basic_string<Char> m_pattern;
     std::vector<Placeholder> m_placeholders;
     Levels m_levels;

include/slimlog/util/unicode.h

@@ -262,6 +262,18 @@ constexpr auto to_ascii(Char chr) -> char
     return chr <= std::numeric_limits<unsigned char>::max() ? static_cast<char>(chr) : '\0';
 }
 
+/**
+ * @brief Converts a null-terminated multibyte string to a singlebyte character sequence.
+ *
+ * Destination buffer has to be capable of storing at least @p codepoints + 1 characters
+ * including null terminator.
+ *
+ * @tparam Char Character type of the destination string.
+ * @param dest Pointer to destination buffer for the converted string.
+ * @param data Source multi-byte string to be converted.
+ * @param codepoints Number of codepoints to be written to the destination string.
+ * @return Number of characters written including null terminator.
+ */
 template<typename Char>
 constexpr auto from_multibyte(Char* dest, std::string_view data, std::size_t codepoints)
 {
@@ -271,7 +283,7 @@ constexpr auto from_multibyte(Char* dest, std::string_view data, std::size_t cod
     if constexpr (std::is_same_v<Char, wchar_t>) {
         std::mbstate_t state = {};
 #if defined(_WIN32) and defined(__STDC_WANT_SECURE_LIB__)
-        if (mbsrtowcs_s(&written, dest, codepoints + 1, &source, _TRUNCATE, &state) != 0) {
+        if (mbsrtowcs_s(&written, dest, codepoints, &source, codepoints - 1, &state) != 0) {
             throw std::runtime_error("mbsrtowcs_s(): conversion error");
         }
 #else
@@ -280,8 +292,7 @@ constexpr auto from_multibyte(Char* dest, std::string_view data, std::size_t cod
         if (written == static_cast<std::size_t>(-1)) {
             throw std::runtime_error("std::mbsrtowcs(): conversion error");
         }
-        *std::next(dest, codepoints) = '\0';
-        ++written;
+        *std::next(dest, written++) = '\0';
 #endif
     } else {
         Char wchr;
@@ -298,7 +309,7 @@ constexpr auto from_multibyte(Char* dest, std::string_view data, std::size_t cod
                 throw std::runtime_error("std::mbrtocN(): conversion error");
                 break;
             case -2:
-                // Incomplete but valid character, skip it
+                // Incomplete but valid character, go further
                 break;
             case -3:
                 // Next character from surrogate pair was processed
@@ -316,7 +327,7 @@ constexpr auto from_multibyte(Char* dest, std::string_view data, std::size_t cod
                 break;
             }
         }
-        *std::next(dest, codepoints) = '\0';
+        *dest = '\0';
         ++written;
     }
     return written;