string: add recoding from UTF-16 to UTF-32

I know there are system functions for this (and libiconv), but this is
so trivial and quick in our implementation.

UnitTests/compat.hpp

@@ -54,6 +54,17 @@ namespace Assert
 			throw std::runtime_error("not equal");
 	}
 
+	inline void AreEqual(const char32_t* a, const char32_t* b)
+	{
+#ifdef _WIN32
+		if (stdex::strcmp(a, b) != 0)
+			throw std::runtime_error("not equal");
+#else
+		if (wcscmp(reinterpret_cast<const wchar_t*>(a), reinterpret_cast<const wchar_t*>(b)) != 0)
+			throw std::runtime_error("not equal");
+#endif
+	}
+
 	template <class T>
 	void AreNotEqual(const T& a, const T& b)
 	{

UnitTests/main.cpp

@@ -26,6 +26,7 @@ int main(int, const char *[])
 		UnitTests::stream::file_stat();
 		UnitTests::stream::open_close();
 		UnitTests::stream::replicator();
+		UnitTests::string::strncpy();
 		UnitTests::string::sprintf();
 		UnitTests::unicode::charset_encoder();
 		UnitTests::unicode::normalize();

UnitTests/pch.hpp

@@ -107,6 +107,7 @@ namespace UnitTests
 	TEST_CLASS(string)
 	{
 	public:
+		TEST_METHOD(strncpy);
 		TEST_METHOD(sprintf);
 	};
 

UnitTests/string.cpp

@@ -12,6 +12,13 @@ using namespace Microsoft::VisualStudio::CppUnitTestFramework;
 
 namespace UnitTests
 {
+	void string::strncpy()
+	{
+		stdex::utf32_t tmp[0x100];
+		stdex::strncpy(tmp, u"This is a 🐔Test🐮.");
+		Assert::AreEqual(reinterpret_cast<const stdex::utf32_t*>(U"This is a 🐔Test🐮."), tmp);
+	}
+
 	void string::sprintf()
 	{
 		stdex::locale locale(stdex::create_locale(LC_ALL, "en_US.UTF-8"));

include/stdex/string.hpp

@@ -1,4 +1,4 @@
-/*
+/*
 	SPDX-License-Identifier: MIT
 	Copyright © 2016-2024 Amebis
 */
@@ -1636,6 +1636,26 @@ namespace stdex
 		}
 	}
 
+	///
+	/// Recode UTF-16 zero-terminated string to UTF-32
+	///
+	/// \param[in] dst  Destination string
+	/// \param[in] src  Source string
+	///
+	/// \return Number of code units excluding zero terminator in the dst string after the operation.
+	///
+	inline size_t strcpy(
+		_Out_ _Post_maybez_ utf32_t* dst,
+		_In_z_ const utf16_t* src)
+	{
+		stdex_assert(dst);
+		stdex_assert(src);
+		for (size_t j = 0, i = 0; ; ++j, ++i) {
+			if ((dst[j] = (is_surrogate_pair(&src[i]) ? surrogate_pair_to_ucs4(&src[i++]) : static_cast<utf32_t>(src[i]))) == 0)
+				return j;
+		}
+	}
+
 	///
 	/// Copy zero-terminated string
 	///
@@ -1690,6 +1710,35 @@ namespace stdex
 		}
 	}
 
+	///
+	/// Recode UTF-16 zero-terminated string to UTF-32
+	///
+	/// \param[in] dst        Destination string
+	/// \param[in] count_dst  Destination string code unit count limit
+	/// \param[in] src        Source string
+	/// \param[in] count_src  Source string code unit count limit
+	///
+	/// \return Number of code units excluding zero terminator in the dst string after the operation.
+	///
+	inline size_t strncpy(
+		_Out_writes_(count_dst) _Post_maybez_ utf32_t* dst, _In_ size_t count_dst,
+		_In_reads_or_z_opt_(count_src) const utf16_t* src, _In_ size_t count_src)
+	{
+		stdex_assert(dst || !count_dst);
+		stdex_assert(src || !count_src);
+		for (size_t j = 0, i = 0; ; ++j, ++i)
+		{
+			if (j >= count_dst)
+				return j;
+			if (i >= count_src) {
+				dst[j] = 0;
+				return j;
+			}
+			if ((dst[j] = (i + 1 < count_src && is_surrogate_pair(&src[i]) ? surrogate_pair_to_ucs4(&src[i++]) : static_cast<utf32_t>(src[i]))) == 0)
+				return j;
+		}
+	}
+
 	///
 	/// Copy zero-terminated string
 	///