refactor: write iconv.

- write iconv encoding (not finished).
- rename united_codec to pycodec.

src/yycc/encoding/iconv.hpp

@@ -0,0 +1,58 @@
+#pragma once
+#include "../macro/os_detector.hpp"
+
+#if YYCC_FEAT_ICONV || (YYCC_OS != YYCC_OS_WINDOWS)
+
+#include "../macro/class_copy_move.hpp"
+#include "../patch/expected.hpp"
+#include "../string.hpp"
+#include <memory>
+
+#define NS_YYCC_STRING ::yycc::string
+#define NS_YYCC_PATCH_EXPECTED ::yycc::patch::expected
+
+namespace yycc::encoding::iconv {
+
+    // YYC MARK:
+    // I don't want to include "iconv.h" in there.
+    // One of reasons is that I want to hide all implementation of Iconv.
+    // Another reason is that "iconv.h" defines some annoying macros which intervene some names in this files.
+    // So I introduce PIMPL design mode. Use a pointer to hide all details in class PrivToken.
+
+    using CodeName = NS_YYCC_STRING::u8string_view;
+
+    /// @private
+    class PrivToken;
+
+    class Token {
+    public:
+        Token(const CodeName& from_code, const CodeName& to_code);
+        ~Token();
+        YYCC_DELETE_COPY(Token)
+        YYCC_DEFAULT_MOVE(Token)
+
+        bool is_valid() const;
+        PrivToken* get_inner() const;
+
+    private:
+        std::unique_ptr<PrivToken> inner;
+    };
+
+    /// @private
+    enum class ConvError {
+        InvalidCd, ///< Given token is invalid.
+        NullPointer, ///< Some of essential pointer in argument is nullptr.
+        InvalidMbSeq, ///< An invalid multibyte sequence has been encountered in the input.
+        IncompleteMbSeq, ///< An incomplete multibyte sequence has been encountered in the input.
+    };
+
+    /// @private
+    template<typename T>
+    using ConvResult = NS_YYCC_PATCH_EXPECTED::Expected<T, ConvError>;
+
+} // namespace yycc::encoding::iconv
+
+#undef NS_YYCC_PATCH_EXPECTED
+#undef NS_YYCC_STRING
+
+#endif