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diff --git a/drafts/String.hpp b/drafts/String.hpp
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+/*
+Because the text encoding problem on Windows, here I write some functions
+related to text encoding. The utf-8 and utf-16 conversion function is provided
+by win32 api. However win32 api does not provide any function about charactor
+iteration or index by code point. (At least I haven't found.) I don't use icu
+because it is not easy to build it on Windows and the bundled version in Windows
+(https://docs.microsoft.com/en-us/windows/win32/intl/international-components-for-unicode--icu-)
+is only available after Windows 10 Creators Update.
+
+Luckily, both utf-8 and utf-16 encoding are easy to learn and program with if we
+only do simple iteration rather than do much sophisticated work about
+complicated error situations. (And I learn the internal of the encoding by the
+way.)
+*/
+
+#pragma once
+#include "WinPreConfig.hpp"
+
+#include "cru/common/StringUtil.hpp"
+
+#include <cstdint>
+#include <stdexcept>
+#include <string>
+#include <string_view>
+
+namespace cru::platform::win {
+std::string ToUtf8String(const std::wstring_view& string);
+std::wstring ToUtf16String(const std::string_view& string);
+
+inline bool IsSurrogatePair(wchar_t c) { return c >= 0xD800 && c <= 0xDFFF; }
+
+inline bool IsSurrogatePairLeading(wchar_t c) {
+  return c >= 0xD800 && c <= 0xDBFF;
+}
+
+inline bool IsSurrogatePairTrailing(wchar_t c) {
+  return c >= 0xDC00 && c <= 0xDFFF;
+}
+
+class Utf16Iterator : public Object {
+  static_assert(
+      sizeof(wchar_t) == 2,
+      "Emmm, according to my knowledge, wchar_t should be 2-length on "
+      "Windows. If not, Utf16 will be broken.");
+
+ public:
+  Utf16Iterator(const std::wstring_view& string) : string_(string) {}
+
+  CRU_DEFAULT_COPY(Utf16Iterator)
+  CRU_DEFAULT_MOVE(Utf16Iterator)
+
+  ~Utf16Iterator() = default;
+
+ public:
+  void SetToHead() { position_ = 0; }
+
+  // Advance current position and get next code point. Return k_code_point_end
+  // if there is no next code unit(point). Throw TextEncodeException if decoding
+  // fails.
+  CodePoint Next();
+
+  Index CurrentPosition() const { return this->position_; }
+
+ private:
+  std::wstring_view string_;
+  Index position_ = 0;
+};
+
+Index IndexUtf8ToUtf16(const std::string_view& utf8_string, Index utf8_index,
+                       const std::wstring_view& utf16_string);
+
+Index IndexUtf16ToUtf8(const std::wstring_view& utf16_string, Index utf16_index,
+                       const std::string_view& utf8_string);
+
+}  // namespace cru::platform::win