Add utf16 method to JSI

packages/react-native/ReactCommon/jsi/jsi/decorator.h

@@ -225,6 +225,13 @@ class RuntimeDecorator : public Base, private jsi::Instrumentation {
     return plain_.utf8(s);
   }
 
+  std::u16string utf16(const String& str) override {
+    return plain_.utf16(str);
+  }
+  std::u16string utf16(const PropNameID& sym) override {
+    return plain_.utf16(sym);
+  }
+
   Object createObject() override {
     return plain_.createObject();
   };
@@ -674,6 +681,15 @@ class WithRuntimeDecorator : public RuntimeDecorator<Plain, Base> {
     return RD::utf8(s);
   }
 
+  std::u16string utf16(const String& str) override {
+    Around around{with_};
+    return RD::utf16(str);
+  }
+  std::u16string utf16(const PropNameID& sym) override {
+    Around around{with_};
+    return RD::utf16(sym);
+  }
+
   Value createValueFromJsonUtf8(const uint8_t* json, size_t length) override {
     Around around{with_};
     return RD::createValueFromJsonUtf8(json, length);

packages/react-native/ReactCommon/jsi/jsi/jsi.cpp

@@ -62,6 +62,107 @@ Value callGlobalFunction(Runtime& runtime, const char* name, const Value& arg) {
   return f.call(runtime, arg);
 }
 
+// Given a sequence of UTF8 encoded bytes, advance the input to past where a
+// 32-bit unicode codepoint as been decoded and return the codepoint. If the
+// UTF8 encoding is invalid, then return the value with the unicode replacement
+// character (U+FFFD). This decoder also relies on zero termination at end of
+// the input for bound checks.
+// \param input char pointer pointing to the current character
+// \return Unicode codepoint
+uint32_t decodeUTF8(const char*& input) {
+  uint32_t ch = (unsigned char)input[0];
+  if (ch <= 0x7f) {
+    input += 1;
+    return ch;
+  }
+  uint32_t ret;
+  constexpr uint32_t replacementCharacter = 0xFFFD;
+  if ((ch & 0xE0) == 0xC0) {
+    uint32_t ch1 = (unsigned char)input[1];
+    if ((ch1 & 0xC0) != 0x80) {
+      input += 1;
+      return replacementCharacter;
+    }
+    ret = ((ch & 0x1F) << 6) | (ch1 & 0x3F);
+    input += 2;
+    if (ret <= 0x7F) {
+      return replacementCharacter;
+    }
+  } else if ((ch & 0xF0) == 0xE0) {
+    uint32_t ch1 = (unsigned char)input[1];
+    if ((ch1 & 0x40) != 0 || (ch1 & 0x80) == 0) {
+      input += 1;
+      return replacementCharacter;
+    }
+    uint32_t ch2 = (unsigned char)input[2];
+    if ((ch2 & 0x40) != 0 || (ch2 & 0x80) == 0) {
+      input += 2;
+      return replacementCharacter;
+    }
+    ret = ((ch & 0x0F) << 12) | ((ch1 & 0x3F) << 6) | (ch2 & 0x3F);
+    input += 3;
+    if (ret <= 0x7FF) {
+      return replacementCharacter;
+    }
+  } else if ((ch & 0xF8) == 0xF0) {
+    uint32_t ch1 = (unsigned char)input[1];
+    if ((ch1 & 0x40) != 0 || (ch1 & 0x80) == 0) {
+      input += 1;
+      return replacementCharacter;
+    }
+    uint32_t ch2 = (unsigned char)input[2];
+    if ((ch2 & 0x40) != 0 || (ch2 & 0x80) == 0) {
+      input += 2;
+      return replacementCharacter;
+    }
+    uint32_t ch3 = (unsigned char)input[3];
+    if ((ch3 & 0x40) != 0 || (ch3 & 0x80) == 0) {
+      input += 3;
+      return replacementCharacter;
+    }
+    ret = ((ch & 0x07) << 18) | ((ch1 & 0x3F) << 12) | ((ch2 & 0x3F) << 6) |
+        (ch3 & 0x3F);
+    input += 4;
+    if (ret <= 0xFFFF) {
+      return replacementCharacter;
+    }
+    if (ret > 0x10FFFF) {
+      return replacementCharacter;
+    }
+  } else {
+    input += 1;
+    return replacementCharacter;
+  }
+  return ret;
+}
+
+// Given a valid 32-bit unicode codepoint, encode it as UTF-16 into the output.
+void encodeUTF16(std::u16string& out, uint32_t cp) {
+  if (cp < 0x10000) {
+    out.push_back((uint16_t)cp);
+    return;
+  }
+  cp -= 0x10000;
+  uint16_t highSurrogate = 0xD800 + ((cp >> 10) & 0x3FF);
+  out.push_back(highSurrogate);
+  uint16_t lowSurrogate = 0xDC00 + (cp & 0x3FF);
+  out.push_back(lowSurrogate);
+}
+
+// Convert the UTF8 encoded string into a UTF16 encoded string. If the
+// input is not valid UTF8, the replacement character (U+FFFD) is used to
+// represent the invalid sequence.
+std::u16string convertUTF8ToUTF16(const std::string& utf8) {
+  std::u16string ret;
+  const char* curr = utf8.data();
+  const char* end = curr + utf8.length();
+  while (curr < end) {
+    auto cp = decodeUTF8(curr);
+    encodeUTF16(ret, cp);
+  }
+  return ret;
+}
+
 } // namespace
 
 Buffer::~Buffer() = default;
@@ -147,6 +248,16 @@ Value Runtime::createValueFromJsonUtf8(const uint8_t* json, size_t length) {
   return parseJson.call(*this, String::createFromUtf8(*this, json, length));
 }
 
+std::u16string Runtime::utf16(const PropNameID& sym) {
+  auto utf8Str = utf8(sym);
+  return convertUTF8ToUTF16(utf8Str);
+}
+
+std::u16string Runtime::utf16(const String& str) {
+  auto utf8Str = utf8(str);
+  return convertUTF8ToUTF16(utf8Str);
+}
+
 Pointer& Pointer::operator=(Pointer&& other) {
   if (ptr_) {
     ptr_->invalidate();

packages/react-native/ReactCommon/jsi/jsi/jsi.h

@@ -399,6 +399,9 @@ class JSI_EXPORT Runtime {
       const jsi::Object& obj,
       size_t amount) = 0;
 
+  virtual std::u16string utf16(const String& str);
+  virtual std::u16string utf16(const PropNameID& sym);
+
   // These exist so derived classes can access the private parts of
   // Value, Symbol, String, and Object, which are all friends of Runtime.
   template <typename T>
@@ -501,6 +504,11 @@ class JSI_EXPORT PropNameID : public Pointer {
     return runtime.utf8(*this);
   }
 
+  /// Copies the data in a PropNameID as utf16 into a C++ string.
+  std::u16string utf16(Runtime& runtime) const {
+    return runtime.utf16(*this);
+  }
+
   static bool compare(
       Runtime& runtime,
       const jsi::PropNameID& a,
@@ -651,6 +659,11 @@ class JSI_EXPORT String : public Pointer {
     return runtime.utf8(*this);
   }
 
+  /// Copies the data in a JS string as utf16 into a C++ string.
+  std::u16string utf16(Runtime& runtime) const {
+    return runtime.utf16(*this);
+  }
+
   friend class Runtime;
   friend class Value;
 };

packages/react-native/ReactCommon/jsi/jsi/test/testlib.cpp

@@ -1576,6 +1576,65 @@ TEST_P(JSITest, UTF8ExceptionTest) {
   }
 }
 
+TEST_P(JSITest, UTF16Test) {
+  // This Runtime Decorator is used to test the conversion from UTF-8 to UTF-16
+  // in the default utf16 method for runtimes that do not provide their own
+  // utf16 implementation.
+  class UTF16RD : public RuntimeDecorator<Runtime, Runtime> {
+   public:
+    UTF16RD(Runtime& rt) : RuntimeDecorator(rt) {}
+
+    std::string utf8(const String&) override {
+      return utf8Str;
+    }
+
+    std::u16string utf16(const String& str) override {
+      return Runtime::utf16(str);
+    }
+
+    std::string utf8Str;
+  };
+
+  UTF16RD rd = UTF16RD(rt);
+  String str = String::createFromUtf8(rd, "placeholder");
+
+  rd.utf8Str = "foobar";
+  EXPECT_EQ(str.utf16(rd), u"foobar");
+
+  rd.utf8Str = "你好";
+  EXPECT_EQ(str.utf16(rd), u"你好");
+
+  rd.utf8Str = "👍";
+  EXPECT_EQ(str.utf16(rd), u"👍");
+
+  rd.utf8Str = "foobar👍你好";
+  EXPECT_EQ(str.utf16(rd), u"foobar👍你好");
+
+  // String ended before second byte of the encoding
+  rd.utf8Str = "\xcf";
+  EXPECT_EQ(str.utf16(rd), u"\uFFFD");
+
+  // Third byte should follow the pattern of 0b10xxxxxx
+  rd.utf8Str = "\xef\x8f\x29";
+  EXPECT_EQ(str.utf16(rd), u"\uFFFD\u0029");
+
+  // U+2200 should be encoded in 3 bytes as 0xE2 0x88 0x80, not 4 bytes
+  rd.utf8Str = "\xf0\x82\x88\x80";
+  EXPECT_EQ(str.utf16(rd), u"\uFFFD");
+
+  // Unicode Max Value is U+10FFFF, U+11FFFF is invalid
+  rd.utf8Str = "\xf4\x9f\xbf\xbf";
+  EXPECT_EQ(str.utf16(rd), u"\uFFFD");
+
+  // Missing the third byte of the 3-byte encoding, followed by 'z'
+  rd.utf8Str = "\xe1\xa0\x7a";
+  EXPECT_EQ(str.utf16(rd), u"\uFFFD\u007A");
+
+  // First byte is neither ASCII nor a valid continuation byte
+  rd.utf8Str = "\xea\x7a";
+  EXPECT_EQ(str.utf16(rd), u"\uFFFD\u007A");
+}
+
 INSTANTIATE_TEST_CASE_P(
     Runtimes,
     JSITest,