Adding Jsrt function JsCopyStringOneByte

For scenarios where a string is known to have values fitting in
one byte, this method allows directly copying those values into a
char* buffer rather than having to copy to a uint16_t* buffer
and then to a char* buffer, and treating the string as raw bytes
rather than a valid utf8 encoding.

bin/NativeTests/JsRTApiTest.cpp

@@ -2143,4 +2143,32 @@ namespace JsRTApiTest
         JsRTApiTest::RunWithAttributes(JsRTApiTest::ObjectHasOwnPropertyMethodTest);
     }
 
+    void JsCopyStringOneByteMethodTest(JsRuntimeAttributes attributes, JsRuntimeHandle runtime)
+    {
+        size_t written = 0;
+        char buf[10] = {0};
+        JsValueRef value;
+        REQUIRE(JsCreateStringUtf16(reinterpret_cast<uint16_t*>(_u("0\x10\x80\xa9\uabcd\U000104377")), 8, &value) == JsNoError);
+        REQUIRE(JsCopyStringOneByte(value, 0, -1, nullptr, &written) == JsNoError);
+        CHECK(written == 8);
+        buf[written] = '\xff';
+
+        REQUIRE(JsCopyStringOneByte(value, 0, 10, buf, &written) == JsNoError);
+        CHECK(written == 8);
+        CHECK(buf[0] == '0');
+        CHECK(buf[1] == '\x10');
+        CHECK(buf[2] == '\x80');
+        CHECK(buf[3] == '\xA9');
+        CHECK(buf[4] == '\xcd');
+        CHECK(buf[5] == '\x01');
+        CHECK(buf[6] == '\x37');
+        CHECK(buf[7] == '7');
+        CHECK(buf[8] == '\xff');
+    }
+
+    TEST_CASE("ApiTest_JsCopyStringOneByteMethodTest", "[ApiTest]")
+    {
+        JsRTApiTest::RunWithAttributes(JsRTApiTest::JsCopyStringOneByteMethodTest);
+    }
+
 }

lib/Jsrt/ChakraCore.h

@@ -679,5 +679,43 @@ CHAKRA_API
         _In_ JsPropertyIdRef propertyId,
         _Out_ bool *hasOwnProperty);
 
+/// <summary>
+///     Write JS string value into char string buffer without a null terminator
+/// </summary>
+/// <remarks>
+///     <para>
+///         When size of the `buffer` is unknown,
+///         `buffer` argument can be nullptr.
+///         In that case, `written` argument will return the length needed.
+///     </para>
+///     <para>
+///         When start is out of range or &lt; 0, returns JsErrorInvalidArgument
+///         and `written` will be equal to 0.
+///         If calculated length is 0 (It can be due to string length or `start`
+///         and length combination), then `written` will be equal to 0 and call
+///         returns JsNoError
+///     </para>
+///     <para>
+///         The JS string `value` will be converted one utf16 code point at a time,
+///         and if it has code points that do not fit in one byte, those values
+///         will be truncated.
+///     </para>
+/// </remarks>
+/// <param name="value">JavascriptString value</param>
+/// <param name="start">Start offset of buffer</param>
+/// <param name="length">Number of characters to be written</param>
+/// <param name="buffer">Pointer to buffer</param>
+/// <param name="written">Total number of characters written</param>
+/// <returns>
+///     The code <c>JsNoError</c> if the operation succeeded, a failure code otherwise.
+/// </returns>
+CHAKRA_API
+JsCopyStringOneByte(
+    _In_ JsValueRef value,
+    _In_ int start,
+    _In_ int length,
+    _Out_opt_ char* buffer,
+    _Out_opt_ size_t* written);
+
 #endif // CHAKRACOREBUILD_
 #endif // _CHAKRACORE_H_

lib/Jsrt/Jsrt.cpp

@@ -4181,14 +4181,15 @@ JsErrorCode WriteStringCopy(
         *written = 0;  // init to 0 for default
     }
 
-    const char16* str = nullptr;
-    size_t strLength = 0;
-    JsErrorCode errorCode = JsStringToPointer(value, &str, &strLength);
-    if (errorCode != JsNoError)
+    if (!Js::JavascriptString::Is(value))
     {
-        return errorCode;
+        return JsErrorInvalidArgument;
     }
 
+    Js::JavascriptString *jsString = Js::JavascriptString::FromVar(value);
+    const char16* str = jsString->GetSz();
+    size_t strLength = jsString->GetLength();
+
     if (start < 0 || (size_t)start > strLength)
     {
         return JsErrorInvalidArgument;  // start out of range, no chars written
@@ -4200,7 +4201,7 @@ JsErrorCode WriteStringCopy(
         return JsNoError;  // no chars written
     }
 
-    errorCode = copyFunc(str + start, count, written);
+    JsErrorCode errorCode = copyFunc(str + start, count, written);
     if (errorCode != JsNoError)
     {
         return errorCode;
@@ -4231,10 +4232,6 @@ CHAKRA_API JsCopyStringUtf16(
             {
                 memmove(buffer, src, sizeof(char16) * count);
             }
-            else
-            {
-                *needed = count;
-            }
             return JsNoError;
         });
 }
@@ -4746,4 +4743,27 @@ CHAKRA_API JsHasOwnProperty(_In_ JsValueRef object, _In_ JsPropertyIdRef propert
     });
 }
 
+CHAKRA_API JsCopyStringOneByte(
+    _In_ JsValueRef value,
+    _In_ int start,
+    _In_ int length,
+    _Out_opt_ char* buffer,
+    _Out_opt_ size_t* written)
+{
+    PARAM_NOT_NULL(value);
+    VALIDATE_JSREF(value);
+    return WriteStringCopy(value, start, length, written,
+        [buffer](const char16* src, size_t count, size_t *needed)
+    {
+        if (buffer)
+        {
+            for (size_t i = 0; i < count; i++)
+            {
+                buffer[i] = (char)src[i];
+            }
+        }
+        return JsNoError;
+    });
+}
+
 #endif // CHAKRACOREBUILD_

lib/Jsrt/JsrtCommonExports.inc

@@ -120,4 +120,5 @@
     JsGetWeakReferenceValue
     JsGetAndClearExceptionWithMetadata
     JsHasOwnProperty
+    JsCopyStringOneByte
 #endif