Introduce new API to expose external string resource regardless of encoding.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index ddde388cd4f27c970c29e351333b06cd94b5c026..37d66586aa08f0aea782f8152bc9639e4bef4de8 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -1018,6 +1018,11 @@ class Boolean : public Primitive {
  */
 class String : public Primitive {
  public:
+  enum Encoding {
+    UNKNOWN_ENCODING = 0x1,
+    TWO_BYTE_ENCODING = 0x0,
+    ASCII_ENCODING = 0x4
+  };
   /**
    * Returns the number of characters in this string.
    */
@@ -1181,6 +1186,14 @@ class String : public Primitive {
   };
 
   /**
+   * If the string is an external string, return the ExternalStringResourceBase
+   * regardless of the encoding, otherwise return NULL.  The encoding of the
+   * string is returned in type_out.
+   */
+  inline ExternalStringResourceBase* GetExternalStringResourceBase(
+      Encoding* encoding_out) const;
+
+  /**
    * Get the ExternalStringResource for an external string.  Returns
    * NULL if IsExternal() doesn't return true.
    */
@@ -1226,6 +1239,8 @@ class String : public Primitive {
    * this function should not otherwise delete or modify the resource. Neither
    * should the underlying buffer be deallocated or modified except through the
    * destructor of the external string resource.
+   * The optional encoding argument provides a hint on whether scanning for
+   * ASCII content is necessary.

[drcarney, 2012/09/12 11:27:09]

Looks like this comment accidentally slipped in.

[Yang, 2012/09/12 11:28:50]

Done.

    */
   V8EXPORT static Local<String> NewExternal(ExternalStringResource* resource);
 
@@ -1343,6 +1358,8 @@ class String : public Primitive {
   };
 
  private:
+  V8EXPORT void VerifyExternalStringResourceBase(ExternalStringResourceBase* v,
+                                                 Encoding encoding_out) const;
   V8EXPORT void VerifyExternalStringResource(ExternalStringResource* val) const;
   V8EXPORT static void CheckCast(v8::Value* obj);
 };
@@ -4033,7 +4050,9 @@ class Internals {
   static const int kForeignAddressOffset = kApiPointerSize;
   static const int kJSObjectHeaderSize = 3 * kApiPointerSize;
   static const int kFullStringRepresentationMask = 0x07;
+  static const int kStringEncodingMask = 0x4;
   static const int kExternalTwoByteRepresentationTag = 0x02;
+  static const int kExternalAsciiRepresentationTag = 0x06;
 
   static const int kIsolateStateOffset = 0;
   static const int kIsolateEmbedderDataOffset = 1 * kApiPointerSize;
@@ -4388,6 +4407,26 @@ String::ExternalStringResource* String::GetExternalStringResource() const {
 }
 
 
+String::ExternalStringResourceBase* String::GetExternalStringResourceBase(
+    String::Encoding* encoding_out) const {
+  typedef internal::Object O;
+  typedef internal::Internals I;
+  O* obj = *reinterpret_cast<O**>(const_cast<String*>(this));
+  int type = I::GetInstanceType(obj) & I::kFullStringRepresentationMask;
+  *encoding_out = static_cast<Encoding>(type & I::kStringEncodingMask);
+  ExternalStringResourceBase* resource = NULL;
+  if (type == I::kExternalAsciiRepresentationTag ||
+      type == I::kExternalTwoByteRepresentationTag) {
+    void* value = I::ReadField<void*>(obj, I::kStringResourceOffset);
+    resource = static_cast<ExternalStringResourceBase*>(value);
+  }
+#ifdef V8_ENABLE_CHECKS
+    VerifyExternalStringResourceBase(resource, *encoding_out);
+#endif
+  return resource;
+}
+
+
 bool Value::IsUndefined() const {
 #ifdef V8_ENABLE_CHECKS
   return FullIsUndefined();