Unify var tracking between webkit and the proxy.

ppapi/shared_impl/id_assignment.h

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef PPAPI_SHARED_IMPL_ID_ASSIGNMENT_H_
+#define PPAPI_SHARED_IMPL_ID_ASSIGNMENT_H_
+
+#include <limits>
+
+#include "base/basictypes.h"
+
+namespace ppapi {
+
+enum PPIdType {
+  PP_ID_TYPE_MODULE,
+  PP_ID_TYPE_INSTANCE,
+  PP_ID_TYPE_RESOURCE,
+  PP_ID_TYPE_VAR,
+  PP_ID_TYPE_COUNT

[dmichael (off chromium), 2011/08/08 22:13:59]

Can you comment a little about COUNT not being a real type, and needing to be at
the end? At a glance, I thought 'hey, 5 things don't fit in 2 bits, this is a
bug'

+};
+
+extern const unsigned int kPPIdTypeBits;
+
+extern const int32 kMaxPPIdType;

[dmichael (off chromium), 2011/08/08 22:13:59]

Why does this assume int32, but the functions are templates?

It would be simpler if the functions all used int32 explicitly, if that's the
only type for which they are instantiated...

[brettw, 2011/08/09 17:08:09]

I think this is because this is old code and resources used to be 64 bits. I
think I want to make the var IDs 64 bits to match PP_Var, so I'm inclined to
leave the templates for now.

+
+template <typename T> inline T MakeTypedId(T value, PPIdType type) {
+  return (value << kPPIdTypeBits) | static_cast<T>(type);

[dmichael (off chromium), 2011/08/08 22:13:59]

So you use the least-significant bits for the type? Interesting; I somehow
assumed the opposite. It would be good to have a comment somewhere on the
overall approach and why you're doing this.

+}
+
+template <typename T> inline bool CheckIdType(T id, PPIdType type) {
+  // 0 is a valid resource.

[dmichael (off chromium), 2011/08/08 22:13:59]

Apparently, 0 is also a valid Var, Instance, and Module, too? Or should it only
return true if type == PP_ID_TYPE_RESOURCE when id is 0?

+  if (!id)
+    return true;
+  const T mask = (static_cast<T>(1) << kPPIdTypeBits) - 1;
+  return (id & mask) == type;
+}
+
+}  // namespace ppapi
+
+#endif  // PPAPI_SHARED_IMPL_ID_ASSIGNMENT_H_