mojom: Add associative arrays to the mojom language.

mojo/public/cpp/bindings/lib/map_serialization.h

+// Copyright 2014 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 MOJO_PUBLIC_CPP_BINDINGS_LIB_MAP_SERIALIZATION_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_LIB_MAP_SERIALIZATION_H_
+
+#include "mojo/public/cpp/bindings/lib/array_internal.h"
+#include "mojo/public/cpp/bindings/lib/map_internal.h"
+
+namespace mojo {
+namespace internal {
+
+template <typename MapType, typename DataType,
+          bool move_only = IsMoveOnlyType<MapType>::value>

[yzshen1, 2014/10/03 18:03:38]

nit: the format kMoveOnly is used in map_internal. it is nice to be consistent.

+struct MapSerializer;
+
+template <typename MapType, typename DataType>
+struct MapSerializer<MapType, DataType, false> {
+  static size_t GetHeaderSize() {
+    return sizeof(Array_Data<DataType>);
+  }
+  static size_t GetBaseArraySize(size_t count) {
+    return Align(count * sizeof(DataType));
+  }
+  static size_t GetItemSize(const MapType& item) {
+    return 0;
+  }
+};
+
+template <>
+struct MapSerializer<bool, bool, false> {
+  static size_t GetHeaderSize() {
+    return sizeof(Array_Data<bool>);
+  }
+  static size_t GetBaseArraySize(size_t count) {
+    return Align((count + 7) / 8);
+  }
+  static size_t GetItemSize(bool item) {
+    return 0;
+  }
+};
+
+template <typename H>
+struct MapSerializer<ScopedHandleBase<H>, H, true> {
+  static size_t GetHeaderSize() {
+    return sizeof(Array_Data<H>);
+  }
+  static size_t GetBaseArraySize(size_t count) {
+    return Align(count * sizeof(H));
+  }
+  static size_t GetItemSize(const H& item) {
+    return 0;
+  }
+};
+
+template <typename S>
+struct MapSerializer<S, typename S::Data_*, true> {
+  static size_t GetHeaderSize() {
+    return sizeof(Array_Data<typename S::Data_*>);
+  }
+  static size_t GetBaseArraySize(size_t count) {
+    return count * sizeof(internal::StructPointer<typename S::Data_>);
+  }
+  static size_t GetItemSize(const S& item) {
+    return GetSerializedSize_(item);
+  }
+};
+
+template <>
+struct MapSerializer<String, String_Data*, false> {
+  static size_t GetHeaderSize() {
+    return sizeof(Array_Data<String_Data*>);
+  }
+  static size_t GetBaseArraySize(size_t count) {
+    return count * sizeof(internal::StringPointer);
+  }
+  static size_t GetItemSize(const String& item) {
+    return GetSerializedSize_(item);
+  }
+};
+
+template <typename MapKey, typename MapValue,
+          typename DataKey, typename DataValue>
+struct SizeAccumulator {
+  SizeAccumulator(size_t* data_size) : size(data_size) {}
+  size_t* size;
+
+  void operator()(const MapKey& key, const MapValue& value) {
+    *size += internal::MapSerializer<MapKey, DataKey>::GetItemSize(key);
+    *size += internal::MapSerializer<MapValue, DataValue>::GetItemSize(value);
+  }
+};
+
+}  // namespace internal
+
+template <typename MapKey, typename MapValue>
+inline size_t GetSerializedSize_(const Map<MapKey, MapValue>& input) {

[yzshen1, 2014/10/03 18:03:38]

I probably haven't understood:
I thought it is okay to just compute GetSerializeSize_(KeyArray) +
GetSerializeSize_(MapArray).

Although the |num_bytes| field of an array is not guaranteed to be
8-byte-multiple, calling GetSerializeSize_() on arrays always returns
8-byte-multiple results. Therefore, I think that is sufficient. Have I missed
anything?

(The rationale is that |num_bytes| indicates the actual bytes used by an array,
while GetSerializeSize_() is the bytes reserved, making it 8-byte-multiple
guarantees that the next object starts at 8-byte boundaries.)

[Elliot Glaysher, 2014/10/03 21:11:49]

The problem is that there's no such thing as KeyArray or MapArray at this point.
We have been passed this Map from a struct to get the size that it takes. We
can't decompose the map because it's been given to us const. We probably don't
want to Clone it because that requires the Value type to be Clone()able. (Also,
cloning arbitrarily large data structures just to count the size is the Wrong
Thing.)

Note that this code is only called when you have a Map as a member of a struct.
So I've rewritten this function, splitting it into four computations. I don't
know exactly where to put the align to get the correct answer.

+  if (!input)
+    return 0;
+  typedef typename internal::WrapperTraits<MapKey>::DataType DataKey;
+  typedef typename internal::WrapperTraits<MapValue>::DataType DataValue;
+
+  size_t count = input.size();
+  size_t base_size =
+      internal::MapSerializer<MapKey, DataKey>::GetHeaderSize() +
+      internal::MapSerializer<MapKey, DataKey>::GetBaseArraySize(count) +
+      internal::MapSerializer<MapValue, DataValue>::GetHeaderSize() +
+      internal::MapSerializer<MapValue, DataValue>::GetBaseArraySize(count);
+
+  size_t data_size;
+  internal::SizeAccumulator<MapKey, MapValue, DataKey, DataValue> accumulator(
+      &data_size);
+  input.Iterate(accumulator);
+
+  return base_size + data_size;
+}
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_MAP_SERIALIZATION_H_