mojo: Starting serialization for python bindings.

mojo/public/python/mojo/bindings/serialization.py

+# 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.
+
+"""Utility classes for serialization"""
+
+import struct
+
+
+# Format of a header for a struct or an array.
+HEADER_STRUCT = struct.Struct("=II")
+
+
+class SerializationException(Exception):
+  """Error when strying to serialize a struct."""
+  pass
+
+
+class Serialization(object):
+  """
+  Helper class to serialize/deserialize a struct.
+  """
+  def __init__(self, groups):
+    self._groups = groups
+    self.version = _GetVersion(groups)
+    main_struct = _GetStruct(groups)
+    self.size = HEADER_STRUCT.size + main_struct.size
+    self._struct_per_version = {
+      self.version: main_struct,
+    }
+
+  def _GetMainStruct(self):
+    return self._GetStruct(self.version)
+
+  def _GetStruct(self, version):
+    # If asking for a greater ver
+    version = min(version, self.version)
+    if version not in self._struct_per_version:
+      self._struct_per_version[version] = _GetStruct(_FilterGroups(self._groups,
+                                                                   version))
+    return self._struct_per_version[version]
+
+  def Serialize(self, obj, handle_offset):
+    """
+    Serialize the given obj. handle_offset is the the first value to use when
+    encoding handles.
+    """
+    handles = []
+    data = bytearray(self.size)
+    HEADER_STRUCT.pack_into(data, 0, self.size, self.version)
+    position = HEADER_STRUCT.size
+    to_pack = []
+    for group in self._groups:
+      position = position + NeededPaddingForAlignment(position,
+                                                      group.GetByteSize())
+      (entry, new_handles) = group.Serialize(
+          obj,
+          len(data) - position,
+          data,
+          handle_offset + len(handles))
+      to_pack.append(entry)
+      handles.extend(new_handles)
+      position = position + group.GetByteSize()
+    self._GetMainStruct().pack_into(data, HEADER_STRUCT.size, *to_pack)
+    return (data, handles)
+
+
+def NeededPaddingForAlignment(value, alignment=8):
+  """Returns the padding necessary to align value with the given alignment."""

[sdefresne, 2014/09/15 08:46:54]

nit: this method could return a tuple padding_length, and padding code (i.e. 'x'
* padding_length)

[qsr, 2014/09/15 11:01:53]

 Not really. It is used that way only once, but I use it plenty of time to just
compute a position, and I do not want to generate strings and use memory in
those cases.

[sdefresne, 2014/09/15 11:14:46]

Make sense, thank you for the justification.

[qsr, 2014/09/15 11:42:00]

Acknowledged.

+  if value % alignment:
+    return alignment - (value % alignment)
+  return 0
+
+
+def _GetVersion(groups):
+  return reduce(lambda x, y: x+y, [len(x.descriptors) for x in groups])

[sdefresne, 2014/09/15 08:46:54]

nit: "reduce(lambda x, y: x+y, [...])" can be rewritten as "sum(...)".

[qsr, 2014/09/15 11:01:53]

Done.

+
+
+def _FilterGroups(groups, version):
+  return [group for group in groups if group.version < version]
+
+
+def _GetStruct(groups):
+  index = 0
+  codes = [ '=' ]
+  for group in groups:
+    code = group.GetTypeCode()
+    size = group.GetByteSize()
+    needed_padding = NeededPaddingForAlignment(index, size)
+    if needed_padding:
+      codes.append('x' * needed_padding)
+      index = index + needed_padding
+    codes.append(code)
+    index = index + size
+  end_alignment = index % 8

[sdefresne, 2014/09/15 08:46:54]

nit: you can use NeededPaddingForAlignment here.

[qsr, 2014/09/15 11:01:53]

Done.

+  if end_alignment:
+    codes.append('x' * (8 - end_alignment))
+  return struct.Struct(''.join(codes))