Massive roll-up of changes. See CHANGES.txt.

1 files changed, 647 insertions, 241 deletions

diff --git a/python/google/protobuf/internal/encoder.py b/python/google/protobuf/internal/encoder.py
index 3ec3b2b1..aa05d5b3 100755
--- a/python/google/protobuf/internal/encoder.py
+++ b/python/google/protobuf/internal/encoder.py
@@ -28,253 +28,659 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-"""Class for encoding protocol message primitives.
+"""Code for encoding protocol message primitives.
 
 Contains the logic for encoding every logical protocol field type
 into one of the 5 physical wire types.
+
+This code is designed to push the Python interpreter's performance to the
+limits.
+
+The basic idea is that at startup time, for every field (i.e. every
+FieldDescriptor) we construct two functions:  a "sizer" and an "encoder".  The
+sizer takes a value of this field's type and computes its byte size.  The
+encoder takes a writer function and a value.  It encodes the value into byte
+strings and invokes the writer function to write those strings.  Typically the
+writer function is the write() method of a cStringIO.
+
+We try to do as much work as possible when constructing the writer and the
+sizer rather than when calling them.  In particular:
+* We copy any needed global functions to local variables, so that we do not need
+  to do costly global table lookups at runtime.
+* Similarly, we try to do any attribute lookups at startup time if possible.
+* Every field's tag is encoded to bytes at startup, since it can't change at
+  runtime.
+* Whatever component of the field size we can compute at startup, we do.
+* We *avoid* sharing code if doing so would make the code slower and not sharing
+  does not burden us too much.  For example, encoders for repeated fields do
+  not just call the encoders for singular fields in a loop because this would
+  add an extra function call overhead for every loop iteration; instead, we
+  manually inline the single-value encoder into the loop.
+* If a Python function lacks a return statement, Python actually generates
+  instructions to pop the result of the last statement off the stack, push
+  None onto the stack, and then return that.  If we really don't care what
+  value is returned, then we can save two instructions by returning the
+  result of the last statement.  It looks funny but it helps.
+* We assume that type and bounds checking has happened at a higher level.
 """
 
-__author__ = 'robinson@google.com (Will Robinson)'
+__author__ = 'kenton@google.com (Kenton Varda)'
 
 import struct
-from google.protobuf import message
 from google.protobuf.internal import wire_format
-from google.protobuf.internal import output_stream
-
-
-# Note that much of this code is ported from //net/proto/ProtocolBuffer, and
-# that the interface is strongly inspired by WireFormat from the C++ proto2
-# implementation.
-
-
-class Encoder(object):
-
-  """Encodes logical protocol buffer fields to the wire format."""
-
-  def __init__(self):
-    self._stream = output_stream.OutputStream()
-
-  def ToString(self):
-    """Returns all values encoded in this object as a string."""
-    return self._stream.ToString()
-
-  # Append*NoTag methods.  These are necessary for serializing packed
-  # repeated fields.  The Append*() methods call these methods to do
-  # the actual serialization.
-  def AppendInt32NoTag(self, value):
-    """Appends a 32-bit integer to our buffer, varint-encoded."""
-    self._stream.AppendVarint32(value)
-
-  def AppendInt64NoTag(self, value):
-    """Appends a 64-bit integer to our buffer, varint-encoded."""
-    self._stream.AppendVarint64(value)
-
-  def AppendUInt32NoTag(self, unsigned_value):
-    """Appends an unsigned 32-bit integer to our buffer, varint-encoded."""
-    self._stream.AppendVarUInt32(unsigned_value)
-
-  def AppendUInt64NoTag(self, unsigned_value):
-    """Appends an unsigned 64-bit integer to our buffer, varint-encoded."""
-    self._stream.AppendVarUInt64(unsigned_value)
-
-  def AppendSInt32NoTag(self, value):
-    """Appends a 32-bit integer to our buffer, zigzag-encoded and then
-    varint-encoded.
-    """
-    zigzag_value = wire_format.ZigZagEncode(value)
-    self._stream.AppendVarUInt32(zigzag_value)
-
-  def AppendSInt64NoTag(self, value):
-    """Appends a 64-bit integer to our buffer, zigzag-encoded and then
-    varint-encoded.
-    """
-    zigzag_value = wire_format.ZigZagEncode(value)
-    self._stream.AppendVarUInt64(zigzag_value)
-
-  def AppendFixed32NoTag(self, unsigned_value):
-    """Appends an unsigned 32-bit integer to our buffer, in little-endian
-    byte-order.
-    """
-    self._stream.AppendLittleEndian32(unsigned_value)
-
-  def AppendFixed64NoTag(self, unsigned_value):
-    """Appends an unsigned 64-bit integer to our buffer, in little-endian
-    byte-order.
-    """
-    self._stream.AppendLittleEndian64(unsigned_value)
-
-  def AppendSFixed32NoTag(self, value):
-    """Appends a signed 32-bit integer to our buffer, in little-endian
-    byte-order.
-    """
-    sign = (value & 0x80000000) and -1 or 0
-    if value >> 32 != sign:
-      raise message.EncodeError('SFixed32 out of range: %d' % value)
-    self._stream.AppendLittleEndian32(value & 0xffffffff)
-
-  def AppendSFixed64NoTag(self, value):
-    """Appends a signed 64-bit integer to our buffer, in little-endian
-    byte-order.
-    """
-    sign = (value & 0x8000000000000000) and -1 or 0
-    if value >> 64 != sign:
-      raise message.EncodeError('SFixed64 out of range: %d' % value)
-    self._stream.AppendLittleEndian64(value & 0xffffffffffffffff)
-
-  def AppendFloatNoTag(self, value):
-    """Appends a floating-point number to our buffer."""
-    self._stream.AppendRawBytes(
-        struct.pack(wire_format.FORMAT_FLOAT_LITTLE_ENDIAN, value))
-
-  def AppendDoubleNoTag(self, value):
-    """Appends a double-precision floating-point number to our buffer."""
-    self._stream.AppendRawBytes(
-        struct.pack(wire_format.FORMAT_DOUBLE_LITTLE_ENDIAN, value))
-
-  def AppendBoolNoTag(self, value):
-    """Appends a boolean to our buffer."""
-    self.AppendInt32NoTag(value)
-
-  def AppendEnumNoTag(self, value):
-    """Appends an enum value to our buffer."""
-    self.AppendInt32NoTag(value)
-
-
-  # All the Append*() methods below first append a tag+type pair to the buffer
-  # before appending the specified value.
-
-  def AppendInt32(self, field_number, value):
-    """Appends a 32-bit integer to our buffer, varint-encoded."""
-    self.AppendTag(field_number, wire_format.WIRETYPE_VARINT)
-    self.AppendInt32NoTag(value)
-
-  def AppendInt64(self, field_number, value):
-    """Appends a 64-bit integer to our buffer, varint-encoded."""
-    self.AppendTag(field_number, wire_format.WIRETYPE_VARINT)
-    self.AppendInt64NoTag(value)
-
-  def AppendUInt32(self, field_number, unsigned_value):
-    """Appends an unsigned 32-bit integer to our buffer, varint-encoded."""
-    self.AppendTag(field_number, wire_format.WIRETYPE_VARINT)
-    self.AppendUInt32NoTag(unsigned_value)
-
-  def AppendUInt64(self, field_number, unsigned_value):
-    """Appends an unsigned 64-bit integer to our buffer, varint-encoded."""
-    self.AppendTag(field_number, wire_format.WIRETYPE_VARINT)
-    self.AppendUInt64NoTag(unsigned_value)
-
-  def AppendSInt32(self, field_number, value):
-    """Appends a 32-bit integer to our buffer, zigzag-encoded and then
-    varint-encoded.
-    """
-    self.AppendTag(field_number, wire_format.WIRETYPE_VARINT)
-    self.AppendSInt32NoTag(value)
-
-  def AppendSInt64(self, field_number, value):
-    """Appends a 64-bit integer to our buffer, zigzag-encoded and then
-    varint-encoded.
-    """
-    self.AppendTag(field_number, wire_format.WIRETYPE_VARINT)
-    self.AppendSInt64NoTag(value)
-
-  def AppendFixed32(self, field_number, unsigned_value):
-    """Appends an unsigned 32-bit integer to our buffer, in little-endian
-    byte-order.
-    """
-    self.AppendTag(field_number, wire_format.WIRETYPE_FIXED32)
-    self.AppendFixed32NoTag(unsigned_value)
-
-  def AppendFixed64(self, field_number, unsigned_value):
-    """Appends an unsigned 64-bit integer to our buffer, in little-endian
-    byte-order.
-    """
-    self.AppendTag(field_number, wire_format.WIRETYPE_FIXED64)
-    self.AppendFixed64NoTag(unsigned_value)
-
-  def AppendSFixed32(self, field_number, value):
-    """Appends a signed 32-bit integer to our buffer, in little-endian
-    byte-order.
-    """
-    self.AppendTag(field_number, wire_format.WIRETYPE_FIXED32)
-    self.AppendSFixed32NoTag(value)
-
-  def AppendSFixed64(self, field_number, value):
-    """Appends a signed 64-bit integer to our buffer, in little-endian
-    byte-order.
-    """
-    self.AppendTag(field_number, wire_format.WIRETYPE_FIXED64)
-    self.AppendSFixed64NoTag(value)
-
-  def AppendFloat(self, field_number, value):
-    """Appends a floating-point number to our buffer."""
-    self.AppendTag(field_number, wire_format.WIRETYPE_FIXED32)
-    self.AppendFloatNoTag(value)
-
-  def AppendDouble(self, field_number, value):
-    """Appends a double-precision floating-point number to our buffer."""
-    self.AppendTag(field_number, wire_format.WIRETYPE_FIXED64)
-    self.AppendDoubleNoTag(value)
-
-  def AppendBool(self, field_number, value):
-    """Appends a boolean to our buffer."""
-    self.AppendInt32(field_number, value)
-
-  def AppendEnum(self, field_number, value):
-    """Appends an enum value to our buffer."""
-    self.AppendInt32(field_number, value)
-
-  def AppendString(self, field_number, value):
-    """Appends a length-prefixed unicode string, encoded as UTF-8 to our buffer,
-    with the length varint-encoded.
-    """
-    self.AppendBytes(field_number, value.encode('utf-8'))
-
-  def AppendBytes(self, field_number, value):
-    """Appends a length-prefixed sequence of bytes to our buffer, with the
-    length varint-encoded.
-    """
-    self.AppendTag(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
-    self._stream.AppendVarUInt32(len(value))
-    self._stream.AppendRawBytes(value)
-
-  # TODO(robinson): For AppendGroup() and AppendMessage(), we'd really like to
-  # avoid the extra string copy here.  We can do so if we widen the Message
-  # interface to be able to serialize to a stream in addition to a string.  The
-  # challenge when thinking ahead to the Python/C API implementation of Message
-  # is finding a stream-like Python thing to which we can write raw bytes
-  # from C.  I'm not sure such a thing exists(?).  (array.array is pretty much
-  # what we want, but it's not directly exposed in the Python/C API).
-
-  def AppendGroup(self, field_number, group):
-    """Appends a group to our buffer.
-    """
-    self.AppendTag(field_number, wire_format.WIRETYPE_START_GROUP)
-    self._stream.AppendRawBytes(group.SerializeToString())
-    self.AppendTag(field_number, wire_format.WIRETYPE_END_GROUP)
-
-  def AppendMessage(self, field_number, msg):
-    """Appends a nested message to our buffer.
-    """
-    self.AppendTag(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
-    self._stream.AppendVarUInt32(msg.ByteSize())
-    self._stream.AppendRawBytes(msg.SerializeToString())
-
-  def AppendMessageSetItem(self, field_number, msg):
-    """Appends an item using the message set wire format.
-
-    The message set message looks like this:
-      message MessageSet {
-        repeated group Item = 1 {
-          required int32 type_id = 2;
-          required string message = 3;
-        }
+
+
+def _VarintSize(value):
+  """Compute the size of a varint value."""
+  if value <= 0x7f: return 1
+  if value <= 0x3fff: return 2
+  if value <= 0x1fffff: return 3
+  if value <= 0xfffffff: return 4
+  if value <= 0x7ffffffff: return 5
+  if value <= 0x3ffffffffff: return 6
+  if value <= 0x1ffffffffffff: return 7
+  if value <= 0xffffffffffffff: return 8
+  if value <= 0x7fffffffffffffff: return 9
+  return 10
+
+
+def _SignedVarintSize(value):
+  """Compute the size of a signed varint value."""
+  if value < 0: return 10
+  if value <= 0x7f: return 1
+  if value <= 0x3fff: return 2
+  if value <= 0x1fffff: return 3
+  if value <= 0xfffffff: return 4
+  if value <= 0x7ffffffff: return 5
+  if value <= 0x3ffffffffff: return 6
+  if value <= 0x1ffffffffffff: return 7
+  if value <= 0xffffffffffffff: return 8
+  if value <= 0x7fffffffffffffff: return 9
+  return 10
+
+
+def _TagSize(field_number):
+  """Returns the number of bytes required to serialize a tag with this field
+  number."""
+  # Just pass in type 0, since the type won't affect the tag+type size.
+  return _VarintSize(wire_format.PackTag(field_number, 0))
+
+
+# --------------------------------------------------------------------
+# In this section we define some generic sizers.  Each of these functions
+# takes parameters specific to a particular field type, e.g. int32 or fixed64.
+# It returns another function which in turn takes parameters specific to a
+# particular field, e.g. the field number and whether it is repeated or packed.
+# Look at the next section to see how these are used.
+
+
+def _SimpleSizer(compute_value_size):
+  """A sizer which uses the function compute_value_size to compute the size of
+  each value.  Typically compute_value_size is _VarintSize."""
+
+  def SpecificSizer(field_number, is_repeated, is_packed):
+    tag_size = _TagSize(field_number)
+    if is_packed:
+      local_VarintSize = _VarintSize
+      def PackedFieldSize(value):
+        result = 0
+        for element in value:
+          result += compute_value_size(element)
+        return result + local_VarintSize(result) + tag_size
+      return PackedFieldSize
+    elif is_repeated:
+      def RepeatedFieldSize(value):
+        result = tag_size * len(value)
+        for element in value:
+          result += compute_value_size(element)
+        return result
+      return RepeatedFieldSize
+    else:
+      def FieldSize(value):
+        return tag_size + compute_value_size(value)
+      return FieldSize
+
+  return SpecificSizer
+
+
+def _ModifiedSizer(compute_value_size, modify_value):
+  """Like SimpleSizer, but modify_value is invoked on each value before it is
+  passed to compute_value_size.  modify_value is typically ZigZagEncode."""
+
+  def SpecificSizer(field_number, is_repeated, is_packed):
+    tag_size = _TagSize(field_number)
+    if is_packed:
+      local_VarintSize = _VarintSize
+      def PackedFieldSize(value):
+        result = 0
+        for element in value:
+          result += compute_value_size(modify_value(element))
+        return result + local_VarintSize(result) + tag_size
+      return PackedFieldSize
+    elif is_repeated:
+      def RepeatedFieldSize(value):
+        result = tag_size * len(value)
+        for element in value:
+          result += compute_value_size(modify_value(element))
+        return result
+      return RepeatedFieldSize
+    else:
+      def FieldSize(value):
+        return tag_size + compute_value_size(modify_value(value))
+      return FieldSize
+
+  return SpecificSizer
+
+
+def _FixedSizer(value_size):
+  """Like _SimpleSizer except for a fixed-size field.  The input is the size
+  of one value."""
+
+  def SpecificSizer(field_number, is_repeated, is_packed):
+    tag_size = _TagSize(field_number)
+    if is_packed:
+      local_VarintSize = _VarintSize
+      def PackedFieldSize(value):
+        result = len(value) * value_size
+        return result + local_VarintSize(result) + tag_size
+      return PackedFieldSize
+    elif is_repeated:
+      element_size = value_size + tag_size
+      def RepeatedFieldSize(value):
+        return len(value) * element_size
+      return RepeatedFieldSize
+    else:
+      field_size = value_size + tag_size
+      def FieldSize(value):
+        return field_size
+      return FieldSize
+
+  return SpecificSizer
+
+
+# ====================================================================
+# Here we declare a sizer constructor for each field type.  Each "sizer
+# constructor" is a function that takes (field_number, is_repeated, is_packed)
+# as parameters and returns a sizer, which in turn takes a field value as
+# a parameter and returns its encoded size.
+
+
+Int32Sizer = Int64Sizer = EnumSizer = _SimpleSizer(_SignedVarintSize)
+
+UInt32Sizer = UInt64Sizer = _SimpleSizer(_VarintSize)
+
+SInt32Sizer = SInt64Sizer = _ModifiedSizer(
+    _SignedVarintSize, wire_format.ZigZagEncode)
+
+Fixed32Sizer = SFixed32Sizer = FloatSizer  = _FixedSizer(4)
+Fixed64Sizer = SFixed64Sizer = DoubleSizer = _FixedSizer(8)
+
+BoolSizer = _FixedSizer(1)
+
+
+def StringSizer(field_number, is_repeated, is_packed):
+  """Returns a sizer for a string field."""
+
+  tag_size = _TagSize(field_number)
+  local_VarintSize = _VarintSize
+  local_len = len
+  assert not is_packed
+  if is_repeated:
+    def RepeatedFieldSize(value):
+      result = tag_size * len(value)
+      for element in value:
+        l = local_len(element.encode('utf-8'))
+        result += local_VarintSize(l) + l
+      return result
+    return RepeatedFieldSize
+  else:
+    def FieldSize(value):
+      l = local_len(value.encode('utf-8'))
+      return tag_size + local_VarintSize(l) + l
+    return FieldSize
+
+
+def BytesSizer(field_number, is_repeated, is_packed):
+  """Returns a sizer for a bytes field."""
+
+  tag_size = _TagSize(field_number)
+  local_VarintSize = _VarintSize
+  local_len = len
+  assert not is_packed
+  if is_repeated:
+    def RepeatedFieldSize(value):
+      result = tag_size * len(value)
+      for element in value:
+        l = local_len(element)
+        result += local_VarintSize(l) + l
+      return result
+    return RepeatedFieldSize
+  else:
+    def FieldSize(value):
+      l = local_len(value)
+      return tag_size + local_VarintSize(l) + l
+    return FieldSize
+
+
+def GroupSizer(field_number, is_repeated, is_packed):
+  """Returns a sizer for a group field."""
+
+  tag_size = _TagSize(field_number) * 2
+  assert not is_packed
+  if is_repeated:
+    def RepeatedFieldSize(value):
+      result = tag_size * len(value)
+      for element in value:
+        result += element.ByteSize()
+      return result
+    return RepeatedFieldSize
+  else:
+    def FieldSize(value):
+      return tag_size + value.ByteSize()
+    return FieldSize
+
+
+def MessageSizer(field_number, is_repeated, is_packed):
+  """Returns a sizer for a message field."""
+
+  tag_size = _TagSize(field_number)
+  local_VarintSize = _VarintSize
+  assert not is_packed
+  if is_repeated:
+    def RepeatedFieldSize(value):
+      result = tag_size * len(value)
+      for element in value:
+        l = element.ByteSize()
+        result += local_VarintSize(l) + l
+      return result
+    return RepeatedFieldSize
+  else:
+    def FieldSize(value):
+      l = value.ByteSize()
+      return tag_size + local_VarintSize(l) + l
+    return FieldSize
+
+
+# --------------------------------------------------------------------
+# MessageSet is special.
+
+
+def MessageSetItemSizer(field_number):
+  """Returns a sizer for extensions of MessageSet.
+
+  The message set message looks like this:
+    message MessageSet {
+      repeated group Item = 1 {
+        required int32 type_id = 2;
+        required string message = 3;
+      }
+    }
+  """
+  static_size = (_TagSize(1) * 2 + _TagSize(2) + _VarintSize(field_number) +
+                 _TagSize(3))
+  local_VarintSize = _VarintSize
+
+  def FieldSize(value):
+    l = value.ByteSize()
+    return static_size + local_VarintSize(l) + l
+
+  return FieldSize
+
+
+# ====================================================================
+# Encoders!
+
+
+def _VarintEncoder():
+  """Return an encoder for a basic varint value (does not include tag)."""
+
+  local_chr = chr
+  def EncodeVarint(write, value):
+    bits = value & 0x7f
+    value >>= 7
+    while value:
+      write(local_chr(0x80|bits))
+      bits = value & 0x7f
+      value >>= 7
+    return write(local_chr(bits))
+
+  return EncodeVarint
+
+
+def _SignedVarintEncoder():
+  """Return an encoder for a basic signed varint value (does not include
+  tag)."""
+
+  local_chr = chr
+  def EncodeSignedVarint(write, value):
+    if value < 0:
+      value += (1 << 64)
+    bits = value & 0x7f
+    value >>= 7
+    while value:
+      write(local_chr(0x80|bits))
+      bits = value & 0x7f
+      value >>= 7
+    return write(local_chr(bits))
+
+  return EncodeSignedVarint
+
+
+_EncodeVarint = _VarintEncoder()
+_EncodeSignedVarint = _SignedVarintEncoder()
+
+
+def _VarintBytes(value):
+  """Encode the given integer as a varint and return the bytes.  This is only
+  called at startup time so it doesn't need to be fast."""
+
+  pieces = []
+  _EncodeVarint(pieces.append, value)
+  return "".join(pieces)
+
+
+def TagBytes(field_number, wire_type):
+  """Encode the given tag and return the bytes.  Only called at startup."""
+
+  return _VarintBytes(wire_format.PackTag(field_number, wire_type))
+
+# --------------------------------------------------------------------
+# As with sizers (see above), we have a number of common encoder
+# implementations.
+
+
+def _SimpleEncoder(wire_type, encode_value, compute_value_size):
+  """Return a constructor for an encoder for fields of a particular type.
+
+  Args:
+      wire_type:  The field's wire type, for encoding tags.
+      encode_value:  A function which encodes an individual value, e.g.
+        _EncodeVarint().
+      compute_value_size:  A function which computes the size of an individual
+        value, e.g. _VarintSize().
+  """
+
+  def SpecificEncoder(field_number, is_repeated, is_packed):
+    if is_packed:
+      tag_bytes = TagBytes(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
+      local_EncodeVarint = _EncodeVarint
+      def EncodePackedField(write, value):
+        write(tag_bytes)
+        size = 0
+        for element in value:
+          size += compute_value_size(element)
+        local_EncodeVarint(write, size)
+        for element in value:
+          encode_value(write, element)
+      return EncodePackedField
+    elif is_repeated:
+      tag_bytes = TagBytes(field_number, wire_type)
+      def EncodeRepeatedField(write, value):
+        for element in value:
+          write(tag_bytes)
+          encode_value(write, element)
+      return EncodeRepeatedField
+    else:
+      tag_bytes = TagBytes(field_number, wire_type)
+      def EncodeField(write, value):
+        write(tag_bytes)
+        return encode_value(write, value)
+      return EncodeField
+
+  return SpecificEncoder
+
+
+def _ModifiedEncoder(wire_type, encode_value, compute_value_size, modify_value):
+  """Like SimpleEncoder but additionally invokes modify_value on every value
+  before passing it to encode_value.  Usually modify_value is ZigZagEncode."""
+
+  def SpecificEncoder(field_number, is_repeated, is_packed):
+    if is_packed:
+      tag_bytes = TagBytes(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
+      local_EncodeVarint = _EncodeVarint
+      def EncodePackedField(write, value):
+        write(tag_bytes)
+        size = 0
+        for element in value:
+          size += compute_value_size(modify_value(element))
+        local_EncodeVarint(write, size)
+        for element in value:
+          encode_value(write, modify_value(element))
+      return EncodePackedField
+    elif is_repeated:
+      tag_bytes = TagBytes(field_number, wire_type)
+      def EncodeRepeatedField(write, value):
+        for element in value:
+          write(tag_bytes)
+          encode_value(write, modify_value(element))
+      return EncodeRepeatedField
+    else:
+      tag_bytes = TagBytes(field_number, wire_type)
+      def EncodeField(write, value):
+        write(tag_bytes)
+        return encode_value(write, modify_value(value))
+      return EncodeField
+
+  return SpecificEncoder
+
+
+def _StructPackEncoder(wire_type, format):
+  """Return a constructor for an encoder for a fixed-width field.
+
+  Args:
+      wire_type:  The field's wire type, for encoding tags.
+      format:  The format string to pass to struct.pack().
+  """
+
+  value_size = struct.calcsize(format)
+
+  def SpecificEncoder(field_number, is_repeated, is_packed):
+    local_struct_pack = struct.pack
+    if is_packed:
+      tag_bytes = TagBytes(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
+      local_EncodeVarint = _EncodeVarint
+      def EncodePackedField(write, value):
+        write(tag_bytes)
+        local_EncodeVarint(write, len(value) * value_size)
+        for element in value:
+          write(local_struct_pack(format, element))
+      return EncodePackedField
+    elif is_repeated:
+      tag_bytes = TagBytes(field_number, wire_type)
+      def EncodeRepeatedField(write, value):
+        for element in value:
+          write(tag_bytes)
+          write(local_struct_pack(format, element))
+      return EncodeRepeatedField
+    else:
+      tag_bytes = TagBytes(field_number, wire_type)
+      def EncodeField(write, value):
+        write(tag_bytes)
+        return write(local_struct_pack(format, value))
+      return EncodeField
+
+  return SpecificEncoder
+
+
+# ====================================================================
+# Here we declare an encoder constructor for each field type.  These work
+# very similarly to sizer constructors, described earlier.
+
+
+Int32Encoder = Int64Encoder = EnumEncoder = _SimpleEncoder(
+    wire_format.WIRETYPE_VARINT, _EncodeSignedVarint, _SignedVarintSize)
+
+UInt32Encoder = UInt64Encoder = _SimpleEncoder(
+    wire_format.WIRETYPE_VARINT, _EncodeVarint, _VarintSize)
+
+SInt32Encoder = SInt64Encoder = _ModifiedEncoder(
+    wire_format.WIRETYPE_VARINT, _EncodeVarint, _VarintSize,
+    wire_format.ZigZagEncode)
+
+# Note that Python conveniently guarantees that when using the '<' prefix on
+# formats, they will also have the same size across all platforms (as opposed
+# to without the prefix, where their sizes depend on the C compiler's basic
+# type sizes).
+Fixed32Encoder  = _StructPackEncoder(wire_format.WIRETYPE_FIXED32, '<I')
+Fixed64Encoder  = _StructPackEncoder(wire_format.WIRETYPE_FIXED64, '<Q')
+SFixed32Encoder = _StructPackEncoder(wire_format.WIRETYPE_FIXED32, '<i')
+SFixed64Encoder = _StructPackEncoder(wire_format.WIRETYPE_FIXED64, '<q')
+FloatEncoder    = _StructPackEncoder(wire_format.WIRETYPE_FIXED32, '<f')
+DoubleEncoder   = _StructPackEncoder(wire_format.WIRETYPE_FIXED64, '<d')
+
+
+def BoolEncoder(field_number, is_repeated, is_packed):
+  """Returns an encoder for a boolean field."""
+
+  false_byte = chr(0)
+  true_byte = chr(1)
+  if is_packed:
+    tag_bytes = TagBytes(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
+    local_EncodeVarint = _EncodeVarint
+    def EncodePackedField(write, value):
+      write(tag_bytes)
+      local_EncodeVarint(write, len(value))
+      for element in value:
+        if element:
+          write(true_byte)
+        else:
+          write(false_byte)
+    return EncodePackedField
+  elif is_repeated:
+    tag_bytes = TagBytes(field_number, wire_format.WIRETYPE_VARINT)
+    def EncodeRepeatedField(write, value):
+      for element in value:
+        write(tag_bytes)
+        if element:
+          write(true_byte)
+        else:
+          write(false_byte)
+    return EncodeRepeatedField
+  else:
+    tag_bytes = TagBytes(field_number, wire_format.WIRETYPE_VARINT)
+    def EncodeField(write, value):
+      write(tag_bytes)
+      if value:
+        return write(true_byte)
+      return write(false_byte)
+    return EncodeField
+
+
+def StringEncoder(field_number, is_repeated, is_packed):
+  """Returns an encoder for a string field."""
+
+  tag = TagBytes(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
+  local_EncodeVarint = _EncodeVarint
+  local_len = len
+  assert not is_packed
+  if is_repeated:
+    def EncodeRepeatedField(write, value):
+      for element in value:
+        encoded = element.encode('utf-8')
+        write(tag)
+        local_EncodeVarint(write, local_len(encoded))
+        write(encoded)
+    return EncodeRepeatedField
+  else:
+    def EncodeField(write, value):
+      encoded = value.encode('utf-8')
+      write(tag)
+      local_EncodeVarint(write, local_len(encoded))
+      return write(encoded)
+    return EncodeField
+
+
+def BytesEncoder(field_number, is_repeated, is_packed):
+  """Returns an encoder for a bytes field."""
+
+  tag = TagBytes(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
+  local_EncodeVarint = _EncodeVarint
+  local_len = len
+  assert not is_packed
+  if is_repeated:
+    def EncodeRepeatedField(write, value):
+      for element in value:
+        write(tag)
+        local_EncodeVarint(write, local_len(element))
+        write(element)
+    return EncodeRepeatedField
+  else:
+    def EncodeField(write, value):
+      write(tag)
+      local_EncodeVarint(write, local_len(value))
+      return write(value)
+    return EncodeField
+
+
+def GroupEncoder(field_number, is_repeated, is_packed):
+  """Returns an encoder for a group field."""
+
+  start_tag = TagBytes(field_number, wire_format.WIRETYPE_START_GROUP)
+  end_tag = TagBytes(field_number, wire_format.WIRETYPE_END_GROUP)
+  assert not is_packed
+  if is_repeated:
+    def EncodeRepeatedField(write, value):
+      for element in value:
+        write(start_tag)
+        element._InternalSerialize(write)
+        write(end_tag)
+    return EncodeRepeatedField
+  else:
+    def EncodeField(write, value):
+      write(start_tag)
+      value._InternalSerialize(write)
+      return write(end_tag)
+    return EncodeField
+
+
+def MessageEncoder(field_number, is_repeated, is_packed):
+  """Returns an encoder for a message field."""
+
+  tag = TagBytes(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
+  local_EncodeVarint = _EncodeVarint
+  assert not is_packed
+  if is_repeated:
+    def EncodeRepeatedField(write, value):
+      for element in value:
+        write(tag)
+        local_EncodeVarint(write, element.ByteSize())
+        element._InternalSerialize(write)
+    return EncodeRepeatedField
+  else:
+    def EncodeField(write, value):
+      write(tag)
+      local_EncodeVarint(write, value.ByteSize())
+      return value._InternalSerialize(write)
+    return EncodeField
+
+
+# --------------------------------------------------------------------
+# As before, MessageSet is special.
+
+
+def MessageSetItemEncoder(field_number):
+  """Encoder for extensions of MessageSet.
+
+  The message set message looks like this:
+    message MessageSet {
+      repeated group Item = 1 {
+        required int32 type_id = 2;
+        required string message = 3;
       }
-    """
-    self.AppendTag(1, wire_format.WIRETYPE_START_GROUP)
-    self.AppendInt32(2, field_number)
-    self.AppendMessage(3, msg)
-    self.AppendTag(1, wire_format.WIRETYPE_END_GROUP)
-
-  def AppendTag(self, field_number, wire_type):
-    """Appends a tag containing field number and wire type information."""
-    self._stream.AppendVarUInt32(wire_format.PackTag(field_number, wire_type))
+    }
+  """
+  start_bytes = "".join([
+      TagBytes(1, wire_format.WIRETYPE_START_GROUP),
+      TagBytes(2, wire_format.WIRETYPE_VARINT),
+      _VarintBytes(field_number),
+      TagBytes(3, wire_format.WIRETYPE_LENGTH_DELIMITED)])
+  end_bytes = TagBytes(1, wire_format.WIRETYPE_END_GROUP)
+  local_EncodeVarint = _EncodeVarint
+
+  def EncodeField(write, value):
+    write(start_bytes)
+    local_EncodeVarint(write, value.ByteSize())
+    value._InternalSerialize(write)
+    return write(end_bytes)
+
+  return EncodeField