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# Copyright 2014 Google Inc. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from . import number_types as N
from .number_types import (UOffsetTFlags, SOffsetTFlags, VOffsetTFlags)
from . import encode
from . import packer
from . import compat
from .compat import range_func
from .compat import memoryview_type
from .compat import import_numpy, NumpyRequiredForThisFeature
np = import_numpy()
## @file
## @addtogroup flatbuffers_python_api
## @{
## @cond FLATBUFFERS_INTERNAL
class OffsetArithmeticError(RuntimeError):
"""
Error caused by an Offset arithmetic error. Probably caused by bad
writing of fields. This is considered an unreachable situation in
normal circumstances.
"""
pass
class IsNotNestedError(RuntimeError):
"""
Error caused by using a Builder to write Object data when not inside
an Object.
"""
pass
class IsNestedError(RuntimeError):
"""
Error caused by using a Builder to begin an Object when an Object is
already being built.
"""
pass
class StructIsNotInlineError(RuntimeError):
"""
Error caused by using a Builder to write a Struct at a location that
is not the current Offset.
"""
pass
class BuilderSizeError(RuntimeError):
"""
Error caused by causing a Builder to exceed the hardcoded limit of 2
gigabytes.
"""
pass
class BuilderNotFinishedError(RuntimeError):
"""
Error caused by not calling `Finish` before calling `Output`.
"""
pass
# VtableMetadataFields is the count of metadata fields in each vtable.
VtableMetadataFields = 2
## @endcond
class Builder(object):
""" A Builder is used to construct one or more FlatBuffers.
Typically, Builder objects will be used from code generated by the `flatc`
compiler.
A Builder constructs byte buffers in a last-first manner for simplicity and
performance during reading.
Internally, a Builder is a state machine for creating FlatBuffer objects.
It holds the following internal state:
- Bytes: an array of bytes.
- current_vtable: a list of integers.
- vtables: a list of vtable entries (i.e. a list of list of integers).
Attributes:
Bytes: The internal `bytearray` for the Builder.
finished: A boolean determining if the Builder has been finalized.
"""
## @cond FLATBUFFERS_INTENRAL
__slots__ = ("Bytes", "current_vtable", "head", "minalign", "objectEnd",
"vtables", "nested", "finished")
"""Maximum buffer size constant, in bytes.
Builder will never allow it's buffer grow over this size.
Currently equals 2Gb.
"""
MAX_BUFFER_SIZE = 2**31
## @endcond
def __init__(self, initialSize):
"""Initializes a Builder of size `initial_size`.
The internal buffer is grown as needed.
"""
if not (0 <= initialSize <= Builder.MAX_BUFFER_SIZE):
msg = "flatbuffers: Cannot create Builder larger than 2 gigabytes."
raise BuilderSizeError(msg)
self.Bytes = bytearray(initialSize)
## @cond FLATBUFFERS_INTERNAL
self.current_vtable = None
self.head = UOffsetTFlags.py_type(initialSize)
self.minalign = 1
self.objectEnd = None
self.vtables = []
self.nested = False
## @endcond
self.finished = False
def Output(self):
"""Return the portion of the buffer that has been used for writing data.
This is the typical way to access the FlatBuffer data inside the
builder. If you try to access `Builder.Bytes` directly, you would need
to manually index it with `Head()`, since the buffer is constructed
backwards.
It raises BuilderNotFinishedError if the buffer has not been finished
with `Finish`.
"""
if not self.finished:
raise BuilderNotFinishedError()
return self.Bytes[self.Head():]
## @cond FLATBUFFERS_INTERNAL
def StartObject(self, numfields):
"""StartObject initializes bookkeeping for writing a new object."""
self.assertNotNested()
# use 32-bit offsets so that arithmetic doesn't overflow.
self.current_vtable = [0 for _ in range_func(numfields)]
self.objectEnd = self.Offset()
self.nested = True
def WriteVtable(self):
"""
WriteVtable serializes the vtable for the current object, if needed.
Before writing out the vtable, this checks pre-existing vtables for
equality to this one. If an equal vtable is found, point the object to
the existing vtable and return.
Because vtable values are sensitive to alignment of object data, not
all logically-equal vtables will be deduplicated.
A vtable has the following format:
<VOffsetT: size of the vtable in bytes, including this value>
<VOffsetT: size of the object in bytes, including the vtable offset>
<VOffsetT: offset for a field> * N, where N is the number of fields
in the schema for this type. Includes deprecated fields.
Thus, a vtable is made of 2 + N elements, each VOffsetT bytes wide.
An object has the following format:
<SOffsetT: offset to this object's vtable (may be negative)>
<byte: data>+
"""
# Prepend a zero scalar to the object. Later in this function we'll
# write an offset here that points to the object's vtable:
self.PrependSOffsetTRelative(0)
objectOffset = self.Offset()
existingVtable = None
# Trim trailing 0 offsets.
while self.current_vtable and self.current_vtable[-1] == 0:
self.current_vtable.pop()
# Search backwards through existing vtables, because similar vtables
# are likely to have been recently appended. See
# BenchmarkVtableDeduplication for a case in which this heuristic
# saves about 30% of the time used in writing objects with duplicate
# tables.
i = len(self.vtables) - 1
while i >= 0:
# Find the other vtable, which is associated with `i`:
vt2Offset = self.vtables[i]
vt2Start = len(self.Bytes) - vt2Offset
vt2Len = encode.Get(packer.voffset, self.Bytes, vt2Start)
metadata = VtableMetadataFields * N.VOffsetTFlags.bytewidth
vt2End = vt2Start + vt2Len
vt2 = self.Bytes[vt2Start+metadata:vt2End]
# Compare the other vtable to the one under consideration.
# If they are equal, store the offset and break:
if vtableEqual(self.current_vtable, objectOffset, vt2):
existingVtable = vt2Offset
break
i -= 1
if existingVtable is None:
# Did not find a vtable, so write this one to the buffer.
# Write out the current vtable in reverse , because
# serialization occurs in last-first order:
i = len(self.current_vtable) - 1
while i >= 0:
off = 0
if self.current_vtable[i] != 0:
# Forward reference to field;
# use 32bit number to ensure no overflow:
off = objectOffset - self.current_vtable[i]
self.PrependVOffsetT(off)
i -= 1
# The two metadata fields are written last.
# First, store the object bytesize:
objectSize = UOffsetTFlags.py_type(objectOffset - self.objectEnd)
self.PrependVOffsetT(VOffsetTFlags.py_type(objectSize))
# Second, store the vtable bytesize:
vBytes = len(self.current_vtable) + VtableMetadataFields
vBytes *= N.VOffsetTFlags.bytewidth
self.PrependVOffsetT(VOffsetTFlags.py_type(vBytes))
# Next, write the offset to the new vtable in the
# already-allocated SOffsetT at the beginning of this object:
objectStart = SOffsetTFlags.py_type(len(self.Bytes) - objectOffset)
encode.Write(packer.soffset, self.Bytes, objectStart,
SOffsetTFlags.py_type(self.Offset() - objectOffset))
# Finally, store this vtable in memory for future
# deduplication:
self.vtables.append(self.Offset())
else:
# Found a duplicate vtable.
objectStart = SOffsetTFlags.py_type(len(self.Bytes) - objectOffset)
self.head = UOffsetTFlags.py_type(objectStart)
# Write the offset to the found vtable in the
# already-allocated SOffsetT at the beginning of this object:
encode.Write(packer.soffset, self.Bytes, self.Head(),
SOffsetTFlags.py_type(existingVtable - objectOffset))
self.current_vtable = None
return objectOffset
def EndObject(self):
"""EndObject writes data necessary to finish object construction."""
self.assertNested()
self.nested = False
return self.WriteVtable()
def growByteBuffer(self):
"""Doubles the size of the byteslice, and copies the old data towards
the end of the new buffer (since we build the buffer backwards)."""
if len(self.Bytes) == Builder.MAX_BUFFER_SIZE:
msg = "flatbuffers: cannot grow buffer beyond 2 gigabytes"
raise BuilderSizeError(msg)
newSize = min(len(self.Bytes) * 2, Builder.MAX_BUFFER_SIZE)
if newSize == 0:
newSize = 1
bytes2 = bytearray(newSize)
bytes2[newSize-len(self.Bytes):] = self.Bytes
self.Bytes = bytes2
## @endcond
def Head(self):
"""Get the start of useful data in the underlying byte buffer.
Note: unlike other functions, this value is interpreted as from the
left.
"""
## @cond FLATBUFFERS_INTERNAL
return self.head
## @endcond
## @cond FLATBUFFERS_INTERNAL
def Offset(self):
"""Offset relative to the end of the buffer."""
return UOffsetTFlags.py_type(len(self.Bytes) - self.Head())
def Pad(self, n):
"""Pad places zeros at the current offset."""
for i in range_func(n):
self.Place(0, N.Uint8Flags)
def Prep(self, size, additionalBytes):
"""
Prep prepares to write an element of `size` after `additional_bytes`
have been written, e.g. if you write a string, you need to align
such the int length field is aligned to SizeInt32, and the string
data follows it directly.
If all you need to do is align, `additionalBytes` will be 0.
"""
# Track the biggest thing we've ever aligned to.
if size > self.minalign:
self.minalign = size
# Find the amount of alignment needed such that `size` is properly
# aligned after `additionalBytes`:
alignSize = (~(len(self.Bytes) - self.Head() + additionalBytes)) + 1
alignSize &= (size - 1)
# Reallocate the buffer if needed:
while self.Head() < alignSize+size+additionalBytes:
oldBufSize = len(self.Bytes)
self.growByteBuffer()
updated_head = self.head + len(self.Bytes) - oldBufSize
self.head = UOffsetTFlags.py_type(updated_head)
self.Pad(alignSize)
def PrependSOffsetTRelative(self, off):
"""
PrependSOffsetTRelative prepends an SOffsetT, relative to where it
will be written.
"""
# Ensure alignment is already done:
self.Prep(N.SOffsetTFlags.bytewidth, 0)
if not (off <= self.Offset()):
msg = "flatbuffers: Offset arithmetic error."
raise OffsetArithmeticError(msg)
off2 = self.Offset() - off + N.SOffsetTFlags.bytewidth
self.PlaceSOffsetT(off2)
## @endcond
def PrependUOffsetTRelative(self, off):
"""Prepends an unsigned offset into vector data, relative to where it
will be written.
"""
# Ensure alignment is already done:
self.Prep(N.UOffsetTFlags.bytewidth, 0)
if not (off <= self.Offset()):
msg = "flatbuffers: Offset arithmetic error."
raise OffsetArithmeticError(msg)
off2 = self.Offset() - off + N.UOffsetTFlags.bytewidth
self.PlaceUOffsetT(off2)
## @cond FLATBUFFERS_INTERNAL
def StartVector(self, elemSize, numElems, alignment):
"""
StartVector initializes bookkeeping for writing a new vector.
A vector has the following format:
- <UOffsetT: number of elements in this vector>
- <T: data>+, where T is the type of elements of this vector.
"""
self.assertNotNested()
self.nested = True
self.Prep(N.Uint32Flags.bytewidth, elemSize*numElems)
self.Prep(alignment, elemSize*numElems) # In case alignment > int.
return self.Offset()
## @endcond
def EndVector(self, vectorNumElems):
"""EndVector writes data necessary to finish vector construction."""
self.assertNested()
## @cond FLATBUFFERS_INTERNAL
self.nested = False
## @endcond
# we already made space for this, so write without PrependUint32
self.PlaceUOffsetT(vectorNumElems)
return self.Offset()
def CreateString(self, s, encoding='utf-8', errors='strict'):
"""CreateString writes a null-terminated byte string as a vector."""
self.assertNotNested()
## @cond FLATBUFFERS_INTERNAL
self.nested = True
## @endcond
if isinstance(s, compat.string_types):
x = s.encode(encoding, errors)
elif isinstance(s, compat.binary_types):
x = s
else:
raise TypeError("non-string passed to CreateString")
self.Prep(N.UOffsetTFlags.bytewidth, (len(x)+1)*N.Uint8Flags.bytewidth)
self.Place(0, N.Uint8Flags)
l = UOffsetTFlags.py_type(len(s))
## @cond FLATBUFFERS_INTERNAL
self.head = UOffsetTFlags.py_type(self.Head() - l)
## @endcond
self.Bytes[self.Head():self.Head()+l] = x
return self.EndVector(len(x))
def CreateByteVector(self, x):
"""CreateString writes a byte vector."""
self.assertNotNested()
## @cond FLATBUFFERS_INTERNAL
self.nested = True
## @endcond
if not isinstance(x, compat.binary_types):
raise TypeError("non-byte vector passed to CreateByteVector")
self.Prep(N.UOffsetTFlags.bytewidth, len(x)*N.Uint8Flags.bytewidth)
l = UOffsetTFlags.py_type(len(x))
## @cond FLATBUFFERS_INTERNAL
self.head = UOffsetTFlags.py_type(self.Head() - l)
## @endcond
self.Bytes[self.Head():self.Head()+l] = x
return self.EndVector(len(x))
def CreateNumpyVector(self, x):
"""CreateNumpyVector writes a numpy array into the buffer."""
if np is None:
# Numpy is required for this feature
raise NumpyRequiredForThisFeature("Numpy was not found.")
if not isinstance(x, np.ndarray):
raise TypeError("non-numpy-ndarray passed to CreateNumpyVector")
if x.dtype.kind not in ['b', 'i', 'u', 'f']:
raise TypeError("numpy-ndarray holds elements of unsupported datatype")
if x.ndim > 1:
raise TypeError("multidimensional-ndarray passed to CreateNumpyVector")
self.StartVector(x.itemsize, x.size, x.dtype.alignment)
# Ensure little endian byte ordering
if x.dtype.str[0] == "<":
x_lend = x
else:
x_lend = x.byteswap(inplace=False)
# Calculate total length
l = UOffsetTFlags.py_type(x_lend.itemsize * x_lend.size)
## @cond FLATBUFFERS_INTERNAL
self.head = UOffsetTFlags.py_type(self.Head() - l)
## @endcond
# tobytes ensures c_contiguous ordering
self.Bytes[self.Head():self.Head()+l] = x_lend.tobytes(order='C')
return self.EndVector(x.size)
## @cond FLATBUFFERS_INTERNAL
def assertNested(self):
"""
Check that we are in the process of building an object.
"""
if not self.nested:
raise IsNotNestedError()
def assertNotNested(self):
"""
Check that no other objects are being built while making this
object. If not, raise an exception.
"""
if self.nested:
raise IsNestedError()
def assertStructIsInline(self, obj):
"""
Structs are always stored inline, so need to be created right
where they are used. You'll get this error if you created it
elsewhere.
"""
N.enforce_number(obj, N.UOffsetTFlags)
if obj != self.Offset():
msg = ("flatbuffers: Tried to write a Struct at an Offset that "
"is different from the current Offset of the Builder.")
raise StructIsNotInlineError(msg)
def Slot(self, slotnum):
"""
Slot sets the vtable key `voffset` to the current location in the
buffer.
"""
self.assertNested()
self.current_vtable[slotnum] = self.Offset()
## @endcond
def __Finish(self, rootTable, sizePrefix):
"""Finish finalizes a buffer, pointing to the given `rootTable`."""
N.enforce_number(rootTable, N.UOffsetTFlags)
prepSize = N.UOffsetTFlags.bytewidth
if sizePrefix:
prepSize += N.Int32Flags.bytewidth
self.Prep(self.minalign, prepSize)
self.PrependUOffsetTRelative(rootTable)
if sizePrefix:
size = len(self.Bytes) - self.Head()
N.enforce_number(size, N.Int32Flags)
self.PrependInt32(size)
self.finished = True
return self.Head()
def Finish(self, rootTable):
"""Finish finalizes a buffer, pointing to the given `rootTable`."""
return self.__Finish(rootTable, False)
def FinishSizePrefixed(self, rootTable):
"""
Finish finalizes a buffer, pointing to the given `rootTable`,
with the size prefixed.
"""
return self.__Finish(rootTable, True)
## @cond FLATBUFFERS_INTERNAL
def Prepend(self, flags, off):
self.Prep(flags.bytewidth, 0)
self.Place(off, flags)
def PrependSlot(self, flags, o, x, d):
N.enforce_number(x, flags)
N.enforce_number(d, flags)
if x != d:
self.Prepend(flags, x)
self.Slot(o)
def PrependBoolSlot(self, *args): self.PrependSlot(N.BoolFlags, *args)
def PrependByteSlot(self, *args): self.PrependSlot(N.Uint8Flags, *args)
def PrependUint8Slot(self, *args): self.PrependSlot(N.Uint8Flags, *args)
def PrependUint16Slot(self, *args): self.PrependSlot(N.Uint16Flags, *args)
def PrependUint32Slot(self, *args): self.PrependSlot(N.Uint32Flags, *args)
def PrependUint64Slot(self, *args): self.PrependSlot(N.Uint64Flags, *args)
def PrependInt8Slot(self, *args): self.PrependSlot(N.Int8Flags, *args)
def PrependInt16Slot(self, *args): self.PrependSlot(N.Int16Flags, *args)
def PrependInt32Slot(self, *args): self.PrependSlot(N.Int32Flags, *args)
def PrependInt64Slot(self, *args): self.PrependSlot(N.Int64Flags, *args)
def PrependFloat32Slot(self, *args): self.PrependSlot(N.Float32Flags,
*args)
def PrependFloat64Slot(self, *args): self.PrependSlot(N.Float64Flags,
*args)
def PrependUOffsetTRelativeSlot(self, o, x, d):
"""
PrependUOffsetTRelativeSlot prepends an UOffsetT onto the object at
vtable slot `o`. If value `x` equals default `d`, then the slot will
be set to zero and no other data will be written.
"""
if x != d:
self.PrependUOffsetTRelative(x)
self.Slot(o)
def PrependStructSlot(self, v, x, d):
"""
PrependStructSlot prepends a struct onto the object at vtable slot `o`.
Structs are stored inline, so nothing additional is being added.
In generated code, `d` is always 0.
"""
N.enforce_number(d, N.UOffsetTFlags)
if x != d:
self.assertStructIsInline(x)
self.Slot(v)
## @endcond
def PrependBool(self, x):
"""Prepend a `bool` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.BoolFlags, x)
def PrependByte(self, x):
"""Prepend a `byte` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Uint8Flags, x)
def PrependUint8(self, x):
"""Prepend an `uint8` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Uint8Flags, x)
def PrependUint16(self, x):
"""Prepend an `uint16` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Uint16Flags, x)
def PrependUint32(self, x):
"""Prepend an `uint32` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Uint32Flags, x)
def PrependUint64(self, x):
"""Prepend an `uint64` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Uint64Flags, x)
def PrependInt8(self, x):
"""Prepend an `int8` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Int8Flags, x)
def PrependInt16(self, x):
"""Prepend an `int16` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Int16Flags, x)
def PrependInt32(self, x):
"""Prepend an `int32` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Int32Flags, x)
def PrependInt64(self, x):
"""Prepend an `int64` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Int64Flags, x)
def PrependFloat32(self, x):
"""Prepend a `float32` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Float32Flags, x)
def PrependFloat64(self, x):
"""Prepend a `float64` to the Builder buffer.
Note: aligns and checks for space.
"""
self.Prepend(N.Float64Flags, x)
##############################################################
## @cond FLATBUFFERS_INTERNAL
def PrependVOffsetT(self, x): self.Prepend(N.VOffsetTFlags, x)
def Place(self, x, flags):
"""
Place prepends a value specified by `flags` to the Builder,
without checking for available space.
"""
N.enforce_number(x, flags)
self.head = self.head - flags.bytewidth
encode.Write(flags.packer_type, self.Bytes, self.Head(), x)
def PlaceVOffsetT(self, x):
"""PlaceVOffsetT prepends a VOffsetT to the Builder, without checking
for space.
"""
N.enforce_number(x, N.VOffsetTFlags)
self.head = self.head - N.VOffsetTFlags.bytewidth
encode.Write(packer.voffset, self.Bytes, self.Head(), x)
def PlaceSOffsetT(self, x):
"""PlaceSOffsetT prepends a SOffsetT to the Builder, without checking
for space.
"""
N.enforce_number(x, N.SOffsetTFlags)
self.head = self.head - N.SOffsetTFlags.bytewidth
encode.Write(packer.soffset, self.Bytes, self.Head(), x)
def PlaceUOffsetT(self, x):
"""PlaceUOffsetT prepends a UOffsetT to the Builder, without checking
for space.
"""
N.enforce_number(x, N.UOffsetTFlags)
self.head = self.head - N.UOffsetTFlags.bytewidth
encode.Write(packer.uoffset, self.Bytes, self.Head(), x)
## @endcond
## @cond FLATBUFFERS_INTERNAL
def vtableEqual(a, objectStart, b):
"""vtableEqual compares an unwritten vtable to a written vtable."""
N.enforce_number(objectStart, N.UOffsetTFlags)
if len(a) * N.VOffsetTFlags.bytewidth != len(b):
return False
for i, elem in enumerate(a):
x = encode.Get(packer.voffset, b, i * N.VOffsetTFlags.bytewidth)
# Skip vtable entries that indicate a default value.
if x == 0 and elem == 0:
pass
else:
y = objectStart - elem
if x != y:
return False
return True
## @endcond
## @}