285 lines
9.6 KiB
Plaintext
285 lines
9.6 KiB
Plaintext
// Copyright 2018 Google Inc. All rights reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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import std
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namespace flatbuffers
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class handle:
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buf_:string
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pos_:int
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// More strongly typed than a naked int, at no cost.
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struct offset:
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o:int
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enum sizeof:
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sz_8 = 1
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sz_16 = 2
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sz_32 = 4
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sz_64 = 8
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sz_voffset = 2
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sz_uoffset = 4
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sz_soffset = 4
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sz_metadata_fields = 2
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class builder:
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buf = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
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current_vtable:[int] = []
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head = 0
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minalign = 1
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object_end = 0
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vtables:[int] = []
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nested = false
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finished = false
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// Optionally call this right after creating the builder for a larger initial buffer.
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def Initial(initial_size:int):
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buf = "\x00".repeat_string(initial_size)
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def Start():
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// Get the start of useful data in the underlying byte buffer.
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return buf.length - head
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def Offset():
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// Offset relative to the end of the buffer.
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return offset { head }
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// Returns a copy of the part of the buffer containing only the finished FlatBuffer
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def SizedCopy():
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assert finished
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return buf.substring(Start(), -1)
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def StartNesting():
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assert not nested
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nested = true
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def EndNesting():
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assert nested
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nested = false
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def StartObject(numfields):
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StartNesting()
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current_vtable = map(numfields): 0
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object_end = head
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minalign = 1
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def EndObject():
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EndNesting()
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// Prepend a zero scalar to the object. Later in this function we'll
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// write an offset here that points to the object's vtable:
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PrependInt32(0)
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let object_offset = head
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// Write out new vtable speculatively.
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let vtable_size = (current_vtable.length + sz_metadata_fields) * sz_voffset
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while current_vtable.length:
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let o = current_vtable.pop()
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PrependVOffsetT(if o: object_offset - o else: 0)
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// The two metadata fields are written last.
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// First, store the object bytesize:
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PrependVOffsetT(object_offset - object_end)
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// Second, store the vtable bytesize:
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PrependVOffsetT(vtable_size)
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// Search backwards through existing vtables, because similar vtables
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// are likely to have been recently appended. See
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// BenchmarkVtableDeduplication for a case in which this heuristic
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// saves about 30% of the time used in writing objects with duplicate
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// tables.
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def find_existing_table():
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reverse(vtables) vt2_offset:
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// Find the other vtable:
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let vt2_start = buf.length - vt2_offset
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let vt2_len = buf.read_int16_le(vt2_start)
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// Compare the other vtable to the one under consideration.
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// If they are equal, return the offset:
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if vtable_size == vt2_len and
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not compare_substring(buf, Start(), buf, vt2_start, vtable_size):
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return vt2_offset
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return 0
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let existing_vtable = find_existing_table()
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if existing_vtable:
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// Found a duplicate vtable, remove the one we wrote.
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head = object_offset
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// Write the offset to the found vtable in the
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// already-allocated offset at the beginning of this object:
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buf.write_int32_le(Start(), existing_vtable - object_offset)
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else:
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// Did not find a vtable, so keep the one we wrote.
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// Next, write the offset to the new vtable in the
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// already-allocated offset at the beginning of this object:
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buf.write_int32_le(buf.length - object_offset, head - object_offset)
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// Finally, store this vtable in memory for future
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// deduplication:
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vtables.push(head)
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return offset { object_offset }
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def Pad(n):
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for(n):
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buf, head = buf.write_int8_le_back(head, 0)
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def Prep(size, additional_bytes):
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// Track the biggest thing we've ever aligned to.
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if size > minalign:
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minalign = size
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// Find the amount of alignment needed such that `size` is properly
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// aligned after `additionalBytes`:
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let align_size = ((~(head + additional_bytes)) + 1) & (size - 1)
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Pad(align_size)
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def PrependUOffsetTRelative(off:offset):
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// Prepends an unsigned offset into vector data, relative to where it will be written.
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Prep(sz_uoffset, 0)
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assert off.o <= head
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PlaceUOffsetT(head - off.o + sz_uoffset)
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def StartVector(elem_size, num_elems, alignment):
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// Initializes bookkeeping for writing a new vector.
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StartNesting()
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Prep(sz_32, elem_size * num_elems)
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Prep(alignment, elem_size * num_elems) // In case alignment > int.
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return Offset()
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def EndVector(vector_num_elems):
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EndNesting()
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// we already made space for this, so write without PrependUint32
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PlaceUOffsetT(vector_num_elems)
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return Offset()
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def CreateString(s:string):
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// writes a null-terminated byte string.
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StartNesting()
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Prep(sz_32, s.length + 1)
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buf, head = buf.write_substring_back(head, s, true)
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return EndVector(s.length)
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def CreateByteVector(s:string):
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// writes a non-null-terminated byte string.
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StartNesting()
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Prep(sz_32, s.length)
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buf, head = buf.write_substring_back(head, s, false)
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return EndVector(s.length)
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def Slot(slotnum):
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assert nested
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while current_vtable.length <= slotnum: current_vtable.push(0)
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current_vtable[slotnum] = head
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def __Finish(root_table:offset, size_prefix:int):
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// Finish finalizes a buffer, pointing to the given root_table
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assert not finished
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assert not nested
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var prep_size = sz_32
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if size_prefix:
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prep_size += sz_32
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Prep(minalign, prep_size)
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PrependUOffsetTRelative(root_table)
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if size_prefix:
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PrependInt32(head)
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finished = true
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return Start()
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def Finish(root_table:offset):
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return __Finish(root_table, false)
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def FinishSizePrefixed(root_table:offset):
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return __Finish(root_table, true)
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def PrependBool(x):
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buf, head = buf.write_int8_le_back(head, x)
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def PrependByte(x):
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buf, head = buf.write_int8_le_back(head, x)
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def PrependUint8(x):
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buf, head = buf.write_int8_le_back(head, x)
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def PrependUint16(x):
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Prep(sz_16, 0)
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buf, head = buf.write_int16_le_back(head, x)
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def PrependUint32(x):
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Prep(sz_32, 0)
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buf, head = buf.write_int32_le_back(head, x)
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def PrependUint64(x):
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Prep(sz_64, 0)
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buf, head = buf.write_int64_le_back(head, x)
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def PrependInt8(x):
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buf, head = buf.write_int8_le_back(head, x)
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def PrependInt16(x):
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Prep(sz_16, 0)
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buf, head = buf.write_int16_le_back(head, x)
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def PrependInt32(x):
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Prep(sz_32, 0)
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buf, head = buf.write_int32_le_back(head, x)
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def PrependInt64(x):
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Prep(sz_64, 0)
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buf, head = buf.write_int64_le_back(head, x)
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def PrependFloat32(x):
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Prep(sz_32, 0)
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buf, head = buf.write_float32_le_back(head, x)
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def PrependFloat64(x):
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Prep(sz_64, 0)
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buf, head = buf.write_float64_le_back(head, x)
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def PrependVOffsetT(x):
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Prep(sz_voffset, 0)
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buf, head = buf.write_int16_le_back(head, x)
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def PlaceVOffsetT(x):
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buf, head = buf.write_int16_le_back(head, x)
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def PlaceSOffsetT(x):
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buf, head = buf.write_int32_le_back(head, x)
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def PlaceUOffsetT(x):
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buf, head = buf.write_int32_le_back(head, x)
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def PrependSlot(o:int, x, d, f):
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if x != d:
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f(x)
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Slot(o)
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def PrependBoolSlot(o, x, d): PrependSlot(o, x, d): PrependBool(_)
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def PrependByteSlot(o, x, d): PrependSlot(o, x, d): PrependByte(_)
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def PrependUint8Slot(o, x, d): PrependSlot(o, x, d): PrependUint8(_)
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def PrependUint16Slot(o, x, d): PrependSlot(o, x, d): PrependUint16(_)
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def PrependUint32Slot(o, x, d): PrependSlot(o, x, d): PrependUint32(_)
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def PrependUint64Slot(o, x, d): PrependSlot(o, x, d): PrependUint64(_)
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def PrependInt8Slot(o, x, d): PrependSlot(o, x, d): PrependInt8(_)
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def PrependInt16Slot(o, x, d): PrependSlot(o, x, d): PrependInt16(_)
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def PrependInt32Slot(o, x, d): PrependSlot(o, x, d): PrependInt32(_)
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def PrependInt64Slot(o, x, d): PrependSlot(o, x, d): PrependInt64(_)
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def PrependFloat32Slot(o, x, d): PrependSlot(o, x, d): PrependFloat32(_)
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def PrependFloat64Slot(o, x, d): PrependSlot(o, x, d): PrependFloat64(_)
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def PrependUOffsetTRelativeSlot(o:int, x:offset):
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if x.o:
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PrependUOffsetTRelative(x)
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Slot(o)
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def PrependStructSlot(v:int, x:offset):
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if x.o:
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// Structs are always stored inline, so need to be created right
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// where they are used. You'll get this error if you created it
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// elsewhere.
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assert x.o == head
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Slot(v)
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