package flatbuffers // Builder is a state machine for creating FlatBuffer objects. // Use a Builder to construct object(s) starting from leaf nodes. // // A Builder constructs byte buffers in a last-first manner for simplicity and // performance. type Builder struct { // `Bytes` gives raw access to the buffer. Most users will want to use // FinishedBytes() instead. Bytes []byte minalign int vtable []UOffsetT objectEnd UOffsetT vtables []UOffsetT head UOffsetT nested bool finished bool sharedStrings map[string]UOffsetT } const fileIdentifierLength = 4 // NewBuilder initializes a Builder of size `initial_size`. // The internal buffer is grown as needed. func NewBuilder(initialSize int) *Builder { if initialSize <= 0 { initialSize = 0 } b := &Builder{} b.Bytes = make([]byte, initialSize) b.head = UOffsetT(initialSize) b.minalign = 1 b.vtables = make([]UOffsetT, 0, 16) // sensible default capacity return b } // Reset truncates the underlying Builder buffer, facilitating alloc-free // reuse of a Builder. It also resets bookkeeping data. func (b *Builder) Reset() { if b.Bytes != nil { b.Bytes = b.Bytes[:cap(b.Bytes)] } if b.vtables != nil { b.vtables = b.vtables[:0] } if b.vtable != nil { b.vtable = b.vtable[:0] } b.head = UOffsetT(len(b.Bytes)) b.minalign = 1 b.nested = false b.finished = false } // FinishedBytes returns a pointer to the written data in the byte buffer. // Panics if the builder is not in a finished state (which is caused by calling // `Finish()`). func (b *Builder) FinishedBytes() []byte { b.assertFinished() return b.Bytes[b.Head():] } // StartObject initializes bookkeeping for writing a new object. func (b *Builder) StartObject(numfields int) { b.assertNotNested() b.nested = true // use 32-bit offsets so that arithmetic doesn't overflow. if cap(b.vtable) < numfields || b.vtable == nil { b.vtable = make([]UOffsetT, numfields) } else { b.vtable = b.vtable[:numfields] for i := 0; i < len(b.vtable); i++ { b.vtable[i] = 0 } } b.objectEnd = b.Offset() } // WriteVtable serializes the vtable for the current object, if applicable. // // 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: // // // * 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 SizeVOffsetT bytes wide. // // An object has the following format: // // + func (b *Builder) WriteVtable() (n UOffsetT) { // Prepend a zero scalar to the object. Later in this function we'll // write an offset here that points to the object's vtable: b.PrependSOffsetT(0) objectOffset := b.Offset() existingVtable := UOffsetT(0) // Trim vtable of trailing zeroes. i := len(b.vtable) - 1 for ; i >= 0 && b.vtable[i] == 0; i-- { } b.vtable = b.vtable[:i+1] // 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. for i := len(b.vtables) - 1; i >= 0; i-- { // Find the other vtable, which is associated with `i`: vt2Offset := b.vtables[i] vt2Start := len(b.Bytes) - int(vt2Offset) vt2Len := GetVOffsetT(b.Bytes[vt2Start:]) metadata := VtableMetadataFields * SizeVOffsetT vt2End := vt2Start + int(vt2Len) vt2 := b.Bytes[vt2Start+metadata : vt2End] // Compare the other vtable to the one under consideration. // If they are equal, store the offset and break: if vtableEqual(b.vtable, objectOffset, vt2) { existingVtable = vt2Offset break } } if existingVtable == 0 { // 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: for i := len(b.vtable) - 1; i >= 0; i-- { var off UOffsetT if b.vtable[i] != 0 { // Forward reference to field; // use 32bit number to assert no overflow: off = objectOffset - b.vtable[i] } b.PrependVOffsetT(VOffsetT(off)) } // The two metadata fields are written last. // First, store the object bytesize: objectSize := objectOffset - b.objectEnd b.PrependVOffsetT(VOffsetT(objectSize)) // Second, store the vtable bytesize: vBytes := (len(b.vtable) + VtableMetadataFields) * SizeVOffsetT b.PrependVOffsetT(VOffsetT(vBytes)) // Next, write the offset to the new vtable in the // already-allocated SOffsetT at the beginning of this object: objectStart := SOffsetT(len(b.Bytes)) - SOffsetT(objectOffset) WriteSOffsetT(b.Bytes[objectStart:], SOffsetT(b.Offset())-SOffsetT(objectOffset)) // Finally, store this vtable in memory for future // deduplication: b.vtables = append(b.vtables, b.Offset()) } else { // Found a duplicate vtable. objectStart := SOffsetT(len(b.Bytes)) - SOffsetT(objectOffset) b.head = UOffsetT(objectStart) // Write the offset to the found vtable in the // already-allocated SOffsetT at the beginning of this object: WriteSOffsetT(b.Bytes[b.head:], SOffsetT(existingVtable)-SOffsetT(objectOffset)) } b.vtable = b.vtable[:0] return objectOffset } // EndObject writes data necessary to finish object construction. func (b *Builder) EndObject() UOffsetT { b.assertNested() n := b.WriteVtable() b.nested = false return n } // Doubles the size of the byteslice, and copies the old data towards the // end of the new byteslice (since we build the buffer backwards). func (b *Builder) growByteBuffer() { if (int64(len(b.Bytes)) & int64(0xC0000000)) != 0 { panic("cannot grow buffer beyond 2 gigabytes") } newLen := len(b.Bytes) * 2 if newLen == 0 { newLen = 1 } if cap(b.Bytes) >= newLen { b.Bytes = b.Bytes[:newLen] } else { extension := make([]byte, newLen-len(b.Bytes)) b.Bytes = append(b.Bytes, extension...) } middle := newLen / 2 copy(b.Bytes[middle:], b.Bytes[:middle]) } // Head gives the start of useful data in the underlying byte buffer. // Note: unlike other functions, this value is interpreted as from the left. func (b *Builder) Head() UOffsetT { return b.head } // Offset relative to the end of the buffer. func (b *Builder) Offset() UOffsetT { return UOffsetT(len(b.Bytes)) - b.head } // Pad places zeros at the current offset. func (b *Builder) Pad(n int) { for i := 0; i < n; i++ { b.PlaceByte(0) } } // 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. func (b *Builder) Prep(size, additionalBytes int) { // Track the biggest thing we've ever aligned to. if size > b.minalign { b.minalign = size } // Find the amount of alignment needed such that `size` is properly // aligned after `additionalBytes`: alignSize := (^(len(b.Bytes) - int(b.Head()) + additionalBytes)) + 1 alignSize &= (size - 1) // Reallocate the buffer if needed: for int(b.head) <= alignSize+size+additionalBytes { oldBufSize := len(b.Bytes) b.growByteBuffer() b.head += UOffsetT(len(b.Bytes) - oldBufSize) } b.Pad(alignSize) } // PrependSOffsetT prepends an SOffsetT, relative to where it will be written. func (b *Builder) PrependSOffsetT(off SOffsetT) { b.Prep(SizeSOffsetT, 0) // Ensure alignment is already done. if !(UOffsetT(off) <= b.Offset()) { panic("unreachable: off <= b.Offset()") } off2 := SOffsetT(b.Offset()) - off + SOffsetT(SizeSOffsetT) b.PlaceSOffsetT(off2) } // PrependUOffsetT prepends an UOffsetT, relative to where it will be written. func (b *Builder) PrependUOffsetT(off UOffsetT) { b.Prep(SizeUOffsetT, 0) // Ensure alignment is already done. if !(off <= b.Offset()) { panic("unreachable: off <= b.Offset()") } off2 := b.Offset() - off + UOffsetT(SizeUOffsetT) b.PlaceUOffsetT(off2) } // StartVector initializes bookkeeping for writing a new vector. // // A vector has the following format: // // +, where T is the type of elements of this vector. func (b *Builder) StartVector(elemSize, numElems, alignment int) UOffsetT { b.assertNotNested() b.nested = true b.Prep(SizeUint32, elemSize*numElems) b.Prep(alignment, elemSize*numElems) // Just in case alignment > int. return b.Offset() } // EndVector writes data necessary to finish vector construction. func (b *Builder) EndVector(vectorNumElems int) UOffsetT { b.assertNested() // we already made space for this, so write without PrependUint32 b.PlaceUOffsetT(UOffsetT(vectorNumElems)) b.nested = false return b.Offset() } // CreateSharedString Checks if the string is already written // to the buffer before calling CreateString func (b *Builder) CreateSharedString(s string) UOffsetT { if b.sharedStrings == nil { b.sharedStrings = make(map[string]UOffsetT) } if v, ok := b.sharedStrings[s]; ok { return v } off := b.CreateString(s) b.sharedStrings[s] = off return off } // CreateString writes a null-terminated string as a vector. func (b *Builder) CreateString(s string) UOffsetT { b.assertNotNested() b.nested = true b.Prep(int(SizeUOffsetT), (len(s)+1)*SizeByte) b.PlaceByte(0) l := UOffsetT(len(s)) b.head -= l copy(b.Bytes[b.head:b.head+l], s) return b.EndVector(len(s)) } // CreateByteString writes a byte slice as a string (null-terminated). func (b *Builder) CreateByteString(s []byte) UOffsetT { b.assertNotNested() b.nested = true b.Prep(int(SizeUOffsetT), (len(s)+1)*SizeByte) b.PlaceByte(0) l := UOffsetT(len(s)) b.head -= l copy(b.Bytes[b.head:b.head+l], s) return b.EndVector(len(s)) } // CreateByteVector writes a ubyte vector func (b *Builder) CreateByteVector(v []byte) UOffsetT { b.assertNotNested() b.nested = true b.Prep(int(SizeUOffsetT), len(v)*SizeByte) l := UOffsetT(len(v)) b.head -= l copy(b.Bytes[b.head:b.head+l], v) return b.EndVector(len(v)) } func (b *Builder) assertNested() { // If you get this assert, you're in an object while trying to write // data that belongs outside of an object. // To fix this, write non-inline data (like vectors) before creating // objects. if !b.nested { panic("Incorrect creation order: must be inside object.") } } func (b *Builder) assertNotNested() { // If you hit this, you're trying to construct a Table/Vector/String // during the construction of its parent table (between the MyTableBuilder // and builder.Finish()). // Move the creation of these sub-objects to above the MyTableBuilder to // not get this assert. // Ignoring this assert may appear to work in simple cases, but the reason // it is here is that storing objects in-line may cause vtable offsets // to not fit anymore. It also leads to vtable duplication. if b.nested { panic("Incorrect creation order: object must not be nested.") } } func (b *Builder) assertFinished() { // If you get this assert, you're attempting to get access a buffer // which hasn't been finished yet. Be sure to call builder.Finish() // with your root table. // If you really need to access an unfinished buffer, use the Bytes // buffer directly. if !b.finished { panic("Incorrect use of FinishedBytes(): must call 'Finish' first.") } } // PrependBoolSlot prepends a bool 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. func (b *Builder) PrependBoolSlot(o int, x, d bool) { val := byte(0) if x { val = 1 } def := byte(0) if d { def = 1 } b.PrependByteSlot(o, val, def) } // PrependByteSlot prepends a byte 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. func (b *Builder) PrependByteSlot(o int, x, d byte) { if x != d { b.PrependByte(x) b.Slot(o) } } // PrependUint8Slot prepends a uint8 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. func (b *Builder) PrependUint8Slot(o int, x, d uint8) { if x != d { b.PrependUint8(x) b.Slot(o) } } // PrependUint16Slot prepends a uint16 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. func (b *Builder) PrependUint16Slot(o int, x, d uint16) { if x != d { b.PrependUint16(x) b.Slot(o) } } // PrependUint32Slot prepends a uint32 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. func (b *Builder) PrependUint32Slot(o int, x, d uint32) { if x != d { b.PrependUint32(x) b.Slot(o) } } // PrependUint64Slot prepends a uint64 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. func (b *Builder) PrependUint64Slot(o int, x, d uint64) { if x != d { b.PrependUint64(x) b.Slot(o) } } // PrependInt8Slot prepends a int8 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. func (b *Builder) PrependInt8Slot(o int, x, d int8) { if x != d { b.PrependInt8(x) b.Slot(o) } } // PrependInt16Slot prepends a int16 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. func (b *Builder) PrependInt16Slot(o int, x, d int16) { if x != d { b.PrependInt16(x) b.Slot(o) } } // PrependInt32Slot prepends a int32 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. func (b *Builder) PrependInt32Slot(o int, x, d int32) { if x != d { b.PrependInt32(x) b.Slot(o) } } // PrependInt64Slot prepends a int64 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. func (b *Builder) PrependInt64Slot(o int, x, d int64) { if x != d { b.PrependInt64(x) b.Slot(o) } } // PrependFloat32Slot prepends a float32 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. func (b *Builder) PrependFloat32Slot(o int, x, d float32) { if x != d { b.PrependFloat32(x) b.Slot(o) } } // PrependFloat64Slot prepends a float64 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. func (b *Builder) PrependFloat64Slot(o int, x, d float64) { if x != d { b.PrependFloat64(x) b.Slot(o) } } // PrependUOffsetTSlot 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. func (b *Builder) PrependUOffsetTSlot(o int, x, d UOffsetT) { if x != d { b.PrependUOffsetT(x) b.Slot(o) } } // 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. func (b *Builder) PrependStructSlot(voffset int, x, d UOffsetT) { if x != d { b.assertNested() if x != b.Offset() { panic("inline data write outside of object") } b.Slot(voffset) } } // Slot sets the vtable key `voffset` to the current location in the buffer. func (b *Builder) Slot(slotnum int) { b.vtable[slotnum] = UOffsetT(b.Offset()) } // FinishWithFileIdentifier finalizes a buffer, pointing to the given `rootTable`. // as well as applys a file identifier func (b *Builder) FinishWithFileIdentifier(rootTable UOffsetT, fid []byte) { if fid == nil || len(fid) != fileIdentifierLength { panic("incorrect file identifier length") } // In order to add a file identifier to the flatbuffer message, we need // to prepare an alignment and file identifier length b.Prep(b.minalign, SizeInt32+fileIdentifierLength) for i := fileIdentifierLength - 1; i >= 0; i-- { // place the file identifier b.PlaceByte(fid[i]) } // finish b.Finish(rootTable) } // Finish finalizes a buffer, pointing to the given `rootTable`. func (b *Builder) Finish(rootTable UOffsetT) { b.assertNotNested() b.Prep(b.minalign, SizeUOffsetT) b.PrependUOffsetT(rootTable) b.finished = true } // vtableEqual compares an unwritten vtable to a written vtable. func vtableEqual(a []UOffsetT, objectStart UOffsetT, b []byte) bool { if len(a)*SizeVOffsetT != len(b) { return false } for i := 0; i < len(a); i++ { x := GetVOffsetT(b[i*SizeVOffsetT : (i+1)*SizeVOffsetT]) // Skip vtable entries that indicate a default value. if x == 0 && a[i] == 0 { continue } y := SOffsetT(objectStart) - SOffsetT(a[i]) if SOffsetT(x) != y { return false } } return true } // PrependBool prepends a bool to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependBool(x bool) { b.Prep(SizeBool, 0) b.PlaceBool(x) } // PrependUint8 prepends a uint8 to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependUint8(x uint8) { b.Prep(SizeUint8, 0) b.PlaceUint8(x) } // PrependUint16 prepends a uint16 to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependUint16(x uint16) { b.Prep(SizeUint16, 0) b.PlaceUint16(x) } // PrependUint32 prepends a uint32 to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependUint32(x uint32) { b.Prep(SizeUint32, 0) b.PlaceUint32(x) } // PrependUint64 prepends a uint64 to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependUint64(x uint64) { b.Prep(SizeUint64, 0) b.PlaceUint64(x) } // PrependInt8 prepends a int8 to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependInt8(x int8) { b.Prep(SizeInt8, 0) b.PlaceInt8(x) } // PrependInt16 prepends a int16 to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependInt16(x int16) { b.Prep(SizeInt16, 0) b.PlaceInt16(x) } // PrependInt32 prepends a int32 to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependInt32(x int32) { b.Prep(SizeInt32, 0) b.PlaceInt32(x) } // PrependInt64 prepends a int64 to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependInt64(x int64) { b.Prep(SizeInt64, 0) b.PlaceInt64(x) } // PrependFloat32 prepends a float32 to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependFloat32(x float32) { b.Prep(SizeFloat32, 0) b.PlaceFloat32(x) } // PrependFloat64 prepends a float64 to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependFloat64(x float64) { b.Prep(SizeFloat64, 0) b.PlaceFloat64(x) } // PrependByte prepends a byte to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependByte(x byte) { b.Prep(SizeByte, 0) b.PlaceByte(x) } // PrependVOffsetT prepends a VOffsetT to the Builder buffer. // Aligns and checks for space. func (b *Builder) PrependVOffsetT(x VOffsetT) { b.Prep(SizeVOffsetT, 0) b.PlaceVOffsetT(x) } // PlaceBool prepends a bool to the Builder, without checking for space. func (b *Builder) PlaceBool(x bool) { b.head -= UOffsetT(SizeBool) WriteBool(b.Bytes[b.head:], x) } // PlaceUint8 prepends a uint8 to the Builder, without checking for space. func (b *Builder) PlaceUint8(x uint8) { b.head -= UOffsetT(SizeUint8) WriteUint8(b.Bytes[b.head:], x) } // PlaceUint16 prepends a uint16 to the Builder, without checking for space. func (b *Builder) PlaceUint16(x uint16) { b.head -= UOffsetT(SizeUint16) WriteUint16(b.Bytes[b.head:], x) } // PlaceUint32 prepends a uint32 to the Builder, without checking for space. func (b *Builder) PlaceUint32(x uint32) { b.head -= UOffsetT(SizeUint32) WriteUint32(b.Bytes[b.head:], x) } // PlaceUint64 prepends a uint64 to the Builder, without checking for space. func (b *Builder) PlaceUint64(x uint64) { b.head -= UOffsetT(SizeUint64) WriteUint64(b.Bytes[b.head:], x) } // PlaceInt8 prepends a int8 to the Builder, without checking for space. func (b *Builder) PlaceInt8(x int8) { b.head -= UOffsetT(SizeInt8) WriteInt8(b.Bytes[b.head:], x) } // PlaceInt16 prepends a int16 to the Builder, without checking for space. func (b *Builder) PlaceInt16(x int16) { b.head -= UOffsetT(SizeInt16) WriteInt16(b.Bytes[b.head:], x) } // PlaceInt32 prepends a int32 to the Builder, without checking for space. func (b *Builder) PlaceInt32(x int32) { b.head -= UOffsetT(SizeInt32) WriteInt32(b.Bytes[b.head:], x) } // PlaceInt64 prepends a int64 to the Builder, without checking for space. func (b *Builder) PlaceInt64(x int64) { b.head -= UOffsetT(SizeInt64) WriteInt64(b.Bytes[b.head:], x) } // PlaceFloat32 prepends a float32 to the Builder, without checking for space. func (b *Builder) PlaceFloat32(x float32) { b.head -= UOffsetT(SizeFloat32) WriteFloat32(b.Bytes[b.head:], x) } // PlaceFloat64 prepends a float64 to the Builder, without checking for space. func (b *Builder) PlaceFloat64(x float64) { b.head -= UOffsetT(SizeFloat64) WriteFloat64(b.Bytes[b.head:], x) } // PlaceByte prepends a byte to the Builder, without checking for space. func (b *Builder) PlaceByte(x byte) { b.head -= UOffsetT(SizeByte) WriteByte(b.Bytes[b.head:], x) } // PlaceVOffsetT prepends a VOffsetT to the Builder, without checking for space. func (b *Builder) PlaceVOffsetT(x VOffsetT) { b.head -= UOffsetT(SizeVOffsetT) WriteVOffsetT(b.Bytes[b.head:], x) } // PlaceSOffsetT prepends a SOffsetT to the Builder, without checking for space. func (b *Builder) PlaceSOffsetT(x SOffsetT) { b.head -= UOffsetT(SizeSOffsetT) WriteSOffsetT(b.Bytes[b.head:], x) } // PlaceUOffsetT prepends a UOffsetT to the Builder, without checking for space. func (b *Builder) PlaceUOffsetT(x UOffsetT) { b.head -= UOffsetT(SizeUOffsetT) WriteUOffsetT(b.Bytes[b.head:], x) }