go/src/pkg/gob/encode.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package gob

import (
	"gob";
	"io";
	"math";
	"os";
	"reflect";
	"sync";
	"unsafe";
)

// Step through the indirections on a type to discover the base type.
// Return the number of indirections.
func indirect(t reflect.Type) (rt reflect.Type, count int) {
	rt = t;
	for {
		pt, ok := rt.(reflect.PtrType);
		if !ok {
			break
		}
		rt = pt.Sub();
		count++;
	}
	return;
}

// The global execution state of an instance of the encoder.
// Field numbers are delta encoded and always increase. The field
// number is initialized to -1 so 0 comes out as delta(1). A delta of
// 0 terminates the structure.
type EncState struct {
	w	io.Writer;
	err	os.Error;	// error encountered during encoding;
	fieldnum	int;	// the last field number written.
	buf [16]byte;	// buffer used by the encoder; here to avoid allocation.
}

// Integers encode as a variant of Google's protocol buffer varint (varvarint?).
// The variant is that the continuation bytes have a zero top bit instead of a one.
// That way there's only one bit to clear and the value is a little easier to see if
// you're the unfortunate sort of person who must read the hex to debug.

// EncodeUint writes an encoded unsigned integer to state.w.  Sets state.err.
// If state.err is already non-nil, it does nothing.
func EncodeUint(state *EncState, x uint64) {
	var n int;
	if state.err != nil {
		return
	}
	for n = 0; x > 127; n++ {
		state.buf[n] = uint8(x & 0x7F);
		x >>= 7;
	}
	state.buf[n] = 0x80 | uint8(x);
	var nn int;
	nn, state.err = state.w.Write(state.buf[0:n+1]);
}

// EncodeInt writes an encoded signed integer to state.w.
// The low bit of the encoding says whether to bit complement the (other bits of the) uint to recover the int.
// Sets state.err. If state.err is already non-nil, it does nothing.
func EncodeInt(state *EncState, i int64){
	var x uint64;
	if i < 0 {
		x = uint64(^i << 1) | 1
	} else {
		x = uint64(i << 1)
	}
	EncodeUint(state, uint64(x))
}

type encInstr struct
type encOp func(i *encInstr, state *EncState, p unsafe.Pointer)

// The 'instructions' of the encoding machine
type encInstr struct {
	op	encOp;
	field		int;	// field number
	indir	int;	// how many pointer indirections to reach the value in the struct
	offset	uintptr;	// offset in the structure of the field to encode
}

// Emit a field number and update the state to record its value for delta encoding.
// If the instruction pointer is nil, do nothing
func (state *EncState) update(instr *encInstr) {
	if instr != nil {
		EncodeUint(state, uint64(instr.field - state.fieldnum));
		state.fieldnum = instr.field;
	}
}

// Each encoder is responsible for handling any indirections associated
// with the data structure.  If any pointer so reached is nil, no bytes are written.
// If the data item is zero, no bytes are written.
// Otherwise, the output (for a scalar) is the field number, as an encoded integer,
// followed by the field data in its appropriate format.

func encIndirect(p unsafe.Pointer, indir int) unsafe.Pointer {
	for ; indir > 0; indir-- {
		p = *(*unsafe.Pointer)(p);
		if p == nil {
			return unsafe.Pointer(nil)
		}
	}
	return p
}

func encBool(i *encInstr, state *EncState, p unsafe.Pointer) {
	b := *(*bool)(p);
	if b {
		state.update(i);
		EncodeUint(state, 1);
	}
}

func encInt(i *encInstr, state *EncState, p unsafe.Pointer) {
	v := int64(*(*int)(p));
	if v != 0 {
		state.update(i);
		EncodeInt(state, v);
	}
}

func encUint(i *encInstr, state *EncState, p unsafe.Pointer) {
	v := uint64(*(*uint)(p));
	if v != 0 {
		state.update(i);
		EncodeUint(state, v);
	}
}

func encInt8(i *encInstr, state *EncState, p unsafe.Pointer) {
	v := int64(*(*int8)(p));
	if v != 0 {
		state.update(i);
		EncodeInt(state, v);
	}
}

func encUint8(i *encInstr, state *EncState, p unsafe.Pointer) {
	v := uint64(*(*uint8)(p));
	if v != 0 {
		state.update(i);
		EncodeUint(state, v);
	}
}

func encInt16(i *encInstr, state *EncState, p unsafe.Pointer) {
	v := int64(*(*int16)(p));
	if v != 0 {
		state.update(i);
		EncodeInt(state, v);
	}
}

func encUint16(i *encInstr, state *EncState, p unsafe.Pointer) {
	v := uint64(*(*uint16)(p));
	if v != 0 {
		state.update(i);
		EncodeUint(state, v);
	}
}

func encInt32(i *encInstr, state *EncState, p unsafe.Pointer) {
	v := int64(*(*int32)(p));
	if v != 0 {
		state.update(i);
		EncodeInt(state, v);
	}
}

func encUint32(i *encInstr, state *EncState, p unsafe.Pointer) {
	v := uint64(*(*uint32)(p));
	if v != 0 {
		state.update(i);
		EncodeUint(state, v);
	}
}

func encInt64(i *encInstr, state *EncState, p unsafe.Pointer) {
	v := *(*int64)(p);
	if v != 0 {
		state.update(i);
		EncodeInt(state, v);
	}
}

func encUint64(i *encInstr, state *EncState, p unsafe.Pointer) {
	v := *(*uint64)(p);
	if v != 0 {
		state.update(i);
		EncodeUint(state, v);
	}
}

// Floating-point numbers are transmitted as uint64s holding the bits
// of the underlying representation.  They are sent byte-reversed, with
// the exponent end coming out first, so integer floating point numbers
// (for example) transmit more compactly.  This routine does the
// swizzling.
func floatBits(f float64) uint64 {
	u := math.Float64bits(f);
	var v uint64;
	for i := 0; i < 8; i++ {
		v <<= 8;
		v |= u & 0xFF;
		u >>= 8;
	}
	return v;
}

func encFloat(i *encInstr, state *EncState, p unsafe.Pointer) {
	f := float(*(*float)(p));
	if f != 0 {
		v := floatBits(float64(f));
		state.update(i);
		EncodeUint(state, v);
	}
}

func encFloat32(i *encInstr, state *EncState, p unsafe.Pointer) {
	f := float32(*(*float32)(p));
	if f != 0 {
		v := floatBits(float64(f));
		state.update(i);
		EncodeUint(state, v);
	}
}

func encFloat64(i *encInstr, state *EncState, p unsafe.Pointer) {
	f := *(*float64)(p);
	if f != 0 {
		state.update(i);
		v := floatBits(f);
		EncodeUint(state, v);
	}
}

// Byte arrays are encoded as an unsigned count followed by the raw bytes.
func encUint8Array(i *encInstr, state *EncState, p unsafe.Pointer) {
	b := *(*[]byte)(p);
	if len(b) > 0 {
		state.update(i);
		EncodeUint(state, uint64(len(b)));
		state.w.Write(b);
	}
}

// Strings are encoded as an unsigned count followed by the raw bytes.
func encString(i *encInstr, state *EncState, p unsafe.Pointer) {
	s := *(*string)(p);
	if len(s) > 0 {
		state.update(i);
		EncodeUint(state, uint64(len(s)));
		io.WriteString(state.w, s);
	}
}

// The end of a struct is marked by a delta field number of 0.
func encStructTerminator(i *encInstr, state *EncState, p unsafe.Pointer) {
	EncodeUint(state, 0);
}

// Execution engine

// The encoder engine is an array of instructions indexed by field number of the encoding
// data, typically a struct.  It is executed top to bottom, walking the struct.
type encEngine struct {
	instr	[]encInstr
}

func encodeStruct(engine *encEngine, w io.Writer, basep uintptr) os.Error {
	state := new(EncState);
	state.w = w;
	state.fieldnum = -1;
	for i := 0; i < len(engine.instr); i++ {
		instr := &engine.instr[i];
		p := unsafe.Pointer(basep+instr.offset);
		if instr.indir > 0 {
			if p = encIndirect(p, instr.indir); p == nil {
				state.fieldnum = i;
				continue
			}
		}
		instr.op(instr, state, p);
		if state.err != nil {
			break
		}
	}
	return state.err
}

func encodeArray(w io.Writer, p uintptr, op encOp, elemWid int, length int, elemIndir int) os.Error {
	state := new(EncState);
	state.w = w;
	state.fieldnum = -1;
	EncodeUint(state, uint64(length));
	for i := 0; i < length && state.err == nil; i++ {
		up := unsafe.Pointer(p);
		if elemIndir > 0 {
			if up = encIndirect(up, elemIndir); up == nil {
				state.err = os.ErrorString("encodeArray: nil element");
				break
			}
			p = uintptr(up);
		}
		op(nil, state, unsafe.Pointer(p));
		p += uintptr(elemWid);
	}
	return state.err
}

var encEngineMap = make(map[reflect.Type] *encEngine)
var encOpMap = map[int] encOp {
	 reflect.BoolKind: encBool,
	 reflect.IntKind: encInt,
	 reflect.Int8Kind: encInt8,
	 reflect.Int16Kind: encInt16,
	 reflect.Int32Kind: encInt32,
	 reflect.Int64Kind: encInt64,
	 reflect.UintKind: encUint,
	 reflect.Uint8Kind: encUint8,
	 reflect.Uint16Kind: encUint16,
	 reflect.Uint32Kind: encUint32,
	 reflect.Uint64Kind: encUint64,
	 reflect.FloatKind: encFloat,
	 reflect.Float32Kind: encFloat32,
	 reflect.Float64Kind: encFloat64,
	 reflect.StringKind: encString,
}

func getEncEngine(rt reflect.Type) *encEngine

func encOpFor(typ reflect.Type) encOp {
	op, ok := encOpMap[typ.Kind()];
	if !ok {
		// Special cases
		if typ.Kind() == reflect.ArrayKind {
			atyp := typ.(reflect.ArrayType);
			switch {
			case atyp.Elem().Kind()  == reflect.Uint8Kind:
				op = encUint8Array
			case atyp.IsSlice():
				// Slices have a header; we decode it to find the underlying array.
				elemOp := encOpFor(atyp.Elem());
				_, indir := indirect(atyp.Elem());
				op = func(i *encInstr, state *EncState, p unsafe.Pointer) {
					slice := *(*reflect.SliceHeader)(p);
					if slice.Len == 0 {
						return
					}
					state.update(i);
					state.err = encodeArray(state.w, slice.Data, elemOp, atyp.Elem().Size(), int(slice.Len), indir);
				};
			case !atyp.IsSlice():
				// True arrays have size in the type.
				elemOp := encOpFor(atyp.Elem());
				_, indir := indirect(atyp.Elem());
				op = func(i *encInstr, state *EncState, p unsafe.Pointer) {
					state.update(i);
					state.err = encodeArray(state.w, uintptr(p), elemOp, atyp.Elem().Size(), atyp.Len(), indir);
				};
			}
		}
		if typ.Kind() == reflect.StructKind {
			// Generate a closure that calls out to the engine for the nested type.
			engine := getEncEngine(typ);
			op = func(i *encInstr, state *EncState, p unsafe.Pointer) {
				state.update(i);
				state.err = encodeStruct(engine, state.w, uintptr(p));
			};
		}
	}
	if op == nil {
		panicln("encode can't handle type", typ.String());
	}
	return op
}

// The local Type was compiled from the actual value, so we know
// it's compatible.
// TODO(r): worth checking?  typ is unused here.
func compileEnc(rt reflect.Type, typ Type) *encEngine {
	srt, ok := rt.(reflect.StructType);
	if !ok {
		panicln("TODO: can't handle non-structs");
	}
	engine := new(encEngine);
	engine.instr = make([]encInstr, srt.Len()+1);	// +1 for terminator
	for fieldnum := 0; fieldnum < srt.Len(); fieldnum++ {
		_name, ftyp, _tag, offset := srt.Field(fieldnum);
		// How many indirections to the underlying data?
		indir := 0;
		for {
			pt, ok := ftyp.(reflect.PtrType);
			if !ok {
				break
			}
			ftyp = pt.Sub();
			indir++;
		}
		op := encOpFor(ftyp);
		engine.instr[fieldnum] = encInstr{op, fieldnum, indir, uintptr(offset)};
	}
	engine.instr[srt.Len()] = encInstr{encStructTerminator, 0, 0, 0};
	return engine;
}

// typeLock must be held.
func getEncEngine(rt reflect.Type) *encEngine {
	engine, ok := encEngineMap[rt];
	if !ok {
		engine = compileEnc(rt, newType(rt.Name(), rt));
		encEngineMap[rt] = engine;
	}
	return engine
}

func Encode(w io.Writer, e interface{}) os.Error {
	// Dereference down to the underlying object.
	rt, indir := indirect(reflect.Typeof(e));
	v := reflect.NewValue(e);
	for i := 0; i < indir; i++ {
		v = reflect.Indirect(v);
	}
	if v.Kind() != reflect.StructKind {
		return os.ErrorString("encode can't handle " + v.Type().String())
	}
	typeLock.Lock();
	engine := getEncEngine(rt);
	typeLock.Unlock();
	return encodeStruct(engine, w, uintptr(v.(reflect.StructValue).Addr()));
}