go/src/pkg/gob/decode.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";
	"unsafe";
)

// The global execution state of an instance of the decoder.
type DecState struct {
	r	io.Reader;
	err	os.Error;
	base	uintptr;
	fieldnum	int;	// the last field number read.
	buf [1]byte;	// buffer used by the decoder; here to avoid allocation.
}

// DecodeUint reads an encoded unsigned integer from state.r.
// Sets state.err.  If state.err is already non-nil, it does nothing.
func DecodeUint(state *DecState) (x uint64) {
	if state.err != nil {
		return
	}
	for shift := uint(0);; shift += 7 {
		var n int;
		n, state.err = state.r.Read(&state.buf);
		if n != 1 {
			return 0
		}
		b := uint64(state.buf[0]);
		x |= b << shift;
		if b&0x80 != 0 {
			x &^= 0x80 << shift;
			break
		}
	}
	return x;
}

// DecodeInt reads an encoded signed integer from state.r.
// Sets state.err.  If state.err is already non-nil, it does nothing.
func DecodeInt(state *DecState) int64 {
	x := DecodeUint(state);
	if state.err != nil {
		return 0
	}
	if x & 1 != 0 {
		return ^int64(x>>1)
	}
	return int64(x >> 1)
}

// The 'instructions' of the decoding machine
type decInstr struct {
	op	func(i *decInstr, state *DecState);
	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
}

// Since the encoder writes no zeros, if we arrive at a decoder we have
// a value to extract and store.  The field number has already been read
// (it's how we knew to call this decoder).
// Each decoder is responsible for handling any indirections associated
// with the data structure.  If any pointer so reached is nil, allocation must
// be done.

// Walk the pointer hierarchy, allocating if we find a nil.  Stop one before the end.
func decIndirect(p unsafe.Pointer, indir int) unsafe.Pointer {
	for ; indir > 1; indir-- {
		if *(*unsafe.Pointer)(p) == nil {
			// Allocation required
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(unsafe.Pointer));
		}
		p = *(*unsafe.Pointer)(p);
	}
	return p
}

func decBool(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := int(DecodeInt(state));
	if state.err == nil {
		*(*bool)(p) = v != 0;
	}
}

func decInt(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := int(DecodeInt(state));
	if state.err == nil {
		*(*int)(p) = v;
	}
}

func decUint(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := uint(DecodeUint(state));
	if state.err == nil {
		*(*uint)(p) = v;
	}
}

func decInt8(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := int8(DecodeInt(state));
	if state.err == nil {
		*(*int8)(p) = v;
	}
}

func decUint8(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := uint8(DecodeUint(state));
	if state.err == nil {
		*(*uint8)(p) = v;
	}
}

func decInt16(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := int16(DecodeInt(state));
	if state.err == nil {
		*(*int16)(p) = v;
	}
}

func decUint16(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := uint16(DecodeUint(state));
	if state.err == nil {
		*(*uint16)(p) = v;
	}
}

func decInt32(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := int32(DecodeInt(state));
	if state.err == nil {
		*(*int32)(p) = v;
	}
}

func decUint32(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := uint32(DecodeUint(state));
	if state.err == nil {
		*(*uint32)(p) = v;
	}
}

func decInt64(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := int64(DecodeInt(state));
	if state.err == nil {
		*(*int64)(p) = v;
	}
}

func decUint64(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := uint64(DecodeUint(state));
	if state.err == nil {
		*(*uint64)(p) = 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
// unswizzling.
func floatFromBits(u uint64) float64 {
	var v uint64;
	for i := 0; i < 8; i++ {
		v <<= 8;
		v |= u & 0xFF;
		u >>= 8;
	}
	return math.Float64frombits(v);
}

func decFloat(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := float(floatFromBits(uint64(DecodeUint(state))));
	if state.err == nil {
		*(*float)(p) = v;
	}
}

func decFloat32(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := float32(floatFromBits(uint64(DecodeUint(state))));
	if state.err == nil {
		*(*float32)(p) = v;
	}
}

func decFloat64(i *decInstr, state *DecState) {
	p := unsafe.Pointer(state.base+i.offset);
	if i.indir > 0 {
		if i.indir > 1 {
			p = decIndirect(p, i.indir);
		}
		if *(*unsafe.Pointer)(p) == nil {
			*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int));
			p = *(*unsafe.Pointer)(p);
		}
	}
	v := floatFromBits(uint64(DecodeUint(state)));
	if state.err == nil {
		*(*float64)(p) = v;
	}
}

// Execution engine

// The encoder engine is an array of instructions indexed by field number of the incoming
// data.  It is executed with random access according to field number.
type decEngine struct {
	instr	[]decInstr
}

var decEngineMap = make(map[reflect.Type] *decEngine)
var decOp = map[int] func(*decInstr, *DecState) {
	 reflect.BoolKind: decBool,
	 reflect.IntKind: decInt,
	 reflect.Int8Kind: decInt8,
	 reflect.Int16Kind: decInt16,
	 reflect.Int32Kind: decInt32,
	 reflect.Int64Kind: decInt64,
	 reflect.UintKind: decUint,
	 reflect.Uint8Kind: decUint8,
	 reflect.Uint16Kind: decUint16,
	 reflect.Uint32Kind: decUint32,
	 reflect.Uint64Kind: decUint64,
	 reflect.FloatKind: decFloat,
	 reflect.Float32Kind: decFloat32,
	 reflect.Float64Kind: decFloat64,
}

func compileDec(rt reflect.Type, typ Type) *decEngine {
	srt, ok1 := rt.(reflect.StructType);
	styp, ok2 := typ.(*structType);
	if !ok1 || !ok2 {
		panicln("TODO: can't handle non-structs");
	}
	engine := new(decEngine);
	engine.instr = make([]decInstr, len(styp.field));
	for fieldnum := 0; fieldnum < len(styp.field); fieldnum++ {
		field := styp.field[fieldnum];
		// TODO(r): verify compatibility with corresponding field of data.
		// For now, assume perfect correspondence between struct and gob.
		_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, ok := decOp[ftyp.Kind()];
		if !ok {
			panicln("can't handle decode for type", ftyp.String());
		}
		engine.instr[fieldnum] = decInstr{op, fieldnum, indir, uintptr(offset)};
	}
	return engine;
}


func getDecEngine(rt reflect.Type) *decEngine {
	engine, ok := decEngineMap[rt];
	if !ok {
		return compileDec(rt, newType(rt.Name(), rt));
		decEngineMap[rt] = engine;
	}
	return engine;
}

func (engine *decEngine) decode(r io.Reader, v reflect.Value) os.Error {
	sv, ok := v.(reflect.StructValue);
	if !ok {
		panicln("decoder can't handle non-struct values yet");
	}
	state := new(DecState);
	state.r = r;
	state.base = uintptr(sv.Addr());
	state.fieldnum = -1;
	for state.err == nil {
		delta := int(DecodeUint(state));
		if state.err != nil || delta == 0 {	// struct terminator is zero delta fieldnum
			break
		}
		fieldnum := state.fieldnum + delta;
		if fieldnum >= len(engine.instr) {
			panicln("TODO(r): need to handle unknown data");
		}
		instr := &engine.instr[fieldnum];
		instr.op(instr, state);
		state.fieldnum = fieldnum;
	}
	return state.err
}

func Decode(r io.Reader, e interface{}) os.Error {
	// Dereference down to the underlying object.
	rt := reflect.Typeof(e);
	v := reflect.NewValue(e);
	for {
		pt, ok := rt.(reflect.PtrType);
		if !ok {
			break
		}
		rt = pt.Sub();
		v = reflect.Indirect(v);
	}
	typeLock.Lock();
	engine := getDecEngine(rt);
	typeLock.Unlock();
	return engine.decode(r, v);
}