src/pkg/reflect/value.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 reflect

import (
	"math"
	"runtime"
	"strconv"
	"unsafe"
)

const ptrSize = unsafe.Sizeof((*byte)(nil))
const cannotSet = "cannot set value obtained from unexported struct field"

// TODO: This will have to go away when
// the new gc goes in.
func memmove(adst, asrc unsafe.Pointer, n uintptr) {
	dst := uintptr(adst)
	src := uintptr(asrc)
	switch {
	case src < dst && src+n > dst:
		// byte copy backward
		// careful: i is unsigned
		for i := n; i > 0; {
			i--
			*(*byte)(unsafe.Pointer(dst + i)) = *(*byte)(unsafe.Pointer(src + i))
		}
	case (n|src|dst)&(ptrSize-1) != 0:
		// byte copy forward
		for i := uintptr(0); i < n; i++ {
			*(*byte)(unsafe.Pointer(dst + i)) = *(*byte)(unsafe.Pointer(src + i))
		}
	default:
		// word copy forward
		for i := uintptr(0); i < n; i += ptrSize {
			*(*uintptr)(unsafe.Pointer(dst + i)) = *(*uintptr)(unsafe.Pointer(src + i))
		}
	}
}

// Value is the reflection interface to a Go value.
//
// Not all methods apply to all kinds of values.  Restrictions,
// if any, are noted in the documentation for each method.
// Use the Kind method to find out the kind of value before
// calling kind-specific methods.  Calling a method
// inappropriate to the kind of type causes a run time panic.
//
// The zero Value represents no value.
// Its IsValid method returns false, its Kind method returns Invalid,
// its String method returns "<invalid Value>", and all other methods panic.
// Most functions and methods never return an invalid value.
// If one does, its documentation states the conditions explicitly.
//
// The fields of Value are exported so that clients can copy and
// pass Values around, but they should not be edited or inspected
// directly.  A future language change may make it possible not to
// export these fields while still keeping Values usable as values.
type Value struct {
	Internal       interface{}
	InternalMethod int
}

// A ValueError occurs when a Value method is invoked on
// a Value that does not support it.  Such cases are documented
// in the description of each method.
type ValueError struct {
	Method string
	Kind   Kind
}

func (e *ValueError) String() string {
	if e.Kind == 0 {
		return "reflect: call of " + e.Method + " on zero Value"
	}
	return "reflect: call of " + e.Method + " on " + e.Kind.String() + " Value"
}

// methodName returns the name of the calling method,
// assumed to be two stack frames above.
func methodName() string {
	pc, _, _, _ := runtime.Caller(2)
	f := runtime.FuncForPC(pc)
	if f == nil {
		return "unknown method"
	}
	return f.Name()
}

// An iword is the word that would be stored in an
// interface to represent a given value v.  Specifically, if v is
// bigger than a pointer, its word is a pointer to v's data.
// Otherwise, its word is a zero uintptr with the data stored
// in the leading bytes.
type iword uintptr

func loadIword(p unsafe.Pointer, size uintptr) iword {
	// Run the copy ourselves instead of calling memmove
	// to avoid moving v to the heap.
	w := iword(0)
	switch size {
	default:
		panic("reflect: internal error: loadIword of " + strconv.Itoa(int(size)) + "-byte value")
	case 0:
	case 1:
		*(*uint8)(unsafe.Pointer(&w)) = *(*uint8)(p)
	case 2:
		*(*uint16)(unsafe.Pointer(&w)) = *(*uint16)(p)
	case 3:
		*(*[3]byte)(unsafe.Pointer(&w)) = *(*[3]byte)(p)
	case 4:
		*(*uint32)(unsafe.Pointer(&w)) = *(*uint32)(p)
	case 5:
		*(*[5]byte)(unsafe.Pointer(&w)) = *(*[5]byte)(p)
	case 6:
		*(*[6]byte)(unsafe.Pointer(&w)) = *(*[6]byte)(p)
	case 7:
		*(*[7]byte)(unsafe.Pointer(&w)) = *(*[7]byte)(p)
	case 8:
		*(*uint64)(unsafe.Pointer(&w)) = *(*uint64)(p)
	}
	return w
}

func storeIword(p unsafe.Pointer, w iword, size uintptr) {
	// Run the copy ourselves instead of calling memmove
	// to avoid moving v to the heap.
	switch size {
	default:
		panic("reflect: internal error: storeIword of " + strconv.Itoa(int(size)) + "-byte value")
	case 0:
	case 1:
		*(*uint8)(p) = *(*uint8)(unsafe.Pointer(&w))
	case 2:
		*(*uint16)(p) = *(*uint16)(unsafe.Pointer(&w))
	case 3:
		*(*[3]byte)(p) = *(*[3]byte)(unsafe.Pointer(&w))
	case 4:
		*(*uint32)(p) = *(*uint32)(unsafe.Pointer(&w))
	case 5:
		*(*[5]byte)(p) = *(*[5]byte)(unsafe.Pointer(&w))
	case 6:
		*(*[6]byte)(p) = *(*[6]byte)(unsafe.Pointer(&w))
	case 7:
		*(*[7]byte)(p) = *(*[7]byte)(unsafe.Pointer(&w))
	case 8:
		*(*uint64)(p) = *(*uint64)(unsafe.Pointer(&w))
	}
}

// emptyInterface is the header for an interface{} value.
type emptyInterface struct {
	typ  *runtime.Type
	word iword
}

// nonEmptyInterface is the header for a interface value with methods.
type nonEmptyInterface struct {
	// see ../runtime/iface.c:/Itab
	itab *struct {
		ityp   *runtime.Type // static interface type
		typ    *runtime.Type // dynamic concrete type
		link   unsafe.Pointer
		bad    int32
		unused int32
		fun    [100000]unsafe.Pointer // method table
	}
	word iword
}

// Regarding the implementation of Value:
//
// The Internal interface is a true interface value in the Go sense,
// but it also serves as a (type, address) pair in which one cannot
// be changed separately from the other.  That is, it serves as a way
// to prevent unsafe mutations of the Internal state even though
// we cannot (yet?) hide the field while preserving the ability for
// clients to make copies of Values.
//
// The internal method converts a Value into the expanded internalValue struct.
// If we could avoid exporting fields we'd probably make internalValue the
// definition of Value.
//
// If a Value is addressable (CanAddr returns true), then the Internal
// interface value holds a pointer to the actual field data, and Set stores
// through that pointer.  If a Value is not addressable (CanAddr returns false),
// then the Internal interface value holds the actual value.
//
// In addition to whether a value is addressable, we track whether it was
// obtained by using an unexported struct field.  Such values are allowed
// to be read, mainly to make fmt.Print more useful, but they are not
// allowed to be written.  We call such values read-only.
//
// A Value can be set (via the Set, SetUint, etc. methods) only if it is both
// addressable and not read-only.
//
// The two permission bits - addressable and read-only - are stored in
// the bottom two bits of the type pointer in the interface value.
//
//	ordinary value: Internal = value
//	addressable value: Internal = value, Internal.typ |= flagAddr
//	read-only value: Internal = value, Internal.typ |= flagRO
//	addressable, read-only value: Internal = value, Internal.typ |= flagAddr | flagRO
//
// It is important that the read-only values have the extra bit set
// (as opposed to using the bit to mean writable), because client code
// can grab the interface field and try to use it.  Having the extra bit
// set makes the type pointer compare not equal to any real type,
// so that a client cannot, say, write through v.Internal.(*int).
// The runtime routines that access interface types reject types with
// low bits set.
//
// If a Value fv = v.Method(i), then fv = v with the InternalMethod
// field set to i+1.  Methods are never addressable.
//
// All in all, this is a lot of effort just to avoid making this new API
// depend on a language change we'll probably do anyway, but
// it's helpful to keep the two separate, and much of the logic is
// necessary to implement the Interface method anyway.

const (
	flagAddr uint32 = 1 << iota // holds address of value
	flagRO                      // read-only

	reflectFlags = 3
)

// An internalValue is the unpacked form of a Value.
// The zero Value unpacks to a zero internalValue
type internalValue struct {
	typ       *commonType // type of value
	kind      Kind        // kind of value
	flag      uint32
	word      iword
	addr      unsafe.Pointer
	rcvr      iword
	method    bool
	nilmethod bool
}

func (v Value) internal() internalValue {
	var iv internalValue
	eface := *(*emptyInterface)(unsafe.Pointer(&v.Internal))
	p := uintptr(unsafe.Pointer(eface.typ))
	iv.typ = toCommonType((*runtime.Type)(unsafe.Pointer(p &^ reflectFlags)))
	if iv.typ == nil {
		return iv
	}
	iv.flag = uint32(p & reflectFlags)
	iv.word = eface.word
	if iv.flag&flagAddr != 0 {
		iv.addr = unsafe.Pointer(iv.word)
		iv.typ = iv.typ.Elem().common()
		if iv.typ.size <= ptrSize {
			iv.word = loadIword(iv.addr, iv.typ.size)
		}
	} else {
		if iv.typ.size > ptrSize {
			iv.addr = unsafe.Pointer(iv.word)
		}
	}
	iv.kind = iv.typ.Kind()

	// Is this a method?  If so, iv describes the receiver.
	// Rewrite to describe the method function.
	if v.InternalMethod != 0 {
		// If this Value is a method value (x.Method(i) for some Value x)
		// then we will invoke it using the interface form of the method,
		// which always passes the receiver as a single word.
		// Record that information.
		i := v.InternalMethod - 1
		if iv.kind == Interface {
			it := (*interfaceType)(unsafe.Pointer(iv.typ))
			if i < 0 || i >= len(it.methods) {
				panic("reflect: broken Value")
			}
			m := &it.methods[i]
			if m.pkgPath != nil {
				iv.flag |= flagRO
			}
			iv.typ = toCommonType(m.typ)
			iface := (*nonEmptyInterface)(iv.addr)
			if iface.itab == nil {
				iv.word = 0
				iv.nilmethod = true
			} else {
				iv.word = iword(iface.itab.fun[i])
			}
			iv.rcvr = iface.word
		} else {
			ut := iv.typ.uncommon()
			if ut == nil || i < 0 || i >= len(ut.methods) {
				panic("reflect: broken Value")
			}
			m := &ut.methods[i]
			if m.pkgPath != nil {
				iv.flag |= flagRO
			}
			iv.typ = toCommonType(m.mtyp)
			iv.rcvr = iv.word
			iv.word = iword(m.ifn)
		}
		iv.kind = Func
		iv.method = true
		iv.flag &^= flagAddr
		iv.addr = nil
	}

	return iv
}

// packValue returns a Value with the given flag bits, type, and interface word.
func packValue(flag uint32, typ *runtime.Type, word iword) Value {
	if typ == nil {
		panic("packValue")
	}
	t := uintptr(unsafe.Pointer(typ))
	t |= uintptr(flag)
	eface := emptyInterface{(*runtime.Type)(unsafe.Pointer(t)), word}
	return Value{Internal: *(*interface{})(unsafe.Pointer(&eface))}
}

var dummy struct {
	b bool
	x interface{}
}

// Dummy annotation marking that the value x escapes,
// for use in cases where the reflect code is so clever that
// the compiler cannot follow.
func escapes(x interface{}) {
	if dummy.b {
		dummy.x = x
	}
}

// valueFromAddr returns a Value using the given type and address.
func valueFromAddr(flag uint32, typ Type, addr unsafe.Pointer) Value {
	// TODO(rsc): Eliminate this terrible hack.
	// The escape analysis knows that addr is a pointer
	// but it doesn't see addr get passed to anything
	// that keeps it.  packValue keeps it, but packValue
	// takes a uintptr (iword(addr)), and integers (non-pointers)
	// are assumed not to matter.  The escapes function works
	// because return values always escape (for now).
	escapes(addr)

	if flag&flagAddr != 0 {
		// Addressable, so the internal value is
		// an interface containing a pointer to the real value.
		return packValue(flag, PtrTo(typ).runtimeType(), iword(addr))
	}

	var w iword
	if n := typ.Size(); n <= ptrSize {
		// In line, so the interface word is the actual value.
		w = loadIword(addr, n)
	} else {
		// Not in line: the interface word is the address.
		w = iword(addr)
	}
	return packValue(flag, typ.runtimeType(), w)
}

// valueFromIword returns a Value using the given type and interface word.
func valueFromIword(flag uint32, typ Type, w iword) Value {
	if flag&flagAddr != 0 {
		panic("reflect: internal error: valueFromIword addressable")
	}
	return packValue(flag, typ.runtimeType(), w)
}

func (iv internalValue) mustBe(want Kind) {
	if iv.kind != want {
		panic(&ValueError{methodName(), iv.kind})
	}
}

func (iv internalValue) mustBeExported() {
	if iv.kind == 0 {
		panic(&ValueError{methodName(), iv.kind})
	}
	if iv.flag&flagRO != 0 {
		panic(methodName() + " using value obtained using unexported field")
	}
}

func (iv internalValue) mustBeAssignable() {
	if iv.kind == 0 {
		panic(&ValueError{methodName(), iv.kind})
	}
	// Assignable if addressable and not read-only.
	if iv.flag&flagRO != 0 {
		panic(methodName() + " using value obtained using unexported field")
	}
	if iv.flag&flagAddr == 0 {
		panic(methodName() + " using unaddressable value")
	}
}

// Addr returns a pointer value representing the address of v.
// It panics if CanAddr() returns false.
// Addr is typically used to obtain a pointer to a struct field
// or slice element in order to call a method that requires a
// pointer receiver.
func (v Value) Addr() Value {
	iv := v.internal()
	if iv.flag&flagAddr == 0 {
		panic("reflect.Value.Addr of unaddressable value")
	}
	return valueFromIword(iv.flag&flagRO, PtrTo(iv.typ.toType()), iword(iv.addr))
}

// Bool returns v's underlying value.
// It panics if v's kind is not Bool.
func (v Value) Bool() bool {
	iv := v.internal()
	iv.mustBe(Bool)
	return *(*bool)(unsafe.Pointer(&iv.word))
}

// Bytes returns v's underlying value.
// It panics if v's underlying value is not a slice of bytes.
func (v Value) Bytes() []byte {
	iv := v.internal()
	iv.mustBe(Slice)
	typ := iv.typ.toType()
	if typ.Elem().Kind() != Uint8 {
		panic("reflect.Value.Bytes of non-byte slice")
	}
	return *(*[]byte)(iv.addr)
}

// CanAddr returns true if the value's address can be obtained with Addr.
// Such values are called addressable.  A value is addressable if it is
// an element of a slice, an element of an addressable array,
// a field of an addressable struct, or the result of dereferencing a pointer.
// If CanAddr returns false, calling Addr will panic.
func (v Value) CanAddr() bool {
	iv := v.internal()
	return iv.flag&flagAddr != 0
}

// CanSet returns true if the value of v can be changed.
// A Value can be changed only if it is addressable and was not
// obtained by the use of unexported struct fields.
// If CanSet returns false, calling Set or any type-specific
// setter (e.g., SetBool, SetInt64) will panic.
func (v Value) CanSet() bool {
	iv := v.internal()
	return iv.flag&(flagAddr|flagRO) == flagAddr
}

// Call calls the function v with the input arguments in.
// For example, if len(in) == 3, v.Call(in) represents the Go call v(in[0], in[1], in[2]).
// Call panics if v's Kind is not Func.
// It returns the output results as Values.
// As in Go, each input argument must be assignable to the
// type of the function's corresponding input parameter.
// If v is a variadic function, Call creates the variadic slice parameter
// itself, copying in the corresponding values.
func (v Value) Call(in []Value) []Value {
	iv := v.internal()
	iv.mustBe(Func)
	iv.mustBeExported()
	return iv.call("Call", in)
}

// CallSlice calls the variadic function v with the input arguments in,
// assigning the slice in[len(in)-1] to v's final variadic argument.  
// For example, if len(in) == 3, v.Call(in) represents the Go call v(in[0], in[1], in[2]...).
// Call panics if v's Kind is not Func or if v is not variadic.
// It returns the output results as Values.
// As in Go, each input argument must be assignable to the
// type of the function's corresponding input parameter.
func (v Value) CallSlice(in []Value) []Value {
	iv := v.internal()
	iv.mustBe(Func)
	iv.mustBeExported()
	return iv.call("CallSlice", in)
}

func (iv internalValue) call(method string, in []Value) []Value {
	if iv.word == 0 {
		if iv.nilmethod {
			panic("reflect.Value.Call: call of method on nil interface value")
		}
		panic("reflect.Value.Call: call of nil function")
	}

	isSlice := method == "CallSlice"
	t := iv.typ
	n := t.NumIn()
	if isSlice {
		if !t.IsVariadic() {
			panic("reflect: CallSlice of non-variadic function")
		}
		if len(in) < n {
			panic("reflect: CallSlice with too few input arguments")
		}
		if len(in) > n {
			panic("reflect: CallSlice with too many input arguments")
		}
	} else {
		if t.IsVariadic() {
			n--
		}
		if len(in) < n {
			panic("reflect: Call with too few input arguments")
		}
		if !t.IsVariadic() && len(in) > n {
			panic("reflect: Call with too many input arguments")
		}
	}
	for _, x := range in {
		if x.Kind() == Invalid {
			panic("reflect: " + method + " using zero Value argument")
		}
	}
	for i := 0; i < n; i++ {
		if xt, targ := in[i].Type(), t.In(i); !xt.AssignableTo(targ) {
			panic("reflect: " + method + " using " + xt.String() + " as type " + targ.String())
		}
	}
	if !isSlice && t.IsVariadic() {
		// prepare slice for remaining values
		m := len(in) - n
		slice := MakeSlice(t.In(n), m, m)
		elem := t.In(n).Elem()
		for i := 0; i < m; i++ {
			x := in[n+i]
			if xt := x.Type(); !xt.AssignableTo(elem) {
				panic("reflect: cannot use " + xt.String() + " as type " + elem.String() + " in " + method)
			}
			slice.Index(i).Set(x)
		}
		origIn := in
		in = make([]Value, n+1)
		copy(in[:n], origIn)
		in[n] = slice
	}

	nin := len(in)
	if nin != t.NumIn() {
		panic("reflect.Value.Call: wrong argument count")
	}
	nout := t.NumOut()

	// Compute arg size & allocate.
	// This computation is 5g/6g/8g-dependent
	// and probably wrong for gccgo, but so
	// is most of this function.
	size := uintptr(0)
	if iv.method {
		// extra word for interface value
		size += ptrSize
	}
	for i := 0; i < nin; i++ {
		tv := t.In(i)
		a := uintptr(tv.Align())
		size = (size + a - 1) &^ (a - 1)
		size += tv.Size()
	}
	size = (size + ptrSize - 1) &^ (ptrSize - 1)
	for i := 0; i < nout; i++ {
		tv := t.Out(i)
		a := uintptr(tv.Align())
		size = (size + a - 1) &^ (a - 1)
		size += tv.Size()
	}

	// size must be > 0 in order for &args[0] to be valid.
	// the argument copying is going to round it up to
	// a multiple of ptrSize anyway, so make it ptrSize to begin with.
	if size < ptrSize {
		size = ptrSize
	}

	// round to pointer size
	size = (size + ptrSize - 1) &^ (ptrSize - 1)

	// Copy into args.
	//
	// TODO(rsc): revisit when reference counting happens.
	// The values are holding up the in references for us,
	// but something must be done for the out references.
	// For now make everything look like a pointer by pretending
	// to allocate a []*int.
	args := make([]*int, size/ptrSize)
	ptr := uintptr(unsafe.Pointer(&args[0]))
	off := uintptr(0)
	if iv.method {
		// Hard-wired first argument.
		*(*iword)(unsafe.Pointer(ptr)) = iv.rcvr
		off = ptrSize
	}
	for i, v := range in {
		iv := v.internal()
		iv.mustBeExported()
		targ := t.In(i).(*commonType)
		a := uintptr(targ.align)
		off = (off + a - 1) &^ (a - 1)
		n := targ.size
		addr := unsafe.Pointer(ptr + off)
		iv = convertForAssignment("reflect.Value.Call", addr, targ, iv)
		if iv.addr == nil {
			storeIword(addr, iv.word, n)
		} else {
			memmove(addr, iv.addr, n)
		}
		off += n
	}
	off = (off + ptrSize - 1) &^ (ptrSize - 1)

	// Call.
	call(unsafe.Pointer(iv.word), unsafe.Pointer(ptr), uint32(size))

	// Copy return values out of args.
	//
	// TODO(rsc): revisit like above.
	ret := make([]Value, nout)
	for i := 0; i < nout; i++ {
		tv := t.Out(i)
		a := uintptr(tv.Align())
		off = (off + a - 1) &^ (a - 1)
		ret[i] = valueFromAddr(0, tv, unsafe.Pointer(ptr+off))
		off += tv.Size()
	}

	return ret
}

// Cap returns v's capacity.
// It panics if v's Kind is not Array, Chan, or Slice.
func (v Value) Cap() int {
	iv := v.internal()
	switch iv.kind {
	case Array:
		return iv.typ.Len()
	case Chan:
		return int(chancap(iv.word))
	case Slice:
		return (*SliceHeader)(iv.addr).Cap
	}
	panic(&ValueError{"reflect.Value.Cap", iv.kind})
}

// Close closes the channel v.
// It panics if v's Kind is not Chan.
func (v Value) Close() {
	iv := v.internal()
	iv.mustBe(Chan)
	iv.mustBeExported()
	ch := iv.word
	chanclose(ch)
}

// Complex returns v's underlying value, as a complex128.
// It panics if v's Kind is not Complex64 or Complex128
func (v Value) Complex() complex128 {
	iv := v.internal()
	switch iv.kind {
	case Complex64:
		if iv.addr == nil {
			return complex128(*(*complex64)(unsafe.Pointer(&iv.word)))
		}
		return complex128(*(*complex64)(iv.addr))
	case Complex128:
		return *(*complex128)(iv.addr)
	}
	panic(&ValueError{"reflect.Value.Complex", iv.kind})
}

// Elem returns the value that the interface v contains
// or that the pointer v points to.
// It panics if v's Kind is not Interface or Ptr.
// It returns the zero Value if v is nil.
func (v Value) Elem() Value {
	iv := v.internal()
	return iv.Elem()
}

func (iv internalValue) Elem() Value {
	switch iv.kind {
	case Interface:
		// Empty interface and non-empty interface have different layouts.
		// Convert to empty interface.
		var eface emptyInterface
		if iv.typ.NumMethod() == 0 {
			eface = *(*emptyInterface)(iv.addr)
		} else {
			iface := (*nonEmptyInterface)(iv.addr)
			if iface.itab != nil {
				eface.typ = iface.itab.typ
			}
			eface.word = iface.word
		}
		if eface.typ == nil {
			return Value{}
		}
		return valueFromIword(iv.flag&flagRO, toType(eface.typ), eface.word)

	case Ptr:
		// The returned value's address is v's value.
		if iv.word == 0 {
			return Value{}
		}
		return valueFromAddr(iv.flag&flagRO|flagAddr, iv.typ.Elem(), unsafe.Pointer(iv.word))
	}
	panic(&ValueError{"reflect.Value.Elem", iv.kind})
}

// Field returns the i'th field of the struct v.
// It panics if v's Kind is not Struct or i is out of range.
func (v Value) Field(i int) Value {
	iv := v.internal()
	iv.mustBe(Struct)
	t := iv.typ.toType()
	if i < 0 || i >= t.NumField() {
		panic("reflect: Field index out of range")
	}
	f := t.Field(i)

	// Inherit permission bits from v.
	flag := iv.flag
	// Using an unexported field forces flagRO.
	if f.PkgPath != "" {
		flag |= flagRO
	}
	return valueFromValueOffset(flag, f.Type, iv, f.Offset)
}

// valueFromValueOffset returns a sub-value of outer
// (outer is an array or a struct) with the given flag and type
// starting at the given byte offset into outer.
func valueFromValueOffset(flag uint32, typ Type, outer internalValue, offset uintptr) Value {
	if outer.addr != nil {
		return valueFromAddr(flag, typ, unsafe.Pointer(uintptr(outer.addr)+offset))
	}

	// outer is so tiny it is in line.
	// We have to use outer.word and derive
	// the new word (it cannot possibly be bigger).
	// In line, so not addressable.
	if flag&flagAddr != 0 {
		panic("reflect: internal error: misuse of valueFromValueOffset")
	}
	b := *(*[ptrSize]byte)(unsafe.Pointer(&outer.word))
	for i := uintptr(0); i < typ.Size(); i++ {
		b[i] = b[offset+i]
	}
	for i := typ.Size(); i < ptrSize; i++ {
		b[i] = 0
	}
	w := *(*iword)(unsafe.Pointer(&b))
	return valueFromIword(flag, typ, w)
}

// FieldByIndex returns the nested field corresponding to index.
// It panics if v's Kind is not struct.
func (v Value) FieldByIndex(index []int) Value {
	v.internal().mustBe(Struct)
	for i, x := range index {
		if i > 0 {
			if v.Kind() == Ptr && v.Elem().Kind() == Struct {
				v = v.Elem()
			}
		}
		v = v.Field(x)
	}
	return v
}

// FieldByName returns the struct field with the given name.
// It returns the zero Value if no field was found.
// It panics if v's Kind is not struct.
func (v Value) FieldByName(name string) Value {
	iv := v.internal()
	iv.mustBe(Struct)
	if f, ok := iv.typ.FieldByName(name); ok {
		return v.FieldByIndex(f.Index)
	}
	return Value{}
}

// FieldByNameFunc returns the struct field with a name
// that satisfies the match function.
// It panics if v's Kind is not struct.
// It returns the zero Value if no field was found.
func (v Value) FieldByNameFunc(match func(string) bool) Value {
	v.internal().mustBe(Struct)
	if f, ok := v.Type().FieldByNameFunc(match); ok {
		return v.FieldByIndex(f.Index)
	}
	return Value{}
}

// Float returns v's underlying value, as an float64.
// It panics if v's Kind is not Float32 or Float64
func (v Value) Float() float64 {
	iv := v.internal()
	switch iv.kind {
	case Float32:
		return float64(*(*float32)(unsafe.Pointer(&iv.word)))
	case Float64:
		// If the pointer width can fit an entire float64,
		// the value is in line when stored in an interface.
		if iv.addr == nil {
			return *(*float64)(unsafe.Pointer(&iv.word))
		}
		// Otherwise we have a pointer.
		return *(*float64)(iv.addr)
	}
	panic(&ValueError{"reflect.Value.Float", iv.kind})
}

// Index returns v's i'th element.
// It panics if v's Kind is not Array or Slice or i is out of range.
func (v Value) Index(i int) Value {
	iv := v.internal()
	switch iv.kind {
	default:
		panic(&ValueError{"reflect.Value.Index", iv.kind})
	case Array:
		flag := iv.flag // element flag same as overall array
		t := iv.typ.toType()
		if i < 0 || i > t.Len() {
			panic("reflect: array index out of range")
		}
		typ := t.Elem()
		return valueFromValueOffset(flag, typ, iv, uintptr(i)*typ.Size())

	case Slice:
		// Element flag same as Elem of Ptr.
		// Addressable, possibly read-only.
		flag := iv.flag&flagRO | flagAddr
		s := (*SliceHeader)(iv.addr)
		if i < 0 || i >= s.Len {
			panic("reflect: slice index out of range")
		}
		typ := iv.typ.Elem()
		addr := unsafe.Pointer(s.Data + uintptr(i)*typ.Size())
		return valueFromAddr(flag, typ, addr)
	}

	panic("not reached")
}

// Int returns v's underlying value, as an int64.
// It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64.
func (v Value) Int() int64 {
	iv := v.internal()
	switch iv.kind {
	case Int:
		return int64(*(*int)(unsafe.Pointer(&iv.word)))
	case Int8:
		return int64(*(*int8)(unsafe.Pointer(&iv.word)))
	case Int16:
		return int64(*(*int16)(unsafe.Pointer(&iv.word)))
	case Int32:
		return int64(*(*int32)(unsafe.Pointer(&iv.word)))
	case Int64:
		if iv.addr == nil {
			return *(*int64)(unsafe.Pointer(&iv.word))
		}
		return *(*int64)(iv.addr)
	}
	panic(&ValueError{"reflect.Value.Int", iv.kind})
}

// CanInterface returns true if Interface can be used without panicking.
func (v Value) CanInterface() bool {
	iv := v.internal()
	if iv.kind == Invalid {
		panic(&ValueError{"reflect.Value.CanInterface", iv.kind})
	}
	return v.InternalMethod == 0 && iv.flag&flagRO == 0
}

// Interface returns v's value as an interface{}.
// If v is a method obtained by invoking Value.Method
// (as opposed to Type.Method), Interface cannot return an
// interface value, so it panics.
func (v Value) Interface() interface{} {
	return valueInterface(v, true)
}

func valueInterface(v Value, safe bool) interface{} {
	iv := v.internal()
	return iv.valueInterface(safe)
}

func (iv internalValue) valueInterface(safe bool) interface{} {
	if iv.kind == 0 {
		panic(&ValueError{"reflect.Value.Interface", iv.kind})
	}
	if iv.method {
		panic("reflect.Value.Interface: cannot create interface value for method with bound receiver")
	}

	if safe && iv.flag&flagRO != 0 {
		// Do not allow access to unexported values via Interface,
		// because they might be pointers that should not be 
		// writable or methods or function that should not be callable.
		panic("reflect.Value.Interface: cannot return value obtained from unexported field or method")
	}
	if iv.kind == Interface {
		// Special case: return the element inside the interface.
		// Won't recurse further because an interface cannot contain an interface.
		if iv.IsNil() {
			return nil
		}
		return iv.Elem().Interface()
	}

	// Non-interface value.
	var eface emptyInterface
	eface.typ = iv.typ.runtimeType()
	eface.word = iv.word
	return *(*interface{})(unsafe.Pointer(&eface))
}

// InterfaceData returns the interface v's value as a uintptr pair.
// It panics if v's Kind is not Interface.
func (v Value) InterfaceData() [2]uintptr {
	iv := v.internal()
	iv.mustBe(Interface)
	// We treat this as a read operation, so we allow
	// it even for unexported data, because the caller
	// has to import "unsafe" to turn it into something
	// that can be abused.
	return *(*[2]uintptr)(iv.addr)
}

// IsNil returns true if v is a nil value.
// It panics if v's Kind is not Chan, Func, Interface, Map, Ptr, or Slice.
func (v Value) IsNil() bool {
	return v.internal().IsNil()
}

func (iv internalValue) IsNil() bool {
	switch iv.kind {
	case Chan, Func, Map, Ptr:
		if iv.method {
			panic("reflect: IsNil of method Value")
		}
		return iv.word == 0
	case Interface, Slice:
		// Both interface and slice are nil if first word is 0.
		return *(*uintptr)(iv.addr) == 0
	}
	panic(&ValueError{"reflect.Value.IsNil", iv.kind})
}

// IsValid returns true if v represents a value.
// It returns false if v is the zero Value.
// If IsValid returns false, all other methods except String panic.
// Most functions and methods never return an invalid value.
// If one does, its documentation states the conditions explicitly.
func (v Value) IsValid() bool {
	return v.Internal != nil
}

// Kind returns v's Kind.
// If v is the zero Value (IsValid returns false), Kind returns Invalid.
func (v Value) Kind() Kind {
	return v.internal().kind
}

// Len returns v's length.
// It panics if v's Kind is not Array, Chan, Map, Slice, or String.
func (v Value) Len() int {
	iv := v.internal()
	switch iv.kind {
	case Array:
		return iv.typ.Len()
	case Chan:
		return int(chanlen(iv.word))
	case Map:
		return int(maplen(iv.word))
	case Slice:
		return (*SliceHeader)(iv.addr).Len
	case String:
		return (*StringHeader)(iv.addr).Len
	}
	panic(&ValueError{"reflect.Value.Len", iv.kind})
}

// MapIndex returns the value associated with key in the map v.
// It panics if v's Kind is not Map.
// It returns the zero Value if key is not found in the map or if v represents a nil map.
// As in Go, the key's value must be assignable to the map's key type.
func (v Value) MapIndex(key Value) Value {
	iv := v.internal()
	iv.mustBe(Map)
	typ := iv.typ.toType()

	// Do not require ikey to be exported, so that DeepEqual
	// and other programs can use all the keys returned by
	// MapKeys as arguments to MapIndex.  If either the map
	// or the key is unexported, though, the result will be
	// considered unexported.

	ikey := key.internal()
	ikey = convertForAssignment("reflect.Value.MapIndex", nil, typ.Key(), ikey)
	if iv.word == 0 {
		return Value{}
	}

	flag := (iv.flag | ikey.flag) & flagRO
	elemType := typ.Elem()
	elemWord, ok := mapaccess(typ.runtimeType(), iv.word, ikey.word)
	if !ok {
		return Value{}
	}
	return valueFromIword(flag, elemType, elemWord)
}

// MapKeys returns a slice containing all the keys present in the map,
// in unspecified order.
// It panics if v's Kind is not Map.
// It returns an empty slice if v represents a nil map.
func (v Value) MapKeys() []Value {
	iv := v.internal()
	iv.mustBe(Map)
	keyType := iv.typ.Key()

	flag := iv.flag & flagRO
	m := iv.word
	mlen := int32(0)
	if m != 0 {
		mlen = maplen(m)
	}
	it := mapiterinit(iv.typ.runtimeType(), m)
	a := make([]Value, mlen)
	var i int
	for i = 0; i < len(a); i++ {
		keyWord, ok := mapiterkey(it)
		if !ok {
			break
		}
		a[i] = valueFromIword(flag, keyType, keyWord)
		mapiternext(it)
	}
	return a[:i]
}

// Method returns a function value corresponding to v's i'th method.
// The arguments to a Call on the returned function should not include
// a receiver; the returned function will always use v as the receiver.
// Method panics if i is out of range.
func (v Value) Method(i int) Value {
	iv := v.internal()
	if iv.kind == Invalid {
		panic(&ValueError{"reflect.Value.Method", Invalid})
	}
	if i < 0 || i >= iv.typ.NumMethod() {
		panic("reflect: Method index out of range")
	}
	return Value{v.Internal, i + 1}
}

// NumMethod returns the number of methods in the value's method set.
func (v Value) NumMethod() int {
	iv := v.internal()
	if iv.kind == Invalid {
		panic(&ValueError{"reflect.Value.NumMethod", Invalid})
	}
	return iv.typ.NumMethod()
}

// MethodByName returns a function value corresponding to the method
// of v with the given name.
// The arguments to a Call on the returned function should not include
// a receiver; the returned function will always use v as the receiver.
// It returns the zero Value if no method was found.
func (v Value) MethodByName(name string) Value {
	iv := v.internal()
	if iv.kind == Invalid {
		panic(&ValueError{"reflect.Value.MethodByName", Invalid})
	}
	m, ok := iv.typ.MethodByName(name)
	if ok {
		return Value{v.Internal, m.Index + 1}
	}
	return Value{}
}

// NumField returns the number of fields in the struct v.
// It panics if v's Kind is not Struct.
func (v Value) NumField() int {
	iv := v.internal()
	iv.mustBe(Struct)
	return iv.typ.NumField()
}

// OverflowComplex returns true if the complex128 x cannot be represented by v's type.
// It panics if v's Kind is not Complex64 or Complex128.
func (v Value) OverflowComplex(x complex128) bool {
	iv := v.internal()
	switch iv.kind {
	case Complex64:
		return overflowFloat32(real(x)) || overflowFloat32(imag(x))
	case Complex128:
		return false
	}
	panic(&ValueError{"reflect.Value.OverflowComplex", iv.kind})
}

// OverflowFloat returns true if the float64 x cannot be represented by v's type.
// It panics if v's Kind is not Float32 or Float64.
func (v Value) OverflowFloat(x float64) bool {
	iv := v.internal()
	switch iv.kind {
	case Float32:
		return overflowFloat32(x)
	case Float64:
		return false
	}
	panic(&ValueError{"reflect.Value.OverflowFloat", iv.kind})
}

func overflowFloat32(x float64) bool {
	if x < 0 {
		x = -x
	}
	return math.MaxFloat32 <= x && x <= math.MaxFloat64
}

// OverflowInt returns true if the int64 x cannot be represented by v's type.
// It panics if v's Kind is not Int, Int8, int16, Int32, or Int64.
func (v Value) OverflowInt(x int64) bool {
	iv := v.internal()
	switch iv.kind {
	case Int, Int8, Int16, Int32, Int64:
		bitSize := iv.typ.size * 8
		trunc := (x << (64 - bitSize)) >> (64 - bitSize)
		return x != trunc
	}
	panic(&ValueError{"reflect.Value.OverflowInt", iv.kind})
}

// OverflowUint returns true if the uint64 x cannot be represented by v's type.
// It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
func (v Value) OverflowUint(x uint64) bool {
	iv := v.internal()
	switch iv.kind {
	case Uint, Uintptr, Uint8, Uint16, Uint32, Uint64:
		bitSize := iv.typ.size * 8
		trunc := (x << (64 - bitSize)) >> (64 - bitSize)
		return x != trunc
	}
	panic(&ValueError{"reflect.Value.OverflowUint", iv.kind})
}

// Pointer returns v's value as a uintptr.
// It returns uintptr instead of unsafe.Pointer so that
// code using reflect cannot obtain unsafe.Pointers
// without importing the unsafe package explicitly.
// It panics if v's Kind is not Chan, Func, Map, Ptr, Slice, or UnsafePointer.
func (v Value) Pointer() uintptr {
	iv := v.internal()
	switch iv.kind {
	case Chan, Func, Map, Ptr, UnsafePointer:
		if iv.kind == Func && v.InternalMethod != 0 {
			panic("reflect.Value.Pointer of method Value")
		}
		return uintptr(iv.word)
	case Slice:
		return (*SliceHeader)(iv.addr).Data
	}
	panic(&ValueError{"reflect.Value.Pointer", iv.kind})
}

// Recv receives and returns a value from the channel v.
// It panics if v's Kind is not Chan.
// The receive blocks until a value is ready.
// The boolean value ok is true if the value x corresponds to a send
// on the channel, false if it is a zero value received because the channel is closed.
func (v Value) Recv() (x Value, ok bool) {
	iv := v.internal()
	iv.mustBe(Chan)
	iv.mustBeExported()
	return iv.recv(false)
}

// internal recv, possibly non-blocking (nb)
func (iv internalValue) recv(nb bool) (val Value, ok bool) {
	t := iv.typ.toType()
	if t.ChanDir()&RecvDir == 0 {
		panic("recv on send-only channel")
	}
	ch := iv.word
	if ch == 0 {
		panic("recv on nil channel")
	}
	valWord, selected, ok := chanrecv(iv.typ.runtimeType(), ch, nb)
	if selected {
		val = valueFromIword(0, t.Elem(), valWord)
	}
	return
}

// Send sends x on the channel v.
// It panics if v's kind is not Chan or if x's type is not the same type as v's element type.
// As in Go, x's value must be assignable to the channel's element type.
func (v Value) Send(x Value) {
	iv := v.internal()
	iv.mustBe(Chan)
	iv.mustBeExported()
	iv.send(x, false)
}

// internal send, possibly non-blocking
func (iv internalValue) send(x Value, nb bool) (selected bool) {
	t := iv.typ.toType()
	if t.ChanDir()&SendDir == 0 {
		panic("send on recv-only channel")
	}
	ix := x.internal()
	ix.mustBeExported() // do not let unexported x leak
	ix = convertForAssignment("reflect.Value.Send", nil, t.Elem(), ix)
	ch := iv.word
	if ch == 0 {
		panic("send on nil channel")
	}
	return chansend(iv.typ.runtimeType(), ch, ix.word, nb)
}

// Set assigns x to the value v.
// It panics if CanSet returns false.
// As in Go, x's value must be assignable to v's type.
func (v Value) Set(x Value) {
	iv := v.internal()
	ix := x.internal()

	iv.mustBeAssignable()
	ix.mustBeExported() // do not let unexported x leak

	ix = convertForAssignment("reflect.Set", iv.addr, iv.typ, ix)

	n := ix.typ.size
	if n <= ptrSize {
		storeIword(iv.addr, ix.word, n)
	} else {
		memmove(iv.addr, ix.addr, n)
	}
}

// SetBool sets v's underlying value.
// It panics if v's Kind is not Bool or if CanSet() is false.
func (v Value) SetBool(x bool) {
	iv := v.internal()
	iv.mustBeAssignable()
	iv.mustBe(Bool)
	*(*bool)(iv.addr) = x
}

// SetBytes sets v's underlying value.
// It panics if v's underlying value is not a slice of bytes.
func (v Value) SetBytes(x []byte) {
	iv := v.internal()
	iv.mustBeAssignable()
	iv.mustBe(Slice)
	typ := iv.typ.toType()
	if typ.Elem().Kind() != Uint8 {
		panic("reflect.Value.SetBytes of non-byte slice")
	}
	*(*[]byte)(iv.addr) = x
}

// SetComplex sets v's underlying value to x.
// It panics if v's Kind is not Complex64 or Complex128, or if CanSet() is false.
func (v Value) SetComplex(x complex128) {
	iv := v.internal()
	iv.mustBeAssignable()
	switch iv.kind {
	default:
		panic(&ValueError{"reflect.Value.SetComplex", iv.kind})
	case Complex64:
		*(*complex64)(iv.addr) = complex64(x)
	case Complex128:
		*(*complex128)(iv.addr) = x
	}
}

// SetFloat sets v's underlying value to x.
// It panics if v's Kind is not Float32 or Float64, or if CanSet() is false.
func (v Value) SetFloat(x float64) {
	iv := v.internal()
	iv.mustBeAssignable()
	switch iv.kind {
	default:
		panic(&ValueError{"reflect.Value.SetFloat", iv.kind})
	case Float32:
		*(*float32)(iv.addr) = float32(x)
	case Float64:
		*(*float64)(iv.addr) = x
	}
}

// SetInt sets v's underlying value to x.
// It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64, or if CanSet() is false.
func (v Value) SetInt(x int64) {
	iv := v.internal()
	iv.mustBeAssignable()
	switch iv.kind {
	default:
		panic(&ValueError{"reflect.Value.SetInt", iv.kind})
	case Int:
		*(*int)(iv.addr) = int(x)
	case Int8:
		*(*int8)(iv.addr) = int8(x)
	case Int16:
		*(*int16)(iv.addr) = int16(x)
	case Int32:
		*(*int32)(iv.addr) = int32(x)
	case Int64:
		*(*int64)(iv.addr) = x
	}
}

// SetLen sets v's length to n.
// It panics if v's Kind is not Slice.
func (v Value) SetLen(n int) {
	iv := v.internal()
	iv.mustBeAssignable()
	iv.mustBe(Slice)
	s := (*SliceHeader)(iv.addr)
	if n < 0 || n > int(s.Cap) {
		panic("reflect: slice length out of range in SetLen")
	}
	s.Len = n
}

// SetMapIndex sets the value associated with key in the map v to val.
// It panics if v's Kind is not Map.
// If val is the zero Value, SetMapIndex deletes the key from the map.
// As in Go, key's value must be assignable to the map's key type,
// and val's value must be assignable to the map's value type.
func (v Value) SetMapIndex(key, val Value) {
	iv := v.internal()
	ikey := key.internal()
	ival := val.internal()

	iv.mustBe(Map)
	iv.mustBeExported()

	ikey.mustBeExported()
	ikey = convertForAssignment("reflect.Value.SetMapIndex", nil, iv.typ.Key(), ikey)

	if ival.kind != Invalid {
		ival.mustBeExported()
		ival = convertForAssignment("reflect.Value.SetMapIndex", nil, iv.typ.Elem(), ival)
	}

	mapassign(iv.typ.runtimeType(), iv.word, ikey.word, ival.word, ival.kind != Invalid)
}

// SetUint sets v's underlying value to x.
// It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64, or if CanSet() is false.
func (v Value) SetUint(x uint64) {
	iv := v.internal()
	iv.mustBeAssignable()
	switch iv.kind {
	default:
		panic(&ValueError{"reflect.Value.SetUint", iv.kind})
	case Uint:
		*(*uint)(iv.addr) = uint(x)
	case Uint8:
		*(*uint8)(iv.addr) = uint8(x)
	case Uint16:
		*(*uint16)(iv.addr) = uint16(x)
	case Uint32:
		*(*uint32)(iv.addr) = uint32(x)
	case Uint64:
		*(*uint64)(iv.addr) = x
	case Uintptr:
		*(*uintptr)(iv.addr) = uintptr(x)
	}
}

// SetPointer sets the unsafe.Pointer value v to x.
// It panics if v's Kind is not UnsafePointer.
func (v Value) SetPointer(x unsafe.Pointer) {
	iv := v.internal()
	iv.mustBeAssignable()
	iv.mustBe(UnsafePointer)
	*(*unsafe.Pointer)(iv.addr) = x
}

// SetString sets v's underlying value to x.
// It panics if v's Kind is not String or if CanSet() is false.
func (v Value) SetString(x string) {
	iv := v.internal()
	iv.mustBeAssignable()
	iv.mustBe(String)
	*(*string)(iv.addr) = x
}

// Slice returns a slice of v.
// It panics if v's Kind is not Array or Slice.
func (v Value) Slice(beg, end int) Value {
	iv := v.internal()
	if iv.kind != Array && iv.kind != Slice {
		panic(&ValueError{"reflect.Value.Slice", iv.kind})
	}
	cap := v.Cap()
	if beg < 0 || end < beg || end > cap {
		panic("reflect.Value.Slice: slice index out of bounds")
	}
	var typ Type
	var base uintptr
	switch iv.kind {
	case Array:
		if iv.flag&flagAddr == 0 {
			panic("reflect.Value.Slice: slice of unaddressable array")
		}
		typ = toType((*arrayType)(unsafe.Pointer(iv.typ)).slice)
		base = uintptr(iv.addr)
	case Slice:
		typ = iv.typ.toType()
		base = (*SliceHeader)(iv.addr).Data
	}

	// Declare slice so that gc can see the base pointer in it.
	var x []byte

	// Reinterpret as *SliceHeader to edit.
	s := (*SliceHeader)(unsafe.Pointer(&x))
	s.Data = base + uintptr(beg)*typ.Elem().Size()
	s.Len = end - beg
	s.Cap = end - beg

	return valueFromAddr(iv.flag&flagRO, typ, unsafe.Pointer(&x))
}

// String returns the string v's underlying value, as a string.
// String is a special case because of Go's String method convention.
// Unlike the other getters, it does not panic if v's Kind is not String.
// Instead, it returns a string of the form "<T value>" where T is v's type.
func (v Value) String() string {
	iv := v.internal()
	switch iv.kind {
	case Invalid:
		return "<invalid Value>"
	case String:
		return *(*string)(iv.addr)
	}
	// If you call String on a reflect.Value of other type, it's better to
	// print something than to panic. Useful in debugging.
	return "<" + iv.typ.String() + " Value>"
}

// TryRecv attempts to receive a value from the channel v but will not block.
// It panics if v's Kind is not Chan.
// If the receive cannot finish without blocking, x is the zero Value.
// The boolean ok is true if the value x corresponds to a send
// on the channel, false if it is a zero value received because the channel is closed.
func (v Value) TryRecv() (x Value, ok bool) {
	iv := v.internal()
	iv.mustBe(Chan)
	iv.mustBeExported()
	return iv.recv(true)
}

// TrySend attempts to send x on the channel v but will not block.
// It panics if v's Kind is not Chan.
// It returns true if the value was sent, false otherwise.
// As in Go, x's value must be assignable to the channel's element type.
func (v Value) TrySend(x Value) bool {
	iv := v.internal()
	iv.mustBe(Chan)
	iv.mustBeExported()
	return iv.send(x, true)
}

// Type returns v's type.
func (v Value) Type() Type {
	t := v.internal().typ
	if t == nil {
		panic(&ValueError{"reflect.Value.Type", Invalid})
	}
	return t.toType()
}

// Uint returns v's underlying value, as a uint64.
// It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
func (v Value) Uint() uint64 {
	iv := v.internal()
	switch iv.kind {
	case Uint:
		return uint64(*(*uint)(unsafe.Pointer(&iv.word)))
	case Uint8:
		return uint64(*(*uint8)(unsafe.Pointer(&iv.word)))
	case Uint16:
		return uint64(*(*uint16)(unsafe.Pointer(&iv.word)))
	case Uint32:
		return uint64(*(*uint32)(unsafe.Pointer(&iv.word)))
	case Uintptr:
		return uint64(*(*uintptr)(unsafe.Pointer(&iv.word)))
	case Uint64:
		if iv.addr == nil {
			return *(*uint64)(unsafe.Pointer(&iv.word))
		}
		return *(*uint64)(iv.addr)
	}
	panic(&ValueError{"reflect.Value.Uint", iv.kind})
}

// UnsafeAddr returns a pointer to v's data.
// It is for advanced clients that also import the "unsafe" package.
// It panics if v is not addressable.
func (v Value) UnsafeAddr() uintptr {
	iv := v.internal()
	if iv.kind == Invalid {
		panic(&ValueError{"reflect.Value.UnsafeAddr", iv.kind})
	}
	if iv.flag&flagAddr == 0 {
		panic("reflect.Value.UnsafeAddr of unaddressable value")
	}
	return uintptr(iv.addr)
}

// StringHeader is the runtime representation of a string.
// It cannot be used safely or portably.
type StringHeader struct {
	Data uintptr
	Len  int
}

// SliceHeader is the runtime representation of a slice.
// It cannot be used safely or portably.
type SliceHeader struct {
	Data uintptr
	Len  int
	Cap  int
}

func typesMustMatch(what string, t1, t2 Type) {
	if t1 != t2 {
		panic("reflect: " + what + ": " + t1.String() + " != " + t2.String())
	}
}

// grow grows the slice s so that it can hold extra more values, allocating
// more capacity if needed. It also returns the old and new slice lengths.
func grow(s Value, extra int) (Value, int, int) {
	i0 := s.Len()
	i1 := i0 + extra
	if i1 < i0 {
		panic("reflect.Append: slice overflow")
	}
	m := s.Cap()
	if i1 <= m {
		return s.Slice(0, i1), i0, i1
	}
	if m == 0 {
		m = extra
	} else {
		for m < i1 {
			if i0 < 1024 {
				m += m
			} else {
				m += m / 4
			}
		}
	}
	t := MakeSlice(s.Type(), i1, m)
	Copy(t, s)
	return t, i0, i1
}

// Append appends the values x to a slice s and returns the resulting slice.
// As in Go, each x's value must be assignable to the slice's element type.
func Append(s Value, x ...Value) Value {
	s.internal().mustBe(Slice)
	s, i0, i1 := grow(s, len(x))
	for i, j := i0, 0; i < i1; i, j = i+1, j+1 {
		s.Index(i).Set(x[j])
	}
	return s
}

// AppendSlice appends a slice t to a slice s and returns the resulting slice.
// The slices s and t must have the same element type.
func AppendSlice(s, t Value) Value {
	s.internal().mustBe(Slice)
	t.internal().mustBe(Slice)
	typesMustMatch("reflect.AppendSlice", s.Type().Elem(), t.Type().Elem())
	s, i0, i1 := grow(s, t.Len())
	Copy(s.Slice(i0, i1), t)
	return s
}

// Copy copies the contents of src into dst until either
// dst has been filled or src has been exhausted.
// It returns the number of elements copied.
// Dst and src each must have kind Slice or Array, and
// dst and src must have the same element type.
func Copy(dst, src Value) int {
	idst := dst.internal()
	isrc := src.internal()

	if idst.kind != Array && idst.kind != Slice {
		panic(&ValueError{"reflect.Copy", idst.kind})
	}
	if idst.kind == Array {
		idst.mustBeAssignable()
	}
	idst.mustBeExported()
	if isrc.kind != Array && isrc.kind != Slice {
		panic(&ValueError{"reflect.Copy", isrc.kind})
	}
	isrc.mustBeExported()

	de := idst.typ.Elem()
	se := isrc.typ.Elem()
	typesMustMatch("reflect.Copy", de, se)

	n := dst.Len()
	if sn := src.Len(); n > sn {
		n = sn
	}

	// If sk is an in-line array, cannot take its address.
	// Instead, copy element by element.
	if isrc.addr == nil {
		for i := 0; i < n; i++ {
			dst.Index(i).Set(src.Index(i))
		}
		return n
	}

	// Copy via memmove.
	var da, sa unsafe.Pointer
	if idst.kind == Array {
		da = idst.addr
	} else {
		da = unsafe.Pointer((*SliceHeader)(idst.addr).Data)
	}
	if isrc.kind == Array {
		sa = isrc.addr
	} else {
		sa = unsafe.Pointer((*SliceHeader)(isrc.addr).Data)
	}
	memmove(da, sa, uintptr(n)*de.Size())
	return n
}

/*
 * constructors
 */

// MakeSlice creates a new zero-initialized slice value
// for the specified slice type, length, and capacity.
func MakeSlice(typ Type, len, cap int) Value {
	if typ.Kind() != Slice {
		panic("reflect: MakeSlice of non-slice type")
	}

	// Declare slice so that gc can see the base pointer in it.
	var x []byte

	// Reinterpret as *SliceHeader to edit.
	s := (*SliceHeader)(unsafe.Pointer(&x))
	s.Data = uintptr(unsafe.NewArray(typ.Elem(), cap))
	s.Len = len
	s.Cap = cap

	return valueFromAddr(0, typ, unsafe.Pointer(&x))
}

// MakeChan creates a new channel with the specified type and buffer size.
func MakeChan(typ Type, buffer int) Value {
	if typ.Kind() != Chan {
		panic("reflect: MakeChan of non-chan type")
	}
	if buffer < 0 {
		panic("MakeChan: negative buffer size")
	}
	if typ.ChanDir() != BothDir {
		panic("MakeChan: unidirectional channel type")
	}
	ch := makechan(typ.runtimeType(), uint32(buffer))
	return valueFromIword(0, typ, ch)
}

// MakeMap creates a new map of the specified type.
func MakeMap(typ Type) Value {
	if typ.Kind() != Map {
		panic("reflect: MakeMap of non-map type")
	}
	m := makemap(typ.runtimeType())
	return valueFromIword(0, typ, m)
}

// Indirect returns the value that v points to.
// If v is a nil pointer, Indirect returns a nil Value.
// If v is not a pointer, Indirect returns v.
func Indirect(v Value) Value {
	if v.Kind() != Ptr {
		return v
	}
	return v.Elem()
}

// ValueOf returns a new Value initialized to the concrete value
// stored in the interface i.  ValueOf(nil) returns the zero Value.
func ValueOf(i interface{}) Value {
	if i == nil {
		return Value{}
	}

	// TODO(rsc): Eliminate this terrible hack.
	// In the call to packValue, eface.typ doesn't escape,
	// and eface.word is an integer.  So it looks like
	// i (= eface) doesn't escape.  But really it does,
	// because eface.word is actually a pointer.
	escapes(i)

	// For an interface value with the noAddr bit set,
	// the representation is identical to an empty interface.
	eface := *(*emptyInterface)(unsafe.Pointer(&i))
	return packValue(0, eface.typ, eface.word)
}

// Zero returns a Value representing a zero value for the specified type.
// The result is different from the zero value of the Value struct,
// which represents no value at all.
// For example, Zero(TypeOf(42)) returns a Value with Kind Int and value 0.
func Zero(typ Type) Value {
	if typ == nil {
		panic("reflect: Zero(nil)")
	}
	if typ.Size() <= ptrSize {
		return valueFromIword(0, typ, 0)
	}
	return valueFromAddr(0, typ, unsafe.New(typ))
}

// New returns a Value representing a pointer to a new zero value
// for the specified type.  That is, the returned Value's Type is PtrTo(t).
func New(typ Type) Value {
	if typ == nil {
		panic("reflect: New(nil)")
	}
	ptr := unsafe.New(typ)
	return valueFromIword(0, PtrTo(typ), iword(ptr))
}

// convertForAssignment 
func convertForAssignment(what string, addr unsafe.Pointer, dst Type, iv internalValue) internalValue {
	if iv.method {
		panic(what + ": cannot assign method value to type " + dst.String())
	}

	dst1 := dst.(*commonType)
	if directlyAssignable(dst1, iv.typ) {
		// Overwrite type so that they match.
		// Same memory layout, so no harm done.
		iv.typ = dst1
		return iv
	}
	if implements(dst1, iv.typ) {
		if addr == nil {
			addr = unsafe.Pointer(new(interface{}))
		}
		x := iv.valueInterface(false)
		if dst.NumMethod() == 0 {
			*(*interface{})(addr) = x
		} else {
			ifaceE2I(dst1.runtimeType(), x, addr)
		}
		iv.addr = addr
		iv.word = iword(addr)
		iv.typ = dst1
		return iv
	}

	// Failed.
	panic(what + ": value of type " + iv.typ.String() + " is not assignable to type " + dst.String())
}

// implemented in ../pkg/runtime
func chancap(ch iword) int32
func chanclose(ch iword)
func chanlen(ch iword) int32
func chanrecv(t *runtime.Type, ch iword, nb bool) (val iword, selected, received bool)
func chansend(t *runtime.Type, ch iword, val iword, nb bool) bool

func makechan(typ *runtime.Type, size uint32) (ch iword)
func makemap(t *runtime.Type) iword
func mapaccess(t *runtime.Type, m iword, key iword) (val iword, ok bool)
func mapassign(t *runtime.Type, m iword, key, val iword, ok bool)
func mapiterinit(t *runtime.Type, m iword) *byte
func mapiterkey(it *byte) (key iword, ok bool)
func mapiternext(it *byte)
func maplen(m iword) int32

func call(fn, arg unsafe.Pointer, n uint32)
func ifaceE2I(t *runtime.Type, src interface{}, dst unsafe.Pointer)