libgo: Update to weekly.2011-11-02.

[pf3gnuchains/gcc-fork.git] / libgo / go / 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) Error() 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 {
                typ    *runtime.Type // dynamic concrete type
                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(uintptr(iv.word))
                iv.typ = iv.typ.Elem().common()
                if Kind(iv.typ.kind) == Ptr || Kind(iv.typ.kind) == UnsafePointer {
                        iv.word = loadIword(iv.addr, iv.typ.size)
                }
        } else {
                if Kind(iv.typ.kind) != Ptr && Kind(iv.typ.kind) != UnsafePointer {
                        iv.addr = unsafe.Pointer(uintptr(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(uintptr(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(uintptr(m.tfn))
                }
                if iv.word != 0 {
                        p := new(iword)
                        *p = iv.word
                        iv.word = iword(uintptr(unsafe.Pointer(p)))
                }
                iv.kind = Func
                iv.method = true
                iv.flag &^= flagAddr
                iv.addr = unsafe.Pointer(uintptr(iv.word))
        }

        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(uintptr(addr)))
        }

        var w iword
        if k := typ.Kind(); k == Ptr || k == UnsafePointer {
                // In line, so the interface word is the actual value.
                w = loadIword(addr, typ.Size())
        } else {
                // Not in line: the interface word is the address.
                w = iword(uintptr(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(uintptr(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.addr))
}

// 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()

        if iv.method {
                nin++
        }
        params := make([]unsafe.Pointer, nin)
        delta := 0
        off := 0
        if iv.method {
                // Hard-wired first argument.
                p := new(iword)
                *p = iv.rcvr
                params[0] = unsafe.Pointer(p)
                off = 1
        }

        first_pointer := false
        for i, v := range in {
                siv := v.internal()
                siv.mustBeExported()
                targ := t.In(i).(*commonType)
                siv = convertForAssignment("reflect.Value.Call", nil, targ, siv)
                if siv.addr == nil {
                        p := new(unsafe.Pointer)
                        *p = unsafe.Pointer(uintptr(siv.word))
                        params[off] = unsafe.Pointer(p)
                } else {
                        params[off] = siv.addr
                }
                if i == 0 && Kind(targ.kind) != Ptr && !iv.method && isMethod(iv.typ) {
                        p := new(unsafe.Pointer)
                        *p = params[off]
                        params[off] = unsafe.Pointer(p)
                        first_pointer = true
                }
                off++
        }

        ret := make([]Value, nout)
        results := make([]unsafe.Pointer, nout)
        for i := 0; i < nout; i++ {
                v := New(t.Out(i))
                results[i] = unsafe.Pointer(v.Pointer())
                ret[i] = Indirect(v)
        }

        var pp *unsafe.Pointer
        if len(params) > 0 {
                pp = &params[0]
        }
        var pr *unsafe.Pointer
        if len(results) > 0 {
                pr = &results[0]
        }

        call(t, *(*unsafe.Pointer)(iv.addr), iv.method, first_pointer, pp, pr)

        return ret
}

// gccgo specific test to see if typ is a method.  We can tell by
// looking at the string to see if there is a receiver.  We need this
// because for gccgo all methods take pointer receivers.
func isMethod(t *commonType) bool {
        if Kind(t.kind) != Func {
                return false
        }
        s := *t.string
        parens := 0
        params := 0
        sawRet := false
        for i, c := range s {
                if c == '(' {
                        parens++
                        params++
                } else if c == ')' {
                        parens--
                } else if parens == 0 && c == ' ' && s[i + 1] != '(' && !sawRet {
                        params++
                        sawRet = true
                }
        }
        return params > 2
}

// 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(*(*iword)(iv.addr)))
        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 := *(*iword)(iv.addr)
        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:
                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(uintptr(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)(iv.addr))
        case Float64:
                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)(iv.addr))
        case Int8:
                return int64(*(*int8)(iv.addr))
        case Int16:
                return int64(*(*int16)(iv.addr))
        case Int32:
                return int64(*(*int32)(iv.addr))
        case Int64:
                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 Ptr:
                if iv.method {
                        panic("reflect: IsNil of method Value")
                }
                return iv.word == 0
        case Chan, Func, Map:
                if iv.method {
                        panic("reflect: IsNil of method Value")
                }
                return *(*uintptr)(iv.addr) == 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(*(*iword)(iv.addr)))
        case Map:
                return int(maplen(*(*iword)(iv.addr)))
        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(), *(*iword)(iv.addr), 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 := *(*iword)(iv.addr)
        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 Ptr, UnsafePointer:
                if iv.kind == Func && v.InternalMethod != 0 {
                        panic("reflect.Value.Pointer of method Value")
                }
                return uintptr(iv.word)
        case Chan, Func, Map:
                if iv.kind == Func && v.InternalMethod != 0 {
                        panic("reflect.Value.Pointer of method Value")
                }
                return *(*uintptr)(iv.addr)
        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 := *(*iword)(iv.addr)
        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 := *(*iword)(iv.addr)
        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 Kind(ix.typ.kind) == Ptr || Kind(ix.typ.kind) == UnsafePointer {
                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(), *(*iword)(iv.addr), 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)(iv.addr))
        case Uint8:
                return uint64(*(*uint8)(iv.addr))
        case Uint16:
                return uint64(*(*uint16)(iv.addr))
        case Uint32:
                return uint64(*(*uint32)(iv.addr))
        case Uintptr:
                return uint64(*(*uintptr)(iv.addr))
        case Uint64:
                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.Kind() == Ptr || typ.Kind() == UnsafePointer {
                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(uintptr(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(uintptr(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(typ *commonType, fnaddr unsafe.Pointer, isInterface bool, isMethod bool, params *unsafe.Pointer, results *unsafe.Pointer)
func ifaceE2I(t *runtime.Type, src interface{}, dst unsafe.Pointer)