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// 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 bigEndian = false // can be smarter if we find a big-endian machine
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.
type Value struct {
        // typ holds the type of the value represented by a Value.
        typ *commonType

        // val holds the 1-word representation of the value.
        // If flag's flagIndir bit is set, then val is a pointer to the data.
        // Otherwise val is a word holding the actual data.
        // When the data is smaller than a word, it begins at
        // the first byte (in the memory address sense) of val.
        // We use unsafe.Pointer so that the garbage collector
        // knows that val could be a pointer.
        val unsafe.Pointer

        // flag holds metadata about the value.
        // The lowest bits are flag bits:
        //      - flagRO: obtained via unexported field, so read-only
        //      - flagIndir: val holds a pointer to the data
        //      - flagAddr: v.CanAddr is true (implies flagIndir)
        //      - flagMethod: v is a method value.
        // The next five bits give the Kind of the value.
        // This repeats typ.Kind() except for method values.
        // The remaining 23+ bits give a method number for method values.
        // If flag.kind() != Func, code can assume that flagMethod is unset.
        // If typ.size > ptrSize, code can assume that flagIndir is set.
        flag

        // A method value represents a curried method invocation
        // like r.Read for some receiver r.  The typ+val+flag bits describe
        // the receiver r, but the flag's Kind bits say Func (methods are
        // functions), and the top bits of the flag give the method number
        // in r's type's method table.
}

type flag uintptr

const (
        flagRO flag = 1 << iota
        flagIndir
        flagAddr
        flagMethod
        flagKindShift        = iota
        flagKindWidth        = 5 // there are 27 kinds
        flagKindMask    flag = 1<<flagKindWidth - 1
        flagMethodShift      = flagKindShift + flagKindWidth
)

func (f flag) kind() Kind {
        return Kind((f >> flagKindShift) & flagKindMask)
}

// 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 holds the data stored
// in its leading bytes (so is not a pointer).
// Because the value sometimes holds a pointer, we use
// unsafe.Pointer to represent it, so that if iword appears
// in a struct, the garbage collector knows that might be
// a pointer.
type iword unsafe.Pointer

func (v Value) iword() iword {
        if v.flag&flagIndir != 0 && (v.kind() == Ptr || v.kind() == UnsafePointer) {
                // Have indirect but want direct word.
                return loadIword(v.val, v.typ.size)
        }
        return iword(v.val)
}

// loadIword loads n bytes at p from memory into an iword.
func loadIword(p unsafe.Pointer, n uintptr) iword {
        // Run the copy ourselves instead of calling memmove
        // to avoid moving w to the heap.
        var w iword
        switch n {
        default:
                panic("reflect: internal error: loadIword of " + strconv.Itoa(int(n)) + "-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
}

// storeIword stores n bytes from w into p.
func storeIword(p unsafe.Pointer, w iword, n uintptr) {
        // Run the copy ourselves instead of calling memmove
        // to avoid moving w to the heap.
        switch n {
        default:
                panic("reflect: internal error: storeIword of " + strconv.Itoa(int(n)) + "-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
}

// mustBe panics if f's kind is not expected.
// Making this a method on flag instead of on Value
// (and embedding flag in Value) means that we can write
// the very clear v.mustBe(Bool) and have it compile into
// v.flag.mustBe(Bool), which will only bother to copy the
// single important word for the receiver.
func (f flag) mustBe(expected Kind) {
        k := f.kind()
        if k != expected {
                panic(&ValueError{methodName(), k})
        }
}

// mustBeExported panics if f records that the value was obtained using
// an unexported field.
func (f flag) mustBeExported() {
        if f == 0 {
                panic(&ValueError{methodName(), 0})
        }
        if f&flagRO != 0 {
                panic(methodName() + " using value obtained using unexported field")
        }
}

// mustBeAssignable panics if f records that the value is not assignable,
// which is to say that either it was obtained using an unexported field
// or it is not addressable.
func (f flag) mustBeAssignable() {
        if f == 0 {
                panic(&ValueError{methodName(), Invalid})
        }
        // Assignable if addressable and not read-only.
        if f&flagRO != 0 {
                panic(methodName() + " using value obtained using unexported field")
        }
        if f&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 {
        if v.flag&flagAddr == 0 {
                panic("reflect.Value.Addr of unaddressable value")
        }
        return Value{v.typ.ptrTo(), v.val, (v.flag & flagRO) | flag(Ptr)<<flagKindShift}
}

// Bool returns v's underlying value.
// It panics if v's kind is not Bool.
func (v Value) Bool() bool {
        v.mustBe(Bool)
        if v.flag&flagIndir != 0 {
                return *(*bool)(v.val)
        }
        return *(*bool)(unsafe.Pointer(&v.val))
}

// Bytes returns v's underlying value.
// It panics if v's underlying value is not a slice of bytes.
func (v Value) Bytes() []byte {
        v.mustBe(Slice)
        if v.typ.Elem().Kind() != Uint8 {
                panic("reflect.Value.Bytes of non-byte slice")
        }
        // Slice is always bigger than a word; assume flagIndir.
        return *(*[]byte)(v.val)
}

// 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 {
        return v.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 {
        return v.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 {
        v.mustBe(Func)
        v.mustBeExported()
        return v.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 {
        v.mustBe(Func)
        v.mustBeExported()
        return v.call("CallSlice", in)
}

func (v Value) call(method string, in []Value) []Value {
        // Get function pointer, type.
        t := v.typ
        var (
                fn   unsafe.Pointer
                rcvr iword
        )
        if v.flag&flagMethod != 0 {
                i := int(v.flag) >> flagMethodShift
                if v.typ.Kind() == Interface {
                        tt := (*interfaceType)(unsafe.Pointer(v.typ))
                        if i < 0 || i >= len(tt.methods) {
                                panic("reflect: broken Value")
                        }
                        m := &tt.methods[i]
                        if m.pkgPath != nil {
                                panic(method + " of unexported method")
                        }
                        t = toCommonType(m.typ)
                        iface := (*nonEmptyInterface)(v.val)
                        if iface.itab == nil {
                                panic(method + " of method on nil interface value")
                        }
                        fn = iface.itab.fun[i]
                        rcvr = iface.word
                } else {
                        ut := v.typ.uncommon()
                        if ut == nil || i < 0 || i >= len(ut.methods) {
                                panic("reflect: broken Value")
                        }
                        m := &ut.methods[i]
                        if m.pkgPath != nil {
                                panic(method + " of unexported method")
                        }
                        fn = m.tfn
                        t = toCommonType(m.mtyp)
                        rcvr = v.iword()
                }
        } else if v.flag&flagIndir != 0 {
                fn = *(*unsafe.Pointer)(v.val)
        } else {
                fn = v.val
        }

        if fn == nil {
                panic("reflect.Value.Call: call of nil function")
        }

        isSlice := method == "CallSlice"
        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 v.flag&flagMethod != 0 {
                nin++
        }
        params := make([]unsafe.Pointer, nin)
        off := 0
        if v.flag&flagMethod != 0 {
                // Hard-wired first argument.
                p := new(iword)
                *p = rcvr
                params[0] = unsafe.Pointer(p)
                off = 1
        }
        first_pointer := false
        for i, pv := range in {
                pv.mustBeExported()
                targ := t.In(i).(*commonType)
                pv = pv.assignTo("reflect.Value.Call", targ, nil)
                if pv.flag&flagIndir == 0 {
                        p := new(unsafe.Pointer)
                        *p = pv.val
                        params[off] = unsafe.Pointer(p)
                } else {
                        params[off] = pv.val
                }
                if i == 0 && Kind(targ.kind) != Ptr && v.flag&flagMethod == 0 && isMethod(v.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, fn, v.flag&flagMethod != 0, 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 {
        k := v.kind()
        switch k {
        case Array:
                return v.typ.Len()
        case Chan:
                return int(chancap(*(*iword)(v.iword())))
        case Slice:
                // Slice is always bigger than a word; assume flagIndir.
                return (*SliceHeader)(v.val).Cap
        }
        panic(&ValueError{"reflect.Value.Cap", k})
}

// Close closes the channel v.
// It panics if v's Kind is not Chan.
func (v Value) Close() {
        v.mustBe(Chan)
        v.mustBeExported()
        chanclose(*(*iword)(v.iword()))
}

// 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 {
        k := v.kind()
        switch k {
        case Complex64:
                if v.flag&flagIndir != 0 {
                        return complex128(*(*complex64)(v.val))
                }
                return complex128(*(*complex64)(unsafe.Pointer(&v.val)))
        case Complex128:
                // complex128 is always bigger than a word; assume flagIndir.
                return *(*complex128)(v.val)
        }
        panic(&ValueError{"reflect.Value.Complex", k})
}

// 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 {
        k := v.kind()
        switch k {
        case Interface:
                var (
                        typ *commonType
                        val unsafe.Pointer
                )
                if v.typ.NumMethod() == 0 {
                        eface := (*emptyInterface)(v.val)
                        if eface.typ == nil {
                                // nil interface value
                                return Value{}
                        }
                        typ = toCommonType(eface.typ)
                        val = unsafe.Pointer(eface.word)
                } else {
                        iface := (*nonEmptyInterface)(v.val)
                        if iface.itab == nil {
                                // nil interface value
                                return Value{}
                        }
                        typ = toCommonType(iface.itab.typ)
                        val = unsafe.Pointer(iface.word)
                }
                fl := v.flag & flagRO
                fl |= flag(typ.Kind()) << flagKindShift
                if typ.Kind() != Ptr && typ.Kind() != UnsafePointer {
                        fl |= flagIndir
                }
                return Value{typ, val, fl}

        case Ptr:
                val := v.val
                if v.flag&flagIndir != 0 {
                        val = *(*unsafe.Pointer)(val)
                }
                // The returned value's address is v's value.
                if val == nil {
                        return Value{}
                }
                tt := (*ptrType)(unsafe.Pointer(v.typ))
                typ := toCommonType(tt.elem)
                fl := v.flag&flagRO | flagIndir | flagAddr
                fl |= flag(typ.Kind() << flagKindShift)
                return Value{typ, val, fl}
        }
        panic(&ValueError{"reflect.Value.Elem", k})
}

// 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 {
        v.mustBe(Struct)
        tt := (*structType)(unsafe.Pointer(v.typ))
        if i < 0 || i >= len(tt.fields) {
                panic("reflect: Field index out of range")
        }
        field := &tt.fields[i]
        typ := toCommonType(field.typ)

        // Inherit permission bits from v.
        fl := v.flag & (flagRO | flagIndir | flagAddr)
        // Using an unexported field forces flagRO.
        if field.pkgPath != nil {
                fl |= flagRO
        }
        fl |= flag(typ.Kind()) << flagKindShift

        var val unsafe.Pointer
        switch {
        case fl&flagIndir != 0:
                // Indirect.  Just bump pointer.
                val = unsafe.Pointer(uintptr(v.val) + field.offset)
        case bigEndian:
                // Direct.  Discard leading bytes.
                val = unsafe.Pointer(uintptr(v.val) << (field.offset * 8))
        default:
                // Direct.  Discard leading bytes.
                val = unsafe.Pointer(uintptr(v.val) >> (field.offset * 8))
        }

        return Value{typ, val, fl}
}

// 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.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 {
        v.mustBe(Struct)
        if f, ok := v.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.mustBe(Struct)
        if f, ok := v.typ.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 {
        k := v.kind()
        switch k {
        case Float32:
                if v.flag&flagIndir != 0 {
                        return float64(*(*float32)(v.val))
                }
                return float64(*(*float32)(unsafe.Pointer(&v.val)))
        case Float64:
                if v.flag&flagIndir != 0 {
                        return *(*float64)(v.val)
                }
                return *(*float64)(unsafe.Pointer(&v.val))
        }
        panic(&ValueError{"reflect.Value.Float", k})
}

// 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 {
        k := v.kind()
        switch k {
        case Array:
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                if i < 0 || i > int(tt.len) {
                        panic("reflect: array index out of range")
                }
                typ := toCommonType(tt.elem)
                fl := v.flag & (flagRO | flagIndir | flagAddr) // bits same as overall array
                fl |= flag(typ.Kind()) << flagKindShift
                offset := uintptr(i) * typ.size

                var val unsafe.Pointer
                switch {
                case fl&flagIndir != 0:
                        // Indirect.  Just bump pointer.
                        val = unsafe.Pointer(uintptr(v.val) + offset)
                case bigEndian:
                        // Direct.  Discard leading bytes.
                        val = unsafe.Pointer(uintptr(v.val) << (offset * 8))
                default:
                        // Direct.  Discard leading bytes.
                        val = unsafe.Pointer(uintptr(v.val) >> (offset * 8))
                }
                return Value{typ, val, fl}

        case Slice:
                // Element flag same as Elem of Ptr.
                // Addressable, indirect, possibly read-only.
                fl := flagAddr | flagIndir | v.flag&flagRO
                s := (*SliceHeader)(v.val)
                if i < 0 || i >= s.Len {
                        panic("reflect: slice index out of range")
                }
                tt := (*sliceType)(unsafe.Pointer(v.typ))
                typ := toCommonType(tt.elem)
                fl |= flag(typ.Kind()) << flagKindShift
                val := unsafe.Pointer(s.Data + uintptr(i)*typ.size)
                return Value{typ, val, fl}
        }
        panic(&ValueError{"reflect.Value.Index", k})
}

// 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 {
        k := v.kind()
        var p unsafe.Pointer
        if v.flag&flagIndir != 0 {
                p = v.val
        } else {
                // The escape analysis is good enough that &v.val
                // does not trigger a heap allocation.
                p = unsafe.Pointer(&v.val)
        }
        switch k {
        case Int:
                return int64(*(*int)(p))
        case Int8:
                return int64(*(*int8)(p))
        case Int16:
                return int64(*(*int16)(p))
        case Int32:
                return int64(*(*int32)(p))
        case Int64:
                return int64(*(*int64)(p))
        }
        panic(&ValueError{"reflect.Value.Int", k})
}

// CanInterface returns true if Interface can be used without panicking.
func (v Value) CanInterface() bool {
        if v.flag == 0 {
                panic(&ValueError{"reflect.Value.CanInterface", Invalid})
        }
        return v.flag&(flagMethod|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{} {
        if v.flag == 0 {
                panic(&ValueError{"reflect.Value.Interface", 0})
        }
        if v.flag&flagMethod != 0 {
                panic("reflect.Value.Interface: cannot create interface value for method with bound receiver")
        }

        if safe && v.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")
        }

        k := v.kind()
        if k == Interface {
                // Special case: return the element inside the interface.
                // Empty interface has one layout, all interfaces with
                // methods have a second layout.
                if v.NumMethod() == 0 {
                        return *(*interface{})(v.val)
                }
                return *(*interface {
                        M()
                })(v.val)
        }

        // Non-interface value.
        var eface emptyInterface
        eface.typ = v.typ.runtimeType()
        eface.word = v.iword()
        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 {
        v.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.
        // Interface value is always bigger than a word; assume flagIndir.
        return *(*[2]uintptr)(v.val)
}

// 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 {
        k := v.kind()
        switch k {
        case Chan, Func, Map, Ptr:
                if v.flag&flagMethod != 0 {
                        panic("reflect: IsNil of method Value")
                }
                ptr := v.val
                if v.flag&flagIndir != 0 {
                        ptr = *(*unsafe.Pointer)(ptr)
                }
                return ptr == nil
        case Interface, Slice:
                // Both interface and slice are nil if first word is 0.
                // Both are always bigger than a word; assume flagIndir.
                return *(*unsafe.Pointer)(v.val) == nil
        }
        panic(&ValueError{"reflect.Value.IsNil", k})
}

// 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.flag != 0
}

// Kind returns v's Kind.
// If v is the zero Value (IsValid returns false), Kind returns Invalid.
func (v Value) Kind() Kind {
        return v.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 {
        k := v.kind()
        switch k {
        case Array:
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                return int(tt.len)
        case Chan:
                return int(chanlen(*(*iword)(v.iword())))
        case Map:
                return int(maplen(*(*iword)(v.iword())))
        case Slice:
                // Slice is bigger than a word; assume flagIndir.
                return (*SliceHeader)(v.val).Len
        case String:
                // String is bigger than a word; assume flagIndir.
                return (*StringHeader)(v.val).Len
        }
        panic(&ValueError{"reflect.Value.Len", k})
}

// 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 {
        v.mustBe(Map)
        tt := (*mapType)(unsafe.Pointer(v.typ))

        // Do not require key 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.  This is consistent with the
        // behavior for structs, which allow read but not write
        // of unexported fields.
        key = key.assignTo("reflect.Value.MapIndex", toCommonType(tt.key), nil)

        word, ok := mapaccess(v.typ.runtimeType(), *(*iword)(v.iword()), key.iword())
        if !ok {
                return Value{}
        }
        typ := toCommonType(tt.elem)
        fl := (v.flag | key.flag) & flagRO
        if typ.Kind() != Ptr && typ.Kind() != UnsafePointer {
                fl |= flagIndir
        }
        fl |= flag(typ.Kind()) << flagKindShift
        return Value{typ, unsafe.Pointer(word), fl}
}

// 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 {
        v.mustBe(Map)
        tt := (*mapType)(unsafe.Pointer(v.typ))
        keyType := toCommonType(tt.key)

        fl := v.flag & flagRO
        fl |= flag(keyType.Kind()) << flagKindShift
        if keyType.Kind() != Ptr && keyType.Kind() != UnsafePointer {
                fl |= flagIndir
        }

        m := *(*iword)(v.iword())
        mlen := int32(0)
        if m != nil {
                mlen = maplen(m)
        }
        it := mapiterinit(v.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] = Value{keyType, unsafe.Pointer(keyWord), fl}
                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 {
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.Method", Invalid})
        }
        if v.flag&flagMethod != 0 || i < 0 || i >= v.typ.NumMethod() {
                panic("reflect: Method index out of range")
        }
        fl := v.flag & (flagRO | flagAddr | flagIndir)
        fl |= flag(Func) << flagKindShift
        fl |= flag(i)<<flagMethodShift | flagMethod
        return Value{v.typ, v.val, fl}
}

// NumMethod returns the number of methods in the value's method set.
func (v Value) NumMethod() int {
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.NumMethod", Invalid})
        }
        if v.flag&flagMethod != 0 {
                return 0
        }
        return v.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 {
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.MethodByName", Invalid})
        }
        if v.flag&flagMethod != 0 {
                return Value{}
        }
        m, ok := v.typ.MethodByName(name)
        if !ok {
                return Value{}
        }
        return v.Method(m.Index)
}

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

// 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 {
        k := v.kind()
        switch k {
        case Complex64:
                return overflowFloat32(real(x)) || overflowFloat32(imag(x))
        case Complex128:
                return false
        }
        panic(&ValueError{"reflect.Value.OverflowComplex", k})
}

// 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 {
        k := v.kind()
        switch k {
        case Float32:
                return overflowFloat32(x)
        case Float64:
                return false
        }
        panic(&ValueError{"reflect.Value.OverflowFloat", k})
}

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 {
        k := v.kind()
        switch k {
        case Int, Int8, Int16, Int32, Int64:
                bitSize := v.typ.size * 8
                trunc := (x << (64 - bitSize)) >> (64 - bitSize)
                return x != trunc
        }
        panic(&ValueError{"reflect.Value.OverflowInt", k})
}

// 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 {
        k := v.kind()
        switch k {
        case Uint, Uintptr, Uint8, Uint16, Uint32, Uint64:
                bitSize := v.typ.size * 8
                trunc := (x << (64 - bitSize)) >> (64 - bitSize)
                return x != trunc
        }
        panic(&ValueError{"reflect.Value.OverflowUint", k})
}

// 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 {
        k := v.kind()
        switch k {
        case Chan, Func, Map, Ptr, UnsafePointer:
                if k == Func && v.flag&flagMethod != 0 {
                        panic("reflect.Value.Pointer of method Value")
                }
                p := v.val
                if v.flag&flagIndir != 0 {
                        p = *(*unsafe.Pointer)(p)
                }
                return uintptr(p)
        case Slice:
                return (*SliceHeader)(v.val).Data
        }
        panic(&ValueError{"reflect.Value.Pointer", k})
}

// 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) {
        v.mustBe(Chan)
        v.mustBeExported()
        return v.recv(false)
}

// internal recv, possibly non-blocking (nb).
// v is known to be a channel.
func (v Value) recv(nb bool) (val Value, ok bool) {
        tt := (*chanType)(unsafe.Pointer(v.typ))
        if ChanDir(tt.dir)&RecvDir == 0 {
                panic("recv on send-only channel")
        }
        word, selected, ok := chanrecv(v.typ.runtimeType(), *(*iword)(v.iword()), nb)
        if selected {
                typ := toCommonType(tt.elem)
                fl := flag(typ.Kind()) << flagKindShift
                if typ.Kind() != Ptr && typ.Kind() != UnsafePointer {
                        fl |= flagIndir
                }
                val = Value{typ, unsafe.Pointer(word), fl}
        }
        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) {
        v.mustBe(Chan)
        v.mustBeExported()
        v.send(x, false)
}

// internal send, possibly non-blocking.
// v is known to be a channel.
func (v Value) send(x Value, nb bool) (selected bool) {
        tt := (*chanType)(unsafe.Pointer(v.typ))
        if ChanDir(tt.dir)&SendDir == 0 {
                panic("send on recv-only channel")
        }
        x.mustBeExported()
        x = x.assignTo("reflect.Value.Send", toCommonType(tt.elem), nil)
        return chansend(v.typ.runtimeType(), *(*iword)(v.iword()), x.iword(), 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) {
        v.mustBeAssignable()
        x.mustBeExported() // do not let unexported x leak
        var target *interface{}
        if v.kind() == Interface {
                target = (*interface{})(v.val)
        }
        x = x.assignTo("reflect.Set", v.typ, target)
        if x.flag&flagIndir != 0 {
                memmove(v.val, x.val, v.typ.size)
        } else {
                storeIword(v.val, iword(x.val), v.typ.size)
        }
}

// 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) {
        v.mustBeAssignable()
        v.mustBe(Bool)
        *(*bool)(v.val) = 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) {
        v.mustBeAssignable()
        v.mustBe(Slice)
        if v.typ.Elem().Kind() != Uint8 {
                panic("reflect.Value.SetBytes of non-byte slice")
        }
        *(*[]byte)(v.val) = 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) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetComplex", k})
        case Complex64:
                *(*complex64)(v.val) = complex64(x)
        case Complex128:
                *(*complex128)(v.val) = 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) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetFloat", k})
        case Float32:
                *(*float32)(v.val) = float32(x)
        case Float64:
                *(*float64)(v.val) = 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) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetInt", k})
        case Int:
                *(*int)(v.val) = int(x)
        case Int8:
                *(*int8)(v.val) = int8(x)
        case Int16:
                *(*int16)(v.val) = int16(x)
        case Int32:
                *(*int32)(v.val) = int32(x)
        case Int64:
                *(*int64)(v.val) = x
        }
}

// SetLen sets v's length to n.
// It panics if v's Kind is not Slice.
func (v Value) SetLen(n int) {
        v.mustBeAssignable()
        v.mustBe(Slice)
        s := (*SliceHeader)(v.val)
        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) {
        v.mustBe(Map)
        v.mustBeExported()
        key.mustBeExported()
        tt := (*mapType)(unsafe.Pointer(v.typ))
        key = key.assignTo("reflect.Value.SetMapIndex", toCommonType(tt.key), nil)
        if val.typ != nil {
                val.mustBeExported()
                val = val.assignTo("reflect.Value.SetMapIndex", toCommonType(tt.elem), nil)
        }
        mapassign(v.typ.runtimeType(), *(*iword)(v.iword()), key.iword(), val.iword(), val.typ != nil)
}

// 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) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetUint", k})
        case Uint:
                *(*uint)(v.val) = uint(x)
        case Uint8:
                *(*uint8)(v.val) = uint8(x)
        case Uint16:
                *(*uint16)(v.val) = uint16(x)
        case Uint32:
                *(*uint32)(v.val) = uint32(x)
        case Uint64:
                *(*uint64)(v.val) = x
        case Uintptr:
                *(*uintptr)(v.val) = 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) {
        v.mustBeAssignable()
        v.mustBe(UnsafePointer)
        *(*unsafe.Pointer)(v.val) = 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) {
        v.mustBeAssignable()
        v.mustBe(String)
        *(*string)(v.val) = 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 {
        var (
                cap  int
                typ  *sliceType
                base unsafe.Pointer
        )
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.Slice", k})
        case Array:
                if v.flag&flagAddr == 0 {
                        panic("reflect.Value.Slice: slice of unaddressable array")
                }
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                cap = int(tt.len)
                typ = (*sliceType)(unsafe.Pointer(toCommonType(tt.slice)))
                base = v.val
        case Slice:
                typ = (*sliceType)(unsafe.Pointer(v.typ))
                s := (*SliceHeader)(v.val)
                base = unsafe.Pointer(s.Data)
                cap = s.Cap

        }
        if beg < 0 || end < beg || end > cap {
                panic("reflect.Value.Slice: slice index out of bounds")
        }

        // 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(base) + uintptr(beg)*toCommonType(typ.elem).Size()
        s.Len = end - beg
        s.Cap = cap - beg

        fl := v.flag&flagRO | flagIndir | flag(Slice)<<flagKindShift
        return Value{typ.common(), unsafe.Pointer(&x), fl}
}

// 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 {
        switch k := v.kind(); k {
        case Invalid:
                return "<invalid Value>"
        case String:
                return *(*string)(v.val)
        }
        // If you call String on a reflect.Value of other type, it's better to
        // print something than to panic. Useful in debugging.
        return "<" + v.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) {
        v.mustBe(Chan)
        v.mustBeExported()
        return v.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 {
        v.mustBe(Chan)
        v.mustBeExported()
        return v.send(x, true)
}

// Type returns v's type.
func (v Value) Type() Type {
        f := v.flag
        if f == 0 {
                panic(&ValueError{"reflect.Value.Type", Invalid})
        }
        if f&flagMethod == 0 {
                // Easy case
                return v.typ.toType()
        }

        // Method value.
        // v.typ describes the receiver, not the method type.
        i := int(v.flag) >> flagMethodShift
        if v.typ.Kind() == Interface {
                // Method on interface.
                tt := (*interfaceType)(unsafe.Pointer(v.typ))
                if i < 0 || i >= len(tt.methods) {
                        panic("reflect: broken Value")
                }
                m := &tt.methods[i]
                return toCommonType(m.typ).toType()
        }
        // Method on concrete type.
        ut := v.typ.uncommon()
        if ut == nil || i < 0 || i >= len(ut.methods) {
                panic("reflect: broken Value")
        }
        m := &ut.methods[i]
        return toCommonType(m.mtyp).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 {
        k := v.kind()
        var p unsafe.Pointer
        if v.flag&flagIndir != 0 {
                p = v.val
        } else {
                // The escape analysis is good enough that &v.val
                // does not trigger a heap allocation.
                p = unsafe.Pointer(&v.val)
        }
        switch k {
        case Uint:
                return uint64(*(*uint)(p))
        case Uint8:
                return uint64(*(*uint8)(p))
        case Uint16:
                return uint64(*(*uint16)(p))
        case Uint32:
                return uint64(*(*uint32)(p))
        case Uint64:
                return uint64(*(*uint64)(p))
        case Uintptr:
                return uint64(*(*uintptr)(p))
        }
        panic(&ValueError{"reflect.Value.Uint", k})
}

// 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 {
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.UnsafeAddr", Invalid})
        }
        if v.flag&flagAddr == 0 {
                panic("reflect.Value.UnsafeAddr of unaddressable value")
        }
        return uintptr(v.val)
}

// 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(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.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.mustBe(Slice)
        t.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 {
        dk := dst.kind()
        if dk != Array && dk != Slice {
                panic(&ValueError{"reflect.Copy", dk})
        }
        if dk == Array {
                dst.mustBeAssignable()
        }
        dst.mustBeExported()

        sk := src.kind()
        if sk != Array && sk != Slice {
                panic(&ValueError{"reflect.Copy", sk})
        }
        src.mustBeExported()

        de := dst.typ.Elem()
        se := src.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 src.flag&flagIndir == 0 {
                for i := 0; i < n; i++ {
                        dst.Index(i).Set(src.Index(i))
                }
                return n
        }

        // Copy via memmove.
        var da, sa unsafe.Pointer
        if dk == Array {
                da = dst.val
        } else {
                da = unsafe.Pointer((*SliceHeader)(dst.val).Data)
        }
        if sk == Array {
                sa = src.val
        } else {
                sa = unsafe.Pointer((*SliceHeader)(src.val).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 Value{typ.common(), unsafe.Pointer(&x), flagIndir | flag(Slice)<<flagKindShift}
}

// 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("reflect.MakeChan: negative buffer size")
        }
        if typ.ChanDir() != BothDir {
                panic("reflect.MakeChan: unidirectional channel type")
        }
        ch := makechan(typ.runtimeType(), uint32(buffer))
        return Value{typ.common(), unsafe.Pointer(ch), flagIndir | (flag(Chan) << flagKindShift)}
}

// 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 Value{typ.common(), unsafe.Pointer(m), flagIndir | (flag(Map) << flagKindShift)}
}

// 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))
        typ := toCommonType(eface.typ)
        fl := flag(typ.Kind()) << flagKindShift
        if typ.Kind() != Ptr && typ.Kind() != UnsafePointer {
                fl |= flagIndir
        }
        return Value{typ, unsafe.Pointer(eface.word), fl}
}

// 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)")
        }
        t := typ.common()
        fl := flag(t.Kind()) << flagKindShift
        if t.Kind() == Ptr || t.Kind() == UnsafePointer {
                return Value{t, nil, fl}
        }
        return Value{t, unsafe.New(typ), fl | flagIndir}
}

// 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)
        fl := flag(Ptr) << flagKindShift
        return Value{typ.common().ptrTo(), ptr, fl}
}

// assignTo returns a value v that can be assigned directly to typ.
// It panics if v is not assignable to typ.
// For a conversion to an interface type, target is a suggested scratch space to use.
func (v Value) assignTo(context string, dst *commonType, target *interface{}) Value {
        if v.flag&flagMethod != 0 {
                panic(context + ": cannot assign method value to type " + dst.String())
        }

        switch {
        case directlyAssignable(dst, v.typ):
                // Overwrite type so that they match.
                // Same memory layout, so no harm done.
                v.typ = dst
                fl := v.flag & (flagRO | flagAddr | flagIndir)
                fl |= flag(dst.Kind()) << flagKindShift
                return Value{dst, v.val, fl}

        case implements(dst, v.typ):
                if target == nil {
                        target = new(interface{})
                }
                x := valueInterface(v, false)
                if dst.NumMethod() == 0 {
                        *target = x
                } else {
                        ifaceE2I(dst.runtimeType(), x, unsafe.Pointer(target))
                }
                return Value{dst, unsafe.Pointer(target), flagIndir | flag(Interface)<<flagKindShift}
        }

        // Failed.
        panic(context + ": value of type " + v.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) (m 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)

// 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
        }
}

var dummy struct {
        b bool
        x interface{}
}