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// Copyright 2017, 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 cmp
import (
// Path is a list of PathSteps describing the sequence of operations to get
// from some root type to the current position in the value tree.
// The first Path element is always an operation-less PathStep that exists
// simply to identify the initial type.
// When traversing structs with embedded structs, the embedded struct will
// always be accessed as a field before traversing the fields of the
// embedded struct themselves. That is, an exported field from the
// embedded struct will never be accessed directly from the parent struct.
type Path []PathStep
// PathStep is a union-type for specific operations to traverse
// a value's tree structure. Users of this package never need to implement
// these types as values of this type will be returned by this package.
// Implementations of this interface are
// StructField, SliceIndex, MapIndex, Indirect, TypeAssertion, and Transform.
type PathStep interface {
String() string
// Type is the resulting type after performing the path step.
Type() reflect.Type
// Values is the resulting values after performing the path step.
// The type of each valid value is guaranteed to be identical to Type.
// In some cases, one or both may be invalid or have restrictions:
// • For StructField, both are not interface-able if the current field
// is unexported and the struct type is not explicitly permitted by
// an Exporter to traverse unexported fields.
// • For SliceIndex, one may be invalid if an element is missing from
// either the x or y slice.
// • For MapIndex, one may be invalid if an entry is missing from
// either the x or y map.
// The provided values must not be mutated.
Values() (vx, vy reflect.Value)
var (
_ PathStep = StructField{}
_ PathStep = SliceIndex{}
_ PathStep = MapIndex{}
_ PathStep = Indirect{}
_ PathStep = TypeAssertion{}
_ PathStep = Transform{}
func (pa *Path) push(s PathStep) {
*pa = append(*pa, s)
func (pa *Path) pop() {
*pa = (*pa)[:len(*pa)-1]
// Last returns the last PathStep in the Path.
// If the path is empty, this returns a non-nil PathStep that reports a nil Type.
func (pa Path) Last() PathStep {
return pa.Index(-1)
// Index returns the ith step in the Path and supports negative indexing.
// A negative index starts counting from the tail of the Path such that -1
// refers to the last step, -2 refers to the second-to-last step, and so on.
// If index is invalid, this returns a non-nil PathStep that reports a nil Type.
func (pa Path) Index(i int) PathStep {
if i < 0 {
i = len(pa) + i
if i < 0 || i >= len(pa) {
return pathStep{}
return pa[i]
// String returns the simplified path to a node.
// The simplified path only contains struct field accesses.
// For example:
// MyMap.MySlices.MyField
func (pa Path) String() string {
var ss []string
for _, s := range pa {
if _, ok := s.(StructField); ok {
ss = append(ss, s.String())
return strings.TrimPrefix(strings.Join(ss, ""), ".")
// GoString returns the path to a specific node using Go syntax.
// For example:
// (*root.MyMap["key"].(*mypkg.MyStruct).MySlices)[2][3].MyField
func (pa Path) GoString() string {
var ssPre, ssPost []string
var numIndirect int
for i, s := range pa {
var nextStep PathStep
if i+1 < len(pa) {
nextStep = pa[i+1]
switch s := s.(type) {
case Indirect:
pPre, pPost := "(", ")"
switch nextStep.(type) {
case Indirect:
continue // Next step is indirection, so let them batch up
case StructField:
numIndirect-- // Automatic indirection on struct fields
case nil:
pPre, pPost = "", "" // Last step; no need for parenthesis
if numIndirect > 0 {
ssPre = append(ssPre, pPre+strings.Repeat("*", numIndirect))
ssPost = append(ssPost, pPost)
numIndirect = 0
case Transform:
ssPre = append(ssPre,"(")
ssPost = append(ssPost, ")")
ssPost = append(ssPost, s.String())
for i, j := 0, len(ssPre)-1; i < j; i, j = i+1, j-1 {
ssPre[i], ssPre[j] = ssPre[j], ssPre[i]
return strings.Join(ssPre, "") + strings.Join(ssPost, "")
type pathStep struct {
typ reflect.Type
vx, vy reflect.Value
func (ps pathStep) Type() reflect.Type { return ps.typ }
func (ps pathStep) Values() (vx, vy reflect.Value) { return ps.vx, ps.vy }
func (ps pathStep) String() string {
if ps.typ == nil {
return "<nil>"
s := ps.typ.String()
if s == "" || strings.ContainsAny(s, "{}\n") {
return "root" // Type too simple or complex to print
return fmt.Sprintf("{%s}", s)
// StructField represents a struct field access on a field called Name.
type StructField struct{ *structField }
type structField struct {
name string
idx int
// These fields are used for forcibly accessing an unexported field.
// pvx, pvy, and field are only valid if unexported is true.
unexported bool
mayForce bool // Forcibly allow visibility
paddr bool // Was parent addressable?
pvx, pvy reflect.Value // Parent values (always addressable)
field reflect.StructField // Field information
func (sf StructField) Type() reflect.Type { return sf.typ }
func (sf StructField) Values() (vx, vy reflect.Value) {
if !sf.unexported {
return sf.vx, sf.vy // CanInterface reports true
// Forcibly obtain read-write access to an unexported struct field.
if sf.mayForce {
vx = retrieveUnexportedField(sf.pvx, sf.field, sf.paddr)
vy = retrieveUnexportedField(sf.pvy, sf.field, sf.paddr)
return vx, vy // CanInterface reports true
return sf.vx, sf.vy // CanInterface reports false
func (sf StructField) String() string { return fmt.Sprintf(".%s", }
// Name is the field name.
func (sf StructField) Name() string { return }
// Index is the index of the field in the parent struct type.
// See reflect.Type.Field.
func (sf StructField) Index() int { return sf.idx }
// SliceIndex is an index operation on a slice or array at some index Key.
type SliceIndex struct{ *sliceIndex }
type sliceIndex struct {
xkey, ykey int
isSlice bool // False for reflect.Array
func (si SliceIndex) Type() reflect.Type { return si.typ }
func (si SliceIndex) Values() (vx, vy reflect.Value) { return si.vx, si.vy }
func (si SliceIndex) String() string {
switch {
case si.xkey == si.ykey:
return fmt.Sprintf("[%d]", si.xkey)
case si.ykey == -1:
// [5->?] means "I don't know where X[5] went"
return fmt.Sprintf("[%d->?]", si.xkey)
case si.xkey == -1:
// [?->3] means "I don't know where Y[3] came from"
return fmt.Sprintf("[?->%d]", si.ykey)
// [5->3] means "X[5] moved to Y[3]"
return fmt.Sprintf("[%d->%d]", si.xkey, si.ykey)
// Key is the index key; it may return -1 if in a split state
func (si SliceIndex) Key() int {
if si.xkey != si.ykey {
return -1
return si.xkey
// SplitKeys are the indexes for indexing into slices in the
// x and y values, respectively. These indexes may differ due to the
// insertion or removal of an element in one of the slices, causing
// all of the indexes to be shifted. If an index is -1, then that
// indicates that the element does not exist in the associated slice.
// Key is guaranteed to return -1 if and only if the indexes returned
// by SplitKeys are not the same. SplitKeys will never return -1 for
// both indexes.
func (si SliceIndex) SplitKeys() (ix, iy int) { return si.xkey, si.ykey }
// MapIndex is an index operation on a map at some index Key.
type MapIndex struct{ *mapIndex }
type mapIndex struct {
key reflect.Value
func (mi MapIndex) Type() reflect.Type { return mi.typ }
func (mi MapIndex) Values() (vx, vy reflect.Value) { return mi.vx, mi.vy }
func (mi MapIndex) String() string { return fmt.Sprintf("[%#v]", mi.key) }
// Key is the value of the map key.
func (mi MapIndex) Key() reflect.Value { return mi.key }
// Indirect represents pointer indirection on the parent type.
type Indirect struct{ *indirect }
type indirect struct {
func (in Indirect) Type() reflect.Type { return in.typ }
func (in Indirect) Values() (vx, vy reflect.Value) { return in.vx, in.vy }
func (in Indirect) String() string { return "*" }
// TypeAssertion represents a type assertion on an interface.
type TypeAssertion struct{ *typeAssertion }
type typeAssertion struct {
func (ta TypeAssertion) Type() reflect.Type { return ta.typ }
func (ta TypeAssertion) Values() (vx, vy reflect.Value) { return ta.vx, ta.vy }
func (ta TypeAssertion) String() string { return fmt.Sprintf(".(%v)", ta.typ) }
// Transform is a transformation from the parent type to the current type.
type Transform struct{ *transform }
type transform struct {
trans *transformer
func (tf Transform) Type() reflect.Type { return tf.typ }
func (tf Transform) Values() (vx, vy reflect.Value) { return tf.vx, tf.vy }
func (tf Transform) String() string { return fmt.Sprintf("%s()", }
// Name is the name of the Transformer.
func (tf Transform) Name() string { return }
// Func is the function pointer to the transformer function.
func (tf Transform) Func() reflect.Value { return tf.trans.fnc }
// Option returns the originally constructed Transformer option.
// The == operator can be used to detect the exact option used.
func (tf Transform) Option() Option { return tf.trans }
// pointerPath represents a dual-stack of pointers encountered when
// recursively traversing the x and y values. This data structure supports
// detection of cycles and determining whether the cycles are equal.
// In Go, cycles can occur via pointers, slices, and maps.
// The pointerPath uses a map to represent a stack; where descension into a
// pointer pushes the address onto the stack, and ascension from a pointer
// pops the address from the stack. Thus, when traversing into a pointer from
// reflect.Ptr, reflect.Slice element, or reflect.Map, we can detect cycles
// by checking whether the pointer has already been visited. The cycle detection
// uses a separate stack for the x and y values.
// If a cycle is detected we need to determine whether the two pointers
// should be considered equal. The definition of equality chosen by Equal
// requires two graphs to have the same structure. To determine this, both the
// x and y values must have a cycle where the previous pointers were also
// encountered together as a pair.
// Semantically, this is equivalent to augmenting Indirect, SliceIndex, and
// MapIndex with pointer information for the x and y values.
// Suppose px and py are two pointers to compare, we then search the
// Path for whether px was ever encountered in the Path history of x, and
// similarly so with py. If either side has a cycle, the comparison is only
// equal if both px and py have a cycle resulting from the same PathStep.
// Using a map as a stack is more performant as we can perform cycle detection
// in O(1) instead of O(N) where N is len(Path).
type pointerPath struct {
// mx is keyed by x pointers, where the value is the associated y pointer.
mx map[value.Pointer]value.Pointer
// my is keyed by y pointers, where the value is the associated x pointer.
my map[value.Pointer]value.Pointer
func (p *pointerPath) Init() { = make(map[value.Pointer]value.Pointer) = make(map[value.Pointer]value.Pointer)
// Push indicates intent to descend into pointers vx and vy where
// visited reports whether either has been seen before. If visited before,
// equal reports whether both pointers were encountered together.
// Pop must be called if and only if the pointers were never visited.
// The pointers vx and vy must be a reflect.Ptr, reflect.Slice, or reflect.Map
// and be non-nil.
func (p pointerPath) Push(vx, vy reflect.Value) (equal, visited bool) {
px := value.PointerOf(vx)
py := value.PointerOf(vy)
_, ok1 :=[px]
_, ok2 :=[py]
if ok1 || ok2 {
equal =[px] == py &&[py] == px // Pointers paired together
return equal, true
}[px] = py[py] = px
return false, false
// Pop ascends from pointers vx and vy.
func (p pointerPath) Pop(vx, vy reflect.Value) {
delete(, value.PointerOf(vx))
delete(, value.PointerOf(vy))
// isExported reports whether the identifier is exported.
func isExported(id string) bool {
r, _ := utf8.DecodeRuneInString(id)
return unicode.IsUpper(r)