blob: 087320da7f0f515a54f0bbef192284b14d69e80d [file] [log] [blame]
// 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 determines equality of values.
// This package is intended to be a more powerful and safer alternative to
// reflect.DeepEqual for comparing whether two values are semantically equal.
// It is intended to only be used in tests, as performance is not a goal and
// it may panic if it cannot compare the values. Its propensity towards
// panicking means that its unsuitable for production environments where a
// spurious panic may be fatal.
// The primary features of cmp are:
// - When the default behavior of equality does not suit the test's needs,
// custom equality functions can override the equality operation.
// For example, an equality function may report floats as equal so long as
// they are within some tolerance of each other.
// - Types with an Equal method may use that method to determine equality.
// This allows package authors to determine the equality operation
// for the types that they define.
// - If no custom equality functions are used and no Equal method is defined,
// equality is determined by recursively comparing the primitive kinds on
// both values, much like reflect.DeepEqual. Unlike reflect.DeepEqual,
// unexported fields are not compared by default; they result in panics
// unless suppressed by using an Ignore option (see cmpopts.IgnoreUnexported)
// or explicitly compared using the Exporter option.
package cmp
import (
// TODO(≥go1.18): Use any instead of interface{}.
// Equal reports whether x and y are equal by recursively applying the
// following rules in the given order to x and y and all of their sub-values:
// - Let S be the set of all Ignore, Transformer, and Comparer options that
// remain after applying all path filters, value filters, and type filters.
// If at least one Ignore exists in S, then the comparison is ignored.
// If the number of Transformer and Comparer options in S is non-zero,
// then Equal panics because it is ambiguous which option to use.
// If S contains a single Transformer, then use that to transform
// the current values and recursively call Equal on the output values.
// If S contains a single Comparer, then use that to compare the current values.
// Otherwise, evaluation proceeds to the next rule.
// - If the values have an Equal method of the form "(T) Equal(T) bool" or
// "(T) Equal(I) bool" where T is assignable to I, then use the result of
// x.Equal(y) even if x or y is nil. Otherwise, no such method exists and
// evaluation proceeds to the next rule.
// - Lastly, try to compare x and y based on their basic kinds.
// Simple kinds like booleans, integers, floats, complex numbers, strings,
// and channels are compared using the equivalent of the == operator in Go.
// Functions are only equal if they are both nil, otherwise they are unequal.
// Structs are equal if recursively calling Equal on all fields report equal.
// If a struct contains unexported fields, Equal panics unless an Ignore option
// (e.g., cmpopts.IgnoreUnexported) ignores that field or the Exporter option
// explicitly permits comparing the unexported field.
// Slices are equal if they are both nil or both non-nil, where recursively
// calling Equal on all non-ignored slice or array elements report equal.
// Empty non-nil slices and nil slices are not equal; to equate empty slices,
// consider using cmpopts.EquateEmpty.
// Maps are equal if they are both nil or both non-nil, where recursively
// calling Equal on all non-ignored map entries report equal.
// Map keys are equal according to the == operator.
// To use custom comparisons for map keys, consider using cmpopts.SortMaps.
// Empty non-nil maps and nil maps are not equal; to equate empty maps,
// consider using cmpopts.EquateEmpty.
// Pointers and interfaces are equal if they are both nil or both non-nil,
// where they have the same underlying concrete type and recursively
// calling Equal on the underlying values reports equal.
// Before recursing into a pointer, slice element, or map, the current path
// is checked to detect whether the address has already been visited.
// If there is a cycle, then the pointed at values are considered equal
// only if both addresses were previously visited in the same path step.
func Equal(x, y interface{}, opts ...Option) bool {
s := newState(opts)
s.compareAny(rootStep(x, y))
return s.result.Equal()
// Diff returns a human-readable report of the differences between two values:
// y - x. It returns an empty string if and only if Equal returns true for the
// same input values and options.
// The output is displayed as a literal in pseudo-Go syntax.
// At the start of each line, a "-" prefix indicates an element removed from x,
// a "+" prefix to indicates an element added from y, and the lack of a prefix
// indicates an element common to both x and y. If possible, the output
// uses fmt.Stringer.String or error.Error methods to produce more humanly
// readable outputs. In such cases, the string is prefixed with either an
// 's' or 'e' character, respectively, to indicate that the method was called.
// Do not depend on this output being stable. If you need the ability to
// programmatically interpret the difference, consider using a custom Reporter.
func Diff(x, y interface{}, opts ...Option) string {
s := newState(opts)
// Optimization: If there are no other reporters, we can optimize for the
// common case where the result is equal (and thus no reported difference).
// This avoids the expensive construction of a difference tree.
if len(s.reporters) == 0 {
s.compareAny(rootStep(x, y))
if s.result.Equal() {
return ""
s.result = diff.Result{} // Reset results
r := new(defaultReporter)
s.reporters = append(s.reporters, reporter{r})
s.compareAny(rootStep(x, y))
d := r.String()
if (d == "") != s.result.Equal() {
panic("inconsistent difference and equality results")
return d
// rootStep constructs the first path step. If x and y have differing types,
// then they are stored within an empty interface type.
func rootStep(x, y interface{}) PathStep {
vx := reflect.ValueOf(x)
vy := reflect.ValueOf(y)
// If the inputs are different types, auto-wrap them in an empty interface
// so that they have the same parent type.
var t reflect.Type
if !vx.IsValid() || !vy.IsValid() || vx.Type() != vy.Type() {
t = anyType
if vx.IsValid() {
vvx := reflect.New(t).Elem()
vx = vvx
if vy.IsValid() {
vvy := reflect.New(t).Elem()
vy = vvy
} else {
t = vx.Type()
return &pathStep{t, vx, vy}
type state struct {
// These fields represent the "comparison state".
// Calling statelessCompare must not result in observable changes to these.
result diff.Result // The current result of comparison
curPath Path // The current path in the value tree
curPtrs pointerPath // The current set of visited pointers
reporters []reporter // Optional reporters
// recChecker checks for infinite cycles applying the same set of
// transformers upon the output of itself.
recChecker recChecker
// dynChecker triggers pseudo-random checks for option correctness.
// It is safe for statelessCompare to mutate this value.
dynChecker dynChecker
// These fields, once set by processOption, will not change.
exporters []exporter // List of exporters for structs with unexported fields
opts Options // List of all fundamental and filter options
func newState(opts []Option) *state {
// Always ensure a validator option exists to validate the inputs.
s := &state{opts: Options{validator{}}}
return s
func (s *state) processOption(opt Option) {
switch opt := opt.(type) {
case nil:
case Options:
for _, o := range opt {
case coreOption:
type filtered interface {
isFiltered() bool
if fopt, ok := opt.(filtered); ok && !fopt.isFiltered() {
panic(fmt.Sprintf("cannot use an unfiltered option: %v", opt))
s.opts = append(s.opts, opt)
case exporter:
s.exporters = append(s.exporters, opt)
case reporter:
s.reporters = append(s.reporters, opt)
panic(fmt.Sprintf("unknown option %T", opt))
// statelessCompare compares two values and returns the result.
// This function is stateless in that it does not alter the current result,
// or output to any registered reporters.
func (s *state) statelessCompare(step PathStep) diff.Result {
// We do not save and restore curPath and curPtrs because all of the
// compareX methods should properly push and pop from them.
// It is an implementation bug if the contents of the paths differ from
// when calling this function to when returning from it.
oldResult, oldReporters := s.result, s.reporters
s.result = diff.Result{} // Reset result
s.reporters = nil // Remove reporters to avoid spurious printouts
res := s.result
s.result, s.reporters = oldResult, oldReporters
return res
func (s *state) compareAny(step PathStep) {
// Update the path stack.
defer s.curPath.pop()
for _, r := range s.reporters {
defer r.PopStep()
// Cycle-detection for slice elements (see NOTE in compareSlice).
t := step.Type()
vx, vy := step.Values()
if si, ok := step.(SliceIndex); ok && si.isSlice && vx.IsValid() && vy.IsValid() {
px, py := vx.Addr(), vy.Addr()
if eq, visited := s.curPtrs.Push(px, py); visited {, reportByCycle)
defer s.curPtrs.Pop(px, py)
// Rule 1: Check whether an option applies on this node in the value tree.
if s.tryOptions(t, vx, vy) {
// Rule 2: Check whether the type has a valid Equal method.
if s.tryMethod(t, vx, vy) {
// Rule 3: Compare based on the underlying kind.
switch t.Kind() {
case reflect.Bool: == vy.Bool(), 0)
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64: == vy.Int(), 0)
case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr: == vy.Uint(), 0)
case reflect.Float32, reflect.Float64: == vy.Float(), 0)
case reflect.Complex64, reflect.Complex128: == vy.Complex(), 0)
case reflect.String: == vy.String(), 0)
case reflect.Chan, reflect.UnsafePointer: == vy.Pointer(), 0)
case reflect.Func: && vy.IsNil(), 0)
case reflect.Struct:
s.compareStruct(t, vx, vy)
case reflect.Slice, reflect.Array:
s.compareSlice(t, vx, vy)
case reflect.Map:
s.compareMap(t, vx, vy)
case reflect.Ptr:
s.comparePtr(t, vx, vy)
case reflect.Interface:
s.compareInterface(t, vx, vy)
panic(fmt.Sprintf("%v kind not handled", t.Kind()))
func (s *state) tryOptions(t reflect.Type, vx, vy reflect.Value) bool {
// Evaluate all filters and apply the remaining options.
if opt := s.opts.filter(s, t, vx, vy); opt != nil {
opt.apply(s, vx, vy)
return true
return false
func (s *state) tryMethod(t reflect.Type, vx, vy reflect.Value) bool {
// Check if this type even has an Equal method.
m, ok := t.MethodByName("Equal")
if !ok || !function.IsType(m.Type, function.EqualAssignable) {
return false
eq := s.callTTBFunc(m.Func, vx, vy), reportByMethod)
return true
func (s *state) callTRFunc(f, v reflect.Value, step Transform) reflect.Value {
if !s.dynChecker.Next() {
return f.Call([]reflect.Value{v})[0]
// Run the function twice and ensure that we get the same results back.
// We run in goroutines so that the race detector (if enabled) can detect
// unsafe mutations to the input.
c := make(chan reflect.Value)
go detectRaces(c, f, v)
got := <-c
want := f.Call([]reflect.Value{v})[0]
if step.vx, step.vy = got, want; !s.statelessCompare(step).Equal() {
// To avoid false-positives with non-reflexive equality operations,
// we sanity check whether a value is equal to itself.
if step.vx, step.vy = want, want; !s.statelessCompare(step).Equal() {
return want
panic(fmt.Sprintf("non-deterministic function detected: %s", function.NameOf(f)))
return want
func (s *state) callTTBFunc(f, x, y reflect.Value) bool {
if !s.dynChecker.Next() {
return f.Call([]reflect.Value{x, y})[0].Bool()
// Swapping the input arguments is sufficient to check that
// f is symmetric and deterministic.
// We run in goroutines so that the race detector (if enabled) can detect
// unsafe mutations to the input.
c := make(chan reflect.Value)
go detectRaces(c, f, y, x)
got := <-c
want := f.Call([]reflect.Value{x, y})[0].Bool()
if !got.IsValid() || got.Bool() != want {
panic(fmt.Sprintf("non-deterministic or non-symmetric function detected: %s", function.NameOf(f)))
return want
func detectRaces(c chan<- reflect.Value, f reflect.Value, vs ...reflect.Value) {
var ret reflect.Value
defer func() {
recover() // Ignore panics, let the other call to f panic instead
c <- ret
ret = f.Call(vs)[0]
func (s *state) compareStruct(t reflect.Type, vx, vy reflect.Value) {
var addr bool
var vax, vay reflect.Value // Addressable versions of vx and vy
var mayForce, mayForceInit bool
step := StructField{&structField{}}
for i := 0; i < t.NumField(); i++ {
step.typ = t.Field(i).Type
step.vx = vx.Field(i)
step.vy = vy.Field(i) = t.Field(i).Name
step.idx = i
step.unexported = !isExported(
if step.unexported {
if == "_" {
// Defer checking of unexported fields until later to give an
// Ignore a chance to ignore the field.
if !vax.IsValid() || !vay.IsValid() {
// For retrieveUnexportedField to work, the parent struct must
// be addressable. Create a new copy of the values if
// necessary to make them addressable.
addr = vx.CanAddr() || vy.CanAddr()
vax = makeAddressable(vx)
vay = makeAddressable(vy)
if !mayForceInit {
for _, xf := range s.exporters {
mayForce = mayForce || xf(t)
mayForceInit = true
step.mayForce = mayForce
step.paddr = addr
step.pvx = vax
step.pvy = vay
step.field = t.Field(i)
func (s *state) compareSlice(t reflect.Type, vx, vy reflect.Value) {
isSlice := t.Kind() == reflect.Slice
if isSlice && (vx.IsNil() || vy.IsNil()) { && vy.IsNil(), 0)
// NOTE: It is incorrect to call curPtrs.Push on the slice header pointer
// since slices represents a list of pointers, rather than a single pointer.
// The pointer checking logic must be handled on a per-element basis
// in compareAny.
// A slice header (see reflect.SliceHeader) in Go is a tuple of a starting
// pointer P, a length N, and a capacity C. Supposing each slice element has
// a memory size of M, then the slice is equivalent to the list of pointers:
// [P+i*M for i in range(N)]
// For example, v[:0] and v[:1] are slices with the same starting pointer,
// but they are clearly different values. Using the slice pointer alone
// violates the assumption that equal pointers implies equal values.
step := SliceIndex{&sliceIndex{pathStep: pathStep{typ: t.Elem()}, isSlice: isSlice}}
withIndexes := func(ix, iy int) SliceIndex {
if ix >= 0 {
step.vx, step.xkey = vx.Index(ix), ix
} else {
step.vx, step.xkey = reflect.Value{}, -1
if iy >= 0 {
step.vy, step.ykey = vy.Index(iy), iy
} else {
step.vy, step.ykey = reflect.Value{}, -1
return step
// Ignore options are able to ignore missing elements in a slice.
// However, detecting these reliably requires an optimal differencing
// algorithm, for which diff.Difference is not.
// Instead, we first iterate through both slices to detect which elements
// would be ignored if standing alone. The index of non-discarded elements
// are stored in a separate slice, which diffing is then performed on.
var indexesX, indexesY []int
var ignoredX, ignoredY []bool
for ix := 0; ix < vx.Len(); ix++ {
ignored := s.statelessCompare(withIndexes(ix, -1)).NumDiff == 0
if !ignored {
indexesX = append(indexesX, ix)
ignoredX = append(ignoredX, ignored)
for iy := 0; iy < vy.Len(); iy++ {
ignored := s.statelessCompare(withIndexes(-1, iy)).NumDiff == 0
if !ignored {
indexesY = append(indexesY, iy)
ignoredY = append(ignoredY, ignored)
// Compute an edit-script for slices vx and vy (excluding ignored elements).
edits := diff.Difference(len(indexesX), len(indexesY), func(ix, iy int) diff.Result {
return s.statelessCompare(withIndexes(indexesX[ix], indexesY[iy]))
// Replay the ignore-scripts and the edit-script.
var ix, iy int
for ix < vx.Len() || iy < vy.Len() {
var e diff.EditType
switch {
case ix < len(ignoredX) && ignoredX[ix]:
e = diff.UniqueX
case iy < len(ignoredY) && ignoredY[iy]:
e = diff.UniqueY
e, edits = edits[0], edits[1:]
switch e {
case diff.UniqueX:
s.compareAny(withIndexes(ix, -1))
case diff.UniqueY:
s.compareAny(withIndexes(-1, iy))
s.compareAny(withIndexes(ix, iy))
func (s *state) compareMap(t reflect.Type, vx, vy reflect.Value) {
if vx.IsNil() || vy.IsNil() { && vy.IsNil(), 0)
// Cycle-detection for maps.
if eq, visited := s.curPtrs.Push(vx, vy); visited {, reportByCycle)
defer s.curPtrs.Pop(vx, vy)
// We combine and sort the two map keys so that we can perform the
// comparisons in a deterministic order.
step := MapIndex{&mapIndex{pathStep: pathStep{typ: t.Elem()}}}
for _, k := range value.SortKeys(append(vx.MapKeys(), vy.MapKeys()...)) {
step.vx = vx.MapIndex(k)
step.vy = vy.MapIndex(k)
step.key = k
if !step.vx.IsValid() && !step.vy.IsValid() {
// It is possible for both vx and vy to be invalid if the
// key contained a NaN value in it.
// Even with the ability to retrieve NaN keys in Go 1.12,
// there still isn't a sensible way to compare the values since
// a NaN key may map to multiple unordered values.
// The most reasonable way to compare NaNs would be to compare the
// set of values. However, this is impossible to do efficiently
// since set equality is provably an O(n^2) operation given only
// an Equal function. If we had a Less function or Hash function,
// this could be done in O(n*log(n)) or O(n), respectively.
// Rather than adding complex logic to deal with NaNs, make it
// the user's responsibility to compare such obscure maps.
const help = "consider providing a Comparer to compare the map"
panic(fmt.Sprintf("%#v has map key with NaNs\n%s", s.curPath, help))
func (s *state) comparePtr(t reflect.Type, vx, vy reflect.Value) {
if vx.IsNil() || vy.IsNil() { && vy.IsNil(), 0)
// Cycle-detection for pointers.
if eq, visited := s.curPtrs.Push(vx, vy); visited {, reportByCycle)
defer s.curPtrs.Pop(vx, vy)
vx, vy = vx.Elem(), vy.Elem()
s.compareAny(Indirect{&indirect{pathStep{t.Elem(), vx, vy}}})
func (s *state) compareInterface(t reflect.Type, vx, vy reflect.Value) {
if vx.IsNil() || vy.IsNil() { && vy.IsNil(), 0)
vx, vy = vx.Elem(), vy.Elem()
if vx.Type() != vy.Type() {, 0)
s.compareAny(TypeAssertion{&typeAssertion{pathStep{vx.Type(), vx, vy}}})
func (s *state) report(eq bool, rf resultFlags) {
if rf&reportByIgnore == 0 {
if eq {
rf |= reportEqual
} else {
rf |= reportUnequal
for _, r := range s.reporters {
r.Report(Result{flags: rf})
// recChecker tracks the state needed to periodically perform checks that
// user provided transformers are not stuck in an infinitely recursive cycle.
type recChecker struct{ next int }
// Check scans the Path for any recursive transformers and panics when any
// recursive transformers are detected. Note that the presence of a
// recursive Transformer does not necessarily imply an infinite cycle.
// As such, this check only activates after some minimal number of path steps.
func (rc *recChecker) Check(p Path) {
const minLen = 1 << 16
if == 0 { = minLen
if len(p) < {
} <<= 1
// Check whether the same transformer has appeared at least twice.
var ss []string
m := map[Option]int{}
for _, ps := range p {
if t, ok := ps.(Transform); ok {
t := t.Option()
if m[t] == 1 { // Transformer was used exactly once before
tf := t.(*transformer).fnc.Type()
ss = append(ss, fmt.Sprintf("%v: %v => %v", t, tf.In(0), tf.Out(0)))
if len(ss) > 0 {
const warning = "recursive set of Transformers detected"
const help = "consider using cmpopts.AcyclicTransformer"
set := strings.Join(ss, "\n\t")
panic(fmt.Sprintf("%s:\n\t%s\n%s", warning, set, help))
// dynChecker tracks the state needed to periodically perform checks that
// user provided functions are symmetric and deterministic.
// The zero value is safe for immediate use.
type dynChecker struct{ curr, next int }
// Next increments the state and reports whether a check should be performed.
// Checks occur every Nth function call, where N is a triangular number:
// 0 1 3 6 10 15 21 28 36 45 55 66 78 91 105 120 136 153 171 190 ...
// See
// This sequence ensures that the cost of checks drops significantly as
// the number of functions calls grows larger.
func (dc *dynChecker) Next() bool {
ok := dc.curr ==
if ok {
dc.curr = 0
return ok
// makeAddressable returns a value that is always addressable.
// It returns the input verbatim if it is already addressable,
// otherwise it creates a new value and returns an addressable copy.
func makeAddressable(v reflect.Value) reflect.Value {
if v.CanAddr() {
return v
vc := reflect.New(v.Type()).Elem()
return vc