Source: compare.go in package github.com/google/go-cmp/cmp

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 needs of the test, 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 that have 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 (
	"fmt"
	"reflect"
	"strings"

	"github.com/google/go-cmp/cmp/internal/diff"
	"github.com/google/go-cmp/cmp/internal/flags"
	"github.com/google/go-cmp/cmp/internal/function"
	"github.com/google/go-cmp/cmp/internal/value"
)

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 greater than one, 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 y, a "+" prefix to indicates an element added to 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 = reflect.TypeOf((*interface{})(nil)).Elem()
		if vx.IsValid() {
			vvx := reflect.New(t).Elem()
			vvx.Set(vx)
			vx = vvx
		}
		if vy.IsValid() {
			vvy := reflect.New(t).Elem()
			vvy.Set(vy)
			vy = vvy
		}
	} else {
		t = vx.Type()
	}

	return &pathStep{t, vx, vy}
}

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
}

Always ensure a validator option exists to validate the inputs.

	s := &state{opts: Options{validator{}}}
	s.curPtrs.Init()
	s.processOption(Options(opts))
	return s
}

func (s *state) processOption(opt Option) {
	switch opt := opt.(type) {
	case nil:
	case Options:
		for _, o := range opt {
			s.processOption(o)
		}
	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)
	default:
		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.

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
	s.compareAny(step)
	res := s.result
	s.result, s.reporters = oldResult, oldReporters
	return res
}

Update the path stack.

	s.curPath.push(step)
	defer s.curPath.pop()
	for _, r := range s.reporters {
		r.PushStep(step)
		defer r.PopStep()
	}
	s.recChecker.Check(s.curPath)

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 {
			s.report(eq, reportByCycle)
			return
		}
		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) {
		return
	}

Rule 2: Check whether the type has a valid Equal method.

	if s.tryMethod(t, vx, vy) {
		return
	}

Rule 3: Compare based on the underlying kind.

	switch t.Kind() {
	case reflect.Bool:
		s.report(vx.Bool() == vy.Bool(), 0)
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		s.report(vx.Int() == vy.Int(), 0)
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
		s.report(vx.Uint() == vy.Uint(), 0)
	case reflect.Float32, reflect.Float64:
		s.report(vx.Float() == vy.Float(), 0)
	case reflect.Complex64, reflect.Complex128:
		s.report(vx.Complex() == vy.Complex(), 0)
	case reflect.String:
		s.report(vx.String() == vy.String(), 0)
	case reflect.Chan, reflect.UnsafePointer:
		s.report(vx.Pointer() == vy.Pointer(), 0)
	case reflect.Func:
		s.report(vx.IsNil() && 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)
	default:
		panic(fmt.Sprintf("%v kind not handled", t.Kind()))
	}
}

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
}

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)
	s.report(eq, reportByMethod)
	return true
}

func (s *state) callTRFunc(f, v reflect.Value, step Transform) reflect.Value {
	v = sanitizeValue(v, f.Type().In(0))
	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]

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 {
	x = sanitizeValue(x, f.Type().In(0))
	y = sanitizeValue(y, f.Type().In(1))
	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]
}

sanitizeValue converts nil interfaces of type T to those of type R, assuming that T is assignable to R. Otherwise, it returns the input value as is.

TODO(≥go1.10): Workaround for reflect bug (https://golang.org/issue/22143).

	if !flags.AtLeastGo110 {
		if v.Kind() == reflect.Interface && v.IsNil() && v.Type() != t {
			return reflect.New(t).Elem()
		}
	}
	return v
}

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)
		step.name = t.Field(i).Name
		step.idx = i
		step.unexported = !isExported(step.name)
		if step.unexported {
			if step.name == "_" {
				continue

Defer checking of unexported fields until later to give an Ignore a chance to ignore the field.

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)
		}
		s.compareAny(step)
	}
}

func (s *state) compareSlice(t reflect.Type, vx, vy reflect.Value) {
	isSlice := t.Kind() == reflect.Slice
	if isSlice && (vx.IsNil() || vy.IsNil()) {
		s.report(vx.IsNil() && vy.IsNil(), 0)
		return
	}

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
		default:
			e, edits = edits[0], edits[1:]
		}
		switch e {
		case diff.UniqueX:
			s.compareAny(withIndexes(ix, -1))
			ix++
		case diff.UniqueY:
			s.compareAny(withIndexes(-1, iy))
			iy++
		default:
			s.compareAny(withIndexes(ix, iy))
			ix++
			iy++
		}
	}
}

func (s *state) compareMap(t reflect.Type, vx, vy reflect.Value) {
	if vx.IsNil() || vy.IsNil() {
		s.report(vx.IsNil() && vy.IsNil(), 0)
		return
	}

Cycle-detection for maps.

	if eq, visited := s.curPtrs.Push(vx, vy); visited {
		s.report(eq, reportByCycle)
		return
	}
	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

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))
		}
		s.compareAny(step)
	}
}

func (s *state) comparePtr(t reflect.Type, vx, vy reflect.Value) {
	if vx.IsNil() || vy.IsNil() {
		s.report(vx.IsNil() && vy.IsNil(), 0)
		return
	}

Cycle-detection for pointers.

	if eq, visited := s.curPtrs.Push(vx, vy); visited {
		s.report(eq, reportByCycle)
		return
	}
	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() {
		s.report(vx.IsNil() && vy.IsNil(), 0)
		return
	}
	vx, vy = vx.Elem(), vy.Elem()
	if vx.Type() != vy.Type() {
		s.report(false, 0)
		return
	}
	s.compareAny(TypeAssertion{&typeAssertion{pathStep{vx.Type(), vx, vy}}})
}

func (s *state) report(eq bool, rf resultFlags) {
	if rf&reportByIgnore == 0 {
		if eq {
			s.result.NumSame++
			rf |= reportEqual
		} else {
			s.result.NumDiff++
			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 rc.next == 0 {
		rc.next = minLen
	}
	if len(p) < rc.next {
		return
	}
	rc.next <<= 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)))
			}
			m[t]++
		}
	}
	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 https://en.wikipedia.org/wiki/Triangular_number 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 == dc.next
	if ok {
		dc.curr = 0
		dc.next++
	}
	dc.curr++
	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()
	vc.Set(v)
	return vc