vendor/github.com/google/go-cmp/cmp/compare.go - gddo - Git at Google

 // 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.md 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.
 //
 // 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 explictly compared
 // using the AllowUnexported option.
 package cmp

 import (
 	"fmt"
 	"reflect"

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

 // BUG(dsnet): Maps with keys containing NaN values cannot be properly compared due to
 // the reflection package's inability to retrieve such entries. Equal will panic
 // anytime it comes across a NaN key, but this behavior may change.
 //
 // See https://golang.org/issue/11104 for more details.

 var nothing = reflect.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:
 //
 // • If two values are not of the same type, then they are never equal
 // and the overall result is false.
 //
 // • 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). 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.
 // Pointers are equal if the underlying values they point to are also equal.
 // Interfaces are equal if their underlying concrete values are also equal.
 //
 // Structs are equal if all of their fields are equal. If a struct contains
 // unexported fields, Equal panics unless the AllowUnexported option is used or
 // an Ignore option (e.g., cmpopts.IgnoreUnexported) ignores that field.
 //
 // Arrays, slices, and maps are equal if they are both nil or both non-nil
 // with the same length and the elements at each index or key are equal.
 // Note that a non-nil empty slice and a nil slice are not equal.
 // To equate empty slices and maps, consider using cmpopts.EquateEmpty.
 // Map keys are equal according to the == operator.
 // To use custom comparisons for map keys, consider using cmpopts.SortMaps.
 func Equal(x, y interface{}, opts ...Option) bool {
 	s := newState(opts)
 	s.compareAny(reflect.ValueOf(x), reflect.ValueOf(y))
 	return s.result.Equal()
 }

 // Diff returns a human-readable report of the differences between two values.
 // It returns an empty string if and only if Equal returns true for the same
 // input values and options. The output string will use the "-" symbol to
 // indicate elements removed from x, and the "+" symbol to indicate elements
 // added to y.
 //
 // Do not depend on this output being stable.
 func Diff(x, y interface{}, opts ...Option) string {
 	r := new(defaultReporter)
 	opts = Options{Options(opts), r}
 	eq := Equal(x, y, opts...)
 	d := r.String()
 	if (d == "") != eq {
 		panic("inconsistent difference and equality results")
 	}
 	return d
 }

 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
 	reporter reporter    // Optional reporter used for difference formatting

 	// 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 map[reflect.Type]bool // Set of structs with unexported field visibility
 	opts      Options               // List of all fundamental and filter options
 }

 func newState(opts []Option) *state {
 	s := new(state)
 	for _, opt := range opts {
 		s.processOption(opt)
 	}
 	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 visibleStructs:
 		if s.exporters == nil {
 			s.exporters = make(map[reflect.Type]bool)
 		}
 		for t := range opt {
 			s.exporters[t] = true
 		}
 	case reporter:
 		if s.reporter != nil {
 			panic("difference reporter already registered")
 		}
 		s.reporter = 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.
 func (s *state) statelessCompare(vx, vy reflect.Value) diff.Result {
 	// We do not save and restore the curPath because all of the compareX
 	// methods should properly push and pop from the path.
 	// It is an implementation bug if the contents of curPath differs from
 	// when calling this function to when returning from it.

 	oldResult, oldReporter := s.result, s.reporter
 	s.result = diff.Result{} // Reset result
 	s.reporter = nil         // Remove reporter to avoid spurious printouts
 	s.compareAny(vx, vy)
 	res := s.result
 	s.result, s.reporter = oldResult, oldReporter
 	return res
 }

 func (s *state) compareAny(vx, vy reflect.Value) {
 	// TODO: Support cyclic data structures.

 	// Rule 0: Differing types are never equal.
 	if !vx.IsValid() || !vy.IsValid() {
 		s.report(vx.IsValid() == vy.IsValid(), vx, vy)
 		return
 	}
 	if vx.Type() != vy.Type() {
 		s.report(false, vx, vy) // Possible for path to be empty
 		return
 	}
 	t := vx.Type()
 	if len(s.curPath) == 0 {
 		s.curPath.push(&pathStep{typ: t})
 		defer s.curPath.pop()
 	}
 	vx, vy = s.tryExporting(vx, vy)

 	// Rule 1: Check whether an option applies on this node in the value tree.
 	if s.tryOptions(vx, vy, t) {
 		return
 	}

 	// Rule 2: Check whether the type has a valid Equal method.
 	if s.tryMethod(vx, vy, t) {
 		return
 	}

 	// Rule 3: Recursively descend into each value's underlying kind.
 	switch t.Kind() {
 	case reflect.Bool:
 		s.report(vx.Bool() == vy.Bool(), vx, vy)
 		return
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 		s.report(vx.Int() == vy.Int(), vx, vy)
 		return
 	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
 		s.report(vx.Uint() == vy.Uint(), vx, vy)
 		return
 	case reflect.Float32, reflect.Float64:
 		s.report(vx.Float() == vy.Float(), vx, vy)
 		return
 	case reflect.Complex64, reflect.Complex128:
 		s.report(vx.Complex() == vy.Complex(), vx, vy)
 		return
 	case reflect.String:
 		s.report(vx.String() == vy.String(), vx, vy)
 		return
 	case reflect.Chan, reflect.UnsafePointer:
 		s.report(vx.Pointer() == vy.Pointer(), vx, vy)
 		return
 	case reflect.Func:
 		s.report(vx.IsNil() && vy.IsNil(), vx, vy)
 		return
 	case reflect.Ptr:
 		if vx.IsNil() || vy.IsNil() {
 			s.report(vx.IsNil() && vy.IsNil(), vx, vy)
 			return
 		}
 		s.curPath.push(&indirect{pathStep{t.Elem()}})
 		defer s.curPath.pop()
 		s.compareAny(vx.Elem(), vy.Elem())
 		return
 	case reflect.Interface:
 		if vx.IsNil() || vy.IsNil() {
 			s.report(vx.IsNil() && vy.IsNil(), vx, vy)
 			return
 		}
 		if vx.Elem().Type() != vy.Elem().Type() {
 			s.report(false, vx.Elem(), vy.Elem())
 			return
 		}
 		s.curPath.push(&typeAssertion{pathStep{vx.Elem().Type()}})
 		defer s.curPath.pop()
 		s.compareAny(vx.Elem(), vy.Elem())
 		return
 	case reflect.Slice:
 		if vx.IsNil() || vy.IsNil() {
 			s.report(vx.IsNil() && vy.IsNil(), vx, vy)
 			return
 		}
 		fallthrough
 	case reflect.Array:
 		s.compareArray(vx, vy, t)
 		return
 	case reflect.Map:
 		s.compareMap(vx, vy, t)
 		return
 	case reflect.Struct:
 		s.compareStruct(vx, vy, t)
 		return
 	default:
 		panic(fmt.Sprintf("%v kind not handled", t.Kind()))
 	}
 }

 func (s *state) tryExporting(vx, vy reflect.Value) (reflect.Value, reflect.Value) {
 	if sf, ok := s.curPath[len(s.curPath)-1].(*structField); ok && sf.unexported {
 		if sf.force {
 			// Use unsafe pointer arithmetic to get read-write access to an
 			// unexported field in the struct.
 			vx = unsafeRetrieveField(sf.pvx, sf.field)
 			vy = unsafeRetrieveField(sf.pvy, sf.field)
 		} else {
 			// We are not allowed to export the value, so invalidate them
 			// so that tryOptions can panic later if not explicitly ignored.
 			vx = nothing
 			vy = nothing
 		}
 	}
 	return vx, vy
 }

 func (s *state) tryOptions(vx, vy reflect.Value, t reflect.Type) bool {
 	// If there were no FilterValues, we will not detect invalid inputs,
 	// so manually check for them and append invalid if necessary.
 	// We still evaluate the options since an ignore can override invalid.
 	opts := s.opts
 	if !vx.IsValid() || !vy.IsValid() {
 		opts = Options{opts, invalid{}}
 	}

 	// Evaluate all filters and apply the remaining options.
 	if opt := opts.filter(s, vx, vy, t); opt != nil {
 		return opt.apply(s, vx, vy)
 	}
 	return false
 }

 func (s *state) tryMethod(vx, vy reflect.Value, t reflect.Type) 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)
 	s.report(eq, vx, vy)
 	return true
 }

 func (s *state) callTRFunc(f, v reflect.Value) 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)
 	want := f.Call([]reflect.Value{v})[0]
 	if got := <-c; !s.statelessCompare(got, want).Equal() {
 		// To avoid false-positives with non-reflexive equality operations,
 		// we sanity check whether a value is equal to itself.
 		if !s.statelessCompare(want, want).Equal() {
 			return want
 		}
 		fn := getFuncName(f.Pointer())
 		panic(fmt.Sprintf("non-deterministic function detected: %s", fn))
 	}
 	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)
 	want := f.Call([]reflect.Value{x, y})[0].Bool()
 	if got := <-c; !got.IsValid() || got.Bool() != want {
 		fn := getFuncName(f.Pointer())
 		panic(fmt.Sprintf("non-deterministic or non-symmetric function detected: %s", fn))
 	}
 	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.
 func sanitizeValue(v reflect.Value, t reflect.Type) reflect.Value {
 	// TODO(dsnet): Remove this hacky workaround.
 	// See https://golang.org/issue/22143
 	if v.Kind() == reflect.Interface && v.IsNil() && v.Type() != t {
 		return reflect.New(t).Elem()
 	}
 	return v
 }

 func (s *state) compareArray(vx, vy reflect.Value, t reflect.Type) {
 	step := &sliceIndex{pathStep{t.Elem()}, 0, 0}
 	s.curPath.push(step)

 	// Compute an edit-script for slices vx and vy.
 	es := diff.Difference(vx.Len(), vy.Len(), func(ix, iy int) diff.Result {
 		step.xkey, step.ykey = ix, iy
 		return s.statelessCompare(vx.Index(ix), vy.Index(iy))
 	})

 	// Report the entire slice as is if the arrays are of primitive kind,
 	// and the arrays are different enough.
 	isPrimitive := false
 	switch t.Elem().Kind() {
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
 		reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr,
 		reflect.Bool, reflect.Float32, reflect.Float64, reflect.Complex64, reflect.Complex128:
 		isPrimitive = true
 	}
 	if isPrimitive && es.Dist() > (vx.Len()+vy.Len())/4 {
 		s.curPath.pop() // Pop first since we are reporting the whole slice
 		s.report(false, vx, vy)
 		return
 	}

 	// Replay the edit-script.
 	var ix, iy int
 	for _, e := range es {
 		switch e {
 		case diff.UniqueX:
 			step.xkey, step.ykey = ix, -1
 			s.report(false, vx.Index(ix), nothing)
 			ix++
 		case diff.UniqueY:
 			step.xkey, step.ykey = -1, iy
 			s.report(false, nothing, vy.Index(iy))
 			iy++
 		default:
 			step.xkey, step.ykey = ix, iy
 			if e == diff.Identity {
 				s.report(true, vx.Index(ix), vy.Index(iy))
 			} else {
 				s.compareAny(vx.Index(ix), vy.Index(iy))
 			}
 			ix++
 			iy++
 		}
 	}
 	s.curPath.pop()
 	return
 }

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

 	// We combine and sort the two map keys so that we can perform the
 	// comparisons in a deterministic order.
 	step := &mapIndex{pathStep: pathStep{t.Elem()}}
 	s.curPath.push(step)
 	defer s.curPath.pop()
 	for _, k := range value.SortKeys(append(vx.MapKeys(), vy.MapKeys()...)) {
 		step.key = k
 		vvx := vx.MapIndex(k)
 		vvy := vy.MapIndex(k)
 		switch {
 		case vvx.IsValid() && vvy.IsValid():
 			s.compareAny(vvx, vvy)
 		case vvx.IsValid() && !vvy.IsValid():
 			s.report(false, vvx, nothing)
 		case !vvx.IsValid() && vvy.IsValid():
 			s.report(false, nothing, vvy)
 		default:
 			// It is possible for both vvx and vvy to be invalid if the
 			// key contained a NaN value in it. There is no way in
 			// reflection to be able to retrieve these values.
 			// See https://golang.org/issue/11104
 			panic(fmt.Sprintf("%#v has map key with NaNs", s.curPath))
 		}
 	}
 }

 func (s *state) compareStruct(vx, vy reflect.Value, t reflect.Type) {
 	var vax, vay reflect.Value // Addressable versions of vx and vy

 	step := &structField{}
 	s.curPath.push(step)
 	defer s.curPath.pop()
 	for i := 0; i < t.NumField(); i++ {
 		vvx := vx.Field(i)
 		vvy := vy.Field(i)
 		step.typ = t.Field(i).Type
 		step.name = t.Field(i).Name
 		step.idx = i
 		step.unexported = !isExported(step.name)
 		if step.unexported {
 			// Defer checking of unexported fields until later to give an
 			// Ignore a chance to ignore the field.
 			if !vax.IsValid() || !vay.IsValid() {
 				// For unsafeRetrieveField to work, the parent struct must
 				// be addressable. Create a new copy of the values if
 				// necessary to make them addressable.
 				vax = makeAddressable(vx)
 				vay = makeAddressable(vy)
 			}
 			step.force = s.exporters[t]
 			step.pvx = vax
 			step.pvy = vay
 			step.field = t.Field(i)
 		}
 		s.compareAny(vvx, vvy)
 	}
 }

 // report records the result of a single comparison.
 // It also calls Report if any reporter is registered.
 func (s *state) report(eq bool, vx, vy reflect.Value) {
 	if eq {
 		s.result.NSame++
 	} else {
 		s.result.NDiff++
 	}
 	if s.reporter != nil {
 		s.reporter.Report(vx, vy, eq, s.curPath)
 	}
 }

 // 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
 }
	// 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.md 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.
	//
	// 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 explictly compared
	// using the AllowUnexported option.
	package cmp

	import (
	"fmt"
	"reflect"

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

	// BUG(dsnet): Maps with keys containing NaN values cannot be properly compared due to
	// the reflection package's inability to retrieve such entries. Equal will panic
	// anytime it comes across a NaN key, but this behavior may change.
	//
	// See https://golang.org/issue/11104 for more details.

	var nothing = reflect.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:
	//
	// • If two values are not of the same type, then they are never equal
	// and the overall result is false.
	//
	// • 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). 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.
	// Pointers are equal if the underlying values they point to are also equal.
	// Interfaces are equal if their underlying concrete values are also equal.
	//
	// Structs are equal if all of their fields are equal. If a struct contains
	// unexported fields, Equal panics unless the AllowUnexported option is used or
	// an Ignore option (e.g., cmpopts.IgnoreUnexported) ignores that field.
	//
	// Arrays, slices, and maps are equal if they are both nil or both non-nil
	// with the same length and the elements at each index or key are equal.
	// Note that a non-nil empty slice and a nil slice are not equal.
	// To equate empty slices and maps, consider using cmpopts.EquateEmpty.
	// Map keys are equal according to the == operator.
	// To use custom comparisons for map keys, consider using cmpopts.SortMaps.
	func Equal(x, y interface{}, opts ...Option) bool {
	s := newState(opts)
	s.compareAny(reflect.ValueOf(x), reflect.ValueOf(y))
	return s.result.Equal()
	}

	// Diff returns a human-readable report of the differences between two values.
	// It returns an empty string if and only if Equal returns true for the same
	// input values and options. The output string will use the "-" symbol to
	// indicate elements removed from x, and the "+" symbol to indicate elements
	// added to y.
	//
	// Do not depend on this output being stable.
	func Diff(x, y interface{}, opts ...Option) string {
	r := new(defaultReporter)
	opts = Options{Options(opts), r}
	eq := Equal(x, y, opts...)
	d := r.String()
	if (d == "") != eq {
	panic("inconsistent difference and equality results")
	}
	return d
	}

	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
	reporter reporter // Optional reporter used for difference formatting

	// 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 map[reflect.Type]bool // Set of structs with unexported field visibility
	opts Options // List of all fundamental and filter options
	}

	func newState(opts []Option) *state {
	s := new(state)
	for _, opt := range opts {
	s.processOption(opt)
	}
	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 visibleStructs:
	if s.exporters == nil {
	s.exporters = make(map[reflect.Type]bool)
	}
	for t := range opt {
	s.exporters[t] = true
	}
	case reporter:
	if s.reporter != nil {
	panic("difference reporter already registered")
	}
	s.reporter = 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.
	func (s *state) statelessCompare(vx, vy reflect.Value) diff.Result {
	// We do not save and restore the curPath because all of the compareX
	// methods should properly push and pop from the path.
	// It is an implementation bug if the contents of curPath differs from
	// when calling this function to when returning from it.

	oldResult, oldReporter := s.result, s.reporter
	s.result = diff.Result{} // Reset result
	s.reporter = nil // Remove reporter to avoid spurious printouts
	s.compareAny(vx, vy)
	res := s.result
	s.result, s.reporter = oldResult, oldReporter
	return res
	}

	func (s *state) compareAny(vx, vy reflect.Value) {
	// TODO: Support cyclic data structures.

	// Rule 0: Differing types are never equal.
	if !vx.IsValid() \|\| !vy.IsValid() {
	s.report(vx.IsValid() == vy.IsValid(), vx, vy)
	return
	}
	if vx.Type() != vy.Type() {
	s.report(false, vx, vy) // Possible for path to be empty
	return
	}
	t := vx.Type()
	if len(s.curPath) == 0 {
	s.curPath.push(&pathStep{typ: t})
	defer s.curPath.pop()
	}
	vx, vy = s.tryExporting(vx, vy)

	// Rule 1: Check whether an option applies on this node in the value tree.
	if s.tryOptions(vx, vy, t) {
	return
	}

	// Rule 2: Check whether the type has a valid Equal method.
	if s.tryMethod(vx, vy, t) {
	return
	}

	// Rule 3: Recursively descend into each value's underlying kind.
	switch t.Kind() {
	case reflect.Bool:
	s.report(vx.Bool() == vy.Bool(), vx, vy)
	return
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
	s.report(vx.Int() == vy.Int(), vx, vy)
	return
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
	s.report(vx.Uint() == vy.Uint(), vx, vy)
	return
	case reflect.Float32, reflect.Float64:
	s.report(vx.Float() == vy.Float(), vx, vy)
	return
	case reflect.Complex64, reflect.Complex128:
	s.report(vx.Complex() == vy.Complex(), vx, vy)
	return
	case reflect.String:
	s.report(vx.String() == vy.String(), vx, vy)
	return
	case reflect.Chan, reflect.UnsafePointer:
	s.report(vx.Pointer() == vy.Pointer(), vx, vy)
	return
	case reflect.Func:
	s.report(vx.IsNil() && vy.IsNil(), vx, vy)
	return
	case reflect.Ptr:
	if vx.IsNil() \|\| vy.IsNil() {
	s.report(vx.IsNil() && vy.IsNil(), vx, vy)
	return
	}
	s.curPath.push(&indirect{pathStep{t.Elem()}})
	defer s.curPath.pop()
	s.compareAny(vx.Elem(), vy.Elem())
	return
	case reflect.Interface:
	if vx.IsNil() \|\| vy.IsNil() {
	s.report(vx.IsNil() && vy.IsNil(), vx, vy)
	return
	}
	if vx.Elem().Type() != vy.Elem().Type() {
	s.report(false, vx.Elem(), vy.Elem())
	return
	}
	s.curPath.push(&typeAssertion{pathStep{vx.Elem().Type()}})
	defer s.curPath.pop()
	s.compareAny(vx.Elem(), vy.Elem())
	return
	case reflect.Slice:
	if vx.IsNil() \|\| vy.IsNil() {
	s.report(vx.IsNil() && vy.IsNil(), vx, vy)
	return
	}
	fallthrough
	case reflect.Array:
	s.compareArray(vx, vy, t)
	return
	case reflect.Map:
	s.compareMap(vx, vy, t)
	return
	case reflect.Struct:
	s.compareStruct(vx, vy, t)
	return
	default:
	panic(fmt.Sprintf("%v kind not handled", t.Kind()))
	}
	}

	func (s *state) tryExporting(vx, vy reflect.Value) (reflect.Value, reflect.Value) {
	if sf, ok := s.curPath[len(s.curPath)-1].(*structField); ok && sf.unexported {
	if sf.force {
	// Use unsafe pointer arithmetic to get read-write access to an
	// unexported field in the struct.
	vx = unsafeRetrieveField(sf.pvx, sf.field)
	vy = unsafeRetrieveField(sf.pvy, sf.field)
	} else {
	// We are not allowed to export the value, so invalidate them
	// so that tryOptions can panic later if not explicitly ignored.
	vx = nothing
	vy = nothing
	}
	}
	return vx, vy
	}

	func (s *state) tryOptions(vx, vy reflect.Value, t reflect.Type) bool {
	// If there were no FilterValues, we will not detect invalid inputs,
	// so manually check for them and append invalid if necessary.
	// We still evaluate the options since an ignore can override invalid.
	opts := s.opts
	if !vx.IsValid() \|\| !vy.IsValid() {
	opts = Options{opts, invalid{}}
	}

	// Evaluate all filters and apply the remaining options.
	if opt := opts.filter(s, vx, vy, t); opt != nil {
	return opt.apply(s, vx, vy)
	}
	return false
	}

	func (s *state) tryMethod(vx, vy reflect.Value, t reflect.Type) 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)
	s.report(eq, vx, vy)
	return true
	}

	func (s *state) callTRFunc(f, v reflect.Value) 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)
	want := f.Call([]reflect.Value{v})[0]
	if got := <-c; !s.statelessCompare(got, want).Equal() {
	// To avoid false-positives with non-reflexive equality operations,
	// we sanity check whether a value is equal to itself.
	if !s.statelessCompare(want, want).Equal() {
	return want
	}
	fn := getFuncName(f.Pointer())
	panic(fmt.Sprintf("non-deterministic function detected: %s", fn))
	}
	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)
	want := f.Call([]reflect.Value{x, y})[0].Bool()
	if got := <-c; !got.IsValid() \|\| got.Bool() != want {
	fn := getFuncName(f.Pointer())
	panic(fmt.Sprintf("non-deterministic or non-symmetric function detected: %s", fn))
	}
	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.
	func sanitizeValue(v reflect.Value, t reflect.Type) reflect.Value {
	// TODO(dsnet): Remove this hacky workaround.
	// See https://golang.org/issue/22143
	if v.Kind() == reflect.Interface && v.IsNil() && v.Type() != t {
	return reflect.New(t).Elem()
	}
	return v
	}

	func (s *state) compareArray(vx, vy reflect.Value, t reflect.Type) {
	step := &sliceIndex{pathStep{t.Elem()}, 0, 0}
	s.curPath.push(step)

	// Compute an edit-script for slices vx and vy.
	es := diff.Difference(vx.Len(), vy.Len(), func(ix, iy int) diff.Result {
	step.xkey, step.ykey = ix, iy
	return s.statelessCompare(vx.Index(ix), vy.Index(iy))
	})

	// Report the entire slice as is if the arrays are of primitive kind,
	// and the arrays are different enough.
	isPrimitive := false
	switch t.Elem().Kind() {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
	reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr,
	reflect.Bool, reflect.Float32, reflect.Float64, reflect.Complex64, reflect.Complex128:
	isPrimitive = true
	}
	if isPrimitive && es.Dist() > (vx.Len()+vy.Len())/4 {
	s.curPath.pop() // Pop first since we are reporting the whole slice
	s.report(false, vx, vy)
	return
	}

	// Replay the edit-script.
	var ix, iy int
	for _, e := range es {
	switch e {
	case diff.UniqueX:
	step.xkey, step.ykey = ix, -1
	s.report(false, vx.Index(ix), nothing)
	ix++
	case diff.UniqueY:
	step.xkey, step.ykey = -1, iy
	s.report(false, nothing, vy.Index(iy))
	iy++
	default:
	step.xkey, step.ykey = ix, iy
	if e == diff.Identity {
	s.report(true, vx.Index(ix), vy.Index(iy))
	} else {
	s.compareAny(vx.Index(ix), vy.Index(iy))
	}
	ix++
	iy++
	}
	}
	s.curPath.pop()
	return
	}

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

	// We combine and sort the two map keys so that we can perform the
	// comparisons in a deterministic order.
	step := &mapIndex{pathStep: pathStep{t.Elem()}}
	s.curPath.push(step)
	defer s.curPath.pop()
	for _, k := range value.SortKeys(append(vx.MapKeys(), vy.MapKeys()...)) {
	step.key = k
	vvx := vx.MapIndex(k)
	vvy := vy.MapIndex(k)
	switch {
	case vvx.IsValid() && vvy.IsValid():
	s.compareAny(vvx, vvy)
	case vvx.IsValid() && !vvy.IsValid():
	s.report(false, vvx, nothing)
	case !vvx.IsValid() && vvy.IsValid():
	s.report(false, nothing, vvy)
	default:
	// It is possible for both vvx and vvy to be invalid if the
	// key contained a NaN value in it. There is no way in
	// reflection to be able to retrieve these values.
	// See https://golang.org/issue/11104
	panic(fmt.Sprintf("%#v has map key with NaNs", s.curPath))
	}
	}
	}

	func (s *state) compareStruct(vx, vy reflect.Value, t reflect.Type) {
	var vax, vay reflect.Value // Addressable versions of vx and vy

	step := &structField{}
	s.curPath.push(step)
	defer s.curPath.pop()
	for i := 0; i < t.NumField(); i++ {
	vvx := vx.Field(i)
	vvy := vy.Field(i)
	step.typ = t.Field(i).Type
	step.name = t.Field(i).Name
	step.idx = i
	step.unexported = !isExported(step.name)
	if step.unexported {
	// Defer checking of unexported fields until later to give an
	// Ignore a chance to ignore the field.
	if !vax.IsValid() \|\| !vay.IsValid() {
	// For unsafeRetrieveField to work, the parent struct must
	// be addressable. Create a new copy of the values if
	// necessary to make them addressable.
	vax = makeAddressable(vx)
	vay = makeAddressable(vy)
	}
	step.force = s.exporters[t]
	step.pvx = vax
	step.pvy = vay
	step.field = t.Field(i)
	}
	s.compareAny(vvx, vvy)
	}
	}

	// report records the result of a single comparison.
	// It also calls Report if any reporter is registered.
	func (s *state) report(eq bool, vx, vy reflect.Value) {
	if eq {
	s.result.NSame++
	} else {
	s.result.NDiff++
	}
	if s.reporter != nil {
	s.reporter.Report(vx, vy, eq, s.curPath)
	}
	}

	// 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
	}