Support cyclic types

diff.go

@@ -86,6 +86,7 @@ type Differ struct {
 	FlattenEmbeddedStructs bool
 	ConvertCompatibleTypes bool
 	Filter                 FilterFunc
+	pointersSeen           map[uintptr]map[uintptr]struct{}
 }
 
 // Changelog stores a list of changed items
@@ -127,6 +128,7 @@ func NewDiffer(opts ...func(d *Differ) error) (*Differ, error) {
 	d := Differ{
 		TagName:       "diff",
 		DiscardParent: false,
+		pointersSeen:  make(map[uintptr]map[uintptr]struct{}),
 	}
 
 	for _, opt := range opts {
@@ -222,7 +224,6 @@ func (d *Differ) Diff(a, b interface{}) (Changelog, error) {
 }
 
 func (d *Differ) diff(path []string, a, b reflect.Value, parent interface{}) error {
-
 	//look and see if we need to discard the parent
 	if parent != nil {
 		if d.DiscardParent || reflect.TypeOf(parent).Kind() != reflect.Struct {

diff_pointer.go

@@ -48,6 +48,26 @@ func (d *Differ) diffPtr(path []string, a, b reflect.Value, parent interface{})
 		return nil
 	}
 
+	// If two pointers have already been compared, assume they have no changes
+	// This mirrors how reflect.DeepEqual works, and guarantees termination
+	aSeen, ok := d.pointersSeen[a.Pointer()]
+	if !ok {
+		aSeen = make(map[uintptr]struct{})
+		d.pointersSeen[a.Pointer()] = aSeen
+	}
+	bSeen, ok := d.pointersSeen[b.Pointer()]
+	if !ok {
+		bSeen = make(map[uintptr]struct{})
+		d.pointersSeen[b.Pointer()] = bSeen
+	}
+	_, aok := aSeen[b.Pointer()]
+	_, bok := aSeen[b.Pointer()]
+	if aok || bok {
+		return nil
+	}
+	aSeen[b.Pointer()] = struct{}{}
+	bSeen[a.Pointer()] = struct{}{}
+
 	return d.diff(path, reflect.Indirect(a), reflect.Indirect(b), parent)
 }
 

diff_slice.go

@@ -104,6 +104,7 @@ func (st *sliceTracker) has(s, v reflect.Value, d *Differ) bool {
 		var nd Differ
 		nd.Filter = d.Filter
 		nd.customValueDiffers = d.customValueDiffers
+		nd.pointersSeen = d.pointersSeen
 
 		err := nd.diff([]string{}, x, v, nil)
 		if err != nil {

diff_struct.go

@@ -72,6 +72,7 @@ func (d *Differ) structValues(t string, path []string, a reflect.Value) error {
 	var nd Differ
 	nd.Filter = d.Filter
 	nd.customValueDiffers = d.customValueDiffers
+	nd.pointersSeen = d.pointersSeen
 
 	if t != CREATE && t != DELETE {
 		return ErrInvalidChangeType