expose symm diff iterator state in companion object

utils/iterator.go

@@ -8,23 +8,8 @@ import (
 )
 
 type symmDiffIterator struct {
-	a, b        iterState // Nodes returned are those in b - a and a - b (keys only)
+	a, b iterState // Nodes returned are those in b - a and a - b (keys only)
-	yieldFromA  bool      // Whether next node comes from a
+	SymmDiffAux
-	count       int       // Number of nodes scanned on either trie
-	eqPathIndex int       // Count index of last pair of equal paths, to detect an updated key
-}
-
-// NewSymmetricDifferenceIterator constructs a trie.NodeIterator that iterates over the symmetric difference
-// of elements in a and b, i.e., the elements in a that are not in b, and vice versa.
-// Returns the iterator, and a pointer to an integer recording the number of nodes seen.
-func NewSymmetricDifferenceIterator(a, b trie.NodeIterator) (*symmDiffIterator, *int) {
-	it := &symmDiffIterator{
-		a: iterState{a, true},
-		b: iterState{b, true},
-		// common paths are detected by a distance <=1 from this index, so put it out of reach
-		eqPathIndex: -2,
-	}
-	return it, &it.count
 }
 
 // pairs an iterator with a cache of its valid status
@@ -33,11 +18,49 @@ type iterState struct {
 	valid bool
 }
 
+// SymmDiffAux exposes state specific to symmetric difference iteration, which is not accessible
+// from the NodeIterator interface. This includes the number of nodes seen, whether the current key
+// is common to both A and B, and whether the current node is sourced from A or B.
+type SymmDiffAux struct {
+	yieldFromA  bool // Whether next node comes from a
+	count       int  // Number of nodes scanned on either trie
+	eqPathIndex int  // Count index of last pair of equal paths, to detect an updated key
+}
+
+// NewSymmetricDifferenceIterator constructs a trie.NodeIterator that iterates over the symmetric difference
+// of elements in a and b, i.e., the elements in a that are not in b, and vice versa.
+// Returns the iterator, and a pointer to an auxiliary object for accessing the state not exposed by the NodeIterator interface recording the number of nodes seen.
+func NewSymmetricDifferenceIterator(a, b trie.NodeIterator) (trie.NodeIterator, *SymmDiffAux) {
+	it := &symmDiffIterator{
+		a: iterState{a, true},
+		b: iterState{b, true},
+		// common paths are detected by a distance <=1 between count and this index, so we start at -2
+		SymmDiffAux: SymmDiffAux{eqPathIndex: -2},
+	}
+	return it, &it.SymmDiffAux
+}
+
 func (st *iterState) Next(descend bool) bool {
 	st.valid = st.NodeIterator.Next(descend)
 	return st.valid
 }
 
+// FromA returns true if the current node is sourced from A.
+func (it *SymmDiffAux) FromA() bool {
+	return it.yieldFromA
+}
+
+// CommonPath returns true if a node with the current path exists in each sub-iterator - i.e. it
+// represents an updated node.
+func (it *SymmDiffAux) CommonPath() bool {
+	return it.count-it.eqPathIndex <= 1
+}
+
+// Count returns the number of nodes seen.
+func (it *SymmDiffAux) Count() int {
+	return it.count
+}
+
 func (it *symmDiffIterator) curr() *iterState {
 	if it.yieldFromA {
 		return &it.a
@@ -45,17 +68,6 @@ func (it *symmDiffIterator) curr() *iterState {
 	return &it.b
 }
 
-// FromA returns true if the current node is sourced from A.
-func (it *symmDiffIterator) FromA() bool {
-	return it.yieldFromA
-}
-
-// CommonPath returns true if a node with the current path exists in each sub-iterator - i.e. it
-// represents an updated node.
-func (it *symmDiffIterator) CommonPath() bool {
-	return it.count-it.eqPathIndex <= 1
-}
-
 func (it *symmDiffIterator) Hash() common.Hash {
 	return it.curr().Hash()
 }