Make MaskLeft of NodeID return an explicit copy. (#1612)

Currently we're copying everything twice as do n.Copy().MaskLeft().

merkle/map_verifier.go

@@ -67,7 +67,7 @@ func VerifyMapInclusionProof(treeID int64, leaf *trillian.MapLeaf, expectedRoot
 		// for the branch that we are on before combining it with the neighbor.
 		if len(runningHash) == 0 && len(pElement) != 0 {
 			depth := nID.PrefixLenBits - height
-			emptyBranch := nID.Copy().MaskLeft(depth)
+			emptyBranch := nID.MaskLeft(depth)
 			runningHash = h.HashEmpty(treeID, emptyBranch.Path, height)
 		}
 
@@ -83,7 +83,7 @@ func VerifyMapInclusionProof(treeID int64, leaf *trillian.MapLeaf, expectedRoot
 	}
 	if len(runningHash) == 0 {
 		depth := 0
-		emptyBranch := nID.Copy().MaskLeft(depth)
+		emptyBranch := nID.MaskLeft(depth)
 		runningHash = h.HashEmpty(treeID, emptyBranch.Path, h.BitLen())
 	}
 

storage/types.go

@@ -389,7 +389,7 @@ func (n *NodeID) FlipRightBit(i int) *NodeID {
 // is 0. leftmask is only used to mask the last byte.
 var leftmask = [8]byte{0xFF, 0x80, 0xC0, 0xE0, 0xF0, 0xF8, 0xFC, 0xFE}
 
-// MaskLeft returns NodeID with only the left n bits set
+// MaskLeft returns a new copy of NodeID with only the left n bits set.
 func (n *NodeID) MaskLeft(depth int) *NodeID {
 	r := make([]byte, len(n.Path))
 	if depth > 0 {
@@ -399,11 +399,14 @@ func (n *NodeID) MaskLeft(depth int) *NodeID {
 		// Mask off unwanted bits in the last byte.
 		r[depthBytes-1] = r[depthBytes-1] & leftmask[depth%8]
 	}
+	b := n.PrefixLenBits
 	if depth < n.PrefixLenBits {
-		n.PrefixLenBits = depth
+		b = depth
+	}
+	return &NodeID{
+		PrefixLenBits: b,
+		Path:          r,
 	}
-	n.Path = r
-	return n
 }
 
 // Neighbor returns the same node with the bit at PrefixLenBits flipped.
@@ -425,7 +428,7 @@ func (n *NodeID) Siblings() []NodeID {
 	sibs := make([]NodeID, n.PrefixLenBits)
 	for height := range sibs {
 		depth := n.PrefixLenBits - height
-		sibs[height] = *(n.Copy().MaskLeft(depth).Neighbor())
+		sibs[height] = *(n.MaskLeft(depth).Neighbor())
 	}
 	return sibs
 }