device: remove recursion from insertion and connect parent pointers

This makes the insertion algorithm a bit more efficient, while also now
taking on the additional task of connecting up parent pointers. This
will be handy in the following commit.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/allowedips.go

@@ -14,9 +14,15 @@ import (
 	"unsafe"
 )
 
+type parentIndirection struct {
+	parentBit     **trieEntry
+	parentBitType uint8
+}
+
 type trieEntry struct {
 	peer        *Peer
 	child       [2]*trieEntry
+	parent      parentIndirection
 	cidr        uint8
 	bitAtByte   uint8
 	bitAtShift  uint8
@@ -114,43 +120,45 @@ func (node *trieEntry) maskSelf() {
 	}
 }
 
-func (node *trieEntry) insert(ip net.IP, cidr uint8, peer *Peer) *trieEntry {
+func (node *trieEntry) nodePlacement(ip net.IP, cidr uint8) (parent *trieEntry, exact bool) {
+	for node != nil && node.cidr <= cidr && commonBits(node.bits, ip) >= node.cidr {
+		parent = node
+		if parent.cidr == cidr {
+			exact = true
+			return
+		}
+		bit := node.choose(ip)
+		node = node.child[bit]
+	}
+	return
+}
 
-	// at leaf
-
-	if node == nil {
+func (trie parentIndirection) insert(ip net.IP, cidr uint8, peer *Peer) {
+	if *trie.parentBit == nil {
 		node := &trieEntry{
-			bits:       ip,
 			peer:       peer,
+			parent:     trie,
+			bits:       ip,
 			cidr:       cidr,
 			bitAtByte:  cidr / 8,
 			bitAtShift: 7 - (cidr % 8),
 		}
 		node.maskSelf()
 		node.addToPeerEntries()
-		return node
+		*trie.parentBit = node
+		return
 	}
-
-	// traverse deeper
-
-	common := commonBits(node.bits, ip)
-	if node.cidr <= cidr && common >= node.cidr {
-		if node.cidr == cidr {
-			node.removeFromPeerEntries()
-			node.peer = peer
-			node.addToPeerEntries()
-			return node
-		}
-		bit := node.choose(ip)
-		node.child[bit] = node.child[bit].insert(ip, cidr, peer)
-		return node
+	node, exact := (*trie.parentBit).nodePlacement(ip, cidr)
+	if exact {
+		node.removeFromPeerEntries()
+		node.peer = peer
+		node.addToPeerEntries()
+		return
 	}
 
-	// split node
-
 	newNode := &trieEntry{
-		bits:       ip,
 		peer:       peer,
+		bits:       ip,
 		cidr:       cidr,
 		bitAtByte:  cidr / 8,
 		bitAtShift: 7 - (cidr % 8),
@@ -158,34 +166,61 @@ func (node *trieEntry) insert(ip net.IP, cidr uint8, peer *Peer) *trieEntry {
 	newNode.maskSelf()
 	newNode.addToPeerEntries()
 
+	var down *trieEntry
+	if node == nil {
+		down = *trie.parentBit
+	} else {
+		bit := node.choose(ip)
+		down = node.child[bit]
+		if down == nil {
+			newNode.parent = parentIndirection{&node.child[bit], bit}
+			node.child[bit] = newNode
+			return
+		}
+	}
+	common := commonBits(down.bits, ip)
 	if common < cidr {
 		cidr = common
 	}
-
-	// check for shorter prefix
+	parent := node
 
 	if newNode.cidr == cidr {
-		bit := newNode.choose(node.bits)
-		newNode.child[bit] = node
-		return newNode
+		bit := newNode.choose(down.bits)
+		down.parent = parentIndirection{&newNode.child[bit], bit}
+		newNode.child[bit] = down
+		if parent == nil {
+			newNode.parent = trie
+			*trie.parentBit = newNode
+		} else {
+			bit := parent.choose(newNode.bits)
+			newNode.parent = parentIndirection{&parent.child[bit], bit}
+			parent.child[bit] = newNode
+		}
+		return
 	}
 
-	// create new parent for node & newNode
-
-	parent := &trieEntry{
-		bits:       append([]byte{}, ip...),
-		peer:       nil,
+	node = &trieEntry{
+		bits:       append([]byte{}, newNode.bits...),
 		cidr:       cidr,
 		bitAtByte:  cidr / 8,
 		bitAtShift: 7 - (cidr % 8),
 	}
-	parent.maskSelf()
+	node.maskSelf()
 
-	bit := parent.choose(ip)
-	parent.child[bit] = newNode
-	parent.child[bit^1] = node
-
-	return parent
+	bit := node.choose(down.bits)
+	down.parent = parentIndirection{&node.child[bit], bit}
+	node.child[bit] = down
+	bit = node.choose(newNode.bits)
+	newNode.parent = parentIndirection{&node.child[bit], bit}
+	node.child[bit] = newNode
+	if parent == nil {
+		node.parent = trie
+		*trie.parentBit = node
+	} else {
+		bit := parent.choose(node.bits)
+		node.parent = parentIndirection{&parent.child[bit], bit}
+		parent.child[bit] = node
+	}
 }
 
 func (node *trieEntry) lookup(ip net.IP) *Peer {
@@ -236,9 +271,9 @@ func (table *AllowedIPs) Insert(ip net.IP, cidr uint8, peer *Peer) {
 
 	switch len(ip) {
 	case net.IPv6len:
-		table.IPv6 = table.IPv6.insert(ip, cidr, peer)
+		parentIndirection{&table.IPv6, 2}.insert(ip, cidr, peer)
 	case net.IPv4len:
-		table.IPv4 = table.IPv4.insert(ip, cidr, peer)
+		parentIndirection{&table.IPv4, 2}.insert(ip, cidr, peer)
 	default:
 		panic(errors.New("inserting unknown address type"))
 	}