Add IncludeEmptyNetworks as Networks option

This can be useful when doing things like comparing two databases.

traverse.go

@@ -16,6 +16,7 @@ type netNode struct {
 
 type networkOptions struct {
 	includeAliasedNetworks bool
+	includeEmptyNetworks   bool
 }
 
 var (
@@ -33,21 +34,30 @@ func IncludeAliasedNetworks(networks *networkOptions) {
 	networks.includeAliasedNetworks = true
 }
 
-// Networks returns an iterator that can be used to traverse all networks in
+// IncludeEmptyNetworks is an option for Networks and NetworksWithin
+// that makes them include networks without any data in the iteration.
+func IncludeEmptyNetworks(networks *networkOptions) {
+	networks.includeEmptyNetworks = true
+}
+
+// Networks returns an iterator that can be used to traverse the networks in
 // the database.
 //
 // Please note that a MaxMind DB may map IPv4 networks into several locations
 // in an IPv6 database. This iterator will only iterate over these once by
 // default. To iterate over all the IPv4 network locations, use the
 // [IncludeAliasedNetworks] option.
+//
+// Networks without data are excluded by default. To include them, use
+// [IncludeEmptyNetworks].
 func (r *Reader) Networks(options ...NetworksOption) iter.Seq[Result] {
 	if r.Metadata.IPVersion == 6 {
 		return r.NetworksWithin(allIPv6, options...)
 	}
 	return r.NetworksWithin(allIPv4, options...)
 }
 
-// NetworksWithin returns an iterator that can be used to traverse all networks
+// NetworksWithin returns an iterator that can be used to traverse the networks
 // in the database which are contained in a given prefix.
 //
 // Please note that a MaxMind DB may map IPv4 networks into several locations
@@ -57,6 +67,9 @@ func (r *Reader) Networks(options ...NetworksOption) iter.Seq[Result] {
 //
 // If the provided prefix is contained within a network in the database, the
 // iterator will iterate over exactly one network, the containing network.
+//
+// Networks without data are excluded by default. To include them, use
+// [IncludeEmptyNetworks].
 func (r *Reader) NetworksWithin(prefix netip.Prefix, options ...NetworksOption) iter.Seq[Result] {
 	return func(yield func(Result) bool) {
 		if r.Metadata.IPVersion == 4 && prefix.Addr().Is6() {
@@ -106,7 +119,25 @@ func (r *Reader) NetworksWithin(prefix netip.Prefix, options ...NetworksOption)
 			node := nodes[len(nodes)-1]
 			nodes = nodes[:len(nodes)-1]
 
-			for node.pointer != r.Metadata.NodeCount {
+			for {
+				if node.pointer == r.Metadata.NodeCount {
+					if n.includeEmptyNetworks {
+						ip := node.ip
+						if isInIPv4Subtree(ip) {
+							ip = v6ToV4(ip)
+						}
+
+						ok := yield(Result{
+							ip:        ip,
+							offset:    notFound,
+							prefixLen: uint8(node.bit),
+						})
+						if !ok {
+							return
+						}
+					}
+					break
+				}
 				// This skips IPv4 aliases without hardcoding the networks that the writer
 				// currently aliases.
 				if !n.includeAliasedNetworks && r.ipv4Start != 0 &&

traverse_test.go

@@ -3,6 +3,8 @@ package maxminddb
 import (
 	"fmt"
 	"net/netip"
+	"reflect"
+	"runtime"
 	"strconv"
 	"strings"
 	"testing"
@@ -244,6 +246,43 @@ var tests = []networkTest{
 			"::2:0:58/127",
 		},
 	},
+	{
+		Network:  "1.0.0.0/8",
+		Database: "mixed",
+		Expected: []string{
+			"1.0.0.0/16",
+			"1.1.0.0/24",
+			"1.1.1.0/32",
+			"1.1.1.1/32",
+			"1.1.1.2/31",
+			"1.1.1.4/30",
+			"1.1.1.8/29",
+			"1.1.1.16/28",
+			"1.1.1.32/32",
+			"1.1.1.33/32",
+			"1.1.1.34/31",
+			"1.1.1.36/30",
+			"1.1.1.40/29",
+			"1.1.1.48/28",
+			"1.1.1.64/26",
+			"1.1.1.128/25",
+			"1.1.2.0/23",
+			"1.1.4.0/22",
+			"1.1.8.0/21",
+			"1.1.16.0/20",
+			"1.1.32.0/19",
+			"1.1.64.0/18",
+			"1.1.128.0/17",
+			"1.2.0.0/15",
+			"1.4.0.0/14",
+			"1.8.0.0/13",
+			"1.16.0.0/12",
+			"1.32.0.0/11",
+			"1.64.0.0/10",
+			"1.128.0.0/9",
+		},
+		Options: []NetworksOption{IncludeEmptyNetworks},
+	},
 	{
 		Network:  "1.1.1.16/28",
 		Database: "mixed",
@@ -263,12 +302,16 @@ var tests = []networkTest{
 func TestNetworksWithin(t *testing.T) {
 	for _, v := range tests {
 		for _, recordSize := range []uint{24, 28, 32} {
+			var opts []string
+			for _, o := range v.Options {
+				opts = append(opts, runtime.FuncForPC(reflect.ValueOf(o).Pointer()).Name())
+			}
 			name := fmt.Sprintf(
 				"%s-%d: %s, options: %v",
 				v.Database,
 				recordSize,
 				v.Network,
-				len(v.Options) != 0,
+				opts,
 			)
 			t.Run(name, func(t *testing.T) {
 				fileName := testFile(fmt.Sprintf("MaxMind-DB-test-%s-%d.mmdb", v.Database, recordSize))