You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: structs_test.go
    path of file: ./repos/syncthing/lib/nat
    
    the code of the file until where you have to start completion: // Copyright (C) 2016 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at https://mozilla.org/MPL/2.0/.

package nat

import (
	"net"
	"os"
	"testing"

	"github.com/syncthing/syncthing/lib/config"
	"github.com/syncthing/syncthing/lib/events"
	"github.com/syncthing/syncthing/lib/protocol"
)

func TestMappingValidGateway(t *testing.T) {
	a := net.ParseIP("10.0.0.1")
	b := net.ParseIP("192.168.0.1")
	tests := []struct {
		mappingLocalIP net.IP
		gatewayLocalIP net.IP
		expected       bool
	}{
		// Any of the IPs is nil or unspecified
    
func TestMappingValidGateway(t *testing.T) {
	a := net.ParseIP("10.0.0.1")
	b := net.ParseIP("192.168.0.1")
	tests := []struct {
		mappingLocalIP net.IP
		gatewayLocalIP net.IP
		expected       bool
	}{
		// Any of the IPs is nil or unspecified implies correct
		{nil, nil, true},
		{net.IPv4zero, net.IPv4zero, true},
		{nil, net.IPv4zero, true},
		{net.IPv4zero, nil, true},
		{a, nil, true},
		{b, nil, true},
		{a, net.IPv4zero, true},
		{b, net.IPv4zero, true},
		{nil, a, true},
		{nil, b, true},
		{net.IPv4zero, a, true},
		{net.IPv4zero, b, true},
		// IPs are the same implies correct
		{a, a, true},
		{b, b, true},
		// IPs are specified and different, implies incorrect
		{a, b, false},
		{b, a, false},
	}

	for _, test := range tests {
		m := Mapping{
			address: Address{
				IP: test.mappingLocalIP,
			},
		}
		result := m.validGateway(test.gatewayLocalIP)
		if result != test.expected {
			t.Errorf("Incorrect: local %s gateway %s result %t expected %t", test.mappingLocalIP, test.gatewayLocalIP, result, test.expected)
		}
	}
}