package tun

import (
	"github.com/sagernet/netlink"
	E "github.com/sagernet/sing/common/exceptions"
)

func (m *defaultInterfaceMonitor) checkUpdate() error {
	ruleList, err := netlink.RuleList(netlink.FAMILY_ALL)
	if err != nil {
		return err
	}

	var defaultTableIndex int
	for _, rule := range ruleList {
		if rule.Mask == 0xFFFF {
			defaultTableIndex = rule.Table
		}
	}

	if defaultTableIndex == 0 {
		return E.Extend(ErrNoRoute, "no rule 0xFFFF")
	}

	routes, err := netlink.RouteListFiltered(netlink.FAMILY_ALL, &netlink.Route{Table: defaultTableIndex}, netlink.RT_FILTER_TABLE)
	if err != nil {
		return err
	}

	if len(routes) == 0 {
		return E.Extend(ErrNoRoute, "no route in default table ", defaultTableIndex)
	}

	var link netlink.Link
	link, err = netlink.LinkByIndex(routes[0].LinkIndex)
	if err != nil {
		return err
	}

	oldInterface := m.defaultInterfaceName
	oldIndex := m.defaultInterfaceIndex

	m.defaultInterfaceName = link.Attrs().Name
	m.defaultInterfaceIndex = link.Attrs().Index

	if oldInterface == m.defaultInterfaceName && oldIndex == m.defaultInterfaceIndex {
		return nil
	}

	m.emit()
	return nil
}