package net

import (
	"fmt"
	"net"

	"github.com/vishvananda/netlink"
)

// A network is considered free if it does not overlap any existing
// routes on this host. This is the same approach taken by Docker.
func CheckNetworkFree(subnet *net.IPNet) error {
	routes, err := netlink.RouteList(nil, netlink.FAMILY_V4)
	if err != nil {
		return err
	}
	for _, route := range routes {
		if route.Dst != nil && overlaps(route.Dst, subnet) {
			return fmt.Errorf("network %s would overlap with route %s", subnet, route.Dst)
		}
	}
	return nil
}

// Two networks overlap if the start-point of one is inside the other.
func overlaps(n1, n2 *net.IPNet) bool {
	return n1.Contains(n2.IP) || n2.Contains(n1.IP)
}