// Copyright (c) 2017 Uber Technologies, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package utils

import (
	"errors"
	"net"
)

// This code is borrowed from https://github.com/uber/tchannel-go/blob/dev/localip.go

// scoreAddr scores how likely the given addr is to be a remote address and returns the
// IP to use when listening. Any address which receives a negative score should not be used.
// Scores are calculated as:
// -1 for any unknown IP addresses.
// +300 for IPv4 addresses
// +100 for non-local addresses, extra +100 for "up" interaces.
func scoreAddr(iface net.Interface, addr net.Addr) (int, net.IP) {
	var ip net.IP
	if netAddr, ok := addr.(*net.IPNet); ok {
		ip = netAddr.IP
	} else if netIP, ok := addr.(*net.IPAddr); ok {
		ip = netIP.IP
	} else {
		return -1, nil
	}

	var score int
	if ip.To4() != nil {
		score += 300
	}
	if iface.Flags&net.FlagLoopback == 0 && !ip.IsLoopback() {
		score += 100
		if iface.Flags&net.FlagUp != 0 {
			score += 100
		}
	}
	return score, ip
}

// HostIP tries to find an IP that can be used by other machines to reach this machine.
func HostIP() (net.IP, error) {
	interfaces, err := net.Interfaces()
	if err != nil {
		return nil, err
	}

	bestScore := -1
	var bestIP net.IP
	// Select the highest scoring IP as the best IP.
	for _, iface := range interfaces {
		addrs, err := iface.Addrs()
		if err != nil {
			// Skip this interface if there is an error.
			continue
		}

		for _, addr := range addrs {
			score, ip := scoreAddr(iface, addr)
			if score > bestScore {
				bestScore = score
				bestIP = ip
			}
		}
	}

	if bestScore == -1 {
		return nil, errors.New("no addresses to listen on")
	}

	return bestIP, nil
}