package engine

import (
	"fmt"
	"net"

	"vpnem/internal/config"
)

var localProxyPortCandidates = []int{config.LocalProxyPort, 10808, 10880, 18080, 20800}

func ResolveLocalProxyPort() (int, error) {
	for _, port := range localProxyPortCandidates {
		if localProxyPortAvailable(port) {
			return port, nil
		}
	}
	listener, err := net.Listen("tcp", net.JoinHostPort(config.LocalProxyHost, "0"))
	if err != nil {
		return 0, err
	}
	defer listener.Close()
	addr, ok := listener.Addr().(*net.TCPAddr)
	if !ok {
		return 0, fmt.Errorf("unexpected listener addr type %T", listener.Addr())
	}
	return addr.Port, nil
}

func localProxyPortAvailable(port int) bool {
	listener, err := net.Listen("tcp", net.JoinHostPort(config.LocalProxyHost, fmt.Sprintf("%d", port)))
	if err != nil {
		return false
	}
	_ = listener.Close()
	return true
}