// Copyright 2025 The frp Authors
//
// 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 vnet

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"net"
	"strconv"
	"strings"

	"github.com/vishvananda/netlink"
	"golang.zx2c4.com/wireguard/tun"
)

const (
	baseTunName = "utun"
	defaultMTU  = 1420
)

func openTun(_ context.Context, addr string) (tun.Device, error) {
	name, err := findNextTunName(baseTunName)
	if err != nil {
		name = getFallbackTunName(baseTunName, addr)
	}

	tunDevice, err := tun.CreateTUN(name, defaultMTU)
	if err != nil {
		return nil, fmt.Errorf("failed to create TUN device '%s': %w", name, err)
	}

	actualName, err := tunDevice.Name()
	if err != nil {
		return nil, err
	}

	ifn, err := net.InterfaceByName(actualName)
	if err != nil {
		return nil, err
	}

	link, err := netlink.LinkByName(actualName)
	if err != nil {
		return nil, err
	}

	ip, cidr, err := net.ParseCIDR(addr)
	if err != nil {
		return nil, err
	}
	if err := netlink.AddrAdd(link, &netlink.Addr{
		IPNet: &net.IPNet{
			IP:   ip,
			Mask: cidr.Mask,
		},
	}); err != nil {
		return nil, err
	}

	if err := netlink.LinkSetUp(link); err != nil {
		return nil, err
	}

	if err = addRoutes(ifn, cidr); err != nil {
		return nil, err
	}
	return tunDevice, nil
}

func findNextTunName(basename string) (string, error) {
	interfaces, err := net.Interfaces()
	if err != nil {
		return "", fmt.Errorf("failed to get network interfaces: %w", err)
	}
	maxSuffix := -1

	for _, iface := range interfaces {
		name := iface.Name
		if strings.HasPrefix(name, basename) {
			suffix := name[len(basename):]
			if suffix == "" {
				continue
			}

			numSuffix, err := strconv.Atoi(suffix)
			if err == nil && numSuffix > maxSuffix {
				maxSuffix = numSuffix
			}
		}
	}

	nextSuffix := maxSuffix + 1
	name := fmt.Sprintf("%s%d", basename, nextSuffix)
	return name, nil
}

func addRoutes(ifn *net.Interface, cidr *net.IPNet) error {
	r := netlink.Route{
		Dst:       cidr,
		LinkIndex: ifn.Index,
	}
	if err := netlink.RouteReplace(&r); err != nil {
		return fmt.Errorf("add route to %v error: %v", r.Dst, err)
	}
	return nil
}

// getFallbackTunName generates a deterministic fallback TUN device name
// based on the base name and the provided address string using a hash.
func getFallbackTunName(baseName, addr string) string {
	hasher := sha256.New()
	hasher.Write([]byte(addr))
	hashBytes := hasher.Sum(nil)
	// Use first 4 bytes -> 8 hex chars for brevity, respecting IFNAMSIZ limit.
	shortHash := hex.EncodeToString(hashBytes[:4])
	return fmt.Sprintf("%s%s", baseName, shortHash)
}