//go:build linux
// +build linux

package aghnet

import (
	"bufio"
	"fmt"
	"io"
	"io/fs"
	"net"
	"strings"
	"sync"

	"github.com/AdguardTeam/AdGuardHome/internal/aghos"
	"github.com/AdguardTeam/golibs/log"
	"github.com/AdguardTeam/golibs/netutil"
	"github.com/AdguardTeam/golibs/stringutil"
)

func newARPDB() (arp *arpdbs) {
	// Use the common storage among the implementations.
	ns := &neighs{
		mu: &sync.RWMutex{},
		ns: make([]Neighbor, 0),
	}

	var parseF parseNeighsFunc
	if aghos.IsOpenWrt() {
		parseF = parseArpAWrt
	} else {
		parseF = parseArpA
	}

	return newARPDBs(
		// Try /proc/net/arp first.
		&fsysARPDB{ns: ns, fsys: aghos.RootDirFS(), filename: "proc/net/arp"},
		// Try "arp -a" then.
		&cmdARPDB{parse: parseF, runcmd: rcArpA, ns: ns},
		// Try "ip neigh" finally.
		&cmdARPDB{parse: parseIPNeigh, runcmd: rcIPNeigh, ns: ns},
	)
}

// fsysARPDB accesses the ARP cache file to update the database.
type fsysARPDB struct {
	ns       *neighs
	fsys     fs.FS
	filename string
}

// type check
var _ ARPDB = (*fsysARPDB)(nil)

// Refresh implements the ARPDB interface for *fsysARPDB.
func (arp *fsysARPDB) Refresh() (err error) {
	var f fs.File
	f, err = arp.fsys.Open(arp.filename)
	if err != nil {
		return fmt.Errorf("opening %q: %w", arp.filename, err)
	}

	sc := bufio.NewScanner(f)
	// Skip the header.
	if !sc.Scan() {
		return nil
	} else if err = sc.Err(); err != nil {
		return err
	}

	ns := make([]Neighbor, 0, arp.ns.len())
	for sc.Scan() {
		ln := sc.Text()
		fields := stringutil.SplitTrimmed(ln, " ")
		if len(fields) != 6 {
			continue
		}

		n := Neighbor{}
		if n.IP = net.ParseIP(fields[0]); n.IP == nil || n.IP.IsUnspecified() {
			continue
		} else if n.MAC, err = net.ParseMAC(fields[3]); err != nil {
			continue
		}

		ns = append(ns, n)
	}

	arp.ns.reset(ns)

	return nil
}

// Neighbors implements the ARPDB interface for *fsysARPDB.
func (arp *fsysARPDB) Neighbors() (ns []Neighbor) {
	return arp.ns.clone()
}

// parseArpAWrt parses the output of the "arp -a" command on OpenWrt.  The
// expected input format:
//
//   IP address       HW type     Flags       HW address            Mask     Device
//   192.168.11.98    0x1         0x2         5a:92:df:a9:7e:28     *        wan
//
func parseArpAWrt(sc *bufio.Scanner, lenHint int) (ns []Neighbor) {
	if !sc.Scan() {
		// Skip the header.
		return
	}

	ns = make([]Neighbor, 0, lenHint)
	for sc.Scan() {
		ln := sc.Text()

		fields := strings.Fields(ln)
		if len(fields) < 4 {
			continue
		}

		n := Neighbor{}

		if ip := net.ParseIP(fields[0]); ip == nil || n.IP.IsUnspecified() {
			continue
		} else {
			n.IP = ip
		}

		hwStr := fields[3]
		if mac, err := net.ParseMAC(hwStr); err != nil {
			log.Debug("parsing arp output: %s", err)

			continue
		} else {
			n.MAC = mac
		}

		ns = append(ns, n)
	}

	return ns
}

// parseArpA parses the output of the "arp -a" command on Linux.  The expected
// input format:
//
//   hostname (192.168.1.1) at ab:cd:ef:ab:cd:ef [ether] on enp0s3
//
func parseArpA(sc *bufio.Scanner, lenHint int) (ns []Neighbor) {
	ns = make([]Neighbor, 0, lenHint)
	for sc.Scan() {
		ln := sc.Text()

		fields := strings.Fields(ln)
		if len(fields) < 4 {
			continue
		}

		n := Neighbor{}

		if ipStr := fields[1]; len(ipStr) < 2 {
			continue
		} else if ip := net.ParseIP(ipStr[1 : len(ipStr)-1]); ip == nil {
			continue
		} else {
			n.IP = ip
		}

		hwStr := fields[3]
		if mac, err := net.ParseMAC(hwStr); err != nil {
			log.Debug("parsing arp output: %s", err)

			continue
		} else {
			n.MAC = mac
		}

		host := fields[0]
		if verr := netutil.ValidateDomainName(host); verr != nil {
			log.Debug("parsing arp output: %s", verr)
		} else {
			n.Name = host
		}

		ns = append(ns, n)
	}

	return ns
}

// rcIPNeigh runs "ip neigh".
func rcIPNeigh() (r io.Reader, err error) {
	return runCmd("ip", "neigh")
}

// parseIPNeigh parses the output of the "ip neigh" command on Linux.  The
// expected input format:
//
//   192.168.1.1 dev enp0s3 lladdr ab:cd:ef:ab:cd:ef REACHABLE
//
func parseIPNeigh(sc *bufio.Scanner, lenHint int) (ns []Neighbor) {
	ns = make([]Neighbor, 0, lenHint)
	for sc.Scan() {
		ln := sc.Text()

		fields := strings.Fields(ln)
		if len(fields) < 5 {
			continue
		}

		n := Neighbor{}

		if ip := net.ParseIP(fields[0]); ip == nil {
			continue
		} else {
			n.IP = ip
		}

		if mac, err := net.ParseMAC(fields[4]); err != nil {
			log.Debug("parsing arp output: %s", err)

			continue
		} else {
			n.MAC = mac
		}

		ns = append(ns, n)
	}

	return ns
}