// Package vpn reads this machine's WireGuard tunnels: the interfaces, and the peers on them
// with names put back on.
//
// The live state comes from `wg show all dump`, which is the kernel's own answer and the only
// place handshakes and byte counts exist. The kernel knows nothing about names, though — a
// peer there is a public key — so the configuration is read alongside it purely to label
// them: `# comments` in a wg-quick .conf, or the `description` of a UCI peer section on
// OpenWrt. Everything here is read-only, and no private or pre-shared key is ever read, let
// alone returned.
package vpn

import (
	"bufio"
	"context"
	"net"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"

	"hamlab/internal/priv"
)

// Peer is one end of a tunnel: a device that dials in, or the server this machine dials out
// to. Which it is depends on the tunnel; see Tunnel.Kind.
type Peer struct {
	// Name is what the configuration calls it, where it says at all. The kernel has no idea.
	Name      string `json:"name,omitempty"`
	PublicKey string `json:"public_key"`
	// Endpoint is where its packets come from, empty until it has been heard from.
	Endpoint   string   `json:"endpoint,omitempty"`
	AllowedIPs []string `json:"allowed_ips"`
	// Handshake is the last completed handshake. Zero means never: a peer that is configured
	// but has not connected since this tunnel came up.
	Handshake time.Time `json:"handshake,omitempty"`
	// Online is a handshake inside the last three minutes. WireGuard is silent when idle, so
	// this means "was talking recently", not "is connected" — there is no such thing here.
	Online    bool  `json:"online"`
	Rx        int64 `json:"rx"`
	Tx        int64 `json:"tx"`
	Keepalive int   `json:"keepalive,omitempty"`
	// PSK notes that a pre-shared key is in use, without the key itself going anywhere.
	PSK bool `json:"psk,omitempty"`
}

// Tunnel is one WireGuard interface.
type Tunnel struct {
	Name      string `json:"name"`
	PublicKey string `json:"public_key"`
	// Kind is "server" when peers dial in to this machine, "client" when this machine dials
	// out through a peer, and "" when it is too new or too odd to tell. It is worked out from
	// the shape of the peers, not from anything WireGuard states: a peer that is routed
	// everything and has an endpoint of its own is an exit, and a tunnel with one of those is
	// this machine going out through it.
	Kind       string   `json:"kind"`
	ListenPort int      `json:"listen_port,omitempty"`
	FwMark     string   `json:"fwmark,omitempty"`
	Addrs      []string `json:"addrs"`
	MTU        int      `json:"mtu,omitempty"`
	Peers      []Peer   `json:"peers"`
	// Exit is the peer this machine goes out through, for a client tunnel.
	Exit string `json:"exit,omitempty"`
}

// Report is every tunnel on this machine, and why there are none when there are none.
type Report struct {
	At      time.Time `json:"at"`
	Tunnels []Tunnel  `json:"tunnels"`
	// Tool is false when wireguard-tools is not installed, Root false when the daemon cannot
	// reach root — either one means the peers cannot be read, and the tab should say which.
	Tool bool   `json:"tool"`
	Root bool   `json:"root"`
	Note string `json:"note,omitempty"`
}

const onlineWithin = 3 * time.Minute

// Look reads the tunnels on this machine.
func Look(ctx context.Context) Report {
	r := Report{At: time.Now(), Tunnels: []Tunnel{}, Root: priv.Available()}
	if _, err := exec.LookPath("wg"); err != nil {
		r.Note = "wireguard-tools is not installed here, so there is nothing to read"
		return r
	}
	r.Tool = true
	args := []string{"wg", "show", "all", "dump"}
	if priv.Available() {
		args = priv.Wrap(args...)
	}
	cmd := exec.CommandContext(ctx, args[0], args[1:]...)
	out, err := cmd.Output()
	if err != nil {
		if !r.Root {
			r.Note = "reading WireGuard needs root, and this daemon has no way to it"
		} else {
			r.Note = "wg show failed: " + strings.TrimSpace(err.Error())
		}
		return r
	}
	r.Tunnels = parseDump(string(out))
	names := peerNames()
	for i := range r.Tunnels {
		t := &r.Tunnels[i]
		t.Addrs, t.MTU = ifaceAddrs(t.Name)
		for j := range t.Peers {
			p := &t.Peers[j]
			p.Name = names[p.PublicKey]
			p.Online = !p.Handshake.IsZero() && time.Since(p.Handshake) < onlineWithin
		}
		t.Kind, t.Exit = classify(*t)
		sort.SliceStable(t.Peers, func(a, b int) bool {
			pa, pb := t.Peers[a], t.Peers[b]
			if pa.Online != pb.Online {
				return pa.Online
			}
			if !pa.Handshake.Equal(pb.Handshake) {
				return pa.Handshake.After(pb.Handshake)
			}
			return pa.Name < pb.Name
		})
	}
	if len(r.Tunnels) == 0 {
		r.Note = "no WireGuard interfaces are up on this machine"
	}
	return r
}

// parseDump reads `wg show all dump`. The first line of each interface is the interface
// itself, the rest are its peers; every line is tab-separated and positional.
//
//	interface: name  private-key  public-key  listen-port  fwmark
//	peer:      name  public-key   psk         endpoint     allowed-ips  handshake  rx  tx  keepalive
//
// The private key is in the second column and is read past without being kept.
func parseDump(out string) []Tunnel {
	var tunnels []Tunnel
	byName := map[string]int{}
	sc := bufio.NewScanner(strings.NewReader(out))
	sc.Buffer(make([]byte, 0, 64<<10), 1<<20)
	for sc.Scan() {
		f := strings.Split(sc.Text(), "\t")
		if len(f) < 5 {
			continue
		}
		name := f[0]
		if len(f) == 5 { // an interface line
			port, _ := strconv.Atoi(f[3])
			byName[name] = len(tunnels)
			tunnels = append(tunnels, Tunnel{Name: name, PublicKey: f[2], ListenPort: port, FwMark: off(f[4]), Peers: []Peer{}})
			continue
		}
		if len(f) < 9 {
			continue
		}
		i, ok := byName[name]
		if !ok {
			continue
		}
		hs, _ := strconv.ParseInt(f[5], 10, 64)
		rx, _ := strconv.ParseInt(f[6], 10, 64)
		tx, _ := strconv.ParseInt(f[7], 10, 64)
		ka, _ := strconv.Atoi(f[8])
		p := Peer{PublicKey: f[1], PSK: f[2] != "(none)", Endpoint: off(f[3]), Rx: rx, Tx: tx, Keepalive: ka}
		if f[4] != "(none)" {
			p.AllowedIPs = strings.Split(f[4], ",")
		}
		if hs > 0 {
			p.Handshake = time.Unix(hs, 0)
		}
		tunnels[i].Peers = append(tunnels[i].Peers, p)
	}
	return tunnels
}

// off turns wg's placeholders into an empty string.
func off(v string) string {
	if v == "(none)" || v == "off" {
		return ""
	}
	return v
}

// classify decides whether this machine is the one being dialled or the one dialling. A peer
// that is allowed every address and has an endpoint is a way out to the internet; a tunnel
// with one of those is this machine using it. Otherwise, if anything can reach us here, it is
// a server.
func classify(t Tunnel) (kind, exit string) {
	for _, p := range t.Peers {
		for _, a := range p.AllowedIPs {
			if a == "0.0.0.0/0" || a == "::/0" {
				if p.Endpoint != "" {
					name := p.Name
					if name == "" {
						name = p.Endpoint
					}
					return "client", name
				}
			}
		}
	}
	if t.ListenPort > 0 && len(t.Peers) > 0 {
		return "server", ""
	}
	return "", ""
}

// ifaceAddrs is what the interface carries, and its MTU.
func ifaceAddrs(name string) ([]string, int) {
	addrs := []string{}
	mtu := 0
	ifi, err := net.InterfaceByName(name)
	if err != nil {
		return addrs, mtu
	}
	mtu = ifi.MTU
	got, err := ifi.Addrs()
	if err != nil {
		return addrs, mtu
	}
	for _, a := range got {
		addrs = append(addrs, a.String())
	}
	return addrs, mtu
}

// ---- names ----

// peerNames maps a public key to what the configuration calls that peer. Two shapes are
// understood, which between them cover every machine here: wg-quick's .conf files, where the
// name is a comment, and OpenWrt's UCI, where it is the peer section's description.
func peerNames() map[string]string {
	out := map[string]string{}
	for k, v := range confNames() {
		out[k] = v
	}
	for k, v := range uciNames() { // UCI wins: on OpenWrt it is the live configuration
		out[k] = v
	}
	return out
}

// confNames reads /etc/wireguard/*.conf. In a wg-quick file a peer is named by the comment
// that introduces it, which is the convention every tool that writes these files follows:
//
//	# phone
//	[Peer]
//	PublicKey = …
//
// confDir is where wg-quick keeps its files; a variable so the test can point it elsewhere.
var confDir = "/etc/wireguard"

func confNames() map[string]string {
	out := map[string]string{}
	files, _ := filepath.Glob(filepath.Join(confDir, "*.conf"))
	for _, path := range files {
		f, err := os.Open(path)
		if err != nil {
			continue // unreadable without root, which is the normal case; the kernel still has the keys
		}
		sc := bufio.NewScanner(f)
		// last is the most recent comment line; when a [Peer] block opens, that comment becomes
		// its pending name. Keeping the two apart is what lets each block take the comment that
		// introduces it and ignore any comment written inside it.
		last, pending := "", ""
		inPeer := false
		for sc.Scan() {
			line := strings.TrimSpace(sc.Text())
			switch {
			case strings.EqualFold(line, "[Peer]"):
				inPeer, pending, last = true, last, ""
			case strings.HasPrefix(line, "["):
				inPeer, pending, last = false, "", ""
			case strings.HasPrefix(line, "#"):
				if c := strings.TrimSpace(strings.TrimLeft(line, "#")); c != "" {
					last = c
				}
			case inPeer && strings.HasPrefix(line, "PublicKey"):
				// The separator is the first '='; the key's own base64 padding is the rest of
				// the line, so everything after that first one is the key.
				if _, v, ok := strings.Cut(line, "="); ok && pending != "" {
					out[strings.TrimSpace(v)] = pending
				}
				pending = ""
			}
		}
		f.Close()
	}
	return out
}

// uciNames reads OpenWrt's configuration: each peer is a section with a description and a
// public key. `uci show` is used rather than the file, so includes and overrides are applied.
func uciNames() map[string]string {
	out := map[string]string{}
	if _, err := exec.LookPath("uci"); err != nil {
		return out
	}
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	b, err := exec.CommandContext(ctx, "uci", "-q", "show", "network").Output()
	if err != nil {
		return out
	}
	desc, key := map[string]string{}, map[string]string{}
	for _, line := range strings.Split(string(b), "\n") {
		lhs, rhs, ok := strings.Cut(line, "=")
		if !ok || !strings.HasPrefix(lhs, "network.") {
			continue
		}
		parts := strings.Split(strings.TrimPrefix(lhs, "network."), ".")
		if len(parts) != 2 {
			continue
		}
		section, option := parts[0], parts[1]
		v := strings.Trim(rhs, "'")
		switch option {
		case "description":
			desc[section] = v
		case "public_key":
			key[section] = v
		}
	}
	for section, k := range key {
		if d := desc[section]; d != "" {
			out[k] = d
		}
	}
	return out
}