package encdns

import (
	"bufio"
	"encoding/hex"
	"fmt"
	"net/netip"
	"os"
	"strconv"
	"strings"

	"jarl/internal/model"
)

// Socket is a listening TCP socket on this host.
type Socket struct {
	Addr netip.Addr
	Port int
}

// ListeningSockets reads the host's listening TCP sockets from
// /proc/net/tcp and tcp6 (Linux; nil elsewhere). It needs no privileges
// and binds nothing. Tests replace it.
var ListeningSockets = func() ([]Socket, error) {
	var out []Socket
	for _, f := range []string{"/proc/net/tcp", "/proc/net/tcp6"} {
		socks, err := parseProcNet(f)
		if err != nil {
			if os.IsNotExist(err) {
				continue
			}
			return nil, err
		}
		out = append(out, socks...)
	}
	return out, nil
}

func parseProcNet(path string) ([]Socket, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	var out []Socket
	sc := bufio.NewScanner(f)
	sc.Scan() // header
	for sc.Scan() {
		fields := strings.Fields(sc.Text())
		if len(fields) < 4 || fields[3] != "0A" { // 0A: LISTEN
			continue
		}
		s, err := parseProcAddr(fields[1])
		if err == nil {
			out = append(out, s)
		}
	}
	return out, sc.Err()
}

// parseProcAddr parses "0100007F:0035": the address in 32-bit words of
// host byte order (little-endian here), then the port.
func parseProcAddr(s string) (Socket, error) {
	ha, hp, ok := strings.Cut(s, ":")
	if !ok {
		return Socket{}, fmt.Errorf("bad address %q", s)
	}
	b, err := hex.DecodeString(ha)
	if err != nil || (len(b) != 4 && len(b) != 16) {
		return Socket{}, fmt.Errorf("bad address %q", s)
	}
	for i := 0; i+4 <= len(b); i += 4 {
		b[i], b[i+1], b[i+2], b[i+3] = b[i+3], b[i+2], b[i+1], b[i]
	}
	addr, _ := netip.AddrFromSlice(b)
	port, err := strconv.ParseUint(hp, 16, 16)
	if err != nil {
		return Socket{}, err
	}
	return Socket{Addr: addr.Unmap(), Port: int(port)}, nil
}

// Conflict is something else listening where a listener would go.
type Conflict struct {
	Address string `json:"address"`
	Port    int    `json:"port"`
	What    string `json:"what"` // who it is, when Jarl knows
}

// PortCheck says which of addresses × ports something else listens on
// already. named's own listeners (the setup's, now) and jarld's (known
// listen addresses) are told apart.
func PortCheck(st *model.State, addresses []string, ports []int, jarld []string) ([]Conflict, error) {
	socks, err := ListeningSockets()
	if err != nil {
		return nil, err
	}
	// What named listens on now with DoT or DoH (it will again).
	ours := map[string]bool{}
	for _, l := range Listeners(st) {
		if l.Kind() != "plain" {
			for _, a := range l.Addrs {
				ours[a+"#"+strconv.Itoa(l.EffectivePort())] = true
			}
		}
	}
	jarldOn := map[string]bool{}
	for _, l := range jarld {
		jarldOn[l] = true
	}
	var out []Conflict
	for _, as := range addresses {
		a, err := netip.ParseAddr(as)
		if err != nil {
			continue
		}
		a = a.Unmap()
		for _, p := range ports {
			if ours[a.String()+"#"+strconv.Itoa(p)] {
				continue
			}
			for _, s := range socks {
				if s.Port != p || !(s.Addr == a || s.Addr.IsUnspecified() && s.Addr.Is4() == a.Is4() || s.Addr.IsUnspecified() && s.Addr.Is6()) {
					continue
				}
				what := "another program"
				for l := range jarldOn {
					if strings.HasSuffix(l, ":"+strconv.Itoa(p)) {
						what = "jarld (Jarl's web UI and API)"
					}
				}
				out = append(out, Conflict{Address: a.String(), Port: p, What: what})
				break
			}
		}
	}
	return out, nil
}

// FreeDoHPort returns the first of the usual DoH ports nothing listens on
// at the addresses (0 when none is free).
func FreeDoHPort(st *model.State, addresses []string, jarld []string) int {
	for _, p := range []int{443, 8443, 4443, 9443} {
		if c, err := PortCheck(st, addresses, []int{p}, jarld); err == nil && len(c) == 0 {
			return p
		}
	}
	return 0
}