package proxy

import (
	"bytes"
	"crypto/subtle"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/netip"
	"os"
	"strings"
	"time"

	"sneak.berlin/go/smallwebwaf/internal/bans"
	"sneak.berlin/go/smallwebwaf/internal/config"
	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
	"sneak.berlin/go/smallwebwaf/internal/requestlog"
	"sneak.berlin/go/smallwebwaf/internal/state"
)

// banBodyMaxBytes is the most of the body of a request to add a ban that
// is read; its three fields need far less.
const banBodyMaxBytes = 4 << 10

// permanent is how the log line and the ban endpoint name a ban that
// never ends.
const permanent = "permanent"

var (
	errNotBanToAdd = errors.New(
		"the body is not a JSON object of netblock, duration and reason")
	errNotNetblock = errors.New(
		"is not an address or a netblock, such as 203.0.113.9 or 203.0.113.0/24")
	errMappedNetblock = errors.New(
		"is IPv4-mapped: give the IPv4 netblock, such as 203.0.113.0/24")
	errZone        = errors.New("has a zone, which a netblock cannot have")
	errNotDuration = errors.New(
		"is not a duration above zero, such as 1h or 7d, or permanent")
	errNotAddress = errors.New("is not an address, such as 203.0.113.9")
)

// answerAdmin answers a request for smallwebwaf itself, under
// /_smallwebwaf/, once it has passed the checks. Each endpoint needs a
// token, sent as Authorization: Bearer <token>: the metrics
// SWWAF_METRICS_TOKEN, the others SWWAF_ADMIN_TOKEN. A request without
// it is refused with 401. An endpoint whose token is unset answers 404,
// as any other request under /_smallwebwaf/ does.
func (rq *request) answerAdmin() {
	rq.line.Action = requestlog.ActionAdmin
	rq.startClientResponseTimeout()

	token, answer := rq.endpoint()

	switch {
	case token == "":
		http.Error(rq.out, http.StatusText(http.StatusNotFound), http.StatusNotFound)
	case !hasToken(rq.in, token):
		rq.out.Header().Set("WWW-Authenticate", "Bearer")
		rq.answer(refusal{
			status: http.StatusUnauthorized,
			action: requestlog.ActionAdmin,
		})
	default:
		answer()
	}
}

// endpoint returns the token the request's endpoint needs, and what
// answers the request there; "" when there is no such endpoint.
func (rq *request) endpoint() (string, func()) {
	cfg := rq.h.config
	method, path := rq.in.Method, rq.in.URL.Path

	switch {
	case method == http.MethodGet && path == MetricsPath:
		return cfg.MetricsToken, func() { rq.h.metrics.ServeHTTP(rq.out, rq.in) }
	case method == http.MethodGet && path == BansPath:
		return cfg.AdminToken, rq.listBans
	case method == http.MethodPost && path == BansPath:
		return cfg.AdminToken, rq.addBan
	case method == http.MethodDelete && strings.HasPrefix(path, BansPath+"/"):
		return cfg.AdminToken, rq.liftBans
	case method == http.MethodGet && strings.HasPrefix(path, ClientsPath):
		return cfg.AdminToken, rq.showClient
	default:
		return "", nil
	}
}

// hasToken reports whether r carries token, as Authorization: Bearer
// <token>.
func hasToken(r *http.Request, token string) bool {
	scheme, sent, _ := strings.Cut(r.Header.Get("Authorization"), " ")

	return strings.EqualFold(scheme, "Bearer") &&
		subtle.ConstantTimeCompare([]byte(sent), []byte(token)) == 1
}

// listBans answers GET BansPath with every ban held.
func (rq *request) listBans() {
	rq.answerBans(rq.h.ledger.Snapshot())
}

// banToAdd is the body of POST BansPath.
type banToAdd struct {
	// Netblock is a netblock, or a client's address, which stands for the
	// netblock a ban on that client covers.
	Netblock string `json:"netblock"`
	// Duration is how long the ban lasts, as a setting gives a duration,
	// or permanent.
	Duration string `json:"duration"`
	Reason   string `json:"reason"`
}

// addBan answers POST BansPath: it bans the netblock the body names, as
// an admin, from now for the duration the body gives, with its reason,
// and answers with that ban.
func (rq *request) addBan() {
	// The body must arrive within SWWAF_CLIENT_REQUEST_TIMEOUT, as any
	// other request's must.
	rq.stopReadingBody(rq.clientRequestDeadline())

	toAdd, err := rq.readBanToAdd()
	if refused := rq.refused.Load(); refused != nil {
		rq.answer(*refused) // the body is over SWWAF_REQUEST_MAX_BYTES

		return
	}

	if errors.Is(err, os.ErrDeadlineExceeded) {
		rq.answer(refusal{
			status: http.StatusRequestTimeout,
			action: requestlog.ActionTimedOut,
			limit:  "SWWAF_CLIENT_REQUEST_TIMEOUT",
		})

		return
	}

	var (
		netblock netip.Prefix
		expires  time.Time
		now      = rq.h.now()
	)

	if err == nil {
		netblock, err = rq.h.banNetblock(toAdd.Netblock)
	}

	if err == nil {
		expires, err = expiry(toAdd.Duration, now)
	}

	if err != nil {
		http.Error(rq.out, err.Error(), http.StatusBadRequest)

		return
	}

	ban := rq.h.ledger.BanForAdmin(netblock, now, expires, toAdd.Reason)
	rq.answerBans([]bans.Ban{ban})
}

// readBanToAdd reads the body of POST BansPath: a JSON object with
// nothing but whitespace after it, in at most banBodyMaxBytes.
func (rq *request) readBanToAdd() (banToAdd, error) {
	var body io.ReadCloser = http.NoBody
	if rq.body != nil {
		body = rq.body
	}

	data, err := io.ReadAll(http.MaxBytesReader(nil, body, banBodyMaxBytes))
	if err != nil {
		return banToAdd{}, fmt.Errorf("%w: %w", errNotBanToAdd, err)
	}

	var toAdd banToAdd

	decoder := json.NewDecoder(bytes.NewReader(data))
	decoder.DisallowUnknownFields()

	err = decoder.Decode(&toAdd)
	if err != nil {
		return banToAdd{}, fmt.Errorf("%w: %w", errNotBanToAdd, err)
	}

	// Token returns io.EOF only when nothing but whitespace is left.
	_, err = decoder.Token()
	if !errors.Is(err, io.EOF) {
		return banToAdd{}, fmt.Errorf("%w: more follows the object", errNotBanToAdd)
	}

	return toAdd, nil
}

// banNetblock reads value, a netblock such as 203.0.113.0/24, or a
// client's address, which stands for the netblock a ban on that client
// covers. An IPv4-mapped netblock, such as ::ffff:203.0.113.0/120, is
// refused, since a client's address is looked up as IPv4 and a ban on it
// would refuse nothing, and so is a value with a zone.
func (h *handler) banNetblock(value string) (netip.Prefix, error) {
	netblock, err := netip.ParsePrefix(value)
	if err == nil {
		if netblock.Addr().Is4In6() {
			return netip.Prefix{}, fmt.Errorf("netblock %q %w", value, errMappedNetblock)
		}

		return netblock, nil
	}

	// ParsePrefix refuses a zone, but ParseAddr reads the /48 of
	// 2001:db8::1%x/48 as part of the zone.
	addr, err := netip.ParseAddr(value)
	if err != nil {
		return netip.Prefix{}, fmt.Errorf("netblock %q %w", value, errNotNetblock)
	}

	if addr.Zone() != "" {
		return netip.Prefix{}, fmt.Errorf("netblock %q %w", value, errZone)
	}

	return h.netblock(addr), nil
}

// expiry returns when a ban made at now for duration ends: duration
// later, for a duration as a setting gives one, or zero for permanent.
func expiry(duration string, now time.Time) (time.Time, error) {
	if duration == permanent {
		return time.Time{}, nil
	}

	length, err := config.ParseDurationNotOff(duration)
	if err != nil {
		return time.Time{}, fmt.Errorf("duration %q %w", duration, errNotDuration)
	}

	return now.Add(length), nil
}

// liftBans answers DELETE BansPath/<client>: it lifts every ban active on
// a netblock the client's address is in, and answers with those bans, or
// with 404 when none is active.
func (rq *request) liftBans() {
	client, err := pathAddress(rq.in.URL.Path, BansPath+"/")
	if err != nil {
		http.Error(rq.out, err.Error(), http.StatusBadRequest)

		return
	}

	lifted := rq.h.ledger.Lift(client, rq.h.now())
	if len(lifted) == 0 {
		http.Error(rq.out, "no ban is active on "+client.String(), http.StatusNotFound)

		return
	}

	rq.answerBans(lifted)
}

// clientAnswer is the answer to GET ClientsPath<ip>: the client the
// address is, as clients.json holds it, or null when the table of
// clients does not hold it, and the bans on each netblock the address is
// in, as bans.json lists them.
type clientAnswer struct {
	Client *ratelimit.Client `json:"client"`
	Bans   []state.BanEntry  `json:"bans"`
}

// showClient answers GET ClientsPath<ip> with what smallwebwaf knows of
// the client: its counters, its history, which holds its country as last
// looked up and its offences, and its bans with their notes.
func (rq *request) showClient() {
	addr, err := pathAddress(rq.in.URL.Path, ClientsPath)
	if err != nil {
		http.Error(rq.out, err.Error(), http.StatusBadRequest)

		return
	}

	answer := clientAnswer{Bans: state.BanEntries(rq.h.ledger.Covering(addr))}

	client, seen := rq.h.limiter.Client(clientGroup(addr))
	if seen {
		answer.Client = &client
	}

	rq.answerJSON(answer)
}

// pathAddress reads the client's address that follows prefix in path.
func pathAddress(path, prefix string) (netip.Addr, error) {
	value := strings.TrimPrefix(path, prefix)

	addr, err := netip.ParseAddr(value)
	if err != nil {
		return netip.Addr{}, fmt.Errorf("%q %w", value, errNotAddress)
	}

	return addr.Unmap(), nil
}

// answerBans answers with held under bans, as bans.json lists them.
func (rq *request) answerBans(held []bans.Ban) {
	rq.answerJSON(struct {
		Bans []state.BanEntry `json:"bans"`
	}{state.BanEntries(held)})
}

// answerJSON answers with value as indented JSON.
func (rq *request) answerJSON(value any) {
	body, err := json.MarshalIndent(value, "", "    ")
	if err != nil {
		rq.h.processLog.Error("encoding an answer failed", "error", err.Error())
		http.Error(rq.out, http.StatusText(http.StatusInternalServerError),
			http.StatusInternalServerError)

		return
	}

	rq.out.Header().Set("Content-Type", "application/json")
	_, _ = rq.out.Write(append(body, '\n'))
}