// Package alerts sends alerts on bans, on a source that fails and on a
// file with an error to the webhook SWWAF_ALERT_WEBHOOK_URL names, each
// as one JSON object, as the "Alert webhook schema" section of SPEC.md
// describes. A repeat within SWWAF_ALERT_COOLDOWN is held back, and so is
// an alert past SWWAF_ALERT_MAX_PER_HOUR, for the hour's summary. The
// others wait in a bounded queue, so that a slow or unreachable webhook
// never holds up a request. The state is written to alerts.json and read
// from it by the state package. Nothing logged names the webhook's URL,
// whose path or query can carry a secret.
package alerts

import (
	"bytes"
	"cmp"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"maps"
	"net/http"
	"net/netip"
	"net/url"
	"slices"
	"sync"
	"sync/atomic"
	"time"
)

// The events an alert is for, as SWWAF_ALERT_EVENTS names them.
const (
	// EventBan is a ban smallwebwaf made.
	EventBan = "ban"
	// EventPermanentBan is a permanent ban smallwebwaf made, or a ban it
	// made permanent.
	EventPermanentBan = "permanent_ban"
	// EventWAFBlock, EventAnomaly and EventReputationHit come with the
	// Core Rule Set, the anomaly thresholds and the reputation sources;
	// nothing raises them yet.
	EventWAFBlock      = "waf_block"
	EventAnomaly       = "anomaly"
	EventReputationHit = "reputation_hit"
	// EventSourceFailure is GeoJS failing or refusing smallwebwaf.
	EventSourceFailure = "source_failure"
	// EventFileError is a rule file or state file edited while smallwebwaf
	// runs that does not parse, or a state file that cannot be written.
	EventFileError = "file_error"
	// EventSummary is the summary of the alerts an hour held back past
	// SWWAF_ALERT_MAX_PER_HOUR. SWWAF_ALERT_EVENTS does not name it.
	EventSummary = "summary"
)

// Events returns every event SWWAF_ALERT_EVENTS can name, which is its
// default.
func Events() []string {
	return []string{
		EventBan, EventPermanentBan, EventWAFBlock, EventAnomaly,
		EventReputationHit, EventSourceFailure, EventFileError,
	}
}

const (
	// queueSize is the most alerts that wait to be sent. Past it, the
	// oldest is dropped.
	queueSize = 1000
	// sendTimeout bounds one request to the webhook.
	sendTimeout = 10 * time.Second
	// After a request to the webhook fails, the alert is sent again a
	// second later, and retryDelayFactor times as long after each further
	// failure in a row, up to a minute.
	firstRetryDelay  = time.Second
	retryDelayFactor = 2
	maxRetryDelay    = time.Minute
	// maxAnswerBytes is the most of the webhook's answer that is read.
	maxAnswerBytes = 64 << 10
)

var (
	errStatus = errors.New("the webhook answered")
	// errRefused is a 4xx answer other than 408 and 429: the webhook
	// refuses the alert itself, and would refuse it again.
	errRefused = errors.New("the webhook refused the alert, answering")
)

// Params are what New needs.
type Params struct {
	// WebhookURL is where each alert is posted (SWWAF_ALERT_WEBHOOK_URL),
	// nil while it is unset and no alert is sent. WebhookHeaders are sent
	// with each (SWWAF_ALERT_WEBHOOK_HEADERS).
	WebhookURL     *url.URL
	WebhookHeaders http.Header
	// Events are the events alerts are sent for (SWWAF_ALERT_EVENTS).
	Events []string
	// Cooldown is how long a repeat of an alert is held back
	// (SWWAF_ALERT_COOLDOWN), 0 for no time. MaxPerHour is the most alerts
	// sent in an hour (SWWAF_ALERT_MAX_PER_HOUR), 0 for no limit.
	Cooldown   time.Duration
	MaxPerHour int
	// Instance is SWWAF_INSTANCE_NAME, which every alert gives.
	Instance string
	// Now tells the time of an alert, normally time.Now in UTC.
	Now func() time.Time
	// ProcessLog receives the requests to the webhook that fail.
	ProcessLog *slog.Logger
}

// Alert is one alert, as the webhook is sent it and alerts.json holds it,
// with the fields of the "Alert webhook schema" section of SPEC.md. ASN
// and ASName are empty until AS numbers are looked up.
//
//nolint:tagliatelle // SPEC.md's alert webhook schema names its fields in snake_case
type Alert struct {
	Instance string       `json:"instance"`
	Time     time.Time    `json:"time"`
	Event    string       `json:"event"`
	Client   netip.Addr   `json:"client"`
	Netblock netip.Prefix `json:"netblock"`
	ASN      string       `json:"asn"`
	ASName   string       `json:"as_name"`
	Country  string       `json:"country"`
	// Reason is a short sentence, and Detail what is particular to the
	// event: for a file_error, its "file", and for a source_failure, its
	// "source", which the cooldown tells repeats by.
	Reason string         `json:"reason"`
	Detail map[string]any `json:"detail"`
	// SuppressedRepeats is how many repeats of the alert the cooldown
	// held back since the last one let through.
	SuppressedRepeats int `json:"suppressed_repeats"`
}

// Cooldown is, for an event on a netblock, or about a file or a source,
// when the last alert let through was raised, and how many repeats the
// cooldown has held back since, as alerts.json holds it.
//
//nolint:tagliatelle // the state files use snake_case, as the request log does
type Cooldown struct {
	Event             string       `json:"event"`
	Netblock          netip.Prefix `json:"netblock"`
	File              string       `json:"file,omitempty"`
	Source            string       `json:"source,omitempty"`
	Sent              time.Time    `json:"sent"`
	SuppressedRepeats int          `json:"suppressed_repeats"`
}

// Hour is the hour under way, by the clock, as alerts.json holds it: when
// it started, how many alerts were let through in it, and how many were
// held back in it past MaxPerHour, by event, for its summary.
//
//nolint:tagliatelle // the state files use snake_case, as the request log does
type Hour struct {
	Start    time.Time      `json:"start"`
	Sent     int            `json:"sent"`
	HeldBack map[string]int `json:"held_back"`
}

// State is what alerts.json holds: the cooldowns, the hour under way, and
// the alerts waiting to be sent, oldest first.
type State struct {
	Cooldowns []Cooldown `json:"cooldowns"`
	Hour      Hour       `json:"hour"`
	Waiting   []Alert    `json:"waiting"`
}

// Queue takes the alerts raised, holds back those it must, and sends the
// others to the webhook. It is safe for concurrent use.
type Queue struct {
	params Params
	// httpClient follows no redirect: a redirect is a failure.
	httpClient *http.Client
	// queued receives a value when an alert joins the queue, unless one
	// waits already, so that Run looks at the queue again.
	queued chan struct{}

	mu sync.Mutex
	// cooldowns are the alerts last let through, by event and netblock,
	// file or source.
	cooldowns map[cooldownKey]*Cooldown
	hour      Hour
	// waiting are the alerts waiting to be sent, oldest first.
	waiting []*Alert

	sent, failed, suppressed, dropped atomic.Int64
}

// cooldownKey is what makes an alert a repeat of another: the same event
// on the same netblock, and about the same file or source, as its detail
// names them. Each is empty for an alert without one.
type cooldownKey struct {
	event    string
	netblock netip.Prefix
	file     string
	source   string
}

// cooldownKeyOf returns what makes another alert a repeat of alert.
func cooldownKeyOf(alert *Alert) cooldownKey {
	file, _ := alert.Detail["file"].(string)
	source, _ := alert.Detail["source"].(string)

	return cooldownKey{alert.Event, alert.Netblock, file, source}
}

// New returns a Queue with no alert yet.
func New(params Params) *Queue {
	return &Queue{
		params: params,
		httpClient: &http.Client{
			CheckRedirect: func(*http.Request, []*http.Request) error {
				return http.ErrUseLastResponse
			},
		},
		queued:    make(chan struct{}, 1),
		cooldowns: map[cooldownKey]*Cooldown{},
		hour:      Hour{HeldBack: map[string]int{}},
	}
}

// Raise sends alert, which names its event and what is particular to it,
// unless no webhook is set or SWWAF_ALERT_EVENTS leaves its event out. It
// gives alert the instance and the time. An alert that repeats the last
// one let through less than Cooldown before is held back and counted,
// and the next one let through gives that count. Past MaxPerHour alerts
// let through in the hour under way, by the clock, an alert is held back
// for that hour's summary instead, which is sent once the hour has ended;
// it starts no cooldown, and the repeats held back before it are given by
// the next alert let through. Raise never waits: an alert let through
// joins the queue, from which Run sends it, and with queueSize alerts
// waiting the oldest is dropped.
func (q *Queue) Raise(alert Alert) {
	if q.params.WebhookURL == nil || !slices.Contains(q.params.Events, alert.Event) {
		return
	}

	q.mu.Lock()
	defer q.mu.Unlock()

	now := q.params.Now()
	alert.Instance = q.params.Instance
	alert.Time = now

	if q.repeat(&alert, now) {
		q.suppressed.Add(1)

		return
	}

	q.endHour(now)

	if q.params.MaxPerHour > 0 && q.hour.Sent >= q.params.MaxPerHour {
		q.hour.HeldBack[alert.Event]++
		q.suppressed.Add(1)

		return
	}

	q.startCooldown(&alert, now)
	q.hour.Sent++
	q.queue(&alert)
}

// WouldSend reports whether Raise would let an alert for event on
// netblock through now: a webhook is set, SWWAF_ALERT_EVENTS chooses
// event, no alert for event on netblock was let through less than
// Cooldown before, and fewer than MaxPerHour alerts have been let through
// in the hour under way. Unlike Raise, it counts nothing.
func (q *Queue) WouldSend(event string, netblock netip.Prefix) bool {
	if q.params.WebhookURL == nil || !slices.Contains(q.params.Events, event) {
		return false
	}

	q.mu.Lock()
	defer q.mu.Unlock()

	now := q.params.Now()

	last, found := q.cooldowns[cooldownKey{event: event, netblock: netblock}]
	if q.params.Cooldown > 0 && found && now.Sub(last.Sent) < q.params.Cooldown {
		return false
	}

	q.endHour(now)

	return q.params.MaxPerHour == 0 || q.hour.Sent < q.params.MaxPerHour
}

// Run sends the alerts waiting, oldest first, until ctx is done. An alert
// stays in the queue until the webhook answers it with a 2xx status, or
// refuses it with a 4xx status other than 408 and 429: a refused alert is
// logged, counted as dropped, and given up, so that the next is sent. Any
// other request that fails is logged, and the alert sent again
// firstRetryDelay later, retryDelayFactor times as long after each
// further failure in a row, up to maxRetryDelay. Run also ends each hour
// as Raise does, so that the hour's summary is sent as it ends. With no
// webhook set, it returns at once.
func (q *Queue) Run(ctx context.Context) {
	if q.params.WebhookURL == nil {
		return
	}

	var (
		retryDelay time.Duration
		retryAt    time.Time
	)

	for {
		alert, untilHourEnds := q.next()
		hourEnds := time.NewTimer(untilHourEnds)

		var due <-chan time.Time // nil while no alert waits
		if alert != nil {
			due = time.After(time.Until(retryAt))
		}

		select {
		case <-ctx.Done():
			hourEnds.Stop()

			return
		case <-q.queued:
		case <-hourEnds.C:
			q.mu.Lock()
			q.endHour(q.params.Now())
			q.mu.Unlock()
		case <-due:
			err := q.send(ctx, alert)

			switch {
			case err == nil:
				q.remove(alert)
				q.sent.Add(1)

				retryDelay = 0
				retryAt = time.Time{}
			case errors.Is(err, errRefused):
				q.remove(alert)
				q.failed.Add(1)
				q.dropped.Add(1)

				retryDelay = 0
				retryAt = time.Time{}

				q.params.ProcessLog.Warn("gave up an alert SWWAF_ALERT_WEBHOOK_URL refused",
					"event", alert.Event, "error", err.Error())
			case ctx.Err() == nil: // not cut off as smallwebwaf stops
				q.failed.Add(1)

				retryDelay = min(max(retryDelayFactor*retryDelay, firstRetryDelay),
					maxRetryDelay)
				retryAt = time.Now().Add(retryDelay)

				q.params.ProcessLog.Warn("sending an alert to SWWAF_ALERT_WEBHOOK_URL failed",
					"error", err.Error(), "sending_again_in", retryDelay.String())
			}
		}

		hourEnds.Stop()
	}
}

// Sent is how many alerts the webhook has taken.
func (q *Queue) Sent() int64 {
	return q.sent.Load()
}

// Failed is how many requests to the webhook have failed.
func (q *Queue) Failed() int64 {
	return q.failed.Load()
}

// Suppressed is how many alerts were held back: by the cooldown, and past
// MaxPerHour.
func (q *Queue) Suppressed() int64 {
	return q.suppressed.Load()
}

// Dropped is how many alerts were dropped from a full queue, or given up
// as the webhook refused them.
func (q *Queue) Dropped() int64 {
	return q.dropped.Load()
}

// Snapshot returns the queue's state, as alerts.json holds it, with the
// cooldowns sorted by netblock, then by event, file and source.
func (q *Queue) Snapshot() State {
	q.mu.Lock()
	defer q.mu.Unlock()

	state := State{
		Cooldowns: make([]Cooldown, 0, len(q.cooldowns)),
		Hour:      q.hour,
		Waiting:   make([]Alert, 0, len(q.waiting)),
	}
	state.Hour.HeldBack = maps.Clone(q.hour.HeldBack)

	for _, cooldown := range q.cooldowns {
		state.Cooldowns = append(state.Cooldowns, *cooldown)
	}

	slices.SortFunc(state.Cooldowns, func(a, b Cooldown) int {
		return cmp.Or(a.Netblock.Compare(b.Netblock), cmp.Compare(a.Event, b.Event),
			cmp.Compare(a.File, b.File), cmp.Compare(a.Source, b.Source))
	})

	for _, alert := range q.waiting {
		state.Waiting = append(state.Waiting, *alert)
	}

	return state
}

// Load puts state, read from alerts.json, in place of the queue's state.
// Each cooldown's netblock is masked to its length, so that
// 203.0.113.9/24 is 203.0.113.0/24. Past queueSize alerts waiting, the
// oldest are dropped.
func (q *Queue) Load(state State) {
	q.mu.Lock()
	defer q.mu.Unlock()

	q.cooldowns = map[cooldownKey]*Cooldown{}

	for _, cooldown := range state.Cooldowns {
		cooldown.Netblock = cooldown.Netblock.Masked()
		key := cooldownKey{cooldown.Event, cooldown.Netblock, cooldown.File, cooldown.Source}
		q.cooldowns[key] = &cooldown
	}

	q.hour = state.Hour
	q.hour.HeldBack = maps.Clone(state.Hour.HeldBack)

	if q.hour.HeldBack == nil {
		q.hour.HeldBack = map[string]int{}
	}

	q.waiting = nil

	for _, alert := range state.Waiting {
		q.queue(&alert)
	}
}

// repeat reports whether alert, raised at now, repeats the last one let
// through less than Cooldown before, and counts it if it does.
func (q *Queue) repeat(alert *Alert, now time.Time) bool {
	if q.params.Cooldown == 0 {
		return false
	}

	last, found := q.cooldowns[cooldownKeyOf(alert)]
	if !found || now.Sub(last.Sent) >= q.params.Cooldown {
		return false
	}

	last.SuppressedRepeats++

	return true
}

// startCooldown gives alert, let through at now, the count of the repeats
// held back since the last one let through, and notes alert as the last
// one let through.
func (q *Queue) startCooldown(alert *Alert, now time.Time) {
	if q.params.Cooldown == 0 {
		return
	}

	key := cooldownKeyOf(alert)

	last, found := q.cooldowns[key]
	if found {
		alert.SuppressedRepeats = last.SuppressedRepeats
	}

	q.cooldowns[key] = &Cooldown{
		Event: alert.Event, Netblock: alert.Netblock, File: key.file, Source: key.source,
		Sent: now,
	}
}

// endHour ends the hour under way, if now is past it: it queues that
// hour's summary when alerts were held back in it past MaxPerHour, and
// forgets the cooldowns that have run out with no repeat held back, which
// no alert needs any more.
func (q *Queue) endHour(now time.Time) {
	start := now.Truncate(time.Hour)
	if !start.After(q.hour.Start) {
		return
	}

	heldBack := 0
	for _, count := range q.hour.HeldBack {
		heldBack += count
	}

	if heldBack > 0 {
		q.queue(&Alert{
			Instance: q.params.Instance,
			Time:     now,
			Event:    EventSummary,
			Reason: fmt.Sprintf("%d alerts held back in the hour from %s, past the %d "+
				"an hour SWWAF_ALERT_MAX_PER_HOUR allows", heldBack,
				q.hour.Start.Format(time.RFC3339), q.params.MaxPerHour),
			Detail: map[string]any{
				"hour": q.hour.Start, "count": heldBack, "events": q.hour.HeldBack,
			},
		})
	}

	q.hour = Hour{Start: start, HeldBack: map[string]int{}}

	for key, cooldown := range q.cooldowns {
		if now.Sub(cooldown.Sent) >= q.params.Cooldown && cooldown.SuppressedRepeats == 0 {
			delete(q.cooldowns, key)
		}
	}
}

// queue adds alert to the alerts waiting, first dropping the oldest while
// queueSize wait, and has Run look at the queue again.
func (q *Queue) queue(alert *Alert) {
	if len(q.waiting) == queueSize {
		q.waiting = slices.Delete(q.waiting, 0, 1)
		q.dropped.Add(1)
	}

	q.waiting = append(q.waiting, alert)

	select {
	case q.queued <- struct{}{}:
	default: // a value waits already
	}
}

// next returns the oldest alert waiting, nil when none waits, and how
// long it is until the hour under way ends.
func (q *Queue) next() (*Alert, time.Duration) {
	q.mu.Lock()
	defer q.mu.Unlock()

	var oldest *Alert
	if len(q.waiting) > 0 {
		oldest = q.waiting[0]
	}

	return oldest, q.hour.Start.Add(time.Hour).Sub(q.params.Now())
}

// remove takes alert, which Run has sent or given up, out of the queue,
// unless it has been dropped from it, or Load has replaced the queue,
// since Run took it. Only the oldest alert is ever dropped, so alert is
// the oldest if it is there at all.
func (q *Queue) remove(alert *Alert) {
	q.mu.Lock()
	defer q.mu.Unlock()

	if len(q.waiting) > 0 && q.waiting[0] == alert {
		q.waiting = slices.Delete(q.waiting, 0, 1)
	}
}

// send posts alert to the webhook as JSON, with WebhookHeaders, and
// returns an error unless the webhook answers with a 2xx status: one that
// wraps errRefused for a 4xx status other than 408 and 429. No error
// names the webhook's URL, whose path or query can carry a secret.
func (q *Queue) send(ctx context.Context, alert *Alert) error {
	body, err := json.Marshal(alert)
	if err != nil {
		return fmt.Errorf("encode the alert: %w", err)
	}

	ctx, cancel := context.WithTimeout(ctx, sendTimeout)
	defer cancel()

	req, err := http.NewRequestWithContext(ctx, http.MethodPost,
		q.params.WebhookURL.String(), bytes.NewReader(body))
	if err != nil {
		return fmt.Errorf("make the request: %w", err)
	}

	maps.Copy(req.Header, q.params.WebhookHeaders)
	req.Header.Set("Content-Type", "application/json")

	res, err := q.httpClient.Do(req)
	if err != nil {
		// The client's error names the URL: only what went wrong is kept.
		if urlErr, ok := errors.AsType[*url.Error](err); ok {
			return urlErr.Err
		}

		return err
	}

	defer func() {
		_ = res.Body.Close()
	}()

	// Read, so that the connection can be used again.
	_, _ = io.Copy(io.Discard, io.LimitReader(res.Body, maxAnswerBytes))

	switch status := res.StatusCode; {
	case status >= http.StatusOK && status < http.StatusMultipleChoices:
		return nil
	case status >= http.StatusBadRequest && status < http.StatusInternalServerError &&
		status != http.StatusRequestTimeout && status != http.StatusTooManyRequests:
		return fmt.Errorf("%w %s", errRefused, res.Status)
	default:
		return fmt.Errorf("%w %s", errStatus, res.Status)
	}
}