package api

import (
	"context"
	"database/sql"
	"encoding/json"
	"log"
	"net/http"
	"time"
)

// Values for alerts.resolution_source, recording why an alert left the firing
// state: a real Alertmanager notification, or inference by the sweeper.
const (
	resolutionAlertmanager = "alertmanager"
	resolutionExpiry       = "expiry"
)

// amPayload mirrors the Alertmanager webhook v4 payload.
type amPayload struct {
	Version string `json:"version"`
	Status  string `json:"status"`

	// GroupKey and GroupLabels are how alerts get correlated into incidents.
	// Alertmanager has already done the grouping work according to the group_by
	// routing tree the operator configured, so we adopt its answer instead of
	// inventing a second grouping scheme here.
	GroupKey    string            `json:"groupKey"`
	GroupLabels map[string]string `json:"groupLabels"`

	Alerts []amAlert `json:"alerts"`
}

type amAlert struct {
	Status       string            `json:"status"`
	Labels       map[string]string `json:"labels"`
	Annotations  map[string]string `json:"annotations"`
	StartsAt     time.Time         `json:"startsAt"`
	EndsAt       time.Time         `json:"endsAt"`
	GeneratorURL string            `json:"generatorURL"`
	Fingerprint  string            `json:"fingerprint"`
}

// ingested records what actually happened to one alert of a payload, which is
// what decides whether an incident opens.
type ingested struct {
	id     int64
	name   string
	firing bool

	// newOccurrence marks an alert that transitioned *into* firing: a
	// fingerprint we had never seen, a newer startsAt, or a resolved alert that
	// started again. A repeat_interval re-send of an already-firing alert is
	// none of these, which is what keeps a manually resolved incident closed.
	newOccurrence bool

	// justResolved marks the firing → resolved edge, worth a timeline entry.
	justResolved bool
}

func handleAlertmanagerWebhook(db *sql.DB, notify NotifyConfig) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		var payload amPayload
		if err := decodeJSON(r, &payload); err != nil {
			respond(w, http.StatusBadRequest, errResp("invalid payload"))
			return
		}

		// Alertmanager retries anything that is not 2xx, and a retry of a payload
		// we failed to store is more useful than an error it cannot act on — so
		// failures are logged, not surfaced.
		if err := ingest(r.Context(), db, notify, payload); err != nil {
			log.Printf("webhook ingest (group %q): %v", payload.GroupKey, err)
		}

		w.WriteHeader(http.StatusOK)
	}
}

// ingest stores a payload's alerts and reconciles the incident for its group.
// The whole payload is one transaction: an incident that opened but whose alerts
// failed to link would be a work item nobody could act on.
func ingest(ctx context.Context, db *sql.DB, notify NotifyConfig, payload amPayload) error {
	tx, err := db.BeginTx(ctx, nil)
	if err != nil {
		return err
	}
	defer tx.Rollback() //nolint:errcheck

	accepted, err := upsertAlerts(ctx, tx, payload.Alerts)
	if err != nil {
		return err
	}

	// touched collects every incident this payload affected, so severity and the
	// resolution cascade are recomputed once per incident at the end.
	touched := map[int64]bool{}

	incidentID, err := incidentForGroup(ctx, tx, notify, payload, accepted)
	if err != nil {
		return err
	}
	if incidentID != 0 {
		touched[incidentID] = true
		for _, a := range accepted {
			if !a.firing {
				continue
			}
			if err := linkAlert(ctx, tx, incidentID, a.id); err != nil {
				return err
			}
		}
	}

	for _, a := range accepted {
		if !a.justResolved {
			continue
		}
		id, err := openIncidentForAlert(ctx, tx, a.id)
		if err != nil {
			return err
		}
		if id == 0 {
			continue
		}
		touched[id] = true
		alertID := a.id
		if err := logEvent(ctx, tx, id, evAlertResolved, nil, &alertID, nil); err != nil {
			return err
		}
	}

	for id := range touched {
		if err := refreshSeverity(ctx, tx, id); err != nil {
			return err
		}
		if _, err := resolveIfSettled(ctx, tx, id); err != nil {
			return err
		}
	}

	return tx.Commit()
}

// upsertAlerts stores each alert of a payload and reports what changed. Payloads
// the ordering guard rejected are left out entirely.
func upsertAlerts(ctx context.Context, tx *sql.Tx, alerts []amAlert) ([]ingested, error) {
	now := time.Now().Unix()
	accepted := make([]ingested, 0, len(alerts))

	for _, a := range alerts {
		name := a.Labels["alertname"]
		labelsJSON, _ := json.Marshal(a.Labels)
		annotationsJSON, _ := json.Marshal(a.Annotations)

		// The stored state has to be read before the upsert overwrites it: it is
		// the only way to tell a genuine new occurrence from a re-send.
		var prevStatus string
		var prevStartsAt int64
		existed := true
		switch err := tx.QueryRowContext(ctx,
			"SELECT status, starts_at FROM alerts WHERE fingerprint = ?", a.Fingerprint,
		).Scan(&prevStatus, &prevStartsAt); {
		case err == sql.ErrNoRows:
			existed = false
		case err != nil:
			return nil, err
		}

		// Zero time ("0001-01-01T00:00:00Z") means "no end known" — that is the
		// convention of Alertmanager's ingest API. Outgoing notifications
		// normally carry a real future endsAt instead, which is the watermark
		// the sweeper uses to expire alerts that stop being refreshed.
		var endsAtUnix *int64
		if a.EndsAt.Year() > 1 {
			t := a.EndsAt.Unix()
			endsAtUnix = &t
		}

		var resolutionSource *string
		if a.Status == "resolved" {
			s := resolutionAlertmanager
			resolutionSource = &s
		}

		// The WHERE clause discards payloads that describe an alert instance
		// older than the stored one. Alertmanager retries failed notifications,
		// so a stale firing retry can arrive after the resolved one; it carries
		// the same startsAt, whereas a genuine re-fire carries a newer one.
		// Within a single instance, resolution is terminal.
		if _, err := tx.ExecContext(ctx, `
			INSERT INTO alerts
				(fingerprint, name, status, labels, annotations, starts_at, ends_at,
				 generator_url, received_at, resolution_source)
			VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
			ON CONFLICT(fingerprint) DO UPDATE SET
				status            = excluded.status,
				labels            = excluded.labels,
				annotations       = excluded.annotations,
				starts_at         = excluded.starts_at,
				ends_at           = excluded.ends_at,
				generator_url     = excluded.generator_url,
				-- Load-bearing: advancing received_at on every accepted
				-- payload, re-sends included, is the documented liveness
				-- heartbeat clients and the sweeper both read. Removing it
				-- is a breaking API change — see models.Alert.ReceivedAt.
				received_at       = excluded.received_at,
				resolution_source = excluded.resolution_source,
				-- A re-fire makes the alert current again, so it leaves the archive.
				archived_at       = CASE WHEN excluded.status = 'firing'
				                        THEN NULL ELSE alerts.archived_at END
			WHERE excluded.starts_at > alerts.starts_at
			   OR (excluded.starts_at = alerts.starts_at
			       AND NOT (alerts.status = 'resolved' AND excluded.status = 'firing'))`,
			a.Fingerprint, name, a.Status,
			string(labelsJSON), string(annotationsJSON),
			a.StartsAt.Unix(), endsAtUnix,
			a.GeneratorURL, now, resolutionSource,
		); err != nil {
			return nil, err
		}

		var id int64
		var curStatus string
		var curStartsAt int64
		if err := tx.QueryRowContext(ctx,
			"SELECT id, status, starts_at FROM alerts WHERE fingerprint = ?", a.Fingerprint,
		).Scan(&id, &curStatus, &curStartsAt); err != nil {
			return nil, err
		}

		// The upsert copies status and starts_at straight from the payload, so a
		// row that does not match it is one the ordering guard rejected. A
		// discarded payload describes a past instance and must not touch the
		// incident state either.
		if existed && (curStatus != a.Status || curStartsAt != a.StartsAt.Unix()) {
			continue
		}

		firing := a.Status == "firing"
		accepted = append(accepted, ingested{
			id:            id,
			name:          name,
			firing:        firing,
			newOccurrence: firing && (!existed || a.StartsAt.Unix() > prevStartsAt || prevStatus == "resolved"),
			justResolved:  !firing && existed && prevStatus == "firing",
		})
	}

	return accepted, nil
}

// incidentForGroup returns the open incident that this payload's firing alerts
// belong to, opening one if the group has none. It returns 0 when the payload
// warrants no incident at all.
//
// The rule that matters: a group with no open incident gets a new one only if
// something actually started firing. Without that, a manually resolved incident
// would reappear on the next repeat_interval re-send of an alert that never
// stopped, and manual resolution would be meaningless.
func incidentForGroup(ctx context.Context, tx *sql.Tx, notify NotifyConfig, payload amPayload, accepted []ingested) (int64, error) {
	var firstName string
	anyFiring, anyNew := false, false
	for _, a := range accepted {
		if a.firing {
			if !anyFiring {
				firstName = a.name
			}
			anyFiring = true
		}
		if a.newOccurrence {
			anyNew = true
		}
	}
	if !anyFiring {
		// A payload of nothing but resolutions never opens an incident.
		return 0, nil
	}

	groupKey := payload.GroupKey
	if groupKey == "" {
		// Alertmanager always sends groupKey; a sender that does not still gets
		// one incident per alert name rather than one giant shared incident.
		groupKey = "groupless:" + firstName
	}

	var id int64
	switch err := tx.QueryRowContext(ctx,
		"SELECT id FROM incidents WHERE group_key = ? AND resolved_at IS NULL", groupKey,
	).Scan(&id); {
	case err == nil:
		return id, nil
	case err != sql.ErrNoRows:
		return 0, err
	}

	if !anyNew {
		return 0, nil
	}
	return openIncident(ctx, tx, notify, groupKey, payload.GroupLabels, firstName)
}

// openIncident creates an incident for a group and assigns it to whoever is on
// call today, which is the point at which the schedule stops being decorative.
func openIncident(ctx context.Context, tx *sql.Tx, notify NotifyConfig, groupKey string, groupLabels map[string]string, fallbackName string) (int64, error) {
	onCall, err := currentOnCall(ctx, tx)
	if err != nil {
		return 0, err
	}

	labelsJSON, _ := json.Marshal(groupLabels)
	if groupLabels == nil {
		labelsJSON = []byte("{}")
	}

	res, err := tx.ExecContext(ctx, `
		INSERT INTO incidents (group_key, title, group_labels, status, triggered_at, assigned_to)
		VALUES (?, ?, ?, 'triggered', ?, ?)`,
		groupKey, incidentTitle(groupLabels, fallbackName), string(labelsJSON),
		time.Now().Unix(), onCall)
	if err != nil {
		return 0, err
	}
	id, err := res.LastInsertId()
	if err != nil {
		return 0, err
	}

	if err := logEvent(ctx, tx, id, evTriggered, nil, nil, nil); err != nil {
		return 0, err
	}
	if onCall != nil {
		// On an "assigned" event user_id is the assignee, not the actor.
		if err := logEvent(ctx, tx, id, evAssigned, onCall, nil, nil); err != nil {
			return 0, err
		}
	}

	// Queue the page, but do not send it here: this runs inside the webhook's
	// transaction on a single-connection pool, so an HTTP call would hold up
	// every other request. The notifier picks the row up within a tick.
	if err := enqueueOpened(ctx, tx, notify, id, onCall); err != nil {
		return 0, err
	}
	return id, nil
}

// linkAlert adds an alert to an incident, emitting a timeline entry only the
// first time. Re-sends of an already-linked alert are silent.
func linkAlert(ctx context.Context, tx *sql.Tx, incidentID, alertID int64) error {
	res, err := tx.ExecContext(ctx, `
		INSERT OR IGNORE INTO incident_alerts (incident_id, alert_id, added_at)
		VALUES (?, ?, ?)`, incidentID, alertID, time.Now().Unix())
	if err != nil {
		return err
	}
	if n, _ := res.RowsAffected(); n == 0 {
		return nil
	}
	return logEvent(ctx, tx, incidentID, evAlertAdded, nil, &alertID, nil)
}