package handlers

import (
	"fmt"
	"time"

	"gorm.io/gorm"
	"sneak.berlin/go/webhooker/internal/database"
)

// The spans of the two recent windows the statistics pane reports on:
// the last 10 minutes and the last 24 hours.
const (
	shortWindow = 10 * time.Minute
	longWindow  = 24 * time.Hour
)

// percent turns a fraction into a percentage.
const percent = 100

// WebhookStats holds the figures in the statistics pane at the top of
// the webhook page.
type WebhookStats struct {
	Entrypoints       int
	ActiveEntrypoints int
	Targets           int
	ActiveTargets     int

	// Lifetime counts every event, delivery and failure the webhook
	// has had, and WithinRetention those still stored.
	Lifetime        Counts
	WithinRetention Counts

	// InProgress counts the deliveries still pending or retrying.
	InProgress int64

	// LastEventAt is when the newest event arrived, or nil when none
	// has. Retention does not change it.
	LastEventAt *time.Time

	Last10Minutes RecentWindow
	Last24Hours   RecentWindow
}

// Counts holds a number of events, of deliveries and of failed
// deliveries.
type Counts struct {
	Events     int64
	Deliveries int64
	Failures   int64
}

// RecentWindow holds what happened in one recent window: the events
// received in it, and the deliveries that became delivered or failed in
// it.
type RecentWindow struct {
	Events    int64
	Delivered int64
	Failed    int64
}

// TargetFinished is how many of one target's deliveries became
// delivered, and how many failed, in a recent window.
type TargetFinished struct {
	TargetID  string
	Delivered int64
	Failed    int64
}

// FailurePercent is the share of the deliveries finished in the window
// that failed, or a dash when none finished. Deliveries still pending
// or retrying are not counted either way.
func (w RecentWindow) FailurePercent() string {
	finished := w.Delivered + w.Failed
	if finished == 0 {
		return "—"
	}

	return fmt.Sprintf(
		"%.1f%%", percent*float64(w.Failed)/float64(finished),
	)
}

// loadWebhookStats gathers the figures for the statistics pane from the
// webhook's entrypoints and targets, as the page has already loaded
// them, and from its event database. It returns nil, and logs why, when
// the event database cannot be read.
func (h *Handlers) loadWebhookStats(
	webhookID string,
	entrypoints []database.Entrypoint,
	targets []database.Target,
) *WebhookStats {
	stats := &WebhookStats{
		Entrypoints: len(entrypoints),
		Targets:     len(targets),
	}

	for i := range entrypoints {
		if entrypoints[i].Active {
			stats.ActiveEntrypoints++
		}
	}

	for i := range targets {
		if targets[i].Active {
			stats.ActiveTargets++
		}
	}

	// Opening an event database that does not exist would create it,
	// and it would hold nothing to count.
	if !h.dbMgr.DBExists(webhookID) {
		return stats
	}

	webhookDB, err := h.dbMgr.GetDB(webhookID)
	if err == nil {
		err = readEventStats(webhookDB, time.Now(), stats)
	}

	if err != nil {
		h.log.Error(
			"failed to read webhook statistics",
			"webhook_id", webhookID,
			"error", err,
		)

		return nil
	}

	return stats
}

// readEventStats fills in the figures that come from the webhook's
// event database. None of them reads every stored row: the totals are
// one row for the events and one per target for the deliveries, and
// every other figure is read from an index, over only the rows it
// counts.
func readEventStats(
	db *gorm.DB, now time.Time, stats *WebhookStats,
) error {
	err := readTotals(db, stats)
	if err != nil {
		return err
	}

	err = db.Model(&database.Delivery{}).
		Where("status IN ?", []database.DeliveryStatus{
			database.DeliveryStatusPending,
			database.DeliveryStatusRetrying,
		}).
		Count(&stats.InProgress).Error
	if err != nil {
		return fmt.Errorf("counting deliveries in progress: %w", err)
	}

	stats.Last10Minutes, err = readRecentWindow(
		db, now.Add(-shortWindow),
	)
	if err != nil {
		return err
	}

	stats.Last24Hours, err = readRecentWindow(
		db, now.Add(-longWindow),
	)

	return err
}

// readTotals fills in the lifetime and within-retention figures, and
// when the last event arrived, from the running totals: the events'
// row, and the targets' rows summed.
func readTotals(db *gorm.DB, stats *WebhookStats) error {
	var events database.EventTotals

	err := db.Take(&events).Error
	if err != nil {
		return fmt.Errorf("reading event totals: %w", err)
	}

	var targets []database.TargetTotals

	err = db.Find(&targets).Error
	if err != nil {
		return fmt.Errorf("reading target totals: %w", err)
	}

	stats.Lifetime.Events = events.Events
	stats.WithinRetention.Events = events.Events - events.EventsRemoved
	stats.LastEventAt = events.LastEventAt

	for _, t := range targets {
		stats.Lifetime.Deliveries += t.Deliveries
		stats.Lifetime.Failures += t.Failed
		stats.WithinRetention.Deliveries += t.Deliveries - t.DeliveriesRemoved
		stats.WithinRetention.Failures += t.Failed - t.FailedRemoved
	}

	return nil
}

// readRecentWindow counts the events received, and the deliveries that
// became delivered or failed, since the given time.
func readRecentWindow(
	db *gorm.DB, since time.Time,
) (RecentWindow, error) {
	var w RecentWindow

	err := db.Model(&database.Event{}).
		Where("created_at >= ?", since).
		Count(&w.Events).Error
	if err != nil {
		return w, fmt.Errorf("counting recent events: %w", err)
	}

	byTarget, err := finishedByTarget(db, since)
	if err != nil {
		return w, err
	}

	for _, f := range byTarget {
		w.Delivered += f.Delivered
		w.Failed += f.Failed
	}

	return w, nil
}

// finishedByTarget counts, for each target, the deliveries that became
// delivered and those that failed since the given time, in one query
// over just that window of the deliveries' status index. A target with
// neither is left out.
func finishedByTarget(
	db *gorm.DB, since time.Time,
) ([]TargetFinished, error) {
	var byTarget []TargetFinished

	err := db.Model(&database.Delivery{}).
		Select("target_id, "+
			"count(CASE WHEN status = ? THEN 1 END) AS delivered, "+
			"count(CASE WHEN status = ? THEN 1 END) AS failed",
			database.DeliveryStatusDelivered,
			database.DeliveryStatusFailed).
		Where("status IN ? AND finished_at >= ?",
			[]database.DeliveryStatus{
				database.DeliveryStatusDelivered,
				database.DeliveryStatusFailed,
			}, since).
		Group("target_id").
		Find(&byTarget).Error
	if err != nil {
		return nil, fmt.Errorf(
			"counting deliveries finished by target: %w", err,
		)
	}

	return byTarget, nil
}