// Package scheduler runs an in-process loop that periodically checks every
// domain of every due project schedule, records the resulting status, and
// notifies configured channels on meaningful status transitions.
package scheduler

import (
	"context"
	"fmt"
	"log"
	"time"

	"github.com/google/uuid"

	"github.com/vasyakrg/dns-autoresolver/internal/diff"
	"github.com/vasyakrg/dns-autoresolver/internal/metrics"
	"github.com/vasyakrg/dns-autoresolver/internal/notify"
	"github.com/vasyakrg/dns-autoresolver/internal/store"
)

// Domain check statuses persisted via SchedStore.SetDomainStatus /
// surfaced via GetDomainStatus. "unknown" is the DB default for a domain
// that has never been checked (see migrations/0004_schedule_notify.sql).
const (
	StatusUnknown = "unknown"
	StatusInSync  = "in_sync"
	StatusDrift   = "drift"
	StatusError   = "error"
)

// SchedStore is the narrow store dependency the scheduler needs: due
// schedules, their domains, and per-domain status bookkeeping. Persisting
// the check result itself (check_runs) is the Checker's job — see Checker
// below — not the scheduler's.
type SchedStore interface {
	ListDueSchedules(ctx context.Context, now time.Time) ([]store.Schedule, error)
	TouchScheduleRun(ctx context.Context, projectID uuid.UUID, at time.Time) error
	ListDomains(ctx context.Context, projectID uuid.UUID) ([]store.Domain, error)
	GetDomainStatus(ctx context.Context, domainID uuid.UUID) (string, error)
	SetDomainStatus(ctx context.Context, domainID uuid.UUID, status string) error
	CountDriftDomains(ctx context.Context) (int, error)
}

// Checker computes the diff between a domain's desired template and its
// actual zone state. internal/service.DomainService satisfies this.
type Checker interface {
	Check(ctx context.Context, projectID, domainID uuid.UUID) (diff.Changeset, error)
}

// NotifySender delivers a status-change event to a project's notification
// channels. internal/notify.Dispatcher satisfies this.
type NotifySender interface {
	Send(ctx context.Context, projectID uuid.UUID, ev notify.Event) error
}

// Scheduler drives periodic domain checks for every due project schedule.
type Scheduler struct {
	store    SchedStore
	checker  Checker
	notifier NotifySender
	metrics  *metrics.Metrics
}

// New builds a Scheduler wired with its store, checker, notifier and metrics
// dependencies.
func New(store SchedStore, checker Checker, notifier NotifySender, m *metrics.Metrics) *Scheduler {
	return &Scheduler{store: store, checker: checker, notifier: notifier, metrics: m}
}

// Run ticks every `tick` and calls RunOnce until ctx is cancelled. A failed
// iteration is logged, never fatal — the loop keeps ticking so a transient
// store/provider outage does not permanently stop future checks.
func (s *Scheduler) Run(ctx context.Context, tick time.Duration) {
	ticker := time.NewTicker(tick)
	defer ticker.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case <-ticker.C:
			if err := s.RunOnce(ctx, time.Now()); err != nil {
				log.Printf("scheduler: run once failed: %v", err)
			}
		}
	}
}

// RunOnce performs a single scheduling pass: every due project schedule is
// checked, each of its domains is diffed against its template, its status
// is updated, and channels are notified on a meaningful status transition.
func (s *Scheduler) RunOnce(ctx context.Context, now time.Time) error {
	due, err := s.store.ListDueSchedules(ctx, now)
	if err != nil {
		return fmt.Errorf("list due schedules: %w", err)
	}

	for _, sch := range due {
		domains, err := s.store.ListDomains(ctx, sch.ProjectID)
		if err != nil {
			log.Printf("scheduler: list domains for project %s failed: %v", sch.ProjectID, err)
			continue
		}

		for _, d := range domains {
			s.checkDomain(ctx, sch.ProjectID, d, now)
		}

		if err := s.store.TouchScheduleRun(ctx, sch.ProjectID, now); err != nil {
			log.Printf("scheduler: touch schedule run for project %s failed: %v", sch.ProjectID, err)
		}
	}

	// The real, system-wide count of drift domains — not a local
	// accumulator scoped to this tick's due projects — so the gauge
	// reflects reality even across ticks where different projects are due.
	count, err := s.store.CountDriftDomains(ctx)
	if err != nil {
		log.Printf("scheduler: count drift domains failed: %v", err)
	} else {
		s.metrics.SetDrift(count)
	}
	return nil
}

// checkDomain runs a single domain's check, persists the outcome, and fires
// a notification if the status transition warrants one. It returns the new
// status.
func (s *Scheduler) checkDomain(ctx context.Context, projectID uuid.UUID, d store.Domain, now time.Time) string {
	start := time.Now()
	cs, checkErr := s.checker.Check(ctx, projectID, d.ID)
	dur := time.Since(start)

	newStatus := StatusInSync
	switch {
	case checkErr != nil:
		newStatus = StatusError
	case len(cs.Actionable()) > 0:
		newStatus = StatusDrift
	}
	s.metrics.ObserveCheck(newStatus, dur)

	prev, err := s.store.GetDomainStatus(ctx, d.ID)
	if err != nil {
		log.Printf("scheduler: get domain status for %s failed: %v", d.ID, err)
		prev = StatusUnknown
	}

	// Persisting the check_runs row is the Checker's job: DomainService.Check
	// already calls Recorder.SaveCheckRun internally on every successful
	// check (drift or in_sync). Calling it again here would double-write
	// check_runs history for the same check.

	if err := s.store.SetDomainStatus(ctx, d.ID, newStatus); err != nil {
		log.Printf("scheduler: set domain status for %s failed: %v", d.ID, err)
	}

	if shouldNotify(prev, newStatus) {
		ev := notify.Event{
			Project: projectID.String(),
			Domain:  d.ID.String(),
			Status:  newStatus,
			Summary: summarize(newStatus, cs, checkErr),
			At:      now,
		}
		if err := s.notifier.Send(ctx, projectID, ev); err != nil {
			log.Printf("scheduler: notify send for project %s domain %s failed: %v", projectID, d.ID, err)
		}
		s.metrics.IncNotification("dispatch", newStatus)
	}

	return newStatus
}

// shouldNotify decides whether a prev -> new status transition is worth
// alerting on:
//   - entering drift or error from any other status is always notified;
//   - recovering from drift OR error back to in_sync ("resolved") is
//     notified — including recovery after a provider/check failure;
//   - the initial unknown -> in_sync transition (first successful check of a
//     domain that never drifted or errored) is NOT notified — it is not
//     news, it is the expected steady state.
func shouldNotify(prev, newStatus string) bool {
	if (newStatus == StatusDrift || newStatus == StatusError) && newStatus != prev {
		return true
	}
	if (prev == StatusDrift || prev == StatusError) && newStatus == StatusInSync {
		return true
	}
	return false
}

// summarize builds a short, secret-free human-readable message for an Event.
func summarize(status string, cs diff.Changeset, checkErr error) string {
	if checkErr != nil {
		return fmt.Sprintf("check failed: %v", checkErr)
	}
	if status == StatusDrift {
		return fmt.Sprintf("%d actionable diff(s) detected", len(cs.Actionable()))
	}
	return "zone back in sync with template"
}