// Package service — ReminderService drives the daily sādhana reminders.
//
// A worker calls RunOnce on a fixed tick. For each reminder-enabled user it
// computes the user's *local* clock time (from their IANA timezone) and, if
// it falls in the send window for that reminder AND they haven't done the
// discipline today AND they haven't already been reminded today, sends the
// nudge through every channel (email + push + websocket toast).
//
// "Done today" and the once-per-day dedupe are bucketed by UTC calendar day,
// matching the streak logic; only the *time of day* the reminder fires is
// local. The recency-aware chant message is chosen from the user's most
// recent completed chant day.
package service

import (
	"context"
	"errors"
	"strconv"
	"strings"
	"time"

	// Embed the IANA tz database so time.LoadLocation works regardless of
	// whether the host image ships zoneinfo.
	_ "time/tzdata"

	"github.com/prabhupadasays/spsays-bff/internal/repository"
	"go.uber.org/zap"
)

// ErrInvalidTimezone is returned when a timezone-set request carries a value
// that isn't a loadable IANA zone.
var ErrInvalidTimezone = errors.New("invalid timezone")

const (
	// reminderWindowMin must be >= the worker tick so no target time is
	// missed between ticks; the per-day dedupe prevents a double-send within
	// the overlap.
	reminderWindowMin = 20

	chantMsgStart     = "Chant with Prabhupāda — start building your sādhana streak."
	chantMsgLapsed    = "Just one hour of sādhana can save you from this duḥkhālayam aśāśvatam. Come and build your streak again."
	chantMsgYesterday = "Your sādhana was wonderful yesterday! Let's continue your sādhana today as well."
	quoteMsg          = "Share a verse from the Vāṇī and complete today's sādhana."
)

type ReminderService struct {
	repo      *repository.ReminderRepo
	notif     *NotificationService
	logger    *zap.Logger
	sadhnaURL string
}

func NewReminderService(repo *repository.ReminderRepo, notif *NotificationService, logger *zap.Logger, frontendURL string) *ReminderService {
	return &ReminderService{
		repo:      repo,
		notif:     notif,
		logger:    logger,
		sadhnaURL: strings.TrimRight(frontendURL, "/") + "/sadhna-streak",
	}
}

// Enabled / SetEnabled / SetTimezone back the settings + tz-capture endpoints.
func (s *ReminderService) Enabled(ctx context.Context, userID string) (bool, error) {
	return s.repo.GetEnabled(ctx, userID)
}

func (s *ReminderService) SetEnabled(ctx context.Context, userID string, enabled bool) error {
	return s.repo.SetEnabled(ctx, userID, enabled)
}

// SetTimezone validates the IANA zone before storing so a bad value can never
// break the (timezone-using) candidate scan.
func (s *ReminderService) SetTimezone(ctx context.Context, userID, tz string) error {
	tz = strings.TrimSpace(tz)
	if tz == "" {
		tz = "UTC"
	}
	if _, err := time.LoadLocation(tz); err != nil {
		return ErrInvalidTimezone
	}
	return s.repo.SetTimezone(ctx, userID, tz)
}

// RunOnce evaluates every due candidate against `now` and fires the reminders
// that are due. Errors are logged, never returned — a bad row shouldn't stop
// the rest.
func (s *ReminderService) RunOnce(ctx context.Context, now time.Time) {
	candidates, err := s.repo.DueCandidates(ctx)
	if err != nil {
		s.logger.Warn("reminders: load candidates failed", zap.Error(err))
		return
	}
	utc := now.UTC()
	utcToday := time.Date(utc.Year(), utc.Month(), utc.Day(), 0, 0, 0, 0, time.UTC)

	for _, c := range candidates {
		loc, err := time.LoadLocation(c.TZ)
		if err != nil {
			loc = time.UTC
		}
		local := now.In(loc)
		localMin := local.Hour()*60 + local.Minute()

		// Chant reminder.
		if !c.ChantDoneToday && notRemindedToday(c.LastChantOn, utcToday) {
			if t := parseHHMM(c.ChantTime); t >= 0 && withinWindow(localMin, t) {
				s.notif.SendReminder(ctx, c.UserID, NotifChantReminder,
					"Time for your sādhana 🙏", chantMessage(c.LastChantDate, utcToday), s.sadhnaURL)
				if err := s.repo.MarkChantReminded(ctx, c.UserID, utcToday); err != nil {
					s.logger.Warn("reminders: mark chant failed", zap.String("user", c.UserID), zap.Error(err))
				}
			}
		}

		// Quote reminder.
		if !c.QuoteDoneToday && notRemindedToday(c.LastQuoteOn, utcToday) {
			if t := parseHHMM(c.QuoteTime); t >= 0 && withinWindow(localMin, t) {
				s.notif.SendReminder(ctx, c.UserID, NotifQuoteReminder,
					"Share a verse from the Vāṇī", quoteMsg, s.sadhnaURL)
				if err := s.repo.MarkQuoteReminded(ctx, c.UserID, utcToday); err != nil {
					s.logger.Warn("reminders: mark quote failed", zap.String("user", c.UserID), zap.Error(err))
				}
			}
		}
	}
}

// chantMessage picks the recency-aware nudge from the most recent completed
// chant day (both args are UTC midnight). The candidate is, by construction,
// not done today, so the gap is >= 1 day.
func chantMessage(lastChant *time.Time, utcToday time.Time) string {
	if lastChant == nil {
		return chantMsgStart
	}
	days := int(utcToday.Sub(*lastChant).Hours() / 24)
	switch {
	case days <= 1:
		return chantMsgYesterday // chanted yesterday
	case days <= 7:
		return chantMsgLapsed // within the last week, but not yesterday
	default:
		return chantMsgStart // nothing in over a week
	}
}

// notRemindedToday is true when no reminder of this kind has gone out on the
// current UTC day.
func notRemindedToday(last *time.Time, utcToday time.Time) bool {
	return last == nil || last.Before(utcToday)
}

// withinWindow reports whether localMin falls in [target, target+window),
// handling midnight wrap.
func withinWindow(localMin, target int) bool {
	diff := ((localMin-target)%1440 + 1440) % 1440
	return diff < reminderWindowMin
}

// parseHHMM turns "HH:MM" into minutes-of-day, or -1 if malformed.
func parseHHMM(s string) int {
	parts := strings.SplitN(strings.TrimSpace(s), ":", 2)
	if len(parts) != 2 {
		return -1
	}
	h, err1 := strconv.Atoi(parts[0])
	m, err2 := strconv.Atoi(parts[1])
	if err1 != nil || err2 != nil || h < 0 || h > 23 || m < 0 || m > 59 {
		return -1
	}
	return h*60 + m
}