package core

import (
	"context"
	"fmt"
	"log"
	"strings"
	"time"
)

type cronSpec interface {
	// Get the next time matching this spec that is after the base time.
	GetNextTime(base time.Time) (nextUpdate time.Time)
}

func GetNextSpecTime(lastUpdated time.Time, spec string) (nextUpdate time.Time, err error) {
	var cron cronSpec
	cron, err = ParseCronSpec(spec)
	if err != nil {
		return time.Time{}, err
	}
	return cron.GetNextTime(lastUpdated), nil
}

func ParseCronSpec(spec string) (cron cronSpec, err error) {
	switch {
	case strings.HasPrefix(spec, "every "):
		cron, err = parseEverySpec(spec[len("every "):])
	case strings.HasPrefix(spec, "at "):
		cron, err = parseAtSpec(spec[len("at "):])
	case strings.HasPrefix(spec, "on "):
		cron, err = parseOnSpec(spec[len("on "):])
	default:
		// Interpret "HH:MM" as "at HH:MM"
		cron, err = parseAtSpec(spec)
		if err != nil {
			return nil, fmt.Errorf("unknown spec format: %v", spec)
		}
	}
	return
}

// Return the time that follows the base time most closely.
func nextAfter(base time.Time, nextUpdates []time.Time) (nextUpdate time.Time) {
	for _, next := range nextUpdates {
		if next.After(base) && (nextUpdate.IsZero() || next.Before(nextUpdate)) {
			nextUpdate = next
		}
	}
	return
}

// An every-spec is a Go duration string, e.g. "every 5m"
type everySpec struct {
	duration time.Duration
}

func parseEverySpec(spec string) (*everySpec, error) {
	duration, err := time.ParseDuration(spec)
	if err != nil {
		return nil, err
	}
	return &everySpec{duration}, nil
}

// Get the next time matching the every-spec that is after the base time.
func (spec *everySpec) GetNextTime(base time.Time) (nextUpdate time.Time) {
	next := base.Round(spec.duration)
	if !next.After(base) {
		next = next.Add(spec.duration)
	}
	return next
}

// An at-spec is in the patterm HH:MM[,HH:MM,[...]].
type atSpec struct {
	hours   []int
	minutes []int
}

func parseAtSpec(spec string) (*atSpec, error) {
	var hours, minutes []int
	for _, timeSpec := range strings.Split(spec, ",") {
		var hour, minute int
		_, err := fmt.Sscanf(timeSpec, "%d:%d", &hour, &minute)
		if err != nil {
			return nil, fmt.Errorf("could not parse at-spec %s: %w", timeSpec, err)
		}
		hours = append(hours, hour)
		minutes = append(minutes, minute)
	}
	return &atSpec{hours, minutes}, nil
}

// Get the next time matching the at-spec that is after the base time.
func (spec *atSpec) GetNextTime(base time.Time) (nextUpdate time.Time) {
	nextUpdates := []time.Time{}
	for i, hour := range spec.hours {
		minute := spec.minutes[i]
		// The time instance on the same day as the base time
		specOfDay := time.Date(base.Year(), base.Month(), base.Day(), hour, minute, 0, 0, base.Location())
		// Bump it forward one day if it's before the base time
		if !specOfDay.After(base) {
			specOfDay = specOfDay.Add(24 * time.Hour)
		}
		nextUpdates = append(nextUpdates, specOfDay)
	}
	return nextAfter(base, nextUpdates)
}

var weekdays = map[string]time.Weekday{
	"Sun": time.Sunday,
	"Mon": time.Monday,
	"Tue": time.Tuesday,
	"Wed": time.Wednesday,
	"Thu": time.Thursday,
	"Fri": time.Friday,
	"Sat": time.Saturday,
}

type ymd struct {
	Year  int
	Month time.Month
	Day   int
}

type onSpecDate interface {
	getDates(base time.Time) []ymd
}

type onSpecWeekday struct {
	weekday time.Weekday
}

func (date *onSpecWeekday) getDates(base time.Time) (dates []ymd) {
	// For a weekday, add the next of that weekday 0-6 days ahead and 7-13 days ahead.
	// The first date ensures that we don't miss multiple updates on the same day of the week
	// (e.g. "on Sun at 06:00,18:00") and the second date ensures that we don't get stuck with
	// only a date in the past (e.g. "on Sun at 02:00" when base is Sun 03:00).
	daysForward := (int(date.weekday) - int(base.Weekday()) + 7) % 7 // value in [0,6]
	day0_6 := base.AddDate(0, 0, daysForward)
	dates = append(dates, ymd{day0_6.Year(), day0_6.Month(), day0_6.Day()})
	day7_13 := base.AddDate(0, 0, daysForward+7)
	dates = append(dates, ymd{day7_13.Year(), day7_13.Month(), day7_13.Day()})
	return
}

type onSpecEveryMonth struct {
	day int
}

func (date *onSpecEveryMonth) getDates(base time.Time) (dates []ymd) {
	// For every-month, add the date for the current month and the date for the next month.
	dates = append(dates, ymd{base.Year(), base.Month(), date.day})
	nextMonth := base.AddDate(0, 1, 0)
	dates = append(dates, ymd{nextMonth.Year(), nextMonth.Month(), date.day})
	return
}

type onSpecMonthDay struct {
	month int
	day   int
}

func (date *onSpecMonthDay) getDates(base time.Time) (dates []ymd) {
	// For month/day, add the date for the base year and the next year.
	dates = append(dates, ymd{base.Year(), time.Month(date.month), date.day})
	dates = append(dates, ymd{base.Year() + 1, time.Month(date.month), date.day})
	return
}

// An on-spec is in the pattern DOW[,DOW[...]] where DOW is an abbreviated weekday
// or M/D[,M/D[...]] where M/D is a month and day.
// As a special case, "*/N" matches the Nth day of every month.
// An on-spec may be followed by an at-spec; otherwise, "at 00:00" is implied.
type onSpec struct {
	atSpec atSpec
	dates  []onSpecDate
}

func parseOnSpec(spec string) (*onSpec, error) {
	atPart := "00:00"
	if at := strings.Index(spec, " at "); at > -1 {
		atPart = spec[at+len(" at "):]
		spec = spec[:at]
	}
	atSpec, err := parseAtSpec(atPart)
	if err != nil {
		return nil, fmt.Errorf("could not parse at-spec %v: %w", atSpec, err)
	}

	var dates []onSpecDate
	for _, daySpec := range strings.Split(spec, ",") {
		if weekday, ok := weekdays[daySpec]; ok {
			dates = append(dates, &onSpecWeekday{weekday})
		} else if strings.HasPrefix(daySpec, "*/") {
			var day int
			_, err := fmt.Sscanf(daySpec, "*/%d", &day)
			if err != nil {
				return nil, fmt.Errorf("could not parse month/day %s: %v", daySpec, err)
			}
			dates = append(dates, &onSpecEveryMonth{day})
		} else {
			var month, day int
			_, err := fmt.Sscanf(daySpec, "%d/%d", &month, &day)
			if err != nil {
				return nil, fmt.Errorf("could not parse month/day %s: %v", daySpec, err)
			}
			dates = append(dates, &onSpecMonthDay{month, day})
		}
	}
	return &onSpec{*atSpec, dates}, nil
}

// Get the next time matching the on-spec that is after the base time.
func (spec *onSpec) GetNextTime(base time.Time) (nextUpdate time.Time) {
	// Convert each type of on-spec date spec to a concrete YMD date
	var dates []ymd
	for _, date := range spec.dates {
		dates = append(dates, date.getDates(base)...)
	}

	// Then, for each date, create a datetime for each at-spec.
	nextUpdates := []time.Time{}
	for _, date := range dates {
		for i, atHour := range spec.atSpec.hours {
			atMinute := spec.atSpec.minutes[i]
			nextUpdates = append(nextUpdates, time.Date(date.Year, date.Month, date.Day, atHour, atMinute, 0, 0, base.Location()))
		}
	}
	return nextAfter(base, nextUpdates)
}

type Schedule struct {
	Source      string
	Spec        string
	LastUpdated time.Time
}

func getFetchSchedules(db DB) (schedules []Schedule, err error) {
	res, err := db.Query(`
		select envs.source, envs.value, sources.lastUpdated
		from envs
		inner join sources on envs.source = sources.name
		where envs.name = 'INTAKE_FETCH'
	`)
	if err != nil {
		return nil, fmt.Errorf("failed to get source fetch specs: %v", err)
	}
	for res.Next() {
		var source, value string
		var lastUpdated int
		if err = res.Scan(&source, &value, &lastUpdated); err != nil {
			return nil, fmt.Errorf("failed to scan source fetch spec: %v", err)
		}
		schedules = append(schedules, Schedule{source, value, time.Unix(int64(lastUpdated), 0).UTC()})
	}
	return
}

func fetchReadySources(db DB) {
	log.Printf("checking for sources to update")
	schedules, err := getFetchSchedules(db)
	if err != nil {
		log.Printf("error: could not get fetch specs: %v", err)
		return
	}
	now := time.Now().UTC()
	for _, schedule := range schedules {
		nextUpdate, err := GetNextSpecTime(schedule.LastUpdated, schedule.Spec)
		if err != nil {
			log.Printf("error: could not determine next update for %s: %v", schedule.Source, err)
			continue
		}
		if nextUpdate.After(now) {
			// TODO log this at a debug log level
			// log.Printf(
			// 	"%s: last update %s, next update %s",
			// 	schedule.Source,
			// 	schedule.LastUpdated.Format("2006-01-02 15:04:05"),
			// 	nextUpdate.Format("2006-01-02 15:04:05"),
			// )
			continue
		}
		log.Printf("%s: fetching", schedule.Source)

		duration := DefaultTimeout
		timeout, err := GetSourceTimeout(db, schedule.Source)
		if err != nil {
			log.Printf("error: failed to get source timeout, using default: %v", err)
		} else if timeout != "" {
			duration, err = time.ParseDuration(timeout)
			if err != nil {
				log.Printf("error: invalid duration: %v", err)
				duration = DefaultTimeout
			}
		}

		state, envs, argv, err := GetSourceActionInputs(db, schedule.Source, "fetch")
		if err != nil {
			log.Printf("error: failed to load data for %s: %v", schedule.Source, err)
			continue
		}

		items, newState, errItem, err := Execute(schedule.Source, argv, envs, state, "", duration)
		if err != nil {
			AddErrorItem(db, errItem)
			log.Printf("error: failed to execute fetch: %v", err)
			continue
		}

		added, deleted, err := UpdateWithFetchedItems(db, schedule.Source, newState, items, time.Now())
		if err != nil {
			log.Printf("error: failed to update: %v", err)
			continue
		}
		log.Printf("%s added %d items, updated %d items, and deleted %d items", schedule.Source, added, len(items)-added, deleted)
	}
}

func BackgroundFetch(ctx context.Context, db DB, sleep time.Duration) error {
	log.Print("starting fetch monitor")
	fetchReadySources(db)
	tick := time.NewTicker(sleep)
	for {
		select {
		case <-ctx.Done():
			log.Printf("ending fetch monitor")
			return ctx.Err()
		case <-tick.C:
			fetchReadySources(db)
		}
	}
}