package main

import (
	"cmp"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"os"
	"slices"
	"strconv"
	"strings"
	"sync"
	"time"
)

const (
	// scheduleFileName is the file of the schedules in the directory of the
	// agent.
	scheduleFileName = "schedules.json"

	// scheduleTextBytes is the most bytes of the text of a schedule.
	scheduleTextBytes = 4096

	// scheduleAtLayout is the layout of the time of a one-time schedule.
	scheduleAtLayout = "2006-01-02 15:04"

	// scheduleTimeLayout is the layout of the times in the answers of the
	// tools and in the log.
	scheduleTimeLayout = "Monday 2006-01-02 15:04 -07:00"

	// scheduleSource is the source of the work events of the runs.
	scheduleSource = "schedule"

	// scheduleCheck is the longest wait of Run before it looks at the times
	// again, so that it sees a suspend of the machine or a change of the
	// clock.
	scheduleCheck = time.Minute

	// scheduleLate is how long after its time a run is late.
	scheduleLate = time.Minute

	// scheduleEmptyReply is the last line of the message of a run.
	scheduleEmptyReply = "If nothing in this task needs the user, end your turn with an empty reply."

	// scheduleLateLine tells the agent that a run is late.
	scheduleLateLine = "This run is late: the daemon was stopped at that time."
)

// openSchedules reads the schedules of the file at path. A missing file holds
// none. The owner names its agent. Schedules without an owner take this name. A
// different stored owner is an error. The clock gives the time and the time
// zone of the schedules, and logger gets one line for each run.
func openSchedules(path, owner string, clock func() time.Time, logger *slog.Logger) (*Schedules, error) {
	s := &Schedules{
		path:    path,
		owner:   owner,
		clock:   clock,
		logger:  logger,
		check:   scheduleCheck,
		changed: make(chan struct{}, 1),
	}

	data, err := os.ReadFile(path)
	if errors.Is(err, os.ErrNotExist) {
		return s, nil
	}

	if err != nil {
		return nil, fmt.Errorf("read schedules: %w", err)
	}

	err = json.Unmarshal(data, &s.file)
	if err != nil {
		return nil, fmt.Errorf("%s: %w", path, err)
	}

	changed := false
	for i := range s.file.Schedules {
		schedule := &s.file.Schedules[i]
		if schedule.Owner == "" {
			schedule.Owner = owner
			changed = true
		}

		if schedule.Owner != owner {
			return nil, fmt.Errorf("schedule %d belongs to %q, not %q", schedule.ID, schedule.Owner, owner)
		}

		if schedule.Cron == "" {
			continue
		}

		_, err = scheduleParseCron(schedule.Cron)
		if err != nil {
			return nil, fmt.Errorf("%s: schedule %d: %w", path, schedule.ID, err)
		}
	}

	if changed {
		err = s.save()
		if err != nil {
			return nil, fmt.Errorf("record schedule owner: %w", err)
		}
	}

	return s, nil
}

// Schedules holds the schedules of the agent: tasks that start a turn at one
// time or at the times of a cron expression. Each change writes the file whole
// before it returns, so a schedule survives a restart. The mutex protects the
// schedules, because the tools change them from a turn while Run reads them.
// changed tells Run that a set or a remove changed the times.
type Schedules struct {
	path    string
	owner   string
	clock   func() time.Time
	logger  *slog.Logger
	check   time.Duration
	changed chan struct{}

	mutex sync.Mutex
	file  ScheduleFile
}

// Run starts the runs of the due schedules in l until ctx ends. It waits until
// the first due time, at most check at a time, and also wakes at a set or a
// remove. So a schedule that came due while the daemon was stopped runs at the
// start. It returns the error of a write of the log or of the file.
func (s *Schedules) Run(ctx context.Context, l *TurnLoop) error {
	for {
		err := s.deliver(ctx, l)
		if err != nil {
			return err
		}

		var (
			timer   *time.Timer
			timeout <-chan time.Time
		)

		wait, found := s.Wait(s.clock())
		if found {
			timer = time.NewTimer(min(wait, s.check))
			timeout = timer.C
		}

		select {
		case <-ctx.Done():
			return nil
		case <-s.changed:
		case <-timeout:
		}

		if timer != nil {
			timer.Stop()
		}
	}
}

// Set adds a schedule with text and returns the answer of the tool. Exactly one
// of at and cron is not nil. at is a local time in scheduleAtLayout after now,
// and it goes through scheduleWallTime, so a time in the hour that the clock
// repeats is its first occurrence. cron is an expression of scheduleParseCron.
func (s *Schedules) Set(text string, at, cron *string) (string, error) {
	text = strings.TrimSpace(text)
	if text == "" || len(text) > scheduleTextBytes {
		return "", fmt.Errorf("the text must have 1 to %d bytes", scheduleTextBytes)
	}

	now := s.clock()
	schedule := Schedule{
		Text:  text,
		Owner: s.owner,
	}

	switch {
	case (at == nil) == (cron == nil):
		return "", errors.New("give either at or cron, and null for the other")
	case at != nil:
		parsed, err := time.Parse(scheduleAtLayout, *at)
		if err != nil {
			return "", fmt.Errorf("at must be a local time such as %s", now.Format(scheduleAtLayout))
		}

		schedule.Due = scheduleWallTime(parsed.Year(), parsed.Month(), parsed.Day(), parsed.Hour(), parsed.Minute(), now.Location())
		if !schedule.Due.After(now) {
			return "", fmt.Errorf("the time %s is not after now, %s", schedule.Due.Format(scheduleTimeLayout), now.Format(scheduleTimeLayout))
		}
	default:
		parsed, err := scheduleParseCron(*cron)
		if err != nil {
			return "", err
		}

		next, found := parsed.Next(now)
		if !found {
			return "", fmt.Errorf("the cron expression %q matches no time in %d years", *cron, scheduleCronYears)
		}

		schedule.Cron = parsed.String()
		schedule.Due = next
	}

	s.mutex.Lock()
	defer s.mutex.Unlock()

	s.file.Next++
	schedule.ID = s.file.Next
	s.file.Schedules = append(s.file.Schedules, schedule)
	err := s.save()
	if err != nil {
		return "", err
	}

	s.change()

	return fmt.Sprintf("Schedule %d set. Next: %s.", schedule.ID, schedule.Due.In(now.Location()).Format(scheduleTimeLayout)), nil
}

// List returns the answer of the tool: one line for each schedule, by due time,
// with its ID, its next time, its cron expression or once, and its text on one
// line.
func (s *Schedules) List() string {
	location := s.clock().Location()
	schedules := s.sorted()
	if len(schedules) == 0 {
		return "No schedules."
	}

	lines := make([]string, 0, len(schedules))
	for _, schedule := range schedules {
		lines = append(lines, strconv.Itoa(schedule.ID)+" | "+schedule.Due.In(location).Format(scheduleTimeLayout)+" | "+cmp.Or(schedule.Cron, "once")+" | "+strings.Join(strings.Fields(schedule.Text), " "))
	}

	return strings.Join(lines, "\n")
}

// Len returns the number of schedules.
func (s *Schedules) Len() int {
	s.mutex.Lock()
	defer s.mutex.Unlock()

	return len(s.file.Schedules)
}

// Remove removes the schedule id and returns the answer of the tool.
func (s *Schedules) Remove(id int) (string, error) {
	s.mutex.Lock()
	defer s.mutex.Unlock()

	index := slices.IndexFunc(s.file.Schedules, func(schedule Schedule) bool {
		return schedule.ID == id
	})

	if index < 0 {
		return "", fmt.Errorf("no schedule %d", id)
	}

	s.file.Schedules = slices.Delete(s.file.Schedules, index, index+1)
	err := s.save()
	if err != nil {
		return "", err
	}

	s.change()

	return fmt.Sprintf("Schedule %d removed.", id), nil
}

// deliver logs the message of each due schedule in l, in due order, as a work
// message of scheduleSource, and wakes the loop once with all of them. A
// schedule ends its run with Done after its message is durable in the log. A
// crash between the two makes the run repeat once. Each run logs one line with
// the ID of the schedule and, for a late run, how late it is. The line holds no
// text of the schedule.
func (s *Schedules) deliver(ctx context.Context, l *TurnLoop) error {
	if l.agent.config.Name != s.owner {
		return fmt.Errorf("schedule owner %q does not match agent %q", s.owner, l.agent.config.Name)
	}

	now := s.clock()

	var turns []Message

	for _, schedule := range s.Due(now) {
		message, err := l.Work(scheduleSource, schedule.Message(now), "")
		if err != nil {
			return fmt.Errorf("log a schedule: %w", err)
		}

		// A stop of the daemon during the flush is no failure: the
		// schedule stays due and runs at the next start.
		err = l.Flush(ctx)
		if ctx.Err() != nil {
			return nil
		}

		if err != nil {
			return fmt.Errorf("log a schedule: %w", err)
		}

		err = s.Done(schedule, now)
		if err != nil {
			return err
		}

		attrs := []any{"id", schedule.ID}
		if schedule.late(now) {
			attrs = append(attrs, "late", now.Sub(schedule.Due))
		}

		s.logger.Info("schedule ran", attrs...)

		turns = append(turns, message)
	}

	l.Wake(turns...)

	return nil
}

// Due returns the schedules whose time is not after now, by due time.
func (s *Schedules) Due(now time.Time) []Schedule {
	var due []Schedule

	for _, schedule := range s.sorted() {
		if !schedule.Due.After(now) {
			due = append(due, schedule)
		}
	}

	return due
}

// Done ends the run of schedule at now: a one-time schedule goes, and a
// repeating schedule gets the next time after now. So a repeating schedule that
// missed several times runs once. When a remove took the schedule first, Done
// changes nothing.
func (s *Schedules) Done(schedule Schedule, now time.Time) error {
	s.mutex.Lock()
	defer s.mutex.Unlock()

	index := slices.IndexFunc(s.file.Schedules, func(held Schedule) bool {
		return held.ID == schedule.ID
	})

	if index < 0 {
		return nil
	}

	if schedule.Cron == "" {
		s.file.Schedules = slices.Delete(s.file.Schedules, index, index+1)

		return s.save()
	}

	cron, err := scheduleParseCron(schedule.Cron)
	if err != nil {
		return fmt.Errorf("schedule %d: %w", schedule.ID, err)
	}

	next, found := cron.Next(now)
	if !found {
		s.file.Schedules = slices.Delete(s.file.Schedules, index, index+1)

		return s.save()
	}

	s.file.Schedules[index].Due = next

	return s.save()
}

// Wait returns how long after now the first schedule is due, or false when
// there is no schedule.
func (s *Schedules) Wait(now time.Time) (time.Duration, bool) {
	schedules := s.sorted()
	if len(schedules) == 0 {
		return 0, false
	}

	return max(schedules[0].Due.Sub(now), 0), true
}

// change wakes Run after a set or a remove.
func (s *Schedules) change() {
	select {
	case s.changed <- struct{}{}:
	default:
	}
}

// sorted returns a copy of the schedules by due time, then by ID.
func (s *Schedules) sorted() []Schedule {
	s.mutex.Lock()

	schedules := slices.Clone(s.file.Schedules)
	s.mutex.Unlock()

	slices.SortFunc(schedules, func(a, b Schedule) int {
		return cmp.Or(a.Due.Compare(b.Due), cmp.Compare(a.ID, b.ID))
	})

	return schedules
}

// save writes the file whole, with mode 0600. The caller holds the mutex.
func (s *Schedules) save() error {
	data, err := json.Marshal(s.file)
	if err == nil {
		err = writeFileAtomic(s.path, append(data, '\n'))
	}

	if err != nil {
		return fmt.Errorf("save schedules: %w", err)
	}

	return nil
}

// ScheduleFile is the content of the file of the schedules: the ID of the last
// schedule, so that an ID is never used again, and the schedules.
type ScheduleFile struct {
	Next      int        `json:"next"`
	Schedules []Schedule `json:"schedules"`
}

// Schedule is one schedule: its owner, its ID, the task for the agent, the cron
// expression of a repeating schedule, empty for a one-time schedule, and its
// next time.
type Schedule struct {
	Owner string    `json:"owner"`
	ID    int       `json:"id"`
	Text  string    `json:"text"`
	Cron  string    `json:"cron"`
	Due   time.Time `json:"due"`
}

// Message returns the text of a run at now for the log, with the times in the
// location of now. The first line starts with [Schedule ID] and gives the due
// time and once or the cron expression. Then come the text, the line about a
// late run when now is more than scheduleLate after the due time, and the line
// about the empty reply.
func (s Schedule) Message(now time.Time) string {
	lines := []string{
		"[Schedule " + strconv.Itoa(s.ID) + "] " + s.Due.In(now.Location()).Format(scheduleTimeLayout) + ", " + s.repeats(),
		s.Text,
	}

	if s.late(now) {
		lines = append(lines, scheduleLateLine)
	}

	return strings.Join(append(lines, scheduleEmptyReply), "\n")
}

// late tells if a run at now is late: more than scheduleLate after the due
// time.
func (s Schedule) late(now time.Time) bool {
	return now.Sub(s.Due) > scheduleLate
}

// repeats returns once for a one-time schedule, or else repeats and the cron
// expression.
func (s Schedule) repeats() string {
	if s.Cron == "" {
		return "once"
	}

	return "repeats " + s.Cron
}