package cron

import (
	"errors"
	"fmt"
	"strconv"
	"strings"
	"time"
)

// schedule is a parsed five-field cron expression as sets.
type schedule struct {
	min, hour, dom, mon, dow [64]bool
	domStar, dowStar         bool // "*" in dom/dow changes the match rule
	reboot                   bool
}

var monthNames = map[string]int{"jan": 1, "feb": 2, "mar": 3, "apr": 4, "may": 5, "jun": 6, "jul": 7, "aug": 8, "sep": 9, "oct": 10, "nov": 11, "dec": 12}
var dayNames = map[string]int{"sun": 0, "mon": 1, "tue": 2, "wed": 3, "thu": 4, "fri": 5, "sat": 6}

var aliases = map[string]string{
	"@yearly": "0 0 1 1 *", "@annually": "0 0 1 1 *", "@monthly": "0 0 1 * *", "@weekly": "0 0 * * 0",
	"@daily": "0 0 * * *", "@midnight": "0 0 * * *", "@hourly": "0 * * * *",
}

func parseSchedule(expr string) (*schedule, error) {
	expr = strings.TrimSpace(expr)
	if expr == "@reboot" {
		return &schedule{reboot: true}, nil
	}
	if a, ok := aliases[strings.ToLower(expr)]; ok {
		expr = a
	}
	f := strings.Fields(expr)
	if len(f) != 5 {
		return nil, errors.New("expected 5 fields")
	}
	s := &schedule{}
	var err error
	if s.min, err = parseField(f[0], 0, 59, nil); err != nil {
		return nil, fmt.Errorf("minute: %w", err)
	}
	if s.hour, err = parseField(f[1], 0, 23, nil); err != nil {
		return nil, fmt.Errorf("hour: %w", err)
	}
	if s.dom, err = parseField(f[2], 1, 31, nil); err != nil {
		return nil, fmt.Errorf("day of month: %w", err)
	}
	if s.mon, err = parseField(f[3], 1, 12, monthNames); err != nil {
		return nil, fmt.Errorf("month: %w", err)
	}
	if s.dow, err = parseField(f[4], 0, 7, dayNames); err != nil {
		return nil, fmt.Errorf("day of week: %w", err)
	}
	if s.dow[7] { // 7 == Sunday
		s.dow[0] = true
	}
	s.domStar = f[2] == "*"
	s.dowStar = f[4] == "*"
	return s, nil
}

func parseField(field string, lo, hi int, names map[string]int) ([64]bool, error) {
	var set [64]bool
	atom := func(s string) (int, error) {
		if names != nil {
			if v, ok := names[strings.ToLower(s)]; ok {
				return v, nil
			}
		}
		v, err := strconv.Atoi(s)
		if err != nil {
			return 0, fmt.Errorf("bad value %q", s)
		}
		return v, nil
	}
	for _, part := range strings.Split(field, ",") {
		step := 1
		if i := strings.Index(part, "/"); i >= 0 {
			v, err := strconv.Atoi(part[i+1:])
			if err != nil || v <= 0 {
				return set, fmt.Errorf("bad step in %q", part)
			}
			step, part = v, part[:i]
		}
		a, b := lo, hi
		switch {
		case part == "*":
		case strings.Contains(part, "-"):
			x, y, _ := strings.Cut(part, "-")
			var err error
			if a, err = atom(x); err != nil {
				return set, err
			}
			if b, err = atom(y); err != nil {
				return set, err
			}
		default:
			v, err := atom(part)
			if err != nil {
				return set, err
			}
			a = v
			if step == 1 {
				b = v
			}
		}
		if a < lo || b > hi || a > b {
			return set, fmt.Errorf("out of range: %q", part)
		}
		for v := a; v <= b; v += step {
			set[v] = true
		}
	}
	return set, nil
}

func (s *schedule) dayMatches(t time.Time) bool {
	if !s.mon[int(t.Month())] {
		return false
	}
	dom, dow := s.dom[t.Day()], s.dow[int(t.Weekday())]
	switch {
	case s.domStar && s.dowStar:
		return true
	case s.domStar:
		return dow
	case s.dowStar:
		return dom
	default: // both restricted: cron runs when either matches
		return dom || dow
	}
}

// NextRun is the first time strictly after `from` that a cron expression fires. Exported so
// anything that writes a cron line — the ZFS snapshot schedules — can say when it runs next
// using the same evaluator that reads everyone else's.
func NextRun(expr string, from time.Time) (time.Time, bool) { return nextRun(expr, from) }

// ValidSchedule reports whether a cron expression parses.
func ValidSchedule(expr string) error {
	_, err := parseSchedule(expr)
	return err
}

// nextRun finds the first time strictly after `from` that the schedule fires.
func nextRun(expr string, from time.Time) (time.Time, bool) {
	s, err := parseSchedule(expr)
	if err != nil || s.reboot {
		return time.Time{}, false
	}
	t := from.Truncate(time.Minute).Add(time.Minute)
	day := time.Date(t.Year(), t.Month(), t.Day(), 0, 0, 0, 0, t.Location())
	for i := 0; i < 366*2; i++ {
		if s.dayMatches(day) {
			for h := 0; h < 24; h++ {
				if !s.hour[h] {
					continue
				}
				for mi := 0; mi < 60; mi++ {
					if !s.min[mi] {
						continue
					}
					c := time.Date(day.Year(), day.Month(), day.Day(), h, mi, 0, 0, day.Location())
					if !c.Before(t) {
						return c, true
					}
				}
			}
		}
		day = day.AddDate(0, 0, 1)
	}
	return time.Time{}, false
}