package webui

import (
	"time"

	"github.com/teambition/rrule-go"

	"mailgoserver/internal/db"
)

// expandOccurrences returns every occurrence of e falling within [rangeStart,
// rangeEnd) — a single-element slice (or none, if outside the range) for a
// non-recurring event, or every matching occurrence for a recurring one (rrule-go's
// own expansion, seeded with the event's own start time as DTSTART since e.RRule only
// ever stores the bare "FREQ=..." value, not a full RRULE property with its own
// DTSTART). Display-only: an occurrence's time is never itself stored — editing or
// deleting always acts on the whole series (see caldav.go/webmail_calendar.go; a
// single-occurrence RFC 5545 RECURRENCE-ID override is out of scope for this pass).
func expandOccurrences(e db.CalendarEvent, rangeStart, rangeEnd time.Time) []time.Time {
	if e.RRule == "" {
		if e.StartAt.Before(rangeEnd) && e.EndAt.After(rangeStart) {
			return []time.Time{e.StartAt}
		}
		return nil
	}
	opt, err := rrule.StrToROption(e.RRule)
	if err != nil {
		return nil
	}
	opt.Dtstart = e.StartAt
	rule, err := rrule.NewRRule(*opt)
	if err != nil {
		return nil
	}
	// Between's window needs to start far enough back to catch an occurrence whose
	// start falls before rangeStart but whose duration still overlaps it (a
	// multi-hour recurring event straddling midnight) — shift the query window back
	// by the event's own duration rather than assuming same-day occurrences.
	duration := e.EndAt.Sub(e.StartAt)
	return rule.Between(rangeStart.Add(-duration), rangeEnd, true)
}