package models

import (
	"sort"
	"time"
)

// ComputeNextOccurrence returns the next occurrence date after due, given a
// recurrence pattern. weekdays is only consulted when freq == "weekly"; nil
// or empty means "same weekday as due, every interval weeks." Unknown freq
// values return due unchanged. interval < 1 is treated as 1.
//
// Monthly/yearly rollover uses Go's standard AddDate overflow behavior (a
// due date of Jan 31 + 1 month becomes Mar 3, not clamped to Feb's last
// day) -- this is an accepted simplification, and the drift is permanent:
// each call computes from the previous call's actual result, not an
// original anchor day, so a drifted date locks onto its new day-of-month
// going forward. Only anchor days 29-31 are ever affected; every month has
// at least 28 days, so any anchor day <= 28 never drifts.
func ComputeNextOccurrence(due time.Time, freq string, interval int, weekdays []int) time.Time {
	if interval < 1 {
		interval = 1
	}
	switch freq {
	case "daily":
		return due.AddDate(0, 0, interval)
	case "weekly":
		return nextWeeklyOccurrence(due, interval, weekdays)
	case "monthly":
		return due.AddDate(0, interval, 0)
	case "yearly":
		return due.AddDate(interval, 0, 0)
	default:
		return due
	}
}

func nextWeeklyOccurrence(due time.Time, interval int, weekdays []int) time.Time {
	if len(weekdays) == 0 {
		return due.AddDate(0, 0, 7*interval)
	}
	sorted := append([]int(nil), weekdays...)
	sort.Ints(sorted)
	dueWeekday := int(due.Weekday())

	for _, wd := range sorted {
		if wd > dueWeekday {
			return due.AddDate(0, 0, wd-dueWeekday)
		}
	}
	// Wrapped past the last active weekday this week: land on the first
	// active weekday, (interval-1) whole weeks further out than the
	// immediate next week (interval=1 means "next week", interval=2 means
	// "skip a week", etc).
	daysToNextWeekStart := 7 - dueWeekday
	return due.AddDate(0, 0, daysToNextWeekStart+sorted[0]+7*(interval-1))
}