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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))
}
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