/**
 * Centralized date-string utilities.
 *
 * All date values in the app use the 'YYYY-MM-DD' (wall-clock) format.
 * These helpers are the single source of truth for formatting and
 * arithmetic on that format, replacing ad-hoc inline code that was
 * duplicated across organize.js, recur.js, useCalendar.js, and ical-io.js.
 */

/** Pad a number to 2 digits with leading zero. */
export function pad2(n) {
  return String(n).padStart(2, '0')
}

/** Convert a JS Date to 'YYYY-MM-DD' (local wall-clock). */
export function toISODate(date) {
  return `${date.getFullYear()}-${pad2(date.getMonth() + 1)}-${pad2(date.getDate())}`
}

/** Parse 'YYYY-MM-DD' into { year, month, day } (month is 1-based). */
export function parseISODate(s) {
  const [y, m, d] = s.split('-').map(Number)
  return { year: y, month: m, day: d }
}

/** Build a local JS Date at midnight from 'YYYY-MM-DD'. */
export function toDate(isoDate) {
  return new Date(isoDate + 'T00:00:00')
}

/** Add `days` days to an ISO date string, return new ISO string. */
export function addDays(isoDate, days) {
  const d = toDate(isoDate)
  d.setDate(d.getDate() + days)
  return toISODate(d)
}

/** Whole-day difference between two ISO date strings (b - a). */
export function daysBetween(isoA, isoB) {
  return Math.round((toDate(isoB) - toDate(isoA)) / (24 * 3600 * 1000))
}

/** Today's date as 'YYYY-MM-DD' (local). */
export function todayISO() {
  return toISODate(new Date())
}

/**
 * Current UTC timestamp in compact RFC 5545 form (e.g. '20260711T085008Z').
 * Used for DTSTAMP on new events.
 */
export function nowCompactTimestamp() {
  return new Date().toISOString().replace(/[-:]/g, '').split('.')[0] + 'Z'
}

/** Format a JS Date's time as 'HH:MM'. */
export function toHHMM(date) {
  return `${pad2(date.getHours())}:${pad2(date.getMinutes())}`
}