calendar/date-utils.js

// date-utils.js — Date handling utilities for the calendar
const monthAbbr = ['jan','feb','mar','apr','may','jun','jul','aug','sep','oct','nov','dec']
const DAY_MS = 86400000
const WEEK_MS = 7 * DAY_MS

/**
 * Get ISO week information for a given date
 * @param {Date} date - The date to get week info for
 * @returns {Object} Object containing week number and year
 */
const isoWeekInfo = date => {
  const d = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()))
  const day = d.getUTCDay() || 7
  d.setUTCDate(d.getUTCDate() + 4 - day)
  const year = d.getUTCFullYear()
  const yearStart = new Date(Date.UTC(year, 0, 1))
  const diffDays = Math.floor((d - yearStart) / DAY_MS) + 1
  return { week: Math.ceil(diffDays / 7), year }
}

/**
 * Convert a Date object to a local date string (YYYY-MM-DD format)
 * @param {Date} date - The date to convert (defaults to new Date())
 * @returns {string} Date string in YYYY-MM-DD format
 */
function toLocalString(date = new Date()) {
  const pad = n => String(Math.floor(Math.abs(n))).padStart(2, '0')
  return `${date.getFullYear()}-${pad(date.getMonth() + 1)}-${pad(date.getDate())}`
}

/**
 * Convert a local date string (YYYY-MM-DD) to a Date object
 * @param {string} dateString - Date string in YYYY-MM-DD format
 * @returns {Date} Date object
 */
function fromLocalString(dateString) {
  const [year, month, day] = dateString.split('-').map(Number)
  return new Date(year, month - 1, day)
}

/**
 * Get the index of Monday for a given date (0-6, where Monday = 0)
 * @param {Date} d - The date
 * @returns {number} Monday index (0-6)
 */
const mondayIndex = d => (d.getDay() + 6) % 7

/**
 * Pad a number with leading zeros to make it 2 digits
 * @param {number} n - Number to pad
 * @returns {string} Padded string
 */
const pad = n => String(n).padStart(2, '0')

/**
 * Calculate number of days between two date strings (inclusive)
 * @param {string} aStr - First date string (YYYY-MM-DD)
 * @param {string} bStr - Second date string (YYYY-MM-DD)
 * @returns {number} Number of days inclusive
 */
function daysInclusive(aStr, bStr) {
  const a = fromLocalString(aStr)
  const b = fromLocalString(bStr)
  const A = new Date(a.getFullYear(), a.getMonth(), a.getDate()).getTime()
  const B = new Date(b.getFullYear(), b.getMonth(), b.getDate()).getTime()
  return Math.floor(Math.abs(B - A) / DAY_MS) + 1
}

/**
 * Add days to a date string
 * @param {string} str - Date string in YYYY-MM-DD format
 * @param {number} n - Number of days to add (can be negative)
 * @returns {string} New date string
 */
function addDaysStr(str, n) {
  const d = fromLocalString(str)
  d.setDate(d.getDate() + n)
  return toLocalString(d)
}

/**
 * Get localized weekday names starting from Monday
 * @returns {Array<string>} Array of localized weekday names
 */
function getLocalizedWeekdayNames() {
  const res = []
  const base = new Date(2025, 0, 6) // A Monday
  for (let i = 0; i < 7; i++) {
    const d = new Date(base)
    d.setDate(base.getDate() + i)
    res.push(d.toLocaleDateString(undefined, { weekday: 'short' }))
  }
  return res
}

/**
 * Get localized month name
 * @param {number} idx - Month index (0-11)
 * @param {boolean} short - Whether to return short name
 * @returns {string} Localized month name
 */
function getLocalizedMonthName(idx, short = false) {
  const d = new Date(2025, idx, 1)
  return d.toLocaleDateString(undefined, { month: short ? 'short' : 'long' })
}

/**
 * Format a date range for display
 * @param {Date} startDate - Start date
 * @param {Date} endDate - End date
 * @returns {string} Formatted date range string
 */
function formatDateRange(startDate, endDate) {
  if (toLocalString(startDate) === toLocalString(endDate)) return toLocalString(startDate)
  const startISO = toLocalString(startDate)
  const endISO = toLocalString(endDate)
  const [sy, sm] = startISO.split('-')
  const [ey, em, ed] = endISO.split('-')
  if (sy === ey && sm === em) return `${startISO}/${ed}`
  if (sy === ey) return `${startISO}/${em}-${ed}`
  return `${startISO}/${endISO}`
}

/**
 * Compute lunar phase symbol for the four main phases on a given date.
 * Returns one of: 🌑 (new), 🌓 (first quarter), 🌕 (full), 🌗 (last quarter), or '' otherwise.
 * Uses an approximate algorithm with a fixed epoch.
 */
function lunarPhaseSymbol(date) {
  // Reference new moon: 2000-01-06 18:14 UTC (J2000 era), often used in approximations
  const ref = Date.UTC(2000, 0, 6, 18, 14, 0)
  const synodic = 29.530588853 // days
  // Use UTC noon of given date to reduce timezone edge effects
  const dUTC = Date.UTC(date.getFullYear(), date.getMonth(), date.getDate(), 12, 0, 0)
  const daysSince = (dUTC - ref) / DAY_MS
  const phase = ((daysSince / synodic) % 1 + 1) % 1
  const phases = [
    { t: 0.0, s: '🌑' }, // New Moon
    { t: 0.25, s: '🌓' }, // First Quarter
    { t: 0.5, s: '🌕' }, // Full Moon
    { t: 0.75, s: '🌗' }  // Last Quarter
  ]
  // threshold in days from exact phase to still count for this date
  const thresholdDays = 0.5 // ±12 hours
  for (const p of phases) {
    let delta = Math.abs(phase - p.t)
    if (delta > 0.5) delta = 1 - delta
    if (delta * synodic <= thresholdDays) return p.s
  }
  return ''
}

// Export all functions and constants
export {
  monthAbbr,
  DAY_MS,
  WEEK_MS,
  isoWeekInfo,
  toLocalString,
  fromLocalString,
  mondayIndex,
  pad,
  daysInclusive,
  addDaysStr,
  getLocalizedWeekdayNames,
  getLocalizedMonthName,
  formatDateRange
  ,lunarPhaseSymbol
}