/**
 * Date formatting and calculation utilities for Swedish locale (sv-SE).
 * All functions use ISO 8601 format for input (YYYY-MM-DD) and Intl API for display.
 */

/**
 * Format a date for display (e.g. "1 juni 2026" in Swedish).
 * Accepts ISO string or Date object.
 */
export function formatDateForDisplay(date: string | Date): string {
  const d = typeof date === 'string' ? new Date(date + 'T00:00:00Z') : date
  if (!isValidDate(d)) return ''
  const formatter = new Intl.DateTimeFormat('sv-SE', {
    year: 'numeric',
    month: 'long',
    day: 'numeric',
  })
  return formatter.format(d)
}

/**
 * Format a date for HTML input value (e.g. "2026-06-01").
 * Accepts ISO string or Date object.
 */
export function formatDateForInput(date: string | Date): string {
  const d = typeof date === 'string' ? new Date(date + 'T00:00:00Z') : date
  if (!isValidDate(d)) return ''
  const year = d.getUTCFullYear()
  const month = String(d.getUTCMonth() + 1).padStart(2, '0')
  const day = String(d.getUTCDate()).padStart(2, '0')
  return `${year}-${month}-${day}`
}

/**
 * Parse an ISO date string (YYYY-MM-DD) to a Date object.
 */
export function parseInputDate(value: string): Date {
  return new Date(value + 'T00:00:00Z')
}

/**
 * Type guard: check if a value is a valid date.
 */
export function isValidDate(date: any): boolean {
  return date instanceof Date && !isNaN(date.getTime())
}

/**
 * Get the Swedish day name (e.g. "Fredag").
 */
export function getDayName(date: Date): string {
  if (!isValidDate(date)) return ''
  const formatter = new Intl.DateTimeFormat('sv-SE', { weekday: 'long' })
  return formatter.format(date)
}

/**
 * Get the Swedish month name (e.g. "Juni").
 */
export function getMonthName(date: Date): string {
  if (!isValidDate(date)) return ''
  const formatter = new Intl.DateTimeFormat('sv-SE', { month: 'long' })
  return formatter.format(date)
}