import {formatIsoToDisplay} from './dateFormat'

const parseUtc = (iso: string): Date => {
  const [y, m, d] = iso.split('-').map(Number)
  return new Date(Date.UTC(y, m - 1, d))
}

const toIso = (d: Date): string => {
  const y = d.getUTCFullYear()
  const m = String(d.getUTCMonth() + 1).padStart(2, '0')
  const day = String(d.getUTCDate()).padStart(2, '0')
  return `${y}-${m}-${day}`
}

export function mondayOf(iso: string): string {
  const d = parseUtc(iso)
  const dayNum = d.getUTCDay() || 7 // dimanche = 7
  d.setUTCDate(d.getUTCDate() - (dayNum - 1))
  return toIso(d)
}

export function sundayOf(iso: string): string {
  const d = parseUtc(mondayOf(iso))
  d.setUTCDate(d.getUTCDate() + 6)
  return toIso(d)
}

export function toIsoWeek(iso: string): string {
  const d = parseUtc(iso)
  const dayNum = d.getUTCDay() || 7
  d.setUTCDate(d.getUTCDate() + 4 - dayNum) // jeudi de la semaine
  const isoYear = d.getUTCFullYear()
  const yearStart = new Date(Date.UTC(isoYear, 0, 1))
  const week = Math.ceil((((d.getTime() - yearStart.getTime()) / 86400000) + 1) / 7)
  return `${isoYear}-W${String(week).padStart(2, '0')}`
}

export function isoWeekToMonday(week: string): string | null {
  const m = /^(\d{4})-W(\d{2})$/.exec(week)
  if (!m) return null
  const year = Number(m[1])
  const w = Number(m[2])
  if (w < 1 || w > 53) return null
  // Lundi de la semaine 1 = lundi de la semaine contenant le 4 janvier
  const jan4 = new Date(Date.UTC(year, 0, 4))
  const jan4Day = jan4.getUTCDay() || 7
  const monday = new Date(jan4)
  monday.setUTCDate(jan4.getUTCDate() - (jan4Day - 1) + (w - 1) * 7)
  const iso = toIso(monday)
  // Garde-fou : la semaine 53 n'existe pas pour toutes les années
  if (toIsoWeek(iso) !== week) return null
  return iso
}

export function isValidIsoWeek(week: string): boolean {
  return isoWeekToMonday(week) !== null
}

export function formatWeekDisplay(week: string): string {
  const monday = isoWeekToMonday(week)
  if (!monday) return ''
  const sunday = sundayOf(monday)
  const w = Number(week.slice(6))
  const startDdMm = formatIsoToDisplay(monday).slice(0, 5) // "18/05"
  const endFull = formatIsoToDisplay(sunday) // "24/05/2026"
  return `Semaine ${w} (${startDdMm} → ${endFull})`
}