export type CalendarDay = {
  date: Date
  value: string
  inMonth: boolean
  isToday: boolean
  isSelected: boolean
}

export type AnchorRect = Pick<DOMRect, 'left' | 'top' | 'bottom' | 'width' | 'height'>

const DAY_MS = 86_400_000

export function parseLocalDate(value: string): Date | null {
  const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(value)
  if (!match) return null
  const year = Number(match[1])
  const month = Number(match[2]) - 1
  const day = Number(match[3])
  const date = new Date(year, month, day)
  return date.getFullYear() === year && date.getMonth() === month && date.getDate() === day ? date : null
}

export function formatLocalDate(date: Date): string {
  const year = date.getFullYear()
  const month = String(date.getMonth() + 1).padStart(2, '0')
  const day = String(date.getDate()).padStart(2, '0')
  return `${year}-${month}-${day}`
}

export function addCalendarDays(date: Date, amount: number): Date {
  const next = new Date(date.getFullYear(), date.getMonth(), date.getDate())
  next.setDate(next.getDate() + amount)
  return next
}

function addCalendarMonths(date: Date, amount: number): Date {
  const wantedDay = date.getDate()
  const next = new Date(date.getFullYear(), date.getMonth() + amount, 1)
  const finalDay = new Date(next.getFullYear(), next.getMonth() + 1, 0).getDate()
  next.setDate(Math.min(wantedDay, finalDay))
  return next
}

export function moveCalendarFocus(date: Date, key: string): Date {
  if (key === 'ArrowLeft') return addCalendarDays(date, -1)
  if (key === 'ArrowRight') return addCalendarDays(date, 1)
  if (key === 'ArrowUp') return addCalendarDays(date, -7)
  if (key === 'ArrowDown') return addCalendarDays(date, 7)
  if (key === 'PageUp') return addCalendarMonths(date, -1)
  if (key === 'PageDown') return addCalendarMonths(date, 1)
  const mondayIndex = (date.getDay() + 6) % 7
  if (key === 'Home') return addCalendarDays(date, -mondayIndex)
  if (key === 'End') return addCalendarDays(date, 6 - mondayIndex)
  return new Date(date)
}

export function buildCalendarMonth(month: Date, today: Date, selected: Date | null): CalendarDay[] {
  const first = new Date(month.getFullYear(), month.getMonth(), 1)
  const start = addCalendarDays(first, -((first.getDay() + 6) % 7))
  const todayValue = formatLocalDate(today)
  const selectedValue = selected ? formatLocalDate(selected) : ''
  return Array.from({ length: 42 }, (_, index) => {
    const date = addCalendarDays(start, index)
    const value = formatLocalDate(date)
    return { date, value, inMonth: date.getMonth() === month.getMonth(), isToday: value === todayValue, isSelected: value === selectedValue }
  })
}

export function positionCalendarPopover(anchor: AnchorRect, viewportWidth: number, viewportHeight: number, height: number) {
  const width = 328
  const gap = 8
  const edge = 12
  const left = Math.min(Math.max(anchor.left, edge), viewportWidth - width - edge)
  const below = anchor.bottom + gap
  const above = anchor.top - height - gap
  const maxTop = Math.max(edge, viewportHeight - height - edge)
  const top = below + height <= viewportHeight - edge
    ? below
    : above >= edge
      ? above
      : Math.min(Math.max(anchor.bottom - anchor.height + gap, edge), maxTop)
  return { left, top }
}

export const calendarDayDistance = (from: Date, to: Date) => Math.round((Date.UTC(to.getFullYear(), to.getMonth(), to.getDate()) - Date.UTC(from.getFullYear(), from.getMonth(), from.getDate())) / DAY_MS)