import { computed, ref, type ComputedRef, type Ref } from 'vue'

export interface CalendarGridDay {
  date: Date
  isCurrentMonth: boolean
  isDisabled: boolean
}

export interface UseCalendarGridOptions {
  min?: () => string | undefined
  max?: () => string | undefined
  locale?: () => string | undefined
}

export interface UseCalendarGridReturn {
  currentMonth: Ref<Date>
  weekDays: readonly string[]
  monthYear: ComputedRef<string>
  calendarDays: ComputedRef<CalendarGridDay[]>
  isDateDisabled: (date: Date) => boolean
  isSameDay: (date: Date, other: Date | null | undefined) => boolean
  isToday: (date: Date) => boolean
  formatDateValue: (date: Date) => string
  setCurrentMonthFromDate: (date: Date) => void
  prevMonth: () => void
  nextMonth: () => void
}

const WEEK_DAYS = ['Su', 'Mo', 'Tu', 'We', 'Th', 'Fr', 'Sa'] as const

/** Shared calendar grid state for date and datetime pickers. */
export function useCalendarGrid(options: UseCalendarGridOptions = {}): UseCalendarGridReturn {
  const currentMonth = ref(new Date())

  const monthYear = computed(() => {
    return currentMonth.value.toLocaleDateString(options.locale?.() ?? 'en-US', {
      year: 'numeric',
      month: 'long',
    })
  })

  const calendarDays = computed<CalendarGridDay[]>(() => {
    const year = currentMonth.value.getFullYear()
    const month = currentMonth.value.getMonth()
    const firstDay = new Date(year, month, 1)
    const lastDay = new Date(year, month + 1, 0)
    const days: CalendarGridDay[] = []

    const startPadding = firstDay.getDay()
    for (let i = startPadding - 1; i >= 0; i--) {
      const date = new Date(year, month, -i)
      days.push({ date, isCurrentMonth: false, isDisabled: isDateDisabled(date) })
    }

    for (let i = 1; i <= lastDay.getDate(); i++) {
      const date = new Date(year, month, i)
      days.push({ date, isCurrentMonth: true, isDisabled: isDateDisabled(date) })
    }

    const endPadding = 42 - days.length
    for (let i = 1; i <= endPadding; i++) {
      const date = new Date(year, month + 1, i)
      days.push({ date, isCurrentMonth: false, isDisabled: isDateDisabled(date) })
    }

    return days
  })

  function isDateDisabled(date: Date): boolean {
    const minDate = parseBoundaryDate(options.min?.())
    if (minDate && startOfDay(date) < startOfDay(minDate)) return true

    const maxDate = parseBoundaryDate(options.max?.())
    if (maxDate && startOfDay(date) > startOfDay(maxDate)) return true

    return false
  }

  function isSameDay(date: Date, other: Date | null | undefined): boolean {
    if (!other) return false
    return date.toDateString() === other.toDateString()
  }

  function isToday(date: Date): boolean {
    return isSameDay(date, new Date())
  }

  function formatDateValue(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}`
  }

  function setCurrentMonthFromDate(date: Date): void {
    currentMonth.value = new Date(date.getFullYear(), date.getMonth(), 1)
  }

  function prevMonth(): void {
    currentMonth.value = new Date(
      currentMonth.value.getFullYear(),
      currentMonth.value.getMonth() - 1,
      1,
    )
  }

  function nextMonth(): void {
    currentMonth.value = new Date(
      currentMonth.value.getFullYear(),
      currentMonth.value.getMonth() + 1,
      1,
    )
  }

  return {
    currentMonth,
    weekDays: WEEK_DAYS,
    monthYear,
    calendarDays,
    isDateDisabled,
    isSameDay,
    isToday,
    formatDateValue,
    setCurrentMonthFromDate,
    prevMonth,
    nextMonth,
  }
}

function parseBoundaryDate(value: string | undefined): Date | null {
  if (!value) return null
  const date = value.includes('T') ? new Date(value) : new Date(`${value}T00:00:00`)
  return Number.isNaN(date.getTime()) ? null : date
}

function startOfDay(date: Date): Date {
  const copy = new Date(date)
  copy.setHours(0, 0, 0, 0)
  return copy
}