import type { VisibleDateRange } from '../types'

export function parseCalendarDate(value: string | null | undefined): Date | null {
  if (!value) return null

  const parts = /^(\d{4})-(\d{2})-(\d{2})/.exec(value)
  if (!parts) return null

  const year = Number(parts[1])
  const month = Number(parts[2]) - 1
  const day = Number(parts[3])
  const date = new Date(year, month, day)

  return date.getFullYear() === year && date.getMonth() === month && date.getDate() === day
    ? date
    : null
}

export function startOfLocalDay(date: Date): Date {
  const start = new Date(date)
  start.setHours(0, 0, 0, 0)
  return start
}

export function shiftLocalDays(date: Date, days: number): Date {
  const shifted = new Date(date)
  shifted.setDate(shifted.getDate() + days)
  return shifted
}

export function shiftLocalMonths(date: Date, months: number): Date {
  const shifted = new Date(date)
  const targetDay = shifted.getDate()

  shifted.setDate(1)
  shifted.setMonth(shifted.getMonth() + months)

  const lastDayOfTargetMonth = new Date(
    shifted.getFullYear(),
    shifted.getMonth() + 1,
    0,
  ).getDate()
  shifted.setDate(Math.min(targetDay, lastDayOfTargetMonth))

  return shifted
}

export function daysVisibleRange(date: Date, days: number): VisibleDateRange {
  const start = startOfLocalDay(date)
  const end = shiftLocalDays(start, Math.max(1, days))

  return {
    startMs: start.getTime(),
    endMs: end.getTime(),
  }
}

export function monthGridVisibleRange(date: Date): VisibleDateRange {
  const firstDay = new Date(date.getFullYear(), date.getMonth(), 1)
  const gridStart = new Date(firstDay)
  gridStart.setDate(gridStart.getDate() - gridStart.getDay())

  const lastDay = new Date(date.getFullYear(), date.getMonth() + 1, 0)
  const weeks = Math.ceil((firstDay.getDay() + lastDay.getDate()) / 7)
  const gridEnd = shiftLocalDays(gridStart, weeks * 7)

  return {
    startMs: gridStart.getTime(),
    endMs: gridEnd.getTime(),
  }
}