import { type ClassValue, clsx } from "clsx"
import { twMerge } from "tailwind-merge"

const TIMEZONE = "Asia/Shanghai"

export function cn(...inputs: ClassValue[]) {
  return twMerge(clsx(inputs))
}

function getShanghaiOffsetMs(): number {
  const formatter = new Intl.DateTimeFormat("en-US", {
    timeZone: TIMEZONE,
    timeZoneName: "short",
  })
  const parts = formatter.formatToParts(new Date())
  const tzPart = parts.find((p) => p.type === "timeZoneName")
  if (!tzPart) return 8 * 60 * 60 * 1000

  const match = tzPart.value.match(/^([+-])(\d{2}):?(\d{2})$/)
  if (!match) return 8 * 60 * 60 * 1000

  const sign = match[1] === "+" ? 1 : -1
  const offsetHours = parseInt(match[2], 10)
  const offsetMinutes = parseInt(match[3], 10)
  return sign * (offsetHours * 60 + offsetMinutes) * 60 * 1000
}

export function formatDateForDatetimeLocal(date: Date): string {
  const shanghaiOffsetMs = getShanghaiOffsetMs()
  const localTimeMs = date.getTime() + shanghaiOffsetMs
  const localDate = new Date(localTimeMs)

  const year = localDate.getUTCFullYear()
  const month = String(localDate.getUTCMonth() + 1).padStart(2, "0")
  const day = String(localDate.getUTCDate()).padStart(2, "0")
  const hours = String(localDate.getUTCHours()).padStart(2, "0")
  const minutes = String(localDate.getUTCMinutes()).padStart(2, "0")
  return `${year}-${month}-${day}T${hours}:${minutes}`
}

export function parseDateTimeLocalToUTC_v2(year: number, month: number, day: number, hours: number, minutes: number): Date {
  const localTimeMs = Date.UTC(year, month, day, hours, minutes)
  const shanghaiOffsetMs = getShanghaiOffsetMs()
  const utcMs = localTimeMs - shanghaiOffsetMs
  return new Date(utcMs)
}

export function parseDateTimeLocalToUTC(value: string): Date | null {
  if (!value) return null
  const [datePart, timePart] = value.split("T")
  if (!datePart) return null
  const [y, m, d] = datePart.split("-").map(Number)
  const [h = 0, min = 0] = timePart ? timePart.split(":").map(Number) : [0, 0]
  return parseDateTimeLocalToUTC_v2(y, m - 1, d, h, min)
}

export function nowInShanghai(): Date {
  const shanghaiOffsetMs = getShanghaiOffsetMs()
  return new Date(Date.now() + shanghaiOffsetMs)
}