import { addDays, isWeekend, parseISO, format, isWithinInterval } from "date-fns"
import type { WorkdayExceptionData } from "./types"

function isExceptionDay(
  date: Date,
  exceptions: WorkdayExceptionData[]
): boolean {
  return exceptions.some((ex) => {
    const start = parseISO(ex.startDate)
    const end = parseISO(ex.endDate)
    return isWithinInterval(date, { start, end })
  })
}

function isNonWorkday(
  date: Date,
  exceptions: WorkdayExceptionData[] = []
): boolean {
  return isWeekend(date) || isExceptionDay(date, exceptions)
}

export function calculateEndDate(
  startDate: string,
  workdays: number,
  exceptions: WorkdayExceptionData[] = []
): string {
  if (workdays <= 0) return startDate

  let current = parseISO(startDate)
  let remaining = workdays

  if (!isNonWorkday(current, exceptions)) {
    remaining--
  }

  while (remaining > 0) {
    current = addDays(current, 1)
    if (!isNonWorkday(current, exceptions)) {
      remaining--
    }
  }

  return format(current, "yyyy-MM-dd")
}

export function countBusinessDays(
  startDate: string,
  endDate: string,
  exceptions: WorkdayExceptionData[] = []
): number {
  let current = parseISO(startDate)
  const end = parseISO(endDate)
  let count = 0

  while (current <= end) {
    if (!isNonWorkday(current, exceptions)) {
      count++
    }
    current = addDays(current, 1)
  }

  return count
}

export function addBusinessDays(
  date: string,
  days: number,
  exceptions: WorkdayExceptionData[] = []
): string {
  let current = parseISO(date)
  let remaining = Math.abs(days)
  const direction = days >= 0 ? 1 : -1

  while (remaining > 0) {
    current = addDays(current, direction)
    if (!isNonWorkday(current, exceptions)) {
      remaining--
    }
  }

  return format(current, "yyyy-MM-dd")
}