import { isNaN } from "lodash"
import MEMOIZED_FACTDOUBLE from "./MEMOIZED_FACTDOUBLE"
import NUM from "./NUM"

/**
 * Returns double factorial of a number, n (n!!)
 * 
 * View Documentation - https://learn.fulcrumapp.com/dev/expressions/reference/factdouble/
 * @param value (Number, required): postive integer
 * @returns factorial of value
 * @example
 * FACT(5) // returns 15
 */

export default function FACTDOUBLE(value: any): number {
  value = NUM(value)

  if (isNaN(value) || value < 0) { return NaN }

  const n: number = Math.floor(value)

  if (n === 0 || n === 1) {
    return 1
  } else if (MEMOIZED_FACTDOUBLE()[n] > 0) {
    return MEMOIZED_FACTDOUBLE()[n]
  } else {
    MEMOIZED_FACTDOUBLE()[n] = FACTDOUBLE(n - 2) * n
    return MEMOIZED_FACTDOUBLE()[n]
  }
}