/** subtract two numbers
  * @param  {number} param1
  * @param  {number} param2
  * @returns number
  */
 static sub(param1: number, param2: number): number {
     if ((param1) && (is.number(param1) && (param2) && (is.number(param2)))) {
         return param1 - param2;
     } else {
         throw new Error("mathHelper - sub - empty or non numbers specified");
     }
 }

 private isValidUsd (usd: string | number) {
   check(is.not.null(usd), 'USD is not set')
   check(is.not.empty(usd), 'USD is not set')
   check(is.number(Number(usd)), 'USD is not a valid number')
   check(is.above(Number(usd), -1), 'USD must be a positive number')
   return true
 }

 private isValidDeposit (deposit: string | number) {
   check(is.not.null(deposit), 'Deposit is not set')
   check(is.not.empty(deposit), 'Deposit is not set')
   check(is.number(Number(deposit)), 'Deposit is not a valid number')
   check(is.above(Number(deposit), -1), 'Deposit must be a positive number')
   return true
 }

 private isValidCharge  (charge: string | number) {
   check(is.not.null(charge), 'Charge is not set')
   check(is.not.empty(charge), 'Charge is not set')
   check(is.number(Number(charge)), `Charge is not a valid number: ${charge}`)
   check(is.above(Number(charge), -1), 'Charge must be a positive number')
   return true
 }

  private checkDues (currentDue, totalDues) {
    check(is.number(Number(currentDue)), 'Current due is not a number')
    check(is.number(Number(totalDues)), 'Total dues is not a number')
    currentDue = parseInt(currentDue, 10)
    totalDues = parseInt(totalDues, 10)

    // Validations
    if (currentDue > totalDues || currentDue < 1) {
      throw new Error('Current dues can\'t be bigger than total dues or smaller than 1')
    }

    if (totalDues < 0) {
      throw new Error('Total dues can\'t be smaller than 1')
    }

    return true
  }

 private isValidInterestRate (interestRate: number): boolean {
   check(is.number(interestRate), 'Interest rate must be a number')
   check(is.above(interestRate, -1), 'Interest rate must be a positive number')
   return true
 }

 private isValidBalance (balance: number | string) {
   check(is.not.null(balance), 'Balance is not set')
   check(is.not.empty(balance), 'Balance is not set')
   check(is.number(Number(balance)), 'Balance is not a valid number')
   return true
 }