function calc (P, PMT, r, n, t) {
  const rn = (r / 100) / n
  const nt = n * t
  const pn = 12 / n
  const principal = P * Math.pow(1 + rn, nt)
  const series = PMT * (pn * (Math.pow(1 + rn, nt) - 1) / rn)
  return parseInt(principal + series, 10)
}

module.exports = opts => {
  if (typeof opts !== 'object') {
    throw new Error(`Expected 'opts' to be an object, received ${typeof opts}.`)
  }
  return calc(
    opts.initial,
    opts.monthly,
    opts.interest,
    opts.compound,
    opts.years
  )
}

module.exports.verbose = opts => {
  if (typeof opts !== 'object') {
    throw new Error(`Expected 'opts' to be an object, received ${typeof opts}.`)
  }
  return Array(opts.years + 1)
    .fill(null)
    .map((x, idx) =>
      calc(
        opts.initial,
        opts.monthly,
        opts.interest,
        opts.compound,
        idx
      )
    )
}