/** All money is integer paise. Fractional-rupee floats never enter the domain. */
export type Paise = number

export function fromRupees(rupees: number): Paise {
  return Math.round(rupees * 100)
}

/** Indian digit grouping: 12,34,567.89 */
export function formatINR(amount: Paise, opts: { symbol?: boolean } = {}): string {
  const sign = amount < 0 ? '-' : ''
  const abs = Math.abs(Math.round(amount))
  const whole = String(Math.trunc(abs / 100))
  const frac = String(abs % 100).padStart(2, '0')
  const grouped =
    whole.length <= 3
      ? whole
      : whole.slice(0, -3).replace(/\B(?=(\d{2})+(?!\d))/g, ',') + ',' + whole.slice(-3)
  return `${sign}${opts.symbol === false ? '' : '₹'}${grouped}.${frac}`
}

const ONES = [
  '', 'One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten',
  'Eleven', 'Twelve', 'Thirteen', 'Fourteen', 'Fifteen', 'Sixteen', 'Seventeen', 'Eighteen', 'Nineteen',
]
const TENS = ['', '', 'Twenty', 'Thirty', 'Forty', 'Fifty', 'Sixty', 'Seventy', 'Eighty', 'Ninety']

function below100(n: number): string {
  if (n < 20) return ONES[n]!
  const t = TENS[Math.floor(n / 10)]!
  const o = n % 10
  return o === 0 ? t : `${t} ${ONES[o]!}`
}

function below1000(n: number): string {
  const h = Math.floor(n / 100)
  const r = n % 100
  const parts: string[] = []
  if (h > 0) parts.push(`${ONES[h]!} Hundred`)
  if (r > 0) parts.push(below100(r))
  return parts.join(' ')
}

/** Whole non-negative rupees to Indian-numbering words (crore/lakh/thousand). */
function indianWords(rupees: number): string {
  if (rupees === 0) return 'Zero'
  let n = rupees
  const crore = Math.floor(n / 10_000_000); n %= 10_000_000
  const lakh = Math.floor(n / 100_000); n %= 100_000
  const thousand = Math.floor(n / 1000); n %= 1000
  const parts: string[] = []
  if (crore > 0) parts.push(`${below1000(crore)} Crore`)
  if (lakh > 0) parts.push(`${below100(lakh)} Lakh`)
  if (thousand > 0) parts.push(`${below100(thousand)} Thousand`)
  if (n > 0) parts.push(below1000(n))
  return parts.join(' ')
}

/**
 * Integer paise to an Indian-numbering "amount in words" string for GST invoices
 * (e.g. 1,00,00,000 paise → "Rupees One Lakh Only"). Rupees and paise are named
 * separately; "Paisa" is singular. The `formatINR` sibling for the numeric form.
 */
export function amountInWordsINR(paise: Paise): string {
  const sign = paise < 0 ? 'Minus ' : ''
  const abs = Math.abs(Math.round(paise))
  const rupees = Math.trunc(abs / 100)
  const p = abs % 100
  const paiseWord = p === 1 ? 'Paisa' : 'Paise'
  if (rupees > 0 && p > 0) return `${sign}Rupees ${indianWords(rupees)} and ${below100(p)} ${paiseWord} Only`
  if (rupees > 0) return `${sign}Rupees ${indianWords(rupees)} Only`
  if (p > 0) return `${sign}${below100(p)} ${paiseWord} Only`
  return 'Rupees Zero Only'
}