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 twoDigits(n: number): string {
  if (n < 20) return ONES[n];
  const tens = Math.floor(n / 10);
  const ones = n % 10;
  return TENS[tens] + (ones ? " " + ONES[ones] : "");
}

function threeDigits(n: number): string {
  const hundreds = Math.floor(n / 100);
  const rest = n % 100;
  let out = "";
  if (hundreds > 0) out += ONES[hundreds] + " Hundred";
  if (rest > 0) out += (out ? " " : "") + twoDigits(rest);
  return out;
}

/** Whole-number to words using the Indian numbering system (thousand, lakh, crore). */
function wholeToWords(n: number): string {
  if (n === 0) return "Zero";
  const parts: string[] = [];

  const crore = Math.floor(n / 10000000);
  let rest = n % 10000000;
  if (crore > 0) parts.push(`${wholeToWords(crore)} Crore`);

  const lakh = Math.floor(rest / 100000);
  rest %= 100000;
  if (lakh > 0) parts.push(`${twoDigits(lakh)} Lakh`);

  const thousand = Math.floor(rest / 1000);
  rest %= 1000;
  if (thousand > 0) parts.push(`${twoDigits(thousand)} Thousand`);

  if (rest > 0) parts.push(threeDigits(rest));

  return parts.join(" ");
}

/** e.g. 25000 -> "Indian Rupees Twenty Five Thousand Only" */
export function amountInWords(amount: number, currency = "Indian Rupees"): string {
  const value = Number.isFinite(amount) ? Math.abs(amount) : 0;
  const rupees = Math.floor(value);
  const paise = Math.round((value - rupees) * 100);

  let out = "";
  if (currency) out += `${currency} `;
  out += wholeToWords(rupees);
  if (paise > 0) out += ` and ${twoDigits(paise)} Paise`;
  return `${out} Only`;
}