"use strict";

function luhnSum(fullDigits) {
  const digits = fullDigits.split("").map(Number).reverse();
  let sum = 0;
  for (let i = 0; i < digits.length; i++) {
    let d = digits[i];
    if (i % 2 === 1) {
      d *= 2;
      if (d > 9) d -= 9;
    }
    sum += d;
  }
  return sum;
}

function computeCheckDigit(numberWithoutCheck) {
  for (let c = 0; c <= 9; c++) {
    if (luhnSum(numberWithoutCheck + c) % 10 === 0) return c;
  }
  throw new Error("no check digit found (unreachable)");
}

module.exports = {
  slug: "luhn-check-digit",
  title: "Luhn check digit",
  deck_category: "other",
  description: "Validates a number against the Luhn checksum, or computes the Luhn check digit to append to a number.",
  inputs: [
    { name: "mode", type: "string", required: true, description: "Whether to validate a full number or compute a check digit for a number missing one.", enum: ["validate", "compute_check_digit"] },
    { name: "number", type: "string", required: true, description: "Digits only. For validate: the full number including its check digit. For compute_check_digit: the number without its check digit." }
  ],
  outputs: [
    { name: "result", description: "For validate: true or false. For compute_check_digit: the single check digit (0-9)." }
  ],
  steps: [
    "The Luhn algorithm (used for card numbers per ISO/IEC 7812-1): starting from the rightmost digit of the full number (the check digit) and moving left, double every second digit; if doubling produces a number greater than 9, subtract 9 from it.",
    "Sum all the digits (doubled ones after the above adjustment, others unchanged).",
    "The number is valid if that sum is a multiple of 10.",
    "To compute a missing check digit: try each digit 0-9 appended to the given number until the resulting full number's Luhn sum is a multiple of 10; that digit is the check digit."
  ],
  constants: [],
  sources: [
    { kind: "standard", value: "ISO/IEC 7812-1:2017, Identification cards - Identification of issuers - Part 1: Numbering system.", title: "ISO/IEC 7812-1 (Luhn checksum for card numbers)", publisher: "ISO/IEC", retrieved_utc: "2026-09-30T20:01:00Z", note: "Standard specifying the Luhn (mod 10) check algorithm for payment card numbers." }
  ],
  disclaimer: null,
  questionText(inputs) {
    if (inputs.mode === "validate") return `Does the number ${inputs.number} pass the Luhn check?`;
    return `What is the Luhn check digit for ${inputs.number}?`;
  },
  run(inputs) {
    const { mode, number } = inputs || {};
    if (mode !== "validate" && mode !== "compute_check_digit") throw new Error("mode must be validate or compute_check_digit");
    if (typeof number !== "string" || !/^\d+$/.test(number)) throw new Error("number must be a string of digits only");
    if (mode === "validate") {
      const valid = luhnSum(number) % 10 === 0;
      return { answer: `${valid}`, values: { result: valid } };
    }
    const digit = computeCheckDigit(number);
    return { answer: `${digit}`, values: { result: digit } };
  },
  examples: [
    {
      name: "worked example",
      inputs: { mode: "validate", number: "4111111111111111" },
      expected: { answer: "true" },
      check: "4111111111111111 is the widely published Visa test card number, well documented as passing the Luhn check; the algorithm confirms it is valid."
    },
    {
      name: "test vector: compute check digit",
      inputs: { mode: "compute_check_digit", number: "7992739871" },
      expected: { answer: "3" },
      check: "7992739871 with check digit 3 (full number 79927398713) is the widely published worked example used to illustrate the Luhn algorithm; the computed check digit matches that published example."
    }
  ]
};