"use strict";
// Pure Gregorian leap-year rule (Gregorian calendar reform of 1582, as codified in modern
// reference works, cited below). No day-number conversion is needed for this method, no host
// clock, no Date.now().
function isLeapGregorian(y) { return (y % 4 === 0 && y % 100 !== 0) || (y % 400 === 0); }

module.exports = {
  slug: "is-leap-year",
  title: "Is it a leap year?",
  deck_category: "dates",
  description: "Determines whether a given year is a leap year under the Gregorian calendar rule.",
  inputs: [
    { name: "year", type: "integer", required: true, description: "Proleptic Gregorian calendar year" }
  ],
  outputs: [ { name: "is_leap", type: "boolean", description: "True if the year has a February 29" } ],
  steps: [
    "A year is a leap year if it is divisible by 4.",
    "Exception: a year divisible by 100 is NOT a leap year.",
    "Exception to the exception: a year divisible by 400 IS a leap year.",
    "Apply the rules in that order (or equivalently: divisible by 4 and not by 100, OR divisible by 400)."
  ],
  constants: [],
  sources: [
    { kind: "url", value: "https://aa.usno.navy.mil/faq/calendars", title: "Calendars FAQ", publisher: "US Naval Observatory / Astronomical Applications Department", retrieved_utc: "2026-09-30T19:41:35Z", note: "states the Gregorian leap-year rule: divisible by 4, except centuries, unless divisible by 400" }
  ],
  disclaimer: null,
  questionText(inputs) {
    return "Is " + inputs.year + " a leap year?";
  },
  run(inputs) {
    if (!Number.isInteger(inputs.year)) throw new Error("year must be an integer");
    const isLeap = isLeapGregorian(inputs.year);
    return { answer: isLeap ? "yes" : "no", values: { is_leap: isLeap } };
  },
  examples: [
    { name: "worked example", inputs: { year: 2000 },
      expected: { answer: "yes" }, check: "2000 is divisible by 400, so it is a leap year despite being a century year" },
    { name: "test vector: century exception", inputs: { year: 1900 },
      expected: { answer: "no" }, check: "1900 is divisible by 100 but not by 400, so it is not a leap year" },
    { name: "test vector: ordinary leap year", inputs: { year: 2024 },
      expected: { answer: "yes" }, check: "2024 is divisible by 4 and not by 100" }
  ]
};