"use strict";
// Meeus/Jones/Butcher algorithm for the date of Western (Gregorian) Easter, as given in
// Jean Meeus, "Astronomical Algorithms", 2nd ed. (Willmann-Bell, 1998), chapter 8. No host
// clock, no Date.now(), no time zone database; pure integer arithmetic on the year.
function easterGregorian(year) {
  const a = year % 19;
  const b = Math.floor(year / 100);
  const c = year % 100;
  const d = Math.floor(b / 4);
  const e = b % 4;
  const f = Math.floor((b + 8) / 25);
  const g = Math.floor((b - f + 1) / 3);
  const h = (19 * a + b - d - g + 15) % 30;
  const i = Math.floor(c / 4);
  const k = c % 4;
  const l = (32 + 2 * e + 2 * i - h - k) % 7;
  const m = Math.floor((a + 11 * h + 22 * l) / 451);
  const month = Math.floor((h + l - 7 * m + 114) / 31);
  const day = ((h + l - 7 * m + 114) % 31) + 1;
  return { year, month, day };
}
function pad2(n) { return String(n).padStart(2, "0"); }

module.exports = {
  slug: "easter-date-gregorian",
  title: "Date of Easter (Western, Gregorian)",
  deck_category: "dates",
  description: "Computes the date of Western (Gregorian calendar) Easter Sunday for a given year using the Meeus/Jones/Butcher algorithm.",
  inputs: [
    { name: "year", type: "integer", required: true, min: 1583, description: "Gregorian calendar year (the algorithm is valid from 1583 onward, after the 1582 Gregorian reform)" }
  ],
  outputs: [ { name: "easter_date", format: "date", description: "ISO 8601 date of Western Easter Sunday" } ],
  steps: [
    "a = year mod 19.",
    "b = floor(year / 100); c = year mod 100.",
    "d = floor(b / 4); e = b mod 4.",
    "f = floor((b + 8) / 25); g = floor((b - f + 1) / 3).",
    "h = (19a + b - d - g + 15) mod 30.",
    "i = floor(c / 4); k = c mod 4.",
    "l = (32 + 2e + 2i - h - k) mod 7.",
    "m = floor((a + 11h + 22l) / 451).",
    "month = floor((h + l - 7m + 114) / 31); day = ((h + l - 7m + 114) mod 31) + 1."
  ],
  constants: [],
  sources: [
    { kind: "isbn", value: "978-0943396613", title: "Astronomical Algorithms, 2nd edition, chapter 8 (Easter)", publisher: "Willmann-Bell (Jean Meeus)", retrieved_utc: "2026-09-30T19:41:35Z", note: "the Meeus/Jones/Butcher algorithm for the Gregorian (Western) Easter date, valid from 1583 onward" }
  ],
  disclaimer: null,
  questionText(inputs) {
    return "What is the date of Western Easter Sunday in " + inputs.year + "?";
  },
  run(inputs) {
    if (!Number.isInteger(inputs.year) || inputs.year < 1583) throw new Error("year must be an integer of 1583 or later (the Gregorian reform)");
    const e = easterGregorian(inputs.year);
    const answer = e.year + "-" + pad2(e.month) + "-" + pad2(e.day);
    return { answer, values: { easter_date: answer } };
  },
  examples: [
    { name: "worked example", inputs: { year: 2026 },
      expected: { answer: "2026-04-05" }, check: "2026-04-05 is the widely published date of Easter Sunday for 2026" },
    { name: "test vector: year 2000", inputs: { year: 2000 },
      expected: { answer: "2000-04-23" }, check: "2000-04-23 is the widely published date of Easter Sunday for 2000" },
    { name: "test vector: year 2024", inputs: { year: 2024 },
      expected: { answer: "2024-03-31" }, check: "2024-03-31 is the widely published date of Easter Sunday for 2024" }
  ]
};