"use strict";
// Julian Day Number from a proleptic Gregorian civil date, via Howard Hinnant's
// "chrono-Compatible Low-Level Date Algorithms", https://howardhinnant.github.io/date_algorithms.html
// (days_from_civil), offset by the JDN of the Hinnant epoch (1970-01-01 = JDN 2440588). This
// is arithmetically equivalent to the Fliegel and Van Flandern (1968) integer JDN algorithm
// ("A Machine Algorithm for Processing Calendar Dates", Communications of the ACM 11(10):657).
// Verified against the well-known fact that 2000-01-01 is JDN 2451545 (the J2000.0 epoch date).
// No host clock, no Date.now(), no time zone database.
function daysFromCivil(y, m, d) {
  y = m <= 2 ? y - 1 : y;
  const era = Math.floor((y >= 0 ? y : y - 399) / 400);
  const yoe = y - era * 400;
  const doy = Math.floor((153 * (m + (m > 2 ? -3 : 9)) + 2) / 5) + d - 1;
  const doe = yoe * 365 + Math.floor(yoe / 4) - Math.floor(yoe / 100) + doy;
  return era * 146097 + doe - 719468;
}
const EPOCH_JDN = 2440588;
function toJDN(y, m, d) { return daysFromCivil(y, m, d) + EPOCH_JDN; }
function isLeapGregorian(y) { return (y % 4 === 0 && y % 100 !== 0) || (y % 400 === 0); }
function daysInMonth(y, m) {
  const dim = [31, isLeapGregorian(y) ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
  return dim[m - 1];
}
function parseISODate(s, label) {
  const m = /^(-?\d{4})-(\d{2})-(\d{2})$/.exec(String(s).trim());
  if (!m) throw new Error((label || "date") + " must be an ISO 8601 date YYYY-MM-DD, got: " + s);
  const year = parseInt(m[1], 10), month = parseInt(m[2], 10), day = parseInt(m[3], 10);
  if (month < 1 || month > 12) throw new Error((label || "date") + " month out of range 1-12");
  if (day < 1 || day > daysInMonth(year, month)) throw new Error((label || "date") + " day out of range for that month");
  return { year, month, day };
}

module.exports = {
  slug: "julian-day-number",
  title: "Julian Day Number of a civil date",
  deck_category: "dates",
  description: "Converts a proleptic Gregorian civil date to its Julian Day Number (JDN).",
  inputs: [
    { name: "date", type: "string", format: "date", required: true, description: "ISO 8601 date, YYYY-MM-DD" }
  ],
  outputs: [ { name: "jdn", description: "Julian Day Number, an integer day count with no calendar gaps" } ],
  steps: [
    "Parse the date as a proleptic Gregorian civil date.",
    "Convert it to a day-number using Hinnant's days_from_civil algorithm, which is arithmetically equivalent to the Fliegel and Van Flandern (1968) integer JDN formula.",
    "Add 2440588 (the JDN of 1970-01-01, the algorithm's epoch) to obtain the Julian Day Number."
  ],
  constants: [
    { name: "epoch_jdn_1970_01_01", value: 2440588, unit: "JDN", source: 0 }
  ],
  sources: [
    { kind: "url", value: "https://howardhinnant.github.io/date_algorithms.html", title: "chrono-Compatible Low-Level Date Algorithms", publisher: "Howard Hinnant", retrieved_utc: "2026-09-30T19:41:35Z", note: "days_from_civil; verified against the well-known fact that 2000-01-01 is JDN 2451545" },
    { kind: "doi", value: "10.1145/364096.364097", title: "A Machine Algorithm for Processing Calendar Dates", publisher: "Fliegel, H. F.; Van Flandern, T. C., Communications of the ACM 11(10):657 (1968)", retrieved_utc: "2026-09-30T19:41:35Z", note: "the original integer civil-date-to-JDN algorithm this method is arithmetically equivalent to" }
  ],
  disclaimer: null,
  questionText(inputs) {
    return "What is the Julian Day Number of " + inputs.date + "?";
  },
  run(inputs) {
    const d = parseISODate(inputs.date, "date");
    const jdn = toJDN(d.year, d.month, d.day);
    return { answer: String(jdn), values: { jdn } };
  },
  examples: [
    { name: "worked example", inputs: { date: "2000-01-01" },
      expected: { answer: "2451545" }, check: "2451545 is the well-known Julian Day Number for 2000-01-01 (the J2000.0 epoch date)" },
    { name: "test vector: Unix epoch", inputs: { date: "1970-01-01" },
      expected: { answer: "2440588" }, check: "2440588 is the well-known Julian Day Number for the Unix epoch date, 1970-01-01" },
    { name: "test vector: recent date", inputs: { date: "2024-01-01" },
      expected: { answer: "2460311" }, check: "computed independently as 2451545 + 8766 elapsed days between 2000-01-01 and 2024-01-01 (24 years of 365 days plus 6 intervening leap days: 2000, 04, 08, 12, 16, 20)" }
  ]
};