"use strict";
// Day-number kernel: Howard Hinnant, "chrono-Compatible Low-Level Date Algorithms",
// https://howardhinnant.github.io/date_algorithms.html (days_from_civil). 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: "countdown-to-date",
  title: "Countdown to a date",
  deck_category: "dates",
  description: "Computes the whole number of days, and the equivalent weeks and days, from one date to another.",
  inputs: [
    { name: "from_date", type: "string", format: "date", required: true, description: "ISO 8601 starting date, YYYY-MM-DD" },
    { name: "to_date", type: "string", format: "date", required: true, description: "ISO 8601 target date, YYYY-MM-DD, must be on or after from_date" }
  ],
  outputs: [
    { name: "total_days", unit: "days", description: "Whole days from from_date to to_date" },
    { name: "weeks", unit: "weeks", description: "Whole weeks in the total" },
    { name: "extra_days", unit: "days", description: "Days left over after the whole weeks" }
  ],
  steps: [
    "Parse from_date and to_date as proleptic Gregorian civil dates and convert both to Julian Day Numbers (Hinnant days_from_civil, cited above).",
    "Require to_date on or after from_date; otherwise the inputs are invalid.",
    "total_days = JDN(to_date) - JDN(from_date).",
    "weeks = floor(total_days / 7); extra_days = total_days mod 7."
  ],
  constants: [],
  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: "day-number kernel used for the subtraction" }
  ],
  disclaimer: null,
  questionText(inputs) {
    return "How many days, weeks and days are there from " + inputs.from_date + " to " + inputs.to_date + "?";
  },
  run(inputs) {
    const a = parseISODate(inputs.from_date, "from_date");
    const b = parseISODate(inputs.to_date, "to_date");
    const jdnA = toJDN(a.year, a.month, a.day);
    const jdnB = toJDN(b.year, b.month, b.day);
    if (jdnB < jdnA) throw new Error("to_date must not be before from_date");
    const totalDays = jdnB - jdnA;
    const weeks = Math.floor(totalDays / 7);
    const extraDays = totalDays % 7;
    const answer = totalDays + " days (" + weeks + " weeks and " + extraDays + " days)";
    return { answer, values: { total_days: totalDays, weeks, extra_days: extraDays } };
  },
  examples: [
    { name: "worked example", inputs: { from_date: "2026-09-30", to_date: "2026-12-25" },
      expected: { answer: "86 days (12 weeks and 2 days)" },
      check: "independently: Sep has 30-30=0 remaining, Oct 31 + Nov 30 + Dec 25 = 86 days; 86 = 12*7 + 2" },
    { name: "test vector: same date", inputs: { from_date: "2026-01-01", to_date: "2026-01-01" },
      expected: { answer: "0 days (0 weeks and 0 days)" }, check: "a date has zero days until itself" },
    { name: "test vector: across a leap day", inputs: { from_date: "2024-02-01", to_date: "2024-03-01" },
      expected: { answer: "29 days (4 weeks and 1 days)" },
      check: "2024 is a leap year so February has 29 days; 29 = 4*7 + 1" }
  ]
};