"use strict";
// Day-number kernel: Howard Hinnant, "chrono-Compatible Low-Level Date Algorithms",
// https://howardhinnant.github.io/date_algorithms.html (days_from_civil / civil_from_days).
// Verified against 1970-01-01 -> day-number 0 and 2000-01-01 -> day-number 10957 (30 years
// including 7 intervening leap years: 1972, 76, 80, 84, 88, 92, 96). 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;
}
function civilFromDays(z) {
  z += 719468;
  const era = Math.floor((z >= 0 ? z : z - 146096) / 146097);
  const doe = z - era * 146097;
  const yoe = Math.floor((doe - Math.floor(doe / 1460) + Math.floor(doe / 36524) - Math.floor(doe / 146096)) / 365);
  const y = yoe + era * 400;
  const doy = doe - (365 * yoe + Math.floor(yoe / 4) - Math.floor(yoe / 100));
  const mp = Math.floor((5 * doy + 2) / 153);
  const d = doy - Math.floor((153 * mp + 2) / 5) + 1;
  const m = mp + (mp < 10 ? 3 : -9);
  const yy = y + (m <= 2 ? 1 : 0);
  return { year: yy, month: m, day: d };
}
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 };
}
function pad2(n) { return String(n).padStart(2, "0"); }
function toISODate(y, m, d) {
  const ys = y < 0 ? "-" + String(-y).padStart(4, "0") : String(y).padStart(4, "0");
  return ys + "-" + pad2(m) + "-" + pad2(d);
}

module.exports = {
  slug: "add-days-to-date",
  title: "Add days to a date",
  deck_category: "dates",
  description: "Adds (or subtracts, for a negative count) a whole number of days to a calendar date.",
  inputs: [
    { name: "date", type: "string", format: "date", required: true, description: "ISO 8601 starting date, YYYY-MM-DD" },
    { name: "days", type: "integer", required: true, description: "Number of days to add; negative subtracts days" }
  ],
  outputs: [ { name: "result_date", format: "date", description: "The resulting ISO 8601 date" } ],
  steps: [
    "Parse the date as a proleptic Gregorian civil date.",
    "Convert it to a day-number using the Hinnant days_from_civil algorithm (cited above).",
    "Add the signed integer days to the day-number.",
    "Convert the resulting day-number back to a civil (year, month, day) using the inverse civil_from_days algorithm.",
    "Format the result as an ISO 8601 date."
  ],
  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: "days_from_civil / civil_from_days, verified round-trip against 1970-01-01 and 2000-01-01" }
  ],
  disclaimer: null,
  questionText(inputs) {
    const n = inputs.days;
    return "What date is " + Math.abs(n) + (n >= 0 ? " days after " : " days before ") + inputs.date + "?";
  },
  run(inputs) {
    const d = parseISODate(inputs.date, "date");
    if (!Number.isInteger(inputs.days)) throw new Error("days must be an integer");
    const z = daysFromCivil(d.year, d.month, d.day) + inputs.days;
    const r = civilFromDays(z);
    const answer = toISODate(r.year, r.month, r.day);
    return { answer, values: { result_date: answer } };
  },
  examples: [
    { name: "worked example", inputs: { date: "2024-01-01", days: 31 },
      expected: { answer: "2024-02-01" }, check: "January has 31 days, so 31 days after Jan 1 lands on Feb 1" },
    { name: "test vector: leap day backward", inputs: { date: "2024-03-01", days: -1 },
      expected: { answer: "2024-02-29" }, check: "2024 is a leap year, so the day before March 1, 2024 is Feb 29, 2024" },
    { name: "test vector: year rollover", inputs: { date: "2023-12-31", days: 1 },
      expected: { answer: "2024-01-01" }, check: "the day after Dec 31 is always Jan 1 of the next year" }
  ]
};