"use strict";
// Day-number kernel: Howard Hinnant, "chrono-Compatible Low-Level Date Algorithms",
// https://howardhinnant.github.io/date_algorithms.html (days_from_civil). The weekday is
// derived as the day-number modulo 7, calibrated against the known fact that 1970-01-01
// (day-number 0 in this kernel) was a Thursday, and cross-checked against 2000-01-01 (a
// Saturday). 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 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 };
}
const WEEKDAY_NAMES = ["Thursday", "Friday", "Saturday", "Sunday", "Monday", "Tuesday", "Wednesday"];
function weekdayName(y, m, d) {
  const z = daysFromCivil(y, m, d);
  return WEEKDAY_NAMES[((z % 7) + 7) % 7];
}

module.exports = {
  slug: "weekday-of-date",
  title: "Weekday of a date",
  deck_category: "dates",
  description: "Computes the day of the week for any proleptic Gregorian calendar date.",
  inputs: [
    { name: "date", type: "string", format: "date", required: true, description: "ISO 8601 date, YYYY-MM-DD" }
  ],
  outputs: [ { name: "weekday", description: "Name of the day of the week" } ],
  steps: [
    "Parse the date as a proleptic Gregorian civil date (year, month, day).",
    "Compute its day-number using the Hinnant days_from_civil algorithm (equivalent to a Julian Day Number offset), which counts whole days since 1970-01-01.",
    "Take the day-number modulo 7. 1970-01-01 has day-number 0 and was a Thursday, so the remainder indexes into the cycle starting at Thursday.",
    "Map the remainder to a weekday name using that calibrated cycle."
  ],
  constants: [
    { name: "anchor_date", value: "1970-01-01", unit: "date", source: 0 },
    { name: "anchor_weekday", value: "Thursday", unit: "weekday name", 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 day-number kernel; 1970-01-01 is day-number 0 and is independently known to have been a Thursday" }
  ],
  disclaimer: null,
  questionText(inputs) {
    return "What day of the week is " + inputs.date + "?";
  },
  run(inputs) {
    const d = parseISODate(inputs.date, "date");
    const weekday = weekdayName(d.year, d.month, d.day);
    return { answer: weekday, values: { weekday } };
  },
  examples: [
    { name: "worked example", inputs: { date: "2000-01-01" },
      expected: { answer: "Saturday" }, check: "2000-01-01 being a Saturday is a widely documented calendar fact" },
    { name: "test vector: Unix epoch", inputs: { date: "1970-01-01" },
      expected: { answer: "Thursday" }, check: "1970-01-01, the Unix epoch, is independently documented as a Thursday" },
    { name: "test vector: recent date", inputs: { date: "2024-01-01" },
      expected: { answer: "Monday" }, check: "2024-01-01 being a Monday is a widely documented calendar fact" }
  ]
};