"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).
// Weekday parity (to find Saturday/Sunday) uses the day-number modulo 7, calibrated against
// 1970-01-01 (day-number 0) being a Thursday. No host clock, no Date.now(), no time zone
// database, and NO holiday calendar: this method excludes weekends only.
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);
}
// ISO weekday number, Monday=1 .. Sunday=7 (ISO 8601), via day-number mod 7 calibrated
// against 1970-01-01 (day-number 0) being a Thursday (ISO weekday 4).
function isoWeekdayFromZ(z) {
  const idx = ((z % 7) + 7) % 7; // 0=Thursday
  return ((idx + 3) % 7) + 1; // shift so Thursday(idx0) -> 4, Friday(1)->5, ... Wed(6)->3
}
function isWeekend(z) {
  const iso = isoWeekdayFromZ(z);
  return iso === 6 || iso === 7;
}

module.exports = {
  slug: "add-business-days",
  title: "Add business days to a date",
  deck_category: "dates",
  description: "Adds (or subtracts) a count of Monday-to-Friday business days to a date, treating Saturday and Sunday as non-business days and excluding NO holidays.",
  inputs: [
    { name: "date", type: "string", format: "date", required: true, description: "ISO 8601 starting date, YYYY-MM-DD" },
    { name: "business_days", type: "integer", required: true, description: "Number of business days to add; negative subtracts business days" }
  ],
  outputs: [ { name: "result_date", format: "date", description: "The resulting ISO 8601 date" } ],
  steps: [
    "Parse the date as a proleptic Gregorian civil date and convert it to a day-number (Hinnant days_from_civil, cited above).",
    "Business-day convention used here: Monday through Friday are business days; Saturday and Sunday are not. This method does NOT exclude any holidays, only weekends.",
    "Step the day-number one day at a time in the sign of business_days (forward for positive, backward for negative).",
    "Each step, determine the ISO 8601 weekday (Monday=1 .. Sunday=7) from the day-number modulo 7; if it is not Saturday (6) or Sunday (7), count it against the remaining business-day total.",
    "Stop once the requested number of business days has been counted and convert the final day-number back to a civil 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: "day-number kernel used for stepping and weekday parity" },
    { kind: "standard", value: "ISO 8601:2004", title: "Data elements and interchange formats -- Information interchange -- Representation of dates and times", publisher: "International Organization for Standardization", retrieved_utc: "2026-09-30T19:41:35Z", note: "defines weekday numbering Monday=1 through Sunday=7, used to identify the Saturday/Sunday weekend excluded by this method" }
  ],
  disclaimer: null,
  questionText(inputs) {
    const n = inputs.business_days;
    return "What date is " + Math.abs(n) + (n >= 0 ? " business days after " : " business days before ") +
      inputs.date + " (Monday-Friday only, no holidays excluded)?";
  },
  run(inputs) {
    const d = parseISODate(inputs.date, "date");
    if (!Number.isInteger(inputs.business_days)) throw new Error("business_days must be an integer");
    let z = daysFromCivil(d.year, d.month, d.day);
    const step = inputs.business_days >= 0 ? 1 : -1;
    let remaining = Math.abs(inputs.business_days);
    while (remaining > 0) {
      z += step;
      if (!isWeekend(z)) remaining -= 1;
    }
    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: "2026-10-01", business_days: 5 },
      expected: { answer: "2026-10-08" },
      check: "2026-10-01 is a Thursday; counting forward Fri 10-02(1), skip Sat/Sun, Mon 10-05(2), Tue 10-06(3), Wed 10-07(4), Thu 10-08(5)" },
    { name: "test vector: starting on a Saturday", inputs: { date: "2026-10-03", business_days: 1 },
      expected: { answer: "2026-10-05" },
      check: "2026-10-03 is a Saturday; the next business day, skipping Sunday 10-04, is Monday 2026-10-05" },
    { name: "test vector: subtracting across a weekend", inputs: { date: "2026-10-06", business_days: -3 },
      expected: { answer: "2026-10-01" },
      check: "2026-10-06 is a Tuesday; counting back Mon 10-05(1), skip Sun/Sat, Fri 10-02(2), Thu 10-01(3)" }
  ]
};