"use strict";
// Day-number kernel: Howard Hinnant, "chrono-Compatible Low-Level Date Algorithms",
// https://howardhinnant.github.io/date_algorithms.html (days_from_civil). Fixed-offset
// arithmetic only: no time zone database, no host clock, no Date.now().
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})$/.exec(String(s).trim());
  if (!m) throw new Error((label || "date") + " must be an ISO 8601 date YYYY-MM-DD, got: " + s);
  const year = parseInt(m[1], 10), month = parseInt(m[2], 10), day = parseInt(m[3], 10);
  if (month < 1 || month > 12) throw new Error((label || "date") + " month out of range 1-12");
  if (day < 1 || day > daysInMonth(year, month)) throw new Error((label || "date") + " day out of range for that month");
  return { year, month, day };
}
function parseTime(s) {
  const m = /^(\d{2}):(\d{2})(?::(\d{2}))?$/.exec(String(s).trim());
  if (!m) throw new Error("time must be HH:MM or HH:MM:SS, got: " + s);
  const hh = parseInt(m[1], 10), mm = parseInt(m[2], 10), ss = m[3] ? parseInt(m[3], 10) : 0;
  if (hh > 23 || mm > 59 || ss > 59) throw new Error("time fields out of range: " + s);
  return hh * 3600 + mm * 60 + ss;
}
function parseOffset(s) {
  const m = /^([+-])(\d{2}):(\d{2})$/.exec(String(s).trim());
  if (!m) throw new Error("offset must be like -05:00 or +01:00, got: " + s);
  const sign = m[1] === "-" ? -1 : 1;
  const hh = parseInt(m[2], 10), mm = parseInt(m[3], 10);
  if (hh > 23 || mm > 59) throw new Error("offset fields out of range: " + s);
  return sign * (hh * 3600 + mm * 60);
}
function instantSeconds(dateStr, timeStr, offsetStr, label) {
  const d = parseISODate(dateStr, label + "_date");
  const local = parseTime(timeStr);
  const off = parseOffset(offsetStr);
  return daysFromCivil(d.year, d.month, d.day) * 86400 + local - off;
}

module.exports = {
  slug: "flight-duration-across-zones",
  title: "Flight duration across time zones",
  deck_category: "timezones",
  description: "Computes elapsed travel time from a departure local date/time/offset to an arrival local date/time/offset.",
  inputs: [
    { name: "departure_date", type: "string", format: "date", required: true, description: "ISO 8601 departure date, YYYY-MM-DD, local to departure_offset" },
    { name: "departure_time", type: "string", required: true, description: "Departure local wall-clock time, HH:MM or HH:MM:SS" },
    { name: "departure_offset", type: "string", required: true, description: "Fixed UTC offset at departure, e.g. -05:00" },
    { name: "arrival_date", type: "string", format: "date", required: true, description: "ISO 8601 arrival date, YYYY-MM-DD, local to arrival_offset" },
    { name: "arrival_time", type: "string", required: true, description: "Arrival local wall-clock time, HH:MM or HH:MM:SS" },
    { name: "arrival_offset", type: "string", required: true, description: "Fixed UTC offset at arrival, e.g. +01:00" }
  ],
  outputs: [
    { name: "hours", description: "Whole hours of elapsed time" },
    { name: "minutes", description: "Remaining minutes of elapsed time" }
  ],
  steps: [
    "Parse the departure date, time and offset into a single UTC instant, in seconds: day-number (Hinnant days_from_civil, cited above) times 86400, plus local seconds-since-midnight, minus the offset in seconds.",
    "Do the same for the arrival date, time and offset to get the arrival UTC instant.",
    "elapsed_seconds = arrival_utc_instant - departure_utc_instant; require this to be positive.",
    "hours = floor(elapsed_seconds / 3600); minutes = floor((elapsed_seconds mod 3600) / 60)."
  ],
  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 to place both wall-clock times on a single UTC instant line; no time zone database is used" }
  ],
  disclaimer: null,
  questionText(inputs) {
    return "Departing " + inputs.departure_date + " " + inputs.departure_time + " " + inputs.departure_offset +
      " and arriving " + inputs.arrival_date + " " + inputs.arrival_time + " " + inputs.arrival_offset +
      ", how long was the flight?";
  },
  run(inputs) {
    const dep = instantSeconds(inputs.departure_date, inputs.departure_time, inputs.departure_offset, "departure");
    const arr = instantSeconds(inputs.arrival_date, inputs.arrival_time, inputs.arrival_offset, "arrival");
    const elapsed = arr - dep;
    if (elapsed <= 0) throw new Error("arrival instant must be after departure instant");
    const hours = Math.floor(elapsed / 3600);
    const minutes = Math.floor((elapsed % 3600) / 60);
    const answer = hours + "h " + minutes + "m";
    return { answer, values: { hours, minutes } };
  },
  examples: [
    { name: "worked example", inputs: { departure_date: "2026-09-30", departure_time: "08:00:00", departure_offset: "-05:00", arrival_date: "2026-09-30", arrival_time: "21:35:00", arrival_offset: "+01:00" },
      expected: { answer: "7h 35m" },
      check: "08:00 -05:00 is 13:00 UTC; 21:35 +01:00 is 20:35 UTC; 20:35 minus 13:00 is 7 hours 35 minutes" },
    { name: "test vector: overnight, crosses a calendar date", inputs: { departure_date: "2026-03-01", departure_time: "22:00:00", departure_offset: "-05:00", arrival_date: "2026-03-02", arrival_time: "11:30:00", arrival_offset: "+01:00" },
      expected: { answer: "7h 30m" },
      check: "22:00 -05:00 on Mar 1 is 03:00 UTC Mar 2; 11:30 +01:00 on Mar 2 is 10:30 UTC Mar 2; 10:30 minus 03:00 is 7 hours 30 minutes" },
    { name: "test vector: both local times identical but different offsets", inputs: { departure_date: "2026-05-01", departure_time: "09:00:00", departure_offset: "+09:00", arrival_date: "2026-05-01", arrival_time: "09:00:00", arrival_offset: "-08:00" },
      expected: { answer: "17h 0m" },
      check: "09:00 +09:00 is 00:00 UTC; 09:00 -08:00 is 17:00 UTC on the same UTC date; 17:00 minus 00:00 is 17 hours" }
  ]
};