'use strict';

const moment = require('moment-timezone');
const tzLookup = require('tz-lookup');

function isFiniteNumber(value) {
  return typeof value === 'number' && Number.isFinite(value);
}

function parseCompactDateTime(dateTimeString) {
  const match = /^([0-9]{8})T([0-9]{6})$/.exec(dateTimeString || '');
  if (!match) return null;

  return {
    datePart: match[1],
    timePart: match[2]
  };
}

function resolveTimezoneName(lat, lon) {
  if (!isFiniteNumber(lat) || !isFiniteNumber(lon)) return 'UTC';

  try {
    return tzLookup(lat, lon) || 'UTC';
  } catch (err) {
    return 'UTC';
  }
}

function getUtcOffsetMinutesFromLocation(lat, lon, dateLabel) {
  const timezoneName = resolveTimezoneName(lat, lon);
  if (timezoneName === 'UTC') return 0;

  const referenceDate = dateLabel
    ? moment.utc(dateLabel, ['YYYYMMDD', 'YYYY-MM-DD'], true).add(12, 'hours')
    : moment.utc();

  if (!referenceDate.isValid()) return 0;

  return moment.tz(referenceDate.toDate(), timezoneName).utcOffset();
}

function getDateLabelFromDateTime(dateTimeString) {
  if (!dateTimeString) return null;

  const compact = parseCompactDateTime(dateTimeString);
  if (compact) return compact.datePart;

  const isoMoment = moment.utc(dateTimeString, moment.ISO_8601, true);
  if (isoMoment.isValid()) return isoMoment.format('YYYYMMDD');

  return null;
}

function toUtcDateFromAppDateTime(dateTimeString, utcOffsetMinutes = 0) {
  if (!dateTimeString) return null;

  const compact = parseCompactDateTime(dateTimeString);
  if (!compact) {
    const isoMoment = moment.utc(dateTimeString, moment.ISO_8601, true);
    if (isoMoment.isValid()) return isoMoment.toDate();
    return null;
  }

  const hours   = Number(compact.timePart.slice(0, 2));
  const minutes = Number(compact.timePart.slice(2, 4));
  const seconds = Number(compact.timePart.slice(4, 6));
  const utcSecondsOfDay = (hours * 3600) + (minutes * 60) + seconds;

  // AgNav hybrid format:
  //   datePart  = LOCAL mission date assigned by the device (not derived from GPS)
  //   timePart  = GPS UTC time-of-day (already UTC, no conversion needed)
  //
  // timePart is placed directly onto a UTC date.  The only question is: which UTC
  // calendar date?  Compute how timePart translates to local time; if the result
  // crosses midnight the UTC date must be shifted by ±1 day relative to datePart:
  //
  //   localSecondsOfDay = utcSecondsOfDay + offsetSeconds
  //   < 0      → local rolled back to previous day → UTC date = datePart − 1 day
  //   ≥ 86400  → local rolled forward to next day  → UTC date = datePart + 1 day
  //   otherwise→ UTC and local calendar dates align → no shift
  const offsetSeconds = utcOffsetMinutes * 60;
  const localSecondsOfDay = utcSecondsOfDay + offsetSeconds;

  let dateShift = 0;
  if (localSecondsOfDay < 0)      dateShift = -1; // local rolled back  → UTC one day earlier
  if (localSecondsOfDay >= 86400) dateShift = +1; // local rolled forward → UTC one day later

  return moment.utc(compact.datePart, 'YYYYMMDD', true)
    .add(dateShift, 'days')
    .add(utcSecondsOfDay, 'seconds')
    .toDate();
}

function buildApplicationDateFields({ startDateTime, endDateTime, latitude, longitude }) {
  const dateLabel = getDateLabelFromDateTime(startDateTime || endDateTime);
  const utcOffset = getUtcOffsetMinutesFromLocation(latitude, longitude, dateLabel);

  let startDateTimeUTC = toUtcDateFromAppDateTime(startDateTime, utcOffset);
  let endDateTimeUTC = toUtcDateFromAppDateTime(endDateTime, utcOffset);

  // Sanity guard: startDateTimeUTC must never exceed endDateTimeUTC.
  // With the correct dateShift logic above this should not occur for valid data.
  // Kept as a safety net for genuinely corrupt input (e.g. file contains timestamps
  // from two different days with the start record from a later day than the end record).
  if (startDateTimeUTC && endDateTimeUTC && startDateTimeUTC > endDateTimeUTC) {
    startDateTimeUTC = new Date(startDateTimeUTC.getTime() - 86400000);
  }

  return { utcOffset, startDateTimeUTC, endDateTimeUTC };
}

module.exports = {
  resolveTimezoneName,
  getUtcOffsetMinutesFromLocation,
  getDateLabelFromDateTime,
  toUtcDateFromAppDateTime,
  buildApplicationDateFields
};