const MONTHS = Object.freeze({
  Jan: 0,
  Feb: 1,
  Mar: 2,
  Apr: 3,
  May: 4,
  Jun: 5,
  Jul: 6,
  Aug: 7,
  Sep: 8,
  Oct: 9,
  Nov: 10,
  Dec: 11
});

function parseDownloadTrafficLogTime(line) {
  if (!line || typeof line !== 'string') return null;

  const match = line.match(/\[(\d{2})\/([A-Za-z]{3})\/(\d{4}):(\d{2}):(\d{2}):(\d{2})\s*([+-]\d{4})?\]/);
  if (!match) return null;

  const month = MONTHS[match[2]];
  if (month === undefined) return null;

  const year = Number(match[3]);
  const day = Number(match[1]);
  const hour = Number(match[4]);
  const minute = Number(match[5]);
  const second = Number(match[6]);
  if ([year, day, hour, minute, second].some((value) => !Number.isFinite(value))) return null;

  let utcMillis = Date.UTC(year, month, day, hour, minute, second);
  const timezone = match[7];
  if (timezone && /^[+-]\d{4}$/.test(timezone)) {
    const sign = timezone[0] === '+' ? 1 : -1;
    const timezoneHours = Number(timezone.slice(1, 3));
    const timezoneMinutes = Number(timezone.slice(3, 5));
    if (timezoneHours > 23 || timezoneMinutes > 59) return null;
    utcMillis -= sign * ((timezoneHours * 60) + timezoneMinutes) * 60 * 1000;
  }

  const parsed = new Date(utcMillis);
  if (Number.isNaN(parsed.getTime())) return null;

  // Date.UTC normalizes impossible dates; reject them instead of charging the wrong day.
  const offsetMinutes = timezone
    ? (timezone[0] === '+' ? 1 : -1) * ((Number(timezone.slice(1, 3)) * 60) + Number(timezone.slice(3, 5)))
    : 0;
  const localDate = new Date(parsed.getTime() + (offsetMinutes * 60 * 1000));
  if (
    localDate.getUTCFullYear() !== year
    || localDate.getUTCMonth() !== month
    || localDate.getUTCDate() !== day
    || localDate.getUTCHours() !== hour
    || localDate.getUTCMinutes() !== minute
    || localDate.getUTCSeconds() !== second
  ) {
    return null;
  }

  return parsed;
}

function parseDownloadTrafficLine(line, fallbackDate = new Date()) {
  if (!line || typeof line !== 'string') return null;

  const trimmed = line.trim();
  if (!trimmed || !/\bGET\b/i.test(trimmed)) return null;

  const statusMatch = trimmed.match(/"\s*(\d{3})\s+(\d+|-)\b/);
  if (!statusMatch) return null;

  const statusCode = Number(statusMatch[1]);
  const bytesSent = statusMatch[2] === '-' ? 0 : Number(statusMatch[2]);
  if (![200, 206].includes(statusCode) || !Number.isFinite(bytesSent) || bytesSent <= 0) return null;

  let objectKey = null;
  const requestMatch = trimmed.match(/"(?:GET|HEAD)\s+([^" ]+)\s+HTTP\//i);
  if (requestMatch?.[1]) {
    let requestPath = requestMatch[1];
    const queryIndex = requestPath.indexOf('?');
    if (queryIndex >= 0) requestPath = requestPath.slice(0, queryIndex);
    requestPath = requestPath.replace(/^https?:\/\/[^/]+/i, '').replace(/^\/+/, '');
    try {
      requestPath = decodeURIComponent(requestPath);
    } catch {
      // Keep the raw path when the log contains malformed percent encoding.
    }
    objectKey = requestPath || null;
  }

  if (!objectKey) {
    const keyMatch = trimmed.match(/\buser_(\d+)\/[^\s"]+/);
    objectKey = keyMatch?.[0] || null;
  }
  if (!objectKey) return null;

  const userMatch = objectKey.match(/(?:^|\/)user_(\d+)\//);
  if (!userMatch) return null;

  const userId = Number(userMatch[1]);
  if (!Number.isSafeInteger(userId) || userId <= 0) return null;

  return {
    userId,
    bytes: Math.floor(bytesSent),
    objectKey,
    eventAt: parseDownloadTrafficLogTime(trimmed) || fallbackDate
  };
}

module.exports = {
  parseDownloadTrafficLine,
  parseDownloadTrafficLogTime
};