const maxDockerUnixNanoseconds = 9_223_372_036_854_775_807n;
const canonicalNanoseconds = /^(0|[1-9][0-9]*)$/u;
const rfc3339Nano = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(?:\.(\d{1,9}))?(Z|[+-]\d{2}:\d{2})$/u;

/** Docker's UnixNano value is signed 64-bit; never reconstruct it from a JS number. */
export const parseDockerUnixNanoseconds = (valueArg: unknown, contextArg: string): bigint => {
  if (typeof valueArg !== 'string' || !canonicalNanoseconds.test(valueArg)) {
    throw new Error(`${contextArg} must be a canonical nonnegative Unix-nanosecond integer.`);
  }
  const value = BigInt(valueArg);
  if (value > maxDockerUnixNanoseconds) {
    throw new Error(`${contextArg} exceeds Docker's signed 64-bit Unix-nanosecond range.`);
  }
  return value;
};

/** Parse Docker's RFC3339Nano StartedAt without discarding fractional nanoseconds. */
export const parseDockerStartedAtNanoseconds = (valueArg: unknown): bigint => {
  if (typeof valueArg !== 'string') {
    throw new Error('Docker container StartedAt is missing or malformed.');
  }
  const match = rfc3339Nano.exec(valueArg);
  if (!match) {
    throw new Error('Docker container StartedAt is missing or malformed.');
  }
  const [, yearText, monthText, dayText, hourText, minuteText, secondText, fractionalText = '', zone] = match;
  const year = Number(yearText);
  const month = Number(monthText);
  const day = Number(dayText);
  const hour = Number(hourText);
  const minute = Number(minuteText);
  const second = Number(secondText);
  const leapYear = year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);
  const daysInMonth = [31, leapYear ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
  const zoneHour = zone === 'Z' ? 0 : Number(zone.slice(1, 3));
  const zoneMinute = zone === 'Z' ? 0 : Number(zone.slice(4, 6));
  if (
    month < 1 || month > 12 || day < 1 || day > daysInMonth[month - 1]
    || hour > 23 || minute > 59 || second > 59
    || zoneHour > 23 || zoneMinute > 59
  ) {
    throw new Error('Docker container StartedAt is missing or malformed.');
  }
  const wholeSeconds = `${yearText}-${monthText}-${dayText}T${hourText}:${minuteText}:${secondText}${zone}`;
  const milliseconds = Date.parse(wholeSeconds);
  if (!Number.isSafeInteger(milliseconds)) {
    throw new Error('Docker container StartedAt is outside the supported timestamp range.');
  }
  const nanoseconds = BigInt(milliseconds) * 1_000_000n + BigInt(fractionalText.padEnd(9, '0') || '0');
  if (nanoseconds < 0n || nanoseconds > maxDockerUnixNanoseconds) {
    // Includes Docker's 0001-01-01T00:00:00Z sentinel for a never-started container.
    throw new Error('Docker container StartedAt does not identify a started runtime.');
  }
  return nanoseconds;
};