import * as CronParser from 'cron-parser';
import * as x from 'x-value';

export const cronExpressionSymbol = Symbol();

export const CronExpression = x.atomic(cronExpressionSymbol, value => {
  if (typeof value !== 'string') {
    throw new TypeError('Expected a cron expression string.');
  }

  getCronExpressionValue(value);
});

export type CronExpression = x.Nominal<'cron expression', string>;

export function getCronExpressionValue(value: string): string {
  const groups = /^cron\((.+)\)$/.exec(value);

  if (!groups) {
    throw new TypeError(
      'Expected a cron expression string like `cron(0 0 * * *)`.',
    );
  }

  const [, pattern] = groups;

  CronParser.parseExpression(pattern);

  return pattern;
}