import { parseExpression } from "cron-parser";

export function calculateNextScheduledTimestampFromNow(schedule: string, timezone: string | null) {
  return calculateNextScheduledTimestamp(schedule, timezone, new Date());
}

export function calculateNextScheduledTimestamp(
  schedule: string,
  timezone: string | null,
  lastScheduledTimestamp: Date = new Date()
) {
  const nextStep = calculateNextStep(schedule, timezone, lastScheduledTimestamp);

  if (nextStep.getTime() < Date.now()) {
    // If the next step is in the past, we just need to calculate the next step from now
    return calculateNextStep(schedule, timezone, new Date());
  }

  return nextStep;
}

function calculateNextStep(schedule: string, timezone: string | null, currentDate: Date) {
  return parseExpression(schedule, {
    currentDate,
    utc: timezone === null,
    tz: timezone ?? undefined,
  })
    .next()
    .toDate();
}

/**
 * Cron's previous slot relative to `fromTimestamp`. For a continuously-
 * running schedule this equals the actual last fire time; for paused or
 * DST-edge cases it's an approximation. Used only on the recovery path
 * where the actual last fire isn't recoverable from in-flight worker state.
 */
export function previousScheduledTimestamp(
  cron: string,
  timezone: string | null,
  fromTimestamp: Date = new Date()
) {
  return parseExpression(cron, {
    currentDate: fromTimestamp,
    utc: timezone === null,
    tz: timezone ?? undefined,
  })
    .prev()
    .toDate();
}

export function nextScheduledTimestamps(
  cron: string,
  timezone: string | null,
  lastScheduledTimestamp: Date,
  count: number = 1
) {
  const result: Array<Date> = [];
  let nextScheduledTimestamp = lastScheduledTimestamp;

  for (let i = 0; i < count; i++) {
    nextScheduledTimestamp = calculateNextScheduledTimestamp(
      cron,
      timezone,
      nextScheduledTimestamp
    );

    result.push(nextScheduledTimestamp);
  }

  return result;
}