import CronExpressionParser from 'cron-parser';
import { validate } from 'node-cron';
import { ScheduleType } from '../interface/schedule';

const aliases: Record<string, string> = {
  '@yearly': '0 0 0 1 1 *',
  '@annually': '0 0 0 1 1 *',
  '@monthly': '0 0 0 1 * *',
  '@weekly': '0 0 0 * * 0',
  '@daily': '0 0 0 * * *',
  '@midnight': '0 0 0 * * *',
  '@hourly': '0 0 * * * *',
  '@minutely': '0 * * * * *',
};

export interface CronSchedule {
  patterns: string[];
  dayCount: number;
  nthDay: number;
}

// Validation and registration repeatedly see identical rules in large snapshots.
// Cache only immutable syntax, never dates or running tasks, and bound user input.
const scheduleCache = new Map<string, CronSchedule>();
const daysInMonth = [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

// Canonicalize legacy shorthand, aliases, names and numeric-start steps before
// node-cron sees them. Extend legacy aliases explicitly; reject H and bare /N.
export function parseCronSchedule(schedule: unknown): CronSchedule {
  if (typeof schedule !== 'string' || !schedule.trim()) {
    throw new Error('Invalid cron schedule');
  }
  let source = schedule.trim();
  const cacheKey = source;
  const cached = scheduleCache.get(cacheKey);
  if (cached) return cached;
  if (source.startsWith('@')) {
    if (!Object.hasOwn(aliases, source)) throw new Error('Invalid cron alias');
    source = aliases[source];
  }
  let parts = source.split(/\s+/);
  if (
    parts.length > 6 ||
    parts.some((part) => /(^|,)\//.test(part) || /H(?:\(|\/|$)/i.test(part))
  ) {
    throw new Error('Unsupported cron syntax');
  }
  parts = [
    ...['0', '*', '*', '*', '*', '*'].slice(0, 6 - parts.length),
    ...parts,
  ];
  if (
    parts.some(
      (part, index) => index !== 3 && index !== 5 && part.includes('?'),
    )
  ) {
    throw new Error('Question mark is only valid in day fields');
  }
  const expression = CronExpressionParser.parse(parts.join(' '));
  if (!expression.hasNext())
    throw new Error('Cron schedule has no next execution');
  let normalized = parts.map((part) => part.replace(/\?/g, '*'));
  // cron-parser includes Sunday in stepped ranges ending in 7 even when the
  // step does not land on 7. node-cron accepts those ranges with different days.
  if (/(^|,)(?:\d+|[a-z]{3})-0*7\/\d+(?=,|$)/i.test(normalized[5])) {
    normalized[5] = expression.fields.dayOfWeek.values.join(',');
  }
  // Most rules are already supported; only inspect individual fields on failure.
  if (!validate(normalized.join(' '))) {
    const fields = [
      expression.fields.second,
      expression.fields.minute,
      expression.fields.hour,
      expression.fields.dayOfMonth,
      expression.fields.month,
      expression.fields.dayOfWeek,
    ];
    normalized = normalized.map((value, index) => {
      // Preserve supported lists, ranges and names. Expand only fields node-cron
      // cannot consume, such as numeric/name-start steps; never compress them.
      const probe = ['0', '0', '0', '*', '*', '*'];
      probe[index] = value;
      if (validate(probe.join(' '))) return value;
      const values = fields[index].values;
      if (index === 5 && expression.fields.dayOfWeek.nthDay) {
        // The legacy normalizer treated *#N / ?#N as an unrestricted weekday.
        if (values.length === 8) return '*';
        // # applies to a single weekday; normalize its numeric spelling only.
        return values.length === 1
          ? `${values[0]}#${expression.fields.dayOfWeek.nthDay}`
          : value;
      }
      return values.join(',');
    });
  }
  const months = expression.fields.month.values;
  // cron-parser 4 clipped dates outside a single selected month's maximum
  // before deciding whether the day field covered the whole month.
  const dayCount = expression.fields.dayOfMonth.values.filter(
    (day) =>
      months.length !== 1 ||
      typeof day !== 'number' ||
      day <= daysInMonth[months[0] - 1],
  ).length;
  const weekCount = expression.fields.dayOfWeek.values.length;
  // Preserve the legacy count-based wildcard rules, including repeated Sunday
  // and L tokens: an ignored day field must not remain an AND constraint.
  const weekdayPattern = [
    ...normalized.slice(0, 3),
    '*',
    ...normalized.slice(4),
  ].join(' ');
  const patterns =
    weekCount === 8
      ? [[...normalized.slice(0, 5), '*'].join(' ')]
      : dayCount < 31
      ? [[...normalized.slice(0, 5), '*'].join(' '), weekdayPattern]
      : [weekdayPattern];
  const valid = patterns.map((pattern) => validate(pattern));
  if (patterns.length === 2 && !valid[0] && valid[1]) {
    // A date branch can be impossible for all chosen months while the weekday
    // branch still runs. Drop it only if its syntax is valid with any month.
    if (validate([...normalized.slice(0, 4), '*', '*'].join(' '))) {
      patterns.shift();
      valid.shift();
    }
  }
  if (!valid.every(Boolean)) {
    throw new Error('Unsupported cron schedule');
  }
  const parsed = {
    patterns,
    dayCount,
    nthDay: expression.fields.dayOfWeek.nthDay,
  };
  Object.freeze(patterns);
  Object.freeze(parsed);
  if (scheduleCache.size >= 512)
    scheduleCache.delete(scheduleCache.keys().next().value);
  scheduleCache.set(cacheKey, parsed);
  return parsed;
}

export function isValidCronSchedule(schedule: unknown): schedule is string {
  try {
    parseCronSchedule(schedule);
    return true;
  } catch {
    return false;
  }
}

export function getInvalidCronSchedules(cron: {
  schedule?: string;
  extra_schedules?: Array<{ schedule: string }>;
}): unknown[] {
  if (
    cron.schedule?.startsWith(ScheduleType.ONCE) ||
    cron.schedule?.startsWith(ScheduleType.BOOT)
  ) {
    return [];
  }
  return [
    cron.schedule,
    ...(cron.extra_schedules || []).map((x) => x.schedule),
  ].filter((schedule) => !isValidCronSchedule(schedule));
}