/**
 * Closure periods.
 *
 * A vacation period is a source of truth, never expanded into exception rows:
 * the resolver reads it directly. That is why editing one can never orphan
 * anything, and why overlapping periods are worth refusing rather than
 * silently merging — two rows covering the same day would both be "in effect"
 * with no way to say which.
 */

import { civilDaysBetween, compareCivil, type CivilDate } from './civil';

/** Longer than a year is a data-entry accident, not a closure. */
export const MAX_PERIOD_DAYS = 366;

export type Period = {
  startDate: CivilDate;
  endDate: CivilDate;
};

export type NamedPeriod = Period & { id: string; labelFr: string };

/** Inclusive on both ends, as the shop would read it off a note on the door. */
export function countDays(period: Period): number {
  return civilDaysBetween(period.startDate, period.endDate) + 1;
}

export function periodsOverlap(a: Period, b: Period): boolean {
  return compareCivil(a.startDate, b.endDate) <= 0 && compareCivil(b.startDate, a.endDate) <= 0;
}

/** The first existing period a candidate would collide with, if any. */
export function findOverlap(candidate: Period, existing: readonly NamedPeriod[]): NamedPeriod | null {
  return existing.find((period) => periodsOverlap(candidate, period)) ?? null;
}

export function validatePeriod(input: {
  startDate: CivilDate;
  endDate: CivilDate;
  labelFr: string;
}): string[] {
  const errors: string[] = [];

  if (!input.startDate || !input.endDate) {
    errors.push('Indiquez une date de début et une date de fin.');
    return errors;
  }
  if (compareCivil(input.startDate, input.endDate) > 0) {
    errors.push('La date de fin précède la date de début.');
  } else if (countDays(input) > MAX_PERIOD_DAYS) {
    errors.push(`Une période ne peut pas dépasser ${MAX_PERIOD_DAYS} jours.`);
  }
  if (input.labelFr.trim().length === 0) {
    errors.push('Donnez un libellé à cette période (il s’affiche sur l’écran).');
  }

  return errors;
}

/** Periods still to come or in progress, soonest first. */
export function upcomingPeriods<T extends Period>(periods: readonly T[], today: CivilDate): T[] {
  return periods
    .filter((period) => compareCivil(period.endDate, today) >= 0)
    .sort((a, b) => compareCivil(a.startDate, b.startDate));
}