/**
 * Deriving a one-off day from the usual one.
 *
 * "Opening late today" and "closing early today" are the two changes the shop
 * actually makes, and both are relative to whatever that day normally does —
 * including days with a lunch break, where clipping the wrong slot produces
 * hours nobody meant. Pure, so every shape is tested.
 */

import { addCivilDays, compareCivil, minutesOfTime, type CivilDate } from './civil';
import type { Slot } from './types';

/** A range longer than this is a holiday period, not an exception. */
export const MAX_RANGE_DAYS = 92;

/**
 * The day's hours if it opens at `from` instead of its usual time.
 *
 * Slots that finish before the new opening disappear — a shop opening at 14:00
 * does not keep its 10:00-13:00 morning — and the slot straddling it is
 * trimmed rather than dropped.
 */
export function lateOpening(slots: readonly Slot[], from: string): Slot[] {
  const opensAt = minutesOfTime(from);
  return slots
    .filter((slot) => minutesOfTime(slot.close) > opensAt)
    .map((slot) => (minutesOfTime(slot.open) < opensAt ? { ...slot, open: from } : { ...slot }));
}

/**
 * The day's hours if it closes at `until` instead of its usual time.
 *
 * The mirror of `lateOpening`: slots starting after the new closing time go,
 * and the one straddling it is trimmed.
 */
export function earlyClosing(slots: readonly Slot[], until: string): Slot[] {
  const closesAt = minutesOfTime(until);
  return slots
    .filter((slot) => minutesOfTime(slot.open) < closesAt)
    .map((slot) => (minutesOfTime(slot.close) > closesAt ? { ...slot, close: until } : { ...slot }));
}

/**
 * Every date in `[start, end]`, inclusive.
 *
 * Bounded: a typo in a year field should produce an error, not thirty thousand
 * rows.
 */
export function expandRange(start: CivilDate, end: CivilDate): CivilDate[] {
  if (compareCivil(start, end) > 0) {
    return [];
  }
  const dates: CivilDate[] = [];
  let cursor = start;
  while (compareCivil(cursor, end) <= 0) {
    dates.push(cursor);
    if (dates.length > MAX_RANGE_DAYS) {
      return [];
    }
    cursor = addCivilDays(cursor, 1);
  }
  return dates;
}

export type RangeProblem = 'EMPTY' | 'REVERSED' | 'TOO_LONG' | null;

export function checkRange(start: CivilDate, end: CivilDate): RangeProblem {
  if (!start || !end) {
    return 'EMPTY';
  }
  if (compareCivil(start, end) > 0) {
    return 'REVERSED';
  }
  if (expandRange(start, end).length === 0) {
    return 'TOO_LONG';
  }
  return null;
}