Calendar_Frontend/lib/url-state.ts

import { calendarById, calendarBySlug, DEFAULT_VISIBLE_CALENDAR_IDS } from "@/lib/calendars";
import { RANGES, SORTS, type Range, type Sort } from "@/lib/filter";

/**
 * The admin list's view state, kept in the query string so a link reproduces
 * a view: `?cal=public,choir&range=past&sort=start_desc&q=probe`.
 *
 * Defaults are left out of the URL, so the plain `/admin` link means "the
 * default view". Anything unrecognised falls back to its default instead of
 * failing - these URLs get pasted into chats and bookmarked.
 *
 * `cal=` with an empty value is kept as "no calendars selected"; only a
 * missing `cal` means the default selection.
 *
 * The default sort depends on the range: looking back ("Vergangen") starts
 * with the most recent event, everything else with the earliest. A sort that
 * matches its range's default stays out of the URL, so switching ranges
 * carries the right default along; a sort someone picked is kept.
 */

export interface ListState {
  calendarIds: number[];
  range: Range;
  sort: Sort;
  query: string;
}

export const DEFAULT_LIST_STATE: ListState = {
  calendarIds: [...DEFAULT_VISIBLE_CALENDAR_IDS],
  range: "future",
  sort: "start_asc",
  query: "",
};

export const defaultSortFor = (range: Range): Sort => (range === "past" ? "start_desc" : "start_asc");

/**
 * The state after switching to `range`: a sort that was only the old range's
 * default becomes the new range's default; one that someone picked stays.
 */
export const withRange = (state: ListState, range: Range): ListState => ({
  ...state,
  range,
  sort: state.sort === defaultSortFor(state.range) ? defaultSortFor(range) : state.sort,
});

const sameIds = (a: readonly number[], b: readonly number[]): boolean => {
  const sortedB = [...b].sort();
  return a.length === b.length && [...a].sort().every((id, i) => id === sortedB[i]);
};

export const parseListState = (params: URLSearchParams): ListState => {
  const cal = params.get("cal");
  const calendarIds =
    cal === null
      ? [...DEFAULT_LIST_STATE.calendarIds]
      : [
          ...new Set(
            cal
              .split(",")
              .map((slug) => calendarBySlug(slug.trim())?.id)
              .filter((id): id is number => id !== undefined),
          ),
        ];

  const rawRange = params.get("range");
  const rawSort = params.get("sort");
  const range = RANGES.includes(rawRange as Range) ? (rawRange as Range) : DEFAULT_LIST_STATE.range;

  return {
    calendarIds,
    range,
    sort: SORTS.includes(rawSort as Sort) ? (rawSort as Sort) : defaultSortFor(range),
    query: params.get("q") ?? "",
  };
};

export const serializeListState = (state: ListState): URLSearchParams => {
  const params = new URLSearchParams();
  if (!sameIds(state.calendarIds, DEFAULT_LIST_STATE.calendarIds)) {
    params.set(
      "cal",
      state.calendarIds
        .map((id) => calendarById(id)?.slug)
        .filter(Boolean)
        .join(","),
    );
  }
  if (state.range !== DEFAULT_LIST_STATE.range) {
    params.set("range", state.range);
  }
  if (state.sort !== defaultSortFor(state.range)) {
    params.set("sort", state.sort);
  }
  if (state.query.trim() !== "") {
    params.set("q", state.query);
  }
  return params;
};

/**
 * The query string for the address bar, without the leading "?". Commas stay
 * literal (`cal=public,choir`) because these URLs are meant to be read and
 * shared; URLSearchParams would write `%2C`. A comma inside the search text
 * is escaped by hand first, so it cannot be mistaken for a separator - and
 * the parser only splits `cal` anyway.
 */
export const toQueryString = (state: ListState): string =>
  serializeListState(state)
    .toString()
    .replace(/(^|&)cal=([^&]*)/, (_, lead: string, value: string) => `${lead}cal=${value.replace(/%2C/gi, ",")}`);