import { useCallback, useMemo, useState } from 'react';

/**
 * Which rows a person has ticked, and the header checkbox that follows.
 *
 * Shared by the three screens that grew a bulk action — importing from the
 * catalogue, requesting stock, and deciding requests — because the fiddly parts
 * are the same every time and getting them subtly different between screens is
 * how a merchant learns to distrust the tick boxes.
 *
 * ── The part that is easy to get wrong ──────────────────────────────────────
 *
 * A selection is kept against the ROWS CURRENTLY VISIBLE. Filter a list down,
 * tick everything, clear the filter, and press the button: a naive
 * implementation acts on rows the person could not see when they chose. So
 * "select all" only ever covers what is on screen, and `chosen` is intersected
 * with the visible ids before it is handed back.
 *
 * Ids that scroll out of view are NOT dropped from the set, because narrowing a
 * search and widening it again should not silently untick the work. They are
 * simply not acted on while they are hidden.
 */
export interface Selection {
  /** Visible ids that are ticked — what a bulk action should act on. */
  chosen: number[];
  count: number;
  has: (id: number) => boolean;
  toggle: (id: number) => void;
  /** Tick or untick everything currently visible. */
  toggleAll: () => void;
  clear: () => void;
  /** Every visible row is ticked. Drives the header checkbox. */
  allChosen: boolean;
  /** Some but not all — the indeterminate state. */
  someChosen: boolean;
}

export function useSelection(visibleIds: readonly number[]): Selection {
  const [picked, setPicked] = useState<ReadonlySet<number>>(() => new Set());

  const chosen = useMemo(
    () => visibleIds.filter((id) => picked.has(id)),
    [visibleIds, picked],
  );

  const toggle = useCallback((id: number) => {
    setPicked((prev) => {
      const next = new Set(prev);
      if (next.has(id)) {
        next.delete(id);
      } else {
        next.add(id);
      }
      return next;
    });
  }, []);

  const allChosen = visibleIds.length > 0 && chosen.length === visibleIds.length;

  const toggleAll = useCallback(() => {
    setPicked((prev) => {
      const next = new Set(prev);
      const everyVisibleChosen =
        visibleIds.length > 0 && visibleIds.every((id) => next.has(id));
      for (const id of visibleIds) {
        if (everyVisibleChosen) {
          next.delete(id);
        } else {
          next.add(id);
        }
      }
      return next;
    });
  }, [visibleIds]);

  const clear = useCallback(() => setPicked(new Set()), []);

  return {
    chosen,
    count: chosen.length,
    has: (id: number) => picked.has(id),
    toggle,
    toggleAll,
    clear,
    allChosen,
    someChosen: chosen.length > 0 && !allChosen,
  };
}