/**
 * Pure helpers for manual page reordering in the sidebar (issue #45). The
 * server recomputes the moved page's `sort_key` between two neighbours, so the
 * client only has to name them. Given the current order and where the page
 * should land, these compute the `afterId`/`beforeId` the position endpoint
 * expects — kept separate from the component so they are unit-testable.
 */

/** The moved page's new neighbours when it lands at `newIndex` in the list with
 * itself removed. `afterId` is its predecessor (null at the top), `beforeId`
 * its successor (null at the bottom). */
export function neighborsForMove(
  orderedIds: string[],
  movedId: string,
  newIndex: number,
): { afterId: string | null; beforeId: string | null } {
  const without = orderedIds.filter((id) => id !== movedId);
  const clamped = Math.max(0, Math.min(newIndex, without.length));
  return {
    afterId: clamped > 0 ? without[clamped - 1]! : null,
    beforeId: clamped < without.length ? without[clamped]! : null,
  };
}

/** Target index for a drop onto `targetId`: before it, or after it when the
 * pointer is over the item's lower half (so the very bottom is reachable). */
export function dropIndex(
  orderedIds: string[],
  movedId: string,
  targetId: string,
  after: boolean,
): number {
  const without = orderedIds.filter((id) => id !== movedId);
  const base = without.indexOf(targetId);
  if (base === -1) return without.length;
  return after ? base + 1 : base;
}