export type DropPlacement = "before" | "after";

/** Devuelve un orden nuevo o ausencia de cambio; nunca modifica la colección recibida. */
export function reorderItems<T>(
  items: readonly T[],
  sourceIndex: number,
  targetIndex: number,
  placement: DropPlacement
): T[] | undefined {
  if (
    !(Number.isInteger(sourceIndex) && Number.isInteger(targetIndex)) ||
    sourceIndex < 0 ||
    targetIndex < 0 ||
    sourceIndex >= items.length ||
    targetIndex >= items.length
  ) {
    return undefined;
  }

  const insertionIndex = targetIndex + (placement === "after" ? 1 : 0);
  const destination = insertionIndex - (sourceIndex < insertionIndex ? 1 : 0);

  if (sourceIndex === destination) {
    return undefined;
  }

  const next = [...items];
  const removed = next.splice(sourceIndex, 1);

  next.splice(destination, 0, ...removed);

  return next;
}