BillTracker/client/lib/reorder.ts

interface Identified {
  id: string | number;
}

export function moveInArray<T>(items: T[], fromIndex: number, toIndex: number): T[] {
  if (!Array.isArray(items)) return [];
  if (fromIndex === toIndex || fromIndex < 0 || toIndex < 0 || fromIndex >= items.length || toIndex >= items.length) {
    return items;
  }
  const next = [...items];
  const [moved] = next.splice(fromIndex, 1);
  if (moved === undefined) return next; // in-range by the guard above; satisfies noUncheckedIndexedAccess
  next.splice(toIndex, 0, moved);
  return next;
}

export function reorderPayload(items: ReadonlyArray<Identified> | null | undefined): Record<string, number> {
  return Object.fromEntries((items || []).map((item, index) => [item.id, index]));
}

export function movedItemId(
  before: ReadonlyArray<Identified> | null | undefined,
  after: ReadonlyArray<Identified> | null | undefined,
): string | number | null {
  const moved = (after || []).find((item, index) => item.id !== before?.[index]?.id);
  return moved?.id || after?.[0]?.id || null;
}