/**
 * The id one step from `currentId` in board order, wrapping at both ends.
 *
 * @param orderedIds - Pairing ids in board order.
 * @param currentId - The currently focal id to step from.
 * @param direction - +1 for next, -1 for previous.
 * @returns The neighbouring id (wrapped), `currentId` unchanged if it isn't in
 * the list, or null for an empty list.
 */
export function nextFocalId(orderedIds: string[], currentId: string, direction: 1 | -1): string | null {
    if (orderedIds.length === 0) {
        return null;
    }
    const i = orderedIds.indexOf(currentId);
    if (i === -1) {
        return currentId;
    }
    const len = orderedIds.length;
    const next = (i + direction + len) % len;
    return orderedIds[next];
}