/**
 * Pure helpers for the timeline device-filter "All / None" behavior.
 *
 * The timeline auto-selects every device the first time a device list loads so
 * the default view shows all data. The subtle bug this guards against (OPL-2983)
 * is that a naive "auto-select when nothing is selected" effect also re-fires
 * the instant the user clicks "None" — silently re-selecting everything, so the
 * None button appears to do nothing.
 *
 * We disambiguate "not yet initialized" from "user deliberately cleared" by
 * remembering the signature of the device list we last initialized for. An
 * empty selection whose list signature still matches is a deliberate "None";
 * an empty selection for a list we have not initialized yet is a fresh load.
 */

/** Order-independent signature for a device list, keyed by id. */
export function deviceListSignature(deviceIds: readonly string[]): string {
  return [...deviceIds].sort().join(",");
}

/**
 * Decide the initial device selection when a device list appears or changes.
 *
 * Returns `null` when no action is needed — either the list is empty or we have
 * already initialized for this exact list (so an empty selection is a
 * deliberate "None" and must be left alone). Otherwise returns the new
 * signature to remember plus `select`: the ids to auto-select (all of them) for
 * a fresh load, or `null` to record the signature without changing a selection
 * that was restored from the URL/prior state.
 */
export function resolveDeviceAutoSelect(params: {
  deviceIds: readonly string[];
  selectedCount: number;
  initializedSignature: string | null;
}): { signature: string; select: string[] | null } | null {
  const { deviceIds, selectedCount, initializedSignature } = params;
  if (deviceIds.length === 0) return null;
  const signature = deviceListSignature(deviceIds);
  if (signature === initializedSignature) return null;
  return { signature, select: selectedCount > 0 ? null : [...deviceIds] };
}

/**
 * Whether the user has deliberately deselected every device (the timeline
 * should render empty). True only once the list has been initialized — so the
 * brief pre-auto-select render on first load is not mistaken for "None".
 */
export function isNoDevicesSelected(params: {
  deviceCount: number;
  selectedCount: number;
  currentSignature: string;
  initializedSignature: string | null;
}): boolean {
  const { deviceCount, selectedCount, currentSignature, initializedSignature } =
    params;
  return (
    deviceCount > 0 &&
    selectedCount === 0 &&
    initializedSignature === currentSignature
  );
}