export function createPortal(
  children: ReactNodeList,
  containerInfo: any,
  // TODO: figure out the API for cross-renderer implementation.
  implementation: any,
  key: ?string = null,
): ReactPortal {
  return {
    // This tag allow us to uniquely identify this as a React Portal
    $$typeof: REACT_PORTAL_TYPE,
    key: key == null ? null : '' + key,
    children,
    containerInfo,
    implementation,
  };
}

function reconcileSinglePortal(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    portal: ReactPortal,
    expirationTime: ExpirationTime,
  ): Fiber {
    const key = portal.key;
    let child = currentFirstChild;
    while (child !== null) {
      // TODO: If key === null and child.key === null, then this only applies to
      // the first item in the list.
      if (child.key === key) {
        if (
          child.tag === HostPortal &&
          child.stateNode.containerInfo === portal.containerInfo &&
          child.stateNode.implementation === portal.implementation
        ) {
          deleteRemainingChildren(returnFiber, child.sibling);
          const existing = useFiber(
            child,
            portal.children || [],
            expirationTime,
          );
          existing.return = returnFiber;
          return existing;
        } else {
          deleteRemainingChildren(returnFiber, child);
          break;
        }
      } else {
        deleteChild(returnFiber, child);
      }
      child = child.sibling;
    }

    const created = createFiberFromPortal(
      portal,
      returnFiber.mode,
      expirationTime,
    );
    created.return = returnFiber;
    return created;
  }

function updatePortalComponent(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime,
) {
  pushHostContainer(workInProgress, workInProgress.stateNode.containerInfo);
  const nextChildren = workInProgress.pendingProps;
  if (current === null) {
    // Portals are special because we don't append the children during mount
    // but at commit. Therefore we need to track insertions which the normal
    // flow doesn't do during mount. This doesn't happen at the root because
    // the root always starts with a "current" with a null child.
    // TODO: Consider unifying this with how the root works.
    workInProgress.child = reconcileChildFibers(
      workInProgress,
      null,
      nextChildren,
      renderExpirationTime,
    );
  } else {
    reconcileChildren(
      current,
      workInProgress,
      nextChildren,
      renderExpirationTime,
    );
  }
  return workInProgress.child;
}