/**
 * @author e.matsyuk
 */
public class PaginationTool {

    // for first start of items loading then on RecyclerView there are not items and no scrolling
    private static final int EMPTY_LIST_ITEMS_COUNT = 0;
    // default limit for requests
    private static final int DEFAULT_LIMIT = 50;
    // default max attempts to retry loading request
    private static final int MAX_ATTEMPTS_TO_RETRY_LOADING = 3;

    public static <T> Observable<List<T>> paging(RecyclerView recyclerView, PagingListener<T> pagingListener) {
        return paging(recyclerView, pagingListener, DEFAULT_LIMIT, EMPTY_LIST_ITEMS_COUNT, MAX_ATTEMPTS_TO_RETRY_LOADING);
    }

    public static <T> Observable<List<T>> paging(RecyclerView recyclerView, PagingListener<T> pagingListener, int limit) {
        return paging(recyclerView, pagingListener, limit, EMPTY_LIST_ITEMS_COUNT, MAX_ATTEMPTS_TO_RETRY_LOADING);
    }

    public static <T> Observable<List<T>> paging(RecyclerView recyclerView, PagingListener<T> pagingListener, int limit, int emptyListCount) {
        return paging(recyclerView, pagingListener, limit, emptyListCount, MAX_ATTEMPTS_TO_RETRY_LOADING);
    }

    public static <T> Observable<List<T>> paging(RecyclerView recyclerView, PagingListener<T> pagingListener, int limit, int emptyListCount, int retryCount) {
        if (recyclerView == null) {
            throw new PagingException("null recyclerView");
        }
        if (recyclerView.getAdapter() == null) {
            throw new PagingException("null recyclerView adapter");
        }
        if (limit <= 0) {
            throw new PagingException("limit must be greater then 0");
        }
        if (emptyListCount < 0) {
            throw new PagingException("emptyListCount must be not less then 0");
        }
        if (retryCount < 0) {
            throw new PagingException("retryCount must be not less then 0");
        }

        int startNumberOfRetryAttempt = 0;
        return getScrollObservable(recyclerView, limit, emptyListCount)
                .subscribeOn(AndroidSchedulers.mainThread())
                .distinctUntilChanged()
                .observeOn(Schedulers.from(BackgroundExecutor.getSafeBackgroundExecutor()))
                .switchMap(offset -> getPagingObservable(pagingListener, pagingListener.onNextPage(offset), startNumberOfRetryAttempt, offset, retryCount));
    }

    private static Observable<Integer> getScrollObservable(RecyclerView recyclerView, int limit, int emptyListCount) {
        return Observable.create(subscriber -> {
            final RecyclerView.OnScrollListener sl = new RecyclerView.OnScrollListener() {
                @Override
                public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
                    if (!subscriber.isUnsubscribed()) {
                        int position = getLastVisibleItemPosition(recyclerView);
                        int updatePosition = recyclerView.getAdapter().getItemCount() - 1 - (limit / 2);
                        if (position >= updatePosition) {
                            subscriber.onNext(recyclerView.getAdapter().getItemCount());
                        }
                    }
                }
            };
            recyclerView.addOnScrollListener(sl);
            subscriber.add(Subscriptions.create(() -> recyclerView.removeOnScrollListener(sl)));
            if (recyclerView.getAdapter().getItemCount() == emptyListCount) {
                subscriber.onNext(recyclerView.getAdapter().getItemCount());
            }
        });
    }

    private static int getLastVisibleItemPosition(RecyclerView recyclerView) {
        Class recyclerViewLMClass = recyclerView.getLayoutManager().getClass();
        if (recyclerViewLMClass == LinearLayoutManager.class || LinearLayoutManager.class.isAssignableFrom(recyclerViewLMClass)) {
            LinearLayoutManager linearLayoutManager = (LinearLayoutManager)recyclerView.getLayoutManager();
            return linearLayoutManager.findLastVisibleItemPosition();
        } else if (recyclerViewLMClass == StaggeredGridLayoutManager.class || StaggeredGridLayoutManager.class.isAssignableFrom(recyclerViewLMClass)) {
            StaggeredGridLayoutManager staggeredGridLayoutManager = (StaggeredGridLayoutManager)recyclerView.getLayoutManager();
            int[] into = staggeredGridLayoutManager.findLastVisibleItemPositions(null);
            List<Integer> intoList = new ArrayList<>();
            for (int i : into) {
                intoList.add(i);
            }
            return Collections.max(intoList);
        }
        throw new PagingException("Unknown LayoutManager class: " + recyclerViewLMClass.toString());
    }

    private static <T> Observable<List<T>> getPagingObservable(PagingListener<T> listener, Observable<List<T>> observable, int numberOfAttemptToRetry, int offset, int retryCount) {
        return observable.onErrorResumeNext(throwable -> {
            // retry to load new data portion if error occurred
            if (numberOfAttemptToRetry < retryCount) {
                int attemptToRetryInc = numberOfAttemptToRetry + 1;
                return getPagingObservable(listener, listener.onNextPage(offset), attemptToRetryInc, offset, retryCount);
            } else {
                return Observable.empty();
            }
        });
    }

}

/**
 * @author e.matsyuk
 */
public class PagingException extends RuntimeException {

    public PagingException(String detailMessage) {
        super(detailMessage);
    }

}

/**
 * @author e.matsyuk
 */
public interface PagingListener<T> {
    Observable<List<T>> onNextPage(int offset);
}

/**
 * A placeholder fragment containing a simple view.
 */
public class PaginationFragment extends Fragment {

    private final static int LIMIT = 50;
    private PagingRecyclerViewAdapter recyclerViewAdapter;
    private Subscription pagingSubscription;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fmt_pagination, container, false);
        setRetainInstance(true);
        init(rootView, savedInstanceState);
        return rootView;
    }

    @Override
    public void onResume() {
        super.onResume();
    }

    private void init(View view, Bundle savedInstanceState) {
        RecyclerView recyclerView = (RecyclerView) view.findViewById(R.id.RecyclerView);
        GridLayoutManager recyclerViewLayoutManager = new GridLayoutManager(getActivity(), 1);
        recyclerViewLayoutManager.supportsPredictiveItemAnimations();
        // init adapter for the first time
        if (savedInstanceState == null) {
            recyclerViewAdapter = new PagingRecyclerViewAdapter();
            recyclerViewAdapter.setHasStableIds(true);
        }

        recyclerView.setLayoutManager(recyclerViewLayoutManager);
        recyclerView.setAdapter(recyclerViewAdapter);
        // if all items was loaded we don't need Pagination
        if (recyclerViewAdapter.isAllItemsLoaded()) {
            return;
        }
        // RecyclerView pagination
        pagingSubscription = PaginationTool
                .paging(recyclerView, offset -> EmulateResponseManager.getInstance().getEmulateResponse(offset, LIMIT), LIMIT)
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Subscriber<List<Item>>() {
                    @Override
                    public void onCompleted() {
                    }

                    @Override
                    public void onError(Throwable e) {
                    }

                    @Override
                    public void onNext(List<Item> items) {
                        recyclerViewAdapter.addNewItems(items);
                        recyclerViewAdapter.notifyItemInserted(recyclerViewAdapter.getItemCount() - items.size());
                    }
                });
    }

    @Override
    public void onDestroyView() {
        if (pagingSubscription != null && !pagingSubscription.isUnsubscribed()) {
            pagingSubscription.unsubscribe();
        }
        super.onDestroyView();
    }

}

/**
 * @author e.matsyuk
 */
public class PagingRecyclerViewAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> {

    private static final int MAIN_VIEW = 0;

    private List<Item> listElements = new ArrayList<>();
    // after reorientation test this member
    // or one extra request will be sent after each reorientation
    private boolean allItemsLoaded;

    static class MainViewHolder extends RecyclerView.ViewHolder {

        TextView textView;

        public MainViewHolder(View itemView) {
            super(itemView);
            textView = (TextView) itemView.findViewById(R.id.text);
        }
    }

    public void addNewItems(List<Item> items) {
        if (items.size() == 0) {
            allItemsLoaded = true;
            return;
        }
        listElements.addAll(items);
    }

    public boolean isAllItemsLoaded() {
        return allItemsLoaded;
    }

    @Override
    public long getItemId(int position) {
        return getItem(position).getId();
    }

    public Item getItem(int position) {
        return listElements.get(position);
    }

    @Override
    public int getItemCount() {
        return listElements.size();
    }

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        if (viewType == MAIN_VIEW) {
            View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.recycler_item, parent, false);
            return new MainViewHolder(v);
        }
        return null;
    }

    @Override
    public int getItemViewType(int position) {
        return MAIN_VIEW;
    }

    @Override
    public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
        switch (getItemViewType(position)) {
            case MAIN_VIEW:
                onBindTextHolder(holder, position);
                break;
        }
    }

    private void onBindTextHolder(RecyclerView.ViewHolder holder, int position) {
        MainViewHolder mainHolder = (MainViewHolder) holder;
        mainHolder.textView.setText(getItem(position).getItemStr());
    }

}