package com.box.sdk;

import com.eclipsesource.json.Json;
import com.eclipsesource.json.JsonArray;
import com.eclipsesource.json.JsonObject;
import com.eclipsesource.json.JsonValue;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * Common implementation for paging support.
 *
 * @param <T> type of iterated entity
 */
public abstract class BoxResourceIterable<T> implements Iterable<T> {

  /** Parameter for max page size. */
  public static final String PARAMETER_LIMIT = "limit";

  /** Parameter for marker for the beginning of next page. */
  public static final String PARAMETER_MARKER = "marker";

  /** Body Parameter for marker for the beginning of next page. */
  public static final String BODY_PARAMETER_MARKER_NEXT = "next_marker";

  /** Body parameter for page entries. */
  public static final String BODY_PARAMETER_ENTRIES = "entries";

  /** The API connection to be used by the resource. */
  private final BoxAPIConnection api;

  /** To end-point with paging support. */
  private final URL url;

  /** The maximum number of items to return in a page. */
  private final int limit;

  /** The iterator that gets the next items. */
  private final IteratorImpl iterator;

  /**
   * Constructor.
   *
   * @param api the API connection to be used by the resource
   * @param url endpoint with paging support
   * @param limit the maximum number of items to return in a page
   */
  public BoxResourceIterable(BoxAPIConnection api, URL url, int limit) {
    this(api, url, limit, null, null);
  }

  /**
   * Constructor.
   *
   * @param api the API connection to be used by the resource.
   * @param url to endpoint with paging support.
   * @param limit the maximum number of items to return in a page.
   * @param body the body to send to the requested endpoint.
   */
  public BoxResourceIterable(BoxAPIConnection api, URL url, int limit, JsonObject body) {
    this(api, url, limit, body, null);
  }

  /**
   * Constructor.
   *
   * @param api the API connection to be used by the resource.
   * @param url to endpoint with paging support.
   * @param limit the maximum number of items to return in a page.
   * @param marker the marker where the iterator will begin
   */
  public BoxResourceIterable(BoxAPIConnection api, URL url, int limit, String marker) {
    this(api, url, limit, null, marker);
  }

  /**
   * Constructor.
   *
   * @param api the API connection to be used by the resource.
   * @param url to endpoint with paging support.
   * @param limit the maximum number of items to return in a page.
   * @param body the body to send to the requested endpoint.
   * @param marker the marker where the iterator will begin
   */
  public BoxResourceIterable(
      BoxAPIConnection api, URL url, int limit, JsonObject body, String marker) {
    this.api = api;
    this.url = url;
    this.limit = limit;
    this.iterator = new IteratorImpl(marker, body);
  }

  /**
   * Factory to build a new instance for a received JSON item.
   *
   * @param jsonObject of the item
   * @return the item instance
   */
  protected abstract T factory(JsonObject jsonObject);

  /**
   * Builds internal read-only iterator over {@link BoxResource}-s.
   *
   * @return iterator implementation
   * @see Iterable#iterator()
   */
  @Override
  public Iterator<T> iterator() {
    return this.iterator;
  }

  /**
   * Builds internal read-only iterator over {@link BoxResource}-s.
   *
   * @return iterator implementation
   * @see Iterable#iterator()
   */
  public String getNextMarker() {
    return this.iterator.markerNext;
  }

  /** Paging implementation. */
  private class IteratorImpl implements Iterator<T> {

    /**
     * Base 64 encoded string that represents where the paging should being. It should be left blank
     * to begin paging.
     */
    private String markerNext;

    /** Current loaded page. */
    private JsonArray page;

    /** Cursor within the page (index of a next item for read). */
    private int pageCursor;

    /** The body to include in the request. */
    private JsonObject body;

    /**
     * Constructor.
     *
     * @param marker the marker at which the iterator will begin
     * @param body Request body
     */
    IteratorImpl(String marker, JsonObject body) {
      this.markerNext = marker;
      this.body = body;
      this.loadNextPage();
    }

    /** Loads next page. */
    private void loadNextPage() {
      String existingQuery = BoxResourceIterable.this.url.getQuery();
      QueryStringBuilder builder = new QueryStringBuilder(existingQuery);
      builder.appendParam(PARAMETER_LIMIT, BoxResourceIterable.this.limit);
      if (this.markerNext != null) {
        if (this.body != null) {
          this.body.set("marker", this.markerNext);
        } else {
          builder.appendParam(PARAMETER_MARKER, this.markerNext);
        }
      }

      URL url;
      try {
        url = builder.replaceQuery(BoxResourceIterable.this.url);
      } catch (MalformedURLException e) {
        throw new BoxAPIException("Couldn't append a query string to the provided URL.");
      }

      BoxJSONRequest request;
      if (this.body != null) {
        request = new BoxJSONRequest(BoxResourceIterable.this.api, url, "POST");
        request.setBody(this.body.toString());
      } else {
        request = new BoxJSONRequest(BoxResourceIterable.this.api, url, "GET");
      }

      try (BoxJSONResponse response = request.send()) {
        JsonObject pageBody = Json.parse(response.getJSON()).asObject();

        JsonValue markerNextValue = pageBody.get(BODY_PARAMETER_MARKER_NEXT);
        if (markerNextValue != null && markerNextValue.isString()) {
          this.markerNext = markerNextValue.asString();
        } else {
          this.markerNext = null;
        }

        this.page = pageBody.get(BODY_PARAMETER_ENTRIES).asArray();
        this.pageCursor = 0;
      }
    }

    /** {@inheritDoc} */
    @Override
    public boolean hasNext() {
      if (this.pageCursor < this.page.size()) {
        return true;
      }
      if (this.markerNext == null || this.markerNext.isEmpty()) {
        return false;
      }
      this.loadNextPage();
      return !this.page.isEmpty();
    }

    /** {@inheritDoc} */
    @Override
    public T next() {
      if (!this.hasNext()) {
        throw new NoSuchElementException();
      }

      JsonObject entry = this.page.get(this.pageCursor++).asObject();
      return BoxResourceIterable.this.factory(entry);
    }

    /** @throws UnsupportedOperationException Remove is not supported */
    @Override
    public void remove() {
      throw new UnsupportedOperationException();
    }
  }
}