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libp2p is a p2p network SDK implemented in java language.

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package org.tron.p2p.dns.sync;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import lombok.Getter;
import lombok.Setter;
import lombok.extern.slf4j.Slf4j;


@Slf4j(topic = "net")
public class LinkCache {

  @Getter
  Map> backrefs;
  @Getter
  @Setter
  private boolean changed; //if data in backrefs changes, we need to rebuild trees

  public LinkCache() {
    backrefs = new HashMap<>();
    changed = false;
  }

  // check if the urlScheme occurs in other trees
  public boolean isContainInOtherLink(String urlScheme) {
    return backrefs.containsKey(urlScheme) && !backrefs.get(urlScheme).isEmpty();
  }

  /**
   * add the reference to backrefs
   *
   * @param parent the url tree that contains url tree `children`
   * @param children url tree
   */
  public void addLink(String parent, String children) {
    Set refs = backrefs.getOrDefault(children, new HashSet<>());
    if (!refs.contains(parent)) {
      changed = true;
    }
    refs.add(parent);
    backrefs.put(children, refs);
  }

  /**
   * clears all links of the given tree.
   *
   * @param from tree's urlScheme
   * @param keep links contained in this tree
   */
  public void resetLinks(String from, final Set keep) {
    List stk = new ArrayList<>();
    stk.add(from);

    while (!stk.isEmpty()) {
      int size = stk.size();
      String item = stk.get(size - 1);
      stk = stk.subList(0, size - 1);

      Iterator>> it = backrefs.entrySet().iterator();
      while (it.hasNext()) {
        Entry> entry = it.next();
        String r = entry.getKey();
        Set refs = entry.getValue();
        if ((keep != null && keep.contains(r)) || !refs.contains(item)) {
          continue;
        }
        this.changed = true;
        refs.remove(item);
        if (refs.isEmpty()) {
          it.remove();
          stk.add(r);
        }
      }
    }
  }
}