/*
    * Copyright (C) 2016, Google Inc. and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
    */
  
  package org.eclipse.jgit.internal.ketch;
  
  import static org.eclipse.jgit.internal.ketch.KetchReplica.State.AHEAD;
  import static org.eclipse.jgit.internal.ketch.KetchReplica.State.DIVERGENT;
  import static org.eclipse.jgit.internal.ketch.KetchReplica.State.LAGGING;
  import static org.eclipse.jgit.internal.ketch.KetchReplica.State.UNKNOWN;
  import static org.eclipse.jgit.lib.Constants.OBJ_COMMIT;
  
  import java.io.IOException;
  import java.util.Collections;
  import java.util.Map;
  
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.lib.AnyObjectId;
  import org.eclipse.jgit.lib.ObjectId;
  import org.eclipse.jgit.lib.Ref;
  import org.eclipse.jgit.lib.Repository;
  import org.eclipse.jgit.revwalk.RevCommit;
  import org.eclipse.jgit.revwalk.RevWalk;
  import org.eclipse.jgit.transport.ReceiveCommand;
  
  /**
   * A helper to check if a {@link KetchReplica} is ahead or behind the leader.
   */
  class LagCheck implements AutoCloseable {
  	private final KetchReplica replica;
  	private final Repository repo;
  	private RevWalk rw;
  	private ObjectId remoteId;
  
  	LagCheck(KetchReplica replica, Repository repo) {
  		this.replica = replica;
  		this.repo = repo;
  		initRevWalk();
  	}
  
  	private void initRevWalk() {
  		if (rw != null) {
  			rw.close();
  		}
  
  		rw = new RevWalk(repo);
  		rw.setRetainBody(false);
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	public void close() {
  		if (rw != null) {
  			rw.close();
  			rw = null;
  		}
  	}
  
  	ObjectId getRemoteId() {
  		return remoteId;
  	}
  
  	KetchReplica.State check(ObjectId acceptId, ReceiveCommand acceptCmd) {
  		remoteId = acceptId;
  		if (remoteId == null) {
  			// Nothing advertised by the replica, value is unknown.
  			return UNKNOWN;
  		}
  
  		if (AnyObjectId.isEqual(remoteId, ObjectId.zeroId())) {
  			// Replica does not have the txnAccepted reference.
  			return LAGGING;
  		}
  
  		try {
  			RevCommit remote;
  			try {
  				remote = parseRemoteCommit(acceptCmd.getRefName());
  			} catch (RefGoneException gone) {
  				// Replica does not have the txnAccepted reference.
  				return LAGGING;
  			} catch (MissingObjectException notFound) {
  				// Local repository does not know this commit so it cannot
  				// be including the replica's log.
  				return DIVERGENT;
  			}
  
  			RevCommit head = rw.parseCommit(acceptCmd.getNewId());
  			if (rw.isMergedInto(remote, head)) {
  				return LAGGING;
  			}
  
  			// TODO(sop) Check term to see if my leader was deposed.
 			if (rw.isMergedInto(head, remote)) {
 				return AHEAD;
 			}
 			return DIVERGENT;
 		} catch (IOException err) {
 			KetchReplica.log.error(String.format(
 					"Cannot compare %s", //$NON-NLS-1$
 					acceptCmd.getRefName()), err);
 			return UNKNOWN;
 		}
 	}
 
 	private RevCommit parseRemoteCommit(String refName)
 			throws IOException, MissingObjectException, RefGoneException {
 		try {
 			return rw.parseCommit(remoteId);
 		} catch (MissingObjectException notLocal) {
 			// Fall through and try to acquire the object by fetching it.
 		}
 
 		ReplicaFetchRequest fetch = new ReplicaFetchRequest(
 				Collections.singleton(refName),
 				Collections.<ObjectId> emptySet());
 		try {
 			replica.blockingFetch(repo, fetch);
 		} catch (IOException fetchErr) {
 			KetchReplica.log.error(String.format(
 					"Cannot fetch %s (%s) from %s", //$NON-NLS-1$
 					remoteId.abbreviate(8).name(), refName,
 					replica.describeForLog()), fetchErr);
 			throw new MissingObjectException(remoteId, OBJ_COMMIT);
 		}
 
 		Map<String, Ref> adv = fetch.getRefs();
 		if (adv == null) {
 			throw new MissingObjectException(remoteId, OBJ_COMMIT);
 		}
 
 		Ref ref = adv.get(refName);
 		if (ref == null || ref.getObjectId() == null) {
 			throw new RefGoneException();
 		}
 
 		initRevWalk();
 		remoteId = ref.getObjectId();
 		return rw.parseCommit(remoteId);
 	}
 
 	private static class RefGoneException extends Exception {
 		private static final long serialVersionUID = 1L;
 	}
 }