/*
    * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> 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.revwalk;
  
  import java.io.IOException;
  
  import org.eclipse.jgit.errors.IncorrectObjectTypeException;
  import org.eclipse.jgit.errors.MissingObjectException;
  
  /** Sorts commits in topological order. */
  class TopoSortGenerator extends Generator {
  	private static final int TOPO_DELAY = RevWalk.TOPO_DELAY;
  
  	private final FIFORevQueue pending;
  
  	private final int outputType;
  
  	/**
  	 * Create a new sorter and completely spin the generator.
  	 * <p>
  	 * When the constructor completes the supplied generator will have no
  	 * commits remaining, as all of the commits will be held inside of this
  	 * generator's internal buffer.
  	 *
  	 * @param s
  	 *            generator to pull all commits out of, and into this buffer.
  	 * @throws MissingObjectException
  	 * @throws IncorrectObjectTypeException
  	 * @throws IOException
  	 */
  	TopoSortGenerator(Generator s) throws MissingObjectException,
  			IncorrectObjectTypeException, IOException {
  		super(s.firstParent);
  		pending = new FIFORevQueue(firstParent);
  		outputType = s.outputType() | SORT_TOPO;
  		s.shareFreeList(pending);
  		for (;;) {
  			final RevCommit c = s.next();
  			if (c == null) {
  				break;
  			}
  			for (RevCommit p : c.parents) {
  				p.inDegree++;
  				if (firstParent) {
  					break;
  				}
  			}
  			pending.add(c);
  		}
  	}
  
  	@Override
  	int outputType() {
  		return outputType;
  	}
  
  	@Override
  	void shareFreeList(BlockRevQueue q) {
  		q.shareFreeList(pending);
  	}
  
  	@Override
  	RevCommit next() throws MissingObjectException,
  			IncorrectObjectTypeException, IOException {
  		for (;;) {
  			final RevCommit c = pending.next();
  			if (c == null)
  				return null;
  
  			if (c.inDegree > 0) {
  				// At least one of our children is missing. We delay
  				// production until all of our children are output.
  				//
  				c.flags |= TOPO_DELAY;
  				continue;
  			}
  
  			// All of our children have already produced,
  			// so it is OK for us to produce now as well.
  			//
  			for (RevCommit p : c.parents) {
  				if (--p.inDegree == 0 && (p.flags & TOPO_DELAY) != 0) {
  					// This parent tried to come before us, but we are
  					// his last child. unpop the parent so it goes right
  					// behind this child.
  					//
  					p.flags &= ~TOPO_DELAY;
  					pending.unpop(p);
  				}
  				if (firstParent) {
  					break;
 				}
 			}
 			return c;
 		}
 	}
 }