/*
    * 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 java.text.MessageFormat;
  import java.util.LinkedList;
  
  import org.eclipse.jgit.errors.IncorrectObjectTypeException;
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.internal.JGitText;
  
  /**
   * Computes the merge base(s) of the starting commits.
   * <p>
   * This generator is selected if the RevFilter is only
   * {@link org.eclipse.jgit.revwalk.filter.RevFilter#MERGE_BASE}.
   * <p>
   * To compute the merge base we assign a temporary flag to each of the starting
   * commits. The maximum number of starting commits is bounded by the number of
   * free flags available in the RevWalk when the generator is initialized. These
   * flags will be automatically released on the next reset of the RevWalk, but
   * not until then, as they are assigned to commits throughout the history.
   * <p>
   * Several internal flags are reused here for a different purpose, but this
   * should not have any impact as this generator should be run alone, and without
   * any other generators wrapped around it.
   */
  class MergeBaseGenerator extends Generator {
  	private static final int PARSED = RevWalk.PARSED;
  	private static final int IN_PENDING = RevWalk.SEEN;
  	private static final int POPPED = RevWalk.TEMP_MARK;
  	private static final int MERGE_BASE = RevWalk.REWRITE;
  
  	private final RevWalk walker;
  	private final DateRevQueue pending;
  
  	private int branchMask;
  	private int recarryTest;
  	private int recarryMask;
  	private int mergeBaseAncestor = -1;
  	private LinkedList<RevCommit> ret = new LinkedList<>();
  
  	private CarryStack stack;
  
  	MergeBaseGenerator(RevWalk w) {
  		super(w.isFirstParent());
  		walker = w;
  		pending = new DateRevQueue(firstParent);
  	}
  
  	void init(AbstractRevQueue p) throws IOException {
  		try {
  			for (;;) {
  				final RevCommit c = p.next();
  				if (c == null)
  					break;
  				add(c);
  			}
  			// Setup the condition used by carryOntoOne to detect a late
  			// merge base and produce it on the next round.
  			//
  			recarryTest = branchMask | POPPED;
  			recarryMask = branchMask | POPPED | MERGE_BASE;
  			mergeBaseAncestor = walker.allocFlag();
  
  			for (;;) {
  				RevCommit c = _next();
  				if (c == null) {
  					break;
  				}
  				ret.add(c);
  			}
  		} finally {
  			// Always free the flags immediately. This ensures the flags
  			// will be available for reuse when the walk resets.
  			//
  			walker.freeFlag(branchMask | mergeBaseAncestor);
  		}
  	}
  
  	private void add(RevCommit c) {
  		final int flag = walker.allocFlag();
  		branchMask |= flag;
  		if ((c.flags & branchMask) != 0) {
  			// This should never happen. RevWalk ensures we get a
  			// commit admitted to the initial queue only once. If
  			// we see this marks aren't correctly erased.
  			//
  			throw new IllegalStateException(MessageFormat.format(JGitText.get().staleRevFlagsOn, c.name()));
  		}
 		c.flags |= flag;
 		pending.add(c);
 	}
 
 	@Override
 	int outputType() {
 		return 0;
 	}
 
 	private RevCommit _next() throws MissingObjectException,
 			IncorrectObjectTypeException, IOException {
 		for (;;) {
 			final RevCommit c = pending.next();
 			if (c == null) {
 				return null;
 			}
 
 			for (RevCommit p : c.parents) {
 				if ((p.flags & IN_PENDING) != 0)
 					continue;
 				if ((p.flags & PARSED) == 0)
 					p.parseHeaders(walker);
 				p.flags |= IN_PENDING;
 				pending.add(p);
 			}
 
 			int carry = c.flags & branchMask;
 			boolean mb = carry == branchMask;
 			if (mb) {
 				// If we are a merge base make sure our ancestors are
 				// also flagged as being popped, so that they do not
 				// generate to the caller.
 				//
 				carry |= MERGE_BASE | mergeBaseAncestor;
 			}
 			carryOntoHistory(c, carry);
 
 			if ((c.flags & MERGE_BASE) != 0) {
 				// This commit is an ancestor of a merge base we already
 				// popped back to the caller. If everyone in pending is
 				// that way we are done traversing; if not we just need
 				// to move to the next available commit and try again.
 				//
 				if (pending.everbodyHasFlag(MERGE_BASE))
 					return null;
 				continue;
 			}
 			c.flags |= POPPED;
 
 			if (mb) {
 				c.flags |= MERGE_BASE;
 				return c;
 			}
 		}
 	}
 
 	@Override
 	RevCommit next() throws MissingObjectException,
 			IncorrectObjectTypeException, IOException {
 		while (!ret.isEmpty()) {
 			RevCommit commit = ret.remove();
 			if ((commit.flags & mergeBaseAncestor) == 0) {
 				return commit;
 			}
 		}
 		return null;
 	}
 
 	private void carryOntoHistory(RevCommit c, int carry) {
 		stack = null;
 		for (;;) {
 			carryOntoHistoryInnerLoop(c, carry);
 			if (stack == null) {
 				break;
 			}
 			c = stack.c;
 			carry = stack.carry;
 			stack = stack.prev;
 		}
 	}
 
 	private void carryOntoHistoryInnerLoop(RevCommit c, int carry) {
 		for (;;) {
 			RevCommit[] parents = c.parents;
 			if (parents == null || parents.length == 0) {
 				break;
 			}
 
 			int e = parents.length - 1;
 			for (int i = 0; i < e; i++) {
 				RevCommit p = parents[i];
 				if (carryOntoOne(p, carry) == CONTINUE) {
 					// Walking p will be required, buffer p on stack.
 					stack = new CarryStack(stack, p, carry);
 				}
 				// For other results from carryOntoOne:
 				// HAVE_ALL: p has all bits, do nothing to skip that path.
 				// CONTINUE_ON_STACK: callee pushed StackElement for p.
 			}
 
 			c = parents[e];
 			if (carryOntoOne(c, carry) != CONTINUE) {
 				break;
 			}
 		}
 	}
 
 	private static final int CONTINUE = 0;
 	private static final int HAVE_ALL = 1;
 	private static final int CONTINUE_ON_STACK = 2;
 
 	private int carryOntoOne(RevCommit p, int carry) {
 		// If we already had all carried flags, our parents do too.
 		// Return HAVE_ALL to stop caller from running down this leg
 		// of the revision graph any further.
 		//
 		// Otherwise return CONTINUE to ask the caller to walk history.
 		int rc = (p.flags & carry) == carry ? HAVE_ALL : CONTINUE;
 		p.flags |= carry;
 
 		if ((p.flags & recarryMask) == recarryTest) {
 			// We were popped without being a merge base, but we just got
 			// voted to be one. Inject ourselves back at the front of the
 			// pending queue and tell all of our ancestors they are within
 			// the merge base now.
 			p.flags &= ~POPPED;
 			pending.add(p);
 			stack = new CarryStack(stack, p, branchMask | MERGE_BASE);
 			return CONTINUE_ON_STACK;
 		}
 		return rc;
 	}
 
 	private static class CarryStack {
 		final CarryStack prev;
 		final RevCommit c;
 		final int carry;
 
 		CarryStack(CarryStack prev, RevCommit c, int carry) {
 			this.prev = prev;
 			this.c = c;
 			this.carry = carry;
 		}
 	}
 }