/*
 * Copyright (C) 2008-2009, Google Inc.
 * Copyright (C) 2008, Marek Zawirski <marek.zawirski@gmail.com>
 * 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 org.eclipse.jgit.errors.IncorrectObjectTypeException;
import org.eclipse.jgit.errors.MissingObjectException;
import org.eclipse.jgit.internal.JGitText;
import org.eclipse.jgit.revwalk.filter.AndRevFilter;
import org.eclipse.jgit.revwalk.filter.RevFilter;
import org.eclipse.jgit.treewalk.filter.TreeFilter;

/**
 * Initial RevWalk generator that bootstraps a new walk.
 * <p>
 * Initially RevWalk starts with this generator as its chosen implementation.
 * The first request for a RevCommit from the RevWalk instance calls to our
 * {@link #next()} method, and we replace ourselves with the best Generator
 * implementation available based upon the current RevWalk configuration.
 */
class StartGenerator extends Generator {
	private final RevWalk walker;

	StartGenerator(RevWalk w) {
		super(w.isFirstParent());
		walker = w;
	}

	@Override
	int outputType() {
		return 0;
	}

	@Override
	RevCommit next() throws MissingObjectException,
			IncorrectObjectTypeException, IOException {
		Generator g;

		final RevWalk w = walker;
		RevFilter rf = w.getRevFilter();
		final TreeFilter tf = w.getTreeFilter();
		AbstractRevQueue q = walker.queue;

		if (rf == RevFilter.MERGE_BASE) {
			// Computing for merge bases is a special case and does not
			// use the bulk of the generator pipeline.
			//
			if (tf != TreeFilter.ALL) {
				throw new IllegalStateException(MessageFormat.format(
						JGitText.get().cannotCombineTreeFilterWithRevFilter, tf, rf));
			}
			if (w.isFirstParent()) {
				throw new IllegalStateException(
						JGitText.get().cannotFindMergeBaseUsingFirstParent);
			}

			final MergeBaseGenerator mbg = new MergeBaseGenerator(w);
			walker.pending = mbg;
			walker.queue = AbstractRevQueue.EMPTY_QUEUE;
			mbg.init(q);
			return mbg.next();
		}

		final boolean uninteresting = q.anybodyHasFlag(RevWalk.UNINTERESTING);
		boolean boundary = walker.hasRevSort(RevSort.BOUNDARY);

		if (!boundary && walker instanceof ObjectWalk) {
			// The object walker requires boundary support to color
			// trees and blobs at the boundary uninteresting so it
			// does not produce those in the result.
			//
			boundary = true;
		}
		if (boundary && !uninteresting) {
			// If we were not fed uninteresting commits we will never
			// construct a boundary. There is no reason to include the
			// extra overhead associated with that in our pipeline.
			//
			boundary = false;
		}

		final DateRevQueue pending;
		int pendingOutputType = 0;
		if (q instanceof DateRevQueue)
			pending = (DateRevQueue)q;
		else
			pending = new DateRevQueue(q);
		if (tf != TreeFilter.ALL) {
			int rewriteFlag;
			if (w.getRewriteParents()) {
				pendingOutputType |= HAS_REWRITE | NEEDS_REWRITE;
				rewriteFlag = RevWalk.REWRITE;
			} else
				rewriteFlag = 0;
			rf = AndRevFilter.create(new TreeRevFilter(w, tf, rewriteFlag), rf);
		}

		walker.queue = q;

		if (walker instanceof DepthWalk) {
			DepthWalk dw = (DepthWalk) walker;
			g = new DepthGenerator(dw, pending);
		} else {
			g = new PendingGenerator(w, pending, rf, pendingOutputType);

			if (walker.hasRevSort(RevSort.BOUNDARY)) {
				// Because the boundary generator may produce uninteresting
				// commits we cannot allow the pending generator to dispose
				// of them early.
				//
				((PendingGenerator) g).canDispose = false;
			}
		}

		if ((g.outputType() & NEEDS_REWRITE) != 0) {
			// Correction for an upstream NEEDS_REWRITE is to buffer
			// fully and then apply a rewrite generator that can
			// pull through the rewrite chain and produce a dense
			// output graph.
			//
			g = new FIFORevQueue(g);
			g = new RewriteGenerator(g);
		}

		if (walker.hasRevSort(RevSort.TOPO)
				&& (g.outputType() & SORT_TOPO) == 0)
			g = new TopoSortGenerator(g);
		if (walker.hasRevSort(RevSort.REVERSE))
			g = new LIFORevQueue(g);
		if (boundary)
			g = new BoundaryGenerator(w, g);
		else if (uninteresting) {
			// Try to protect ourselves from uninteresting commits producing
			// due to clock skew in the commit time stamps. Delay such that
			// we have a chance at coloring enough of the graph correctly,
			// and then strip any UNINTERESTING nodes that may have leaked
			// through early.
			//
			if (pending.peek() != null)
				g = new DelayRevQueue(g);
			g = new FixUninterestingGenerator(g);
		}

		w.pending = g;
		return g.next();
	}
}