undo-redo/core/src/main/java/dev/kske/undoredo/core/UnlimitedChangeManager.java

package dev.kske.undoredo.core;

import java.util.*;

/**
 * A simple change manager with a linear history model backed by an array list.
 *
 * @param <C> the change type to store in this change manager
 * @author Maximilian K&auml;fer
 * @author Kai S. K. Engelbart
 * @since 0.0.1
 */
public final class UnlimitedChangeManager<C extends Change> implements ChangeManager<C> {

	private final List<C> changes = new ArrayList<>();

	private int	index		= -1;
	private int	markedIndex	= -1;

	/**
	 * @implNote As this change manager uses a linear history model, all changes behind the last
	 *           applied change will be discarded and therefore can be garbage collected.
	 */
	@Override
	public boolean addChange(C change) {
		if (!change.isIdentity()) {
			change.apply();

			// Remove divergent changes
			changes.subList(index + 1, changes.size()).clear();

			changes.add(change);
			++index;
			return true;
		}
		return false;
	}

	@Override
	public Optional<C> getLastChange() {
		return index < 0 ? Optional.empty() : Optional.of(changes.get(index));
	}

	@Override
	public boolean undo() {
		if (isUndoAvailable()) {
			changes.get(index).invert().apply();
			--index;
			return true;
		}
		return false;
	}

	@Override
	public boolean redo() {
		if (isRedoAvailable()) {
			changes.get(index + 1).apply();
			++index;
			return true;
		}
		return false;
	}

	@Override
	public void mark() {
		markedIndex = index;
	}

	@Override
	public void unmark() {
		markedIndex = -1;
	}

	@Override
	public boolean isAtMarkedChange() {
		return markedIndex == index;
	}

	@Override
	public boolean isUndoAvailable() {
		return index > -1;
	}

	@Override
	public boolean isRedoAvailable() {
		return index < changes.size() - 1;
	}

	@Override
	public List<C> getChanges() {
		return Collections.unmodifiableList(changes);
	}
}