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With inspiration from other libraries
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/*
 * Copyright (c) 2012-2017 The ANTLR Project. All rights reserved.
 * Use of this file is governed by the BSD 3-clause license that
 * can be found in the LICENSE.txt file in the project root.
 */

package org.antlr.v4.runtime.tree.pattern;

/**
 * Represents a placeholder tag in a tree pattern. A tag can have any of the
 * following forms.
 *
 * 
 * {@code expr}: An unlabeled placeholder for a parser rule {@code expr}.
 * {@code ID}: An unlabeled placeholder for a token of type {@code ID}.
 * {@code e:expr}: A labeled placeholder for a parser rule {@code expr}.
 * {@code id:ID}: A labeled placeholder for a token of type {@code ID}.
 * 
 *
 * This class does not perform any validation on the tag or label names aside
 * from ensuring that the tag is a non-null, non-empty string.
 */
class TagChunk extends Chunk {
	/**
	 * This is the backing field for {@link #getTag}.
	 */
	private final String tag;
	/**
	 * This is the backing field for {@link #getLabel}.
	 */
	private final String label;

	/**
	 * Construct a new instance of {@link TagChunk} using the specified tag and
	 * no label.
	 *
	 * @param tag The tag, which should be the name of a parser rule or token
	 * type.
	 *
	 * @exception IllegalArgumentException if {@code tag} is {@code null} or
	 * empty.
	 */
	public TagChunk(String tag) {
		this(null, tag);
	}

	/**
	 * Construct a new instance of {@link TagChunk} using the specified label
	 * and tag.
	 *
	 * @param label The label for the tag. If this is {@code null}, the
	 * {@link TagChunk} represents an unlabeled tag.
	 * @param tag The tag, which should be the name of a parser rule or token
	 * type.
	 *
	 * @exception IllegalArgumentException if {@code tag} is {@code null} or
	 * empty.
	 */
	public TagChunk(String label, String tag) {
		if (tag == null || tag.isEmpty()) {
			throw new IllegalArgumentException("tag cannot be null or empty");
		}

		this.label = label;
		this.tag = tag;
	}

	/**
	 * Get the tag for this chunk.
	 *
	 * @return The tag for the chunk.
	 */

	public final String getTag() {
		return tag;
	}

	/**
	 * Get the label, if any, assigned to this chunk.
	 *
	 * @return The label assigned to this chunk, or {@code null} if no label is
	 * assigned to the chunk.
	 */

	public final String getLabel() {
		return label;
	}

	/**
	 * This method returns a text representation of the tag chunk. Labeled tags
	 * are returned in the form {@code label:tag}, and unlabeled tags are
	 * returned as just the tag name.
	 */
	@Override
	public String toString() {
		if (label != null) {
			return label + ":" + tag;
		}

		return tag;
	}
}