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With inspiration from other libraries

There is a newer version: 2.12.15

/*
 * 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.atn;

import org.antlr.v4.runtime.misc.IntervalSet;

/**
 * Utility class to create {@link AtomTransition}, {@link RangeTransition},
 * and {@link SetTransition} appropriately based on the range of the input.
 *
 * To keep the serialized ATN size small, we only inline atom and
 * range transitions for Unicode code points <= U+FFFF.
 *
 * Whenever we encounter a Unicode code point > U+FFFF, we represent that
 * as a set transition (even if it is logically an atom or a range).
 */
public abstract class CodePointTransitions {
	/**
	 * If {@code codePoint} is <= U+FFFF, returns a new {@link AtomTransition}.
	 * Otherwise, returns a new {@link SetTransition}.
	 */
	public static Transition createWithCodePoint(ATNState target, int codePoint) {
		if (Character.isSupplementaryCodePoint(codePoint)) {
			return new SetTransition(target, IntervalSet.of(codePoint));
		}
		else {
			return new AtomTransition(target, codePoint);
		}
	}

	/**
	 * If {@code codePointFrom} and {@code codePointTo} are both
	 * <= U+FFFF, returns a new {@link RangeTransition}.
	 * Otherwise, returns a new {@link SetTransition}.
	 */
	public static Transition createWithCodePointRange(
			ATNState target,
			int codePointFrom,
			int codePointTo) {
		if (Character.isSupplementaryCodePoint(codePointFrom) ||
		    Character.isSupplementaryCodePoint(codePointTo)) {
			return new SetTransition(target, IntervalSet.of(codePointFrom, codePointTo));
		}
		else {
			return new RangeTransition(target, codePointFrom, codePointTo);
		}
	}
}