Case insensitive input stream for ANTLR 4

CaseInsensitiveInputStream.java

/*
 * [The "BSD license"]
 *  Copyright (c) 2012 Terence Parr
 *  Copyright (c) 2012 Sam Harwell
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 *
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 *
 *  1. Redistributions of source code must retain the above copyright
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 *  3. The name of the author may not be used to endorse or promote products
 *     derived from this software without specific prior written permission.
 *
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package org.antlr.v4.runtime;

/**
 *
 * @author Sam Harwell
 */
public class CaseInsensitiveInputStream extends ANTLRInputStream {

	protected char[] lookaheadData;

	public CaseInsensitiveInputStream(String input) {
		super(input);
		lookaheadData = input.toLowerCase().toCharArray();
	}

	@Override
	public int LA(int i) {
		if (i == 0) {
			return 0; // undefined
		}
		if (i < 0) {
			i++; // e.g., translate LA(-1) to use offset i=0; then data[p+0-1]
			if ((p + i - 1) < 0) {
				return IntStream.EOF; // invalid; no char before first char
			}
		}

		if ((p + i - 1) >= n) {
			return IntStream.EOF;
		}

		return lookaheadData[p + i - 1];
	}

}

For truly case-insensitive languages, a case-insensitive lookahead stream causes the ANTLR lexer to behave as though the input contained only lowercase letters, but messaging and other getText() methods return data from the original input string.

This implementation duplicates the data buffer in an effort to improve performance (since calling Character.toLowerCase(char) is quite slow and LA(int) is a performance-critical method). If memory overhead is more important than speed, the lookaheadData buffer could be removed.