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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.parse;

import org.antlr.runtime.Token;
import org.antlr.v4.Tool;
import org.antlr.v4.codegen.CodeGenerator;
import org.antlr.v4.tool.ErrorType;
import org.antlr.v4.tool.Grammar;
import org.antlr.v4.tool.ast.GrammarAST;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/** */
public class TokenVocabParser {
	protected final Grammar g;

	public TokenVocabParser(Grammar g) {
		this.g = g;
	}

	/** Load a vocab file {@code .tokens} and return mapping. */
	public Map load() {
		Map tokens = new LinkedHashMap();
		int maxTokenType = -1;
		File fullFile = getImportedVocabFile();
		FileInputStream fis = null;
		BufferedReader br = null;
		Tool tool = g.tool;
		String vocabName = g.getOptionString("tokenVocab");
		try {
			Pattern tokenDefPattern = Pattern.compile("([^\n]+?)[ \\t]*?=[ \\t]*?([0-9]+)");
			fis = new FileInputStream(fullFile);
			InputStreamReader isr;
			if (tool.grammarEncoding != null) {
				isr = new InputStreamReader(fis, tool.grammarEncoding);
			}
			else {
				isr = new InputStreamReader(fis);
			}

			br = new BufferedReader(isr);
			String tokenDef = br.readLine();
			int lineNum = 1;
			while ( tokenDef!=null ) {
				Matcher matcher = tokenDefPattern.matcher(tokenDef);
				if ( matcher.find() ) {
					String tokenID = matcher.group(1);
					String tokenTypeS = matcher.group(2);
					int tokenType;
					try {
						tokenType = Integer.valueOf(tokenTypeS);
					}
					catch (NumberFormatException nfe) {
						tool.errMgr.toolError(ErrorType.TOKENS_FILE_SYNTAX_ERROR,
											  vocabName + CodeGenerator.VOCAB_FILE_EXTENSION,
											  " bad token type: "+tokenTypeS,
											  lineNum);
						tokenType = Token.INVALID_TOKEN_TYPE;
					}
					tool.log("grammar", "import "+tokenID+"="+tokenType);
					tokens.put(tokenID, tokenType);
					maxTokenType = Math.max(maxTokenType,tokenType);
					lineNum++;
				}
				else {
					if ( tokenDef.length()>0 ) { // ignore blank lines
						tool.errMgr.toolError(ErrorType.TOKENS_FILE_SYNTAX_ERROR,
											  vocabName + CodeGenerator.VOCAB_FILE_EXTENSION,
											  " bad token def: " + tokenDef,
											  lineNum);
					}
				}
				tokenDef = br.readLine();
			}
		}
		catch (FileNotFoundException fnfe) {
			GrammarAST inTree = g.ast.getOptionAST("tokenVocab");
			String inTreeValue = inTree.getToken().getText();
			if ( vocabName.equals(inTreeValue) ) {
				tool.errMgr.grammarError(ErrorType.CANNOT_FIND_TOKENS_FILE_REFD_IN_GRAMMAR,
										 g.fileName,
										 inTree.getToken(),
										 fullFile);
			}
			else { // must be from -D option on cmd-line not token in tree
				tool.errMgr.toolError(ErrorType.CANNOT_FIND_TOKENS_FILE_GIVEN_ON_CMDLINE,
									  fullFile,
									  g.name);
			}
		}
		catch (Exception e) {
			tool.errMgr.toolError(ErrorType.ERROR_READING_TOKENS_FILE,
								  e,
								  fullFile,
								  e.getMessage());
		}
		finally {
			try {
				if ( br!=null ) br.close();
			}
			catch (IOException ioe) {
				tool.errMgr.toolError(ErrorType.ERROR_READING_TOKENS_FILE,
									  ioe,
									  fullFile,
									  ioe.getMessage());
			}
		}
		return tokens;
	}

	/** Return a File descriptor for vocab file.  Look in library or
	 *  in -o output path.  antlr -o foo T.g4 U.g4 where U needs T.tokens
	 *  won't work unless we look in foo too. If we do not find the
	 *  file in the lib directory then must assume that the .tokens file
	 *  is going to be generated as part of this build and we have defined
	 *  .tokens files so that they ALWAYS are generated in the base output
	 *  directory, which means the current directory for the command line tool if there
	 *  was no output directory specified.
	 */
	public File getImportedVocabFile() {
		String vocabName = g.getOptionString("tokenVocab");
		File f = new File(g.tool.libDirectory,
						  File.separator +
						  vocabName +
						  CodeGenerator.VOCAB_FILE_EXTENSION);
		if (f.exists()) {
			return f;
		}

		// We did not find the vocab file in the lib directory, so we need
		// to look for it in the output directory which is where .tokens
		// files are generated (in the base, not relative to the input
		// location.)
		f = new File(g.tool.outputDirectory, vocabName + CodeGenerator.VOCAB_FILE_EXTENSION);
		if ( f.exists() ) {
			return f;
		}
		
		// Still not found? Use the grammar's subfolder then.
		String fileDirectory;

		if (g.fileName.lastIndexOf(File.separatorChar) == -1) {
			// No path is included in the file name, so make the file
			// directory the same as the parent grammar (which might still be just ""
			// but when it is not, we will write the file in the correct place.
			fileDirectory = ".";
		}
		else {
			fileDirectory = g.fileName.substring(0, g.fileName.lastIndexOf(File.separatorChar));
		}
		return new File(fileDirectory, vocabName + CodeGenerator.VOCAB_FILE_EXTENSION);
	}
}