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Java implementation of Devin Cook's GOLD Parser engine
There is a newer version: 5.0.5
package com.creativewidgetworks.goldparser.simple2;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

import com.creativewidgetworks.goldparser.engine.ParserException;
import com.creativewidgetworks.goldparser.parser.GOLDParser;
import com.creativewidgetworks.goldparser.util.FormatHelper;

/**
 * An interpreter for the Simple2 demonstration language that comes
 * with the GOLD Parser. Source files can be executed and parse
 * trees can be generated from the input stream.
 *
 * @author Ralph Iden (http://www.creativewidgetworks.com)
 * @version 5.0.0 
 */
public class Simple2 {
    
    private static final String RESOURCE_FILE = "simple2/simple2";
    
    /**
     * Formats rule handler messages using the simple2.properties resource file
     * @param message key (e.g., error.syntax)
     * @param parameters that will be inserted into any message placeholder entries (0}...
     * @return formated string.
     */
    public static String formatMessage(String message, Object... parameters) {
        return FormatHelper.formatMessage(RESOURCE_FILE, message, parameters);
    }

    /*----------------------------------------------------------------------------*/

    /**
     * Executes a Simple2 program
     * @param sourceCode to interpret
     * @param wantTree true if the parse tree should be generated
     * @return parse tree if generate parse tree open is set
     */
    public String executeProgram(String sourceCode, boolean wantTree) {
        // Use the compiled grammar file inside the jar
        GOLDParser parser = new GOLDParser(
            getClass().getResourceAsStream("/simple2/Simple2.cgt"), // compiled grammar table
            "com.creativewidgetworks.goldparser.simple2",           // rule handler package
            true);                                                  // trim reductions
        
        // Controls whether or not a parse tree is returned or the program executed.
        parser.setGenerateTree(wantTree);
        
        String tree = null;
        try {
            // Parse the source statements to see if it is syntactically correct
            boolean parsedWithoutError = parser.parseSourceStatements(sourceCode);

            // Holds the parse tree if setGenerateTree(true) was called
            tree = parser.getParseTree();
            
            // Either execute the code or print any error message
            if (parsedWithoutError) {
                parser.getCurrentReduction().execute();
            } else {
                System.out.println(parser.getErrorMessage());
            }
        } catch (ParserException e) {
            System.out.println(e.getMessage());
            e.printStackTrace();
        }

        return tree;
    }

    /*----------------------------------------------------------------------------*/
    
    /**
     * Load a source file to be interpreted by Simple2.  
     * @param filename of Simple2 source file
     * @return source code to be interpreted
     * @throws IOException 
     */
    public String loadSourceFile(String filename) throws IOException {
        File file = new File(filename);
        FileInputStream fis = new FileInputStream(file);
        byte[] buf = new byte[(int)file.length()];
        fis.read(buf);
        fis.close();
        return new String(buf);
    }
   
    /*----------------------------------------------------------------------------*/
    
    public static void main(String[] args) throws Exception {
        if (args.length > 0) {
            Simple2 parser = new Simple2();
            String source = parser.loadSourceFile(args[0]);
            boolean wantTree = args.length > 1 && args[1].equalsIgnoreCase("-tree");
            String tree = parser.executeProgram(source, wantTree);
            if (wantTree) {
                System.out.println(tree);
            }
        } else {
            System.out.println("Usage: java -jar simple2.jar  [-tree]");
        }
    }
}