Main.java

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;
import java.io.IOException;

public class Main {
    public static void main(String[] args) {
        String inputFile = "sample_input.txt";
        if (args.length > 0) {
            inputFile = args[0];
        }

        System.out.println("Processing input file: " + inputFile);

        try {
            // Read input file
            CharStream input = CharStreams.fromFileName(inputFile);

            // Instantiate lexer
            HTMLStructureLexer lexer = new HTMLStructureLexer(input);

            // Get token stream
            CommonTokenStream tokens = new CommonTokenStream(lexer);

            // Instantiate parser
            HTMLStructureParser parser = new HTMLStructureParser(tokens);

            // Add standard error listener to output syntax errors clearly
            parser.removeErrorListeners();
            parser.addErrorListener(new BaseErrorListener() {
                @Override
                public void syntaxError(Recognizer<?, ?> recognizer, Object offendingSymbol,
                                        int line, int charPositionInLine, String msg,
                                        RecognitionException e) {
                    System.err.println("Syntax Error at line " + line + ":" + charPositionInLine + " - " + msg);
                }
            });

            // Parse document rule
            ParseTree tree = parser.htmlDoc();

            if (parser.getNumberOfSyntaxErrors() > 0) {
                System.err.println("Parsing finished with " + parser.getNumberOfSyntaxErrors() + " syntax error(s). Exiting.");
                System.exit(1);
            }

            // Create and execute custom visitor
            HTMLStructureBaseVisitorExtended visitor = new HTMLStructureBaseVisitorExtended();
            visitor.visit(tree);

        } catch (IOException e) {
            System.err.println("Error reading input file: " + e.getMessage());
            System.exit(1);
        } catch (Exception e) {
            System.err.println("Unexpected error occurred: " + e.getMessage());
            e.printStackTrace();
            System.exit(1);
        }
    }
}