import java.io.IOException;
import java.io.Reader;
import java.io.StreamTokenizer;
import java.io.StringReader;

/**
 * This class consists of a number of methods that "recognize" strings
 * composed of Tokens that follow the indicated grammar rules for each
 * method.
 * <p>Each method may have one of three outcomes:
 * <ul>
 *   <li>The method may succeed, returning <code>true</code> and
 *      consuming the tokens that make up that particular nonterminal.</li>
 *   <li>The method may fail, returning <code>false</code> and not
 *       consuming any tokens.</li>
 *   <li>(Some methods only) The method may determine that an
 *       unrecoverable error has occurred and throw a
 *       <code>SyntaxException</code></li>.
 * </ul>
 * @author David Matuszek
 * @version Jan 30, 2006.
 */
public class Recognizer {
    StreamTokenizer tokenizer = null;
    
    /**
     * Constructs a Recognizer for the given string.
     * @param text The string to be recognized.
     */
    public Recognizer(String text) {
        Reader reader = new StringReader(text);
        tokenizer = new StreamTokenizer(reader);
        tokenizer.ordinaryChar('-');
        tokenizer.ordinaryChar('/');
        tokenizer.wordChars('_', '_');
        tokenizer.ordinaryChars(10,13);
    }
    
    /**
     * Tries to recognize an &lt;expression&gt;.
     * <pre>&lt;expression&gt; ::= &lt;term&gt; { &lt;add_operator&gt; &lt;expression&gt; }</pre>
     * A <code>SyntaxException</code> will be thrown if the add_operator
     * is present but not followed by a valid &lt;expression&gt;.
     * @return <code>true</code> if an expression is recognized.
     */
    public boolean isExpression() {
        if (!isTerm()) return false;
        while (isAddOperator()) {
            if (!isExpression()) error("Error in expression after '+' or '-'");
        }
        return true;
    }
    
    /**
     * Tries to recognize a &lt;term&gt;.
     * <pre>&lt;term&gt; ::= &lt;factor&gt; { &lt;multiply_operator&gt; &lt;term&gt;}</pre>
     * A <code>SyntaxException</code> will be thrown if the multiply_operator
     * is present but not followed by a valid &lt;term&gt;.
     * @return <code>true</code> if a term is recognized.
     */
    public boolean isTerm() {
        if (!isFactor()) return false;
        while (isMultiplyOperator()) {
            if (!isTerm()) error("No term after '+' or '-'");
        }
        return true;
    }
    
    /**
     * Tries to recognize a &lt;factor&gt;.
     * <pre>&lt;factor&gt; ::= &lt;name&gt;
     *           | &lt;number&gt;
     *           | "(" &lt;expression&gt; ")"</pre>
     * A <code>SyntaxException</code> will be thrown if the opening
     * parenthesis is present but not followed by a valid
     * &lt;expression&gt; and a closing parenthesis.
     * @return <code>true</code> if a factor is recognized.
     */
    public boolean isFactor() {
        if (isVariable()) return true;
        if (number()) return true;
        if (symbol("(")) {
            if (!isExpression()) error("Error in parenthesized expression");
            if (!symbol(")")) error("Unclosed parenthetical expression");
            return true;
       }
       return false;
    }
    
    /**
     * Tries to recognize an &lt;add_operator&gt;.
     * <pre>&lt;add_operator&gt; ::= "+" | "-"</pre>
     * @return <code>true</code> if an addop is recognized.
     */
    public boolean isAddOperator() {
        return symbol("+") || symbol("-");
    }
    
    /**
     * Tries to recognize a &lt;multiply_operator&gt;.
     * <pre>&lt;multiply_operator&gt; ::= "*" | "/" | "%"</pre>
     * @return <code>true</code> if a multiply_operator is recognized.
     */
    public boolean isMultiplyOperator() {
        return symbol("*") || symbol("/") || symbol("%");
    }
    
    /**
     * Tries to recognize a &lt;variable&gt;.
     * <pre>&lt;variable&gt; ::= &lt;NAME&gt;</pre>
     * @return <code>true</code> if a variable is recognized.
     */
    public boolean isVariable() {
        return name();
    }
    
    

//----- Private "helper" methods
    
    /**
   * Tests whether the next token is a number. If it is, the token
   * is consumed, otherwise it is not.
   * 
   * @return <code>true</code> if the next token is a number.
   */
      private boolean number() {
        return nextTokenMatches(Token.Type.NUMBER);
    }
    
    /**
     * Tests whether the next token is a name. If it is, the token
     * is consumed, otherwise it is not.
     * 
     * @return <code>true</code> if the next token is a name.
     */
    private boolean name() {
        return nextTokenMatches(Token.Type.NAME);
    }
    
    /**
     * Tests whether the next token is the expected name. If it is, the token
     * is consumed, otherwise it is not.
     * 
     * @param expectedName The String value of the expected next token.
     * @return <code>true</code> if the next token is a name with the expected value.
     */
    private boolean name(String expectedName) {
        return nextTokenMatches(Token.Type.NAME, expectedName);
    }

    /**
     * Tests whether the next token is the expected keyword. If it is, the token
     * is moved to the stack, otherwise it is not.
     * 
     * @param expectedKeyword The String value of the expected next token.
     * @return <code>true</code> if the next token is a keyword with the expected value.
     */
    private boolean keyword(String expectedKeyword) {
        return nextTokenMatches(Token.Type.KEYWORD, expectedKeyword);
    }
    
    /**
     * Tests whether the next token is the expected symbol. If it is,
     * the token is consumed, otherwise it is not.
     * 
     * @param expectedSymbol The String value of the token we expect
     *    to encounter next.
     * @return <code>true</code> if the next token is the expected symbol.
     */
    boolean symbol(String expectedSymbol) {
        return nextTokenMatches(Token.Type.SYMBOL, expectedSymbol);
    }
    
    /**
     * Tests whether the next token has the expected type. If it does,
     * the token is consumed, otherwise it is not. This method would
     * normally be used only when the token's value is not relevant.
     * 
     * @param type The expected type of the next token.
     * @return <code>true</code> if the next token has the expected type.
     */
    boolean nextTokenMatches(Token.Type type) {
        Token t = nextToken();
        if (t.type == type) return true;
        else pushBack();
        return false;
    }
    
    /**
     * Tests whether the next token has the expected type and value.
     * If it does, the token is consumed, otherwise it is not. This
     * method would normally be used when the token's value is
     * important.
     * 
     * @param type The expected type of the next token.
     * @param value The expected value of the next token; must
     *              not be <code>null</code>.
     * @return <code>true</code> if the next token has the expected type.
     */
    boolean nextTokenMatches(Token.Type type, String value) {
        Token t = nextToken();
        if (type == t.type && value.equals(t.value)) return true;
        else pushBack();
        return false;
    }
    
    /**
     * Returns the next Token.
     * @return The next Token.
     */
    Token nextToken() {
        int code;
        try { code = tokenizer.nextToken(); }
        catch (IOException e) { throw new Error(e); } // Should never happen
        switch (code) {
            case StreamTokenizer.TT_WORD:
                if (Token.KEYWORDS.contains(tokenizer.sval)) {
                    return new Token(Token.Type.KEYWORD, tokenizer.sval);
                }
                else {
                    return new Token(Token.Type.NAME, tokenizer.sval);
                }
            case StreamTokenizer.TT_NUMBER:
                return new Token(Token.Type.NUMBER, ((int) tokenizer.nval) + "");
            case StreamTokenizer.TT_EOL:
                return new Token(Token.Type.EOL, "\n");
            case StreamTokenizer.TT_EOF:
                return new Token(Token.Type.EOF, "EOF");
            default:
                return new Token(Token.Type.SYMBOL, ((char) code) + "");
        }
    }
    
    /**
     * Returns the most recent Token to the tokenizer.
     */
    void pushBack() {
        tokenizer.pushBack();
    }
 
    /**
     * Utility routine to throw a <code>SyntaxException</code> with the
     * given message.
     * @param message The text to put in the <code>SyntaxException</code>.
     */
    private void error(String message) {
        throw new SyntaxException(message);
    }
}