/**
 * Derived class that represents an expression in the SILLY language.
 *   @author Dave Reed
 *   @version 2/8/15
 */
public class Expression {
    private Expression lhs;
    private Token middle;
    private Expression rhs;
    
    /**
     * Creates an expression from the specified TokenStream.
     *   @param input the TokenStream from which the program is read
     */
    public Expression(TokenStream input) throws Exception {        
        Token token1 = input.next();
        if (!token1.toString().equals("(")) {
        	this.lhs = null;
            this.middle = token1;
            this.rhs = null;
            
            if (this.middle.getType() != Token.Type.INTEGER && 
            	this.middle.getType() != Token.Type.STRING &&
            	this.middle.getType() != Token.Type.IDENTIFIER){
                throw new Exception("SYNTAX ERROR: Malformed expression");	
            }
        }
        else {  
            this.lhs = new Expression(input);
            this.middle = input.next();
            this.rhs = new Expression(input);
            input.next();
            
            if (this.middle.getType() != Token.Type.OPERATOR){
                throw new Exception("SYNTAX ERROR: Malformed expression");	
            }
        }
    }
    
    /**
     * Private helper method, called by evaluate.
     */
    private DataValue getValue(Token t) throws Exception {        
    	if (t.getType() == Token.Type.IDENTIFIER) {
            return Interpreter.MEMORY.lookupVariable(t);
        }
        else if (t.getType() == Token.Type.INTEGER) {
            return new IntegerValue(Integer.parseInt(t.toString()));
        }
        else if (t.getType() == Token.Type.STRING) {
            return new StringValue(t.toString());
        }
        else {
        	throw new Exception("RUNTIME ERROR: Invalid data type");
        }
    }
    
    /**
     * Evaluates the current expression involving integers and/or variables.
     *   @param bindings the current variable and subroutine bindings
     *   @return the value represented by the expression
     */
    public DataValue evaluate() throws Exception {
        if (this.lhs == null) {
            return this.getValue(this.middle);
        }
        else {
        	DataValue lhsValue =  this.lhs.evaluate();
        	DataValue rhsValue = this.rhs.evaluate();
        	
        	if (this.middle.toString().equals(".")) {
        		String s1 = lhsValue.getValue().toString();
        		String s2 = rhsValue.getValue().toString();
                if (s1.length() == 0 || s1.charAt(0) != '"') {
                	s1 = '"' + s1 + '"';
                }
                if (s2.length() == 0 || s2.charAt(0) != '"') {
                	s2 = '"' + s2 + '"';
                }
                return new StringValue(s1.substring(0, s1.length()-1) +
                                              s2.substring(1, s2.length()));
    		}
        	else if (lhsValue.getType() == Token.Type.INTEGER &&
        			 rhsValue.getType() == Token.Type.INTEGER) {
                int i1 = ((Integer)(lhsValue.getValue())).intValue();
                int i2 = ((Integer)(rhsValue.getValue())).intValue();  
                
        		if (this.middle.toString().equals("+")) {
        			return new IntegerValue(i1 + i2);
        		}
        		else if (this.middle.toString().equals("-")) {
        			return new IntegerValue(i1 - i2);
        		}
        		else if (this.middle.toString().equals("*")) {
        			return new IntegerValue(i1 * i2);
        		}
        		else if (this.middle.toString().equals("/")) {
        			return new IntegerValue(i1 / i2);
        		}        	}
        	else {
        		throw new Exception("RUNTIME ERROR: illegal operands for " + this.middle);
        	}
        }
        return null;
    }
      
	/**
	 * Converts the current expression into a String.
	 *   @return the String representation of this expression
	 */
	public String toString() {
        if (this.lhs == null) {
            return this.middle.toString();
        }
        else {
            return "( " + this.lhs + " " + this.middle + " " + this.rhs + " )";
        }
    }
}