package function.builtin;

import java.io.FileInputStream;
import java.io.FileNotFoundException;

import function.LispFunction;
import parser.LispParser;
import sexpression.*;

/**
 * <code>LOAD</code> represents the LOAD function in Lisp.
 */
public class LOAD extends LispFunction {
    // The number of arguments that LOAD takes.
    private static final int NUM_ARGS = 1;

    public SExpression call(Cons argList) {
        // retrieve the number of arguments passed to LOAD
        int argListLength = LENGTH.getLength(argList);

        // make sure we have received the proper number of arguments
        if (argListLength != NUM_ARGS) {
            Cons originalSExpr = new Cons(new Symbol("LOAD"), argList);
            String errMsg = "too " +
                            ((argListLength > NUM_ARGS) ? "many" : "few") +
                            " arguments given to LOAD: " + originalSExpr;

            throw new RuntimeException(errMsg);
        }

        SExpression argCar = argList.getCar();

        // make sure the argument is a string
        if (! argCar.stringp()) {
            throw new RuntimeException("LOAD: " + argCar + " is not a string");
        }

        LispString quotedName = (LispString) argCar;
        String fileName = quotedName.toString();

        // remove the surrounding quotes from the file name
        fileName = fileName.substring(1, (fileName.length() - 1));

        return processFile(fileName);
    }

    // Evaluate all the S-expressions found in the file with the specified
    // name.
    //
    // Parameters: fileName - the name of the file to be evaluated
    // Returns: 'T' if the file was processed successfully; 'NIL' otherwise
    private SExpression processFile(String fileName) {
        LispParser parser = null;

        // attempt to create a new 'LispParser' on the specified file
        try {
            parser = new LispParser(new FileInputStream(fileName), fileName);
        } catch (FileNotFoundException e) {
            System.out.println("LOAD: could not open " + fileName);

            return Nil.getUniqueInstance();
        }

        // attempt to evaluate all the S-expressions contained in the file
        while (! parser.isEof()) {
            try {
                SExpression sexpr = parser.getNextSExpression();

                EVAL.eval(sexpr);
            } catch (RuntimeException e) {
                System.out.println("LOAD: " + e.getMessage());

                return Nil.getUniqueInstance();
            }
        }

        // success!
        return Symbol.T;
    }

}