/*
 * Name: Mike Cifelli
 * Course: CIS 443 - Programming Languages
 * Assignment: Lisp Interpreter 2
 */

package eval;

import parser.*;
import java.util.HashMap;

/**
 * A <code>SymbolTable</code> maps symbol names to values.
 */
public class SymbolTable {

    private HashMap<String, SExpression> table;
    private SymbolTable parent;

    /**
     * Create a new symbol table with no parent.
     */
    public SymbolTable() {
        this(null);
    }

    /**
     * Create a new symbol table with the specified parent.
     *
     * @param parent
     *  the parent of this symbol table
     */
    public SymbolTable(SymbolTable parent) {
        this.table = new HashMap<String, SExpression>();
        this.parent = parent;
    }

    /**
     * Determine if the specified symbol name is in this symbol table.
     *
     * @param symbolName
     *  the name of the symbol to look up
     * @return
     *  <code>true</code> if the symbol is in this symbol table;
     *  <code>false</code> otherwise
     */
    public boolean contains(String symbolName) {
        return table.containsKey(symbolName);
    }

    /**
     * Returns the value to which the specified symbol name is mapped in this
     * symbol table.
     *
     * @param symbolName
     *  the name of the symbol whose associated value is to be returned
     * @return
     *  the value to which this symbol table maps <code>symbolName</code>, or
     *  null if no mapping exists
     */
    public SExpression get(String symbolName) {
        return table.get(symbolName);
    }

    /**
     * Associates the specified symbol name with the specified value in this
     * symbol table. If the symbol table previously contained a mapping for
     * this symbol name, the old value has been replaced.
     *
     * @param symbolName
     *  the name of the symbol with which the specified value is to be
     *  associated
     * @param value
     *  the value to be associated with the specified symbol name
     */
    public void put(String symbolName, SExpression value) {
        table.put(symbolName, value);
    }

    /**
     * Returns the parent of this symbol table.
     *
     * @return
     *  the parent of this symbol table
     */
    public SymbolTable getParent() {
        return parent;
    }

}