The main stack in project 3 holds three kinds of items:

• A dollar terminal at the bottom of the stack.
• Terminals (tokens) shifted onto the stack from lookahead.
• Non-terminals (expr's pushed on the stack whenever reductions are performed.)

Add two more values, expr and dollar, to the enumeration of token-type values (only Project 3 uses these values) so each item on the main stack contains two fields:

1. A token-type field whose value is expr, dollar, or the type of a token.

2. A pointer to an entry in the symbol table for the constant or variable which holds the r-value of the stacked item at run time - only the pointer for the dollar item has a null value.

Declare another stack (the handle stack) to hold the handle during each reduce step.

Declare two local integers to help determine precedence relations:

• fmain holds f(topmost terminal on main stack) and
• ghandle holds g(topmost terminal on handle stack)

The accept state occurs when lookahead holds a token in one of the three rightmost columns of the precedence relations table and the main stack only holds an expr and the dollar terminal. If f(dollar) is less than f(of all other rows) and g(OROP) is greater than g(of the three rightmost columns) then the accept state occurs when g(lookahead) < g(OROP) and fmain == f(dollar).

The general form of project 3 is:

symbolptr expr()  
{
    int fmain ; /* f(topmost terminal on main stack) */
    int ghandle ; /* g(topmost terminal on handle stack) */

    Clear Main Stack ;
    Push dollar item onto main stack ;
    fmain = f(dollar) ;
    
    while(fmain > f(dollar) || g(lookahead) >= g(OROP))  
    {                                /*   main while-loop */
        if(fmain <= g(lookahead)) 
        {                                   /* Shift-step */
            Push lookahead and attributes
                onto main stack;
            fmain = f(lookahead) ;
            Match(lookahead);  
        }                            /* End of shift-step */
        else  
        {                                  /* Reduce-step */
            Clear Handle Stack ;
            Pop main stack and push item onto handle stack ;
            if(top of handle stack == expr) 
            {
                Pop main stack and push item 
                    onto handle stack ;
            }
            ghandle = g(top of handle stack) ;
            if(top of main stack != expr) 
            {
                fmain = f(top of main stack) ;
            }
            while((top of main stack == expr) || 
               (fmain == ghandle)) 
            {                  /*  Continue moving handle */
                Pop main stack and push item
                    onto handle stack ;
                if(top of main stack != expr) 
                {
                    fmain = f(top of main stack) ; 
                }
                if(top of handle stack != expr) 
                {
                    ghandle = g(top of handle stack) ; 
                }
            }                      /*  End moving handle  */

            Reduce the handle on the handle stack and 
                push left-hand-side expr of production 
                    onto the main stack  ;
        }                             /*  End reduce-step */

    }                          /* End of main while-loop  */

    return symbol-table-pointer on top of main stack ;
}  /*  End of project 3