Lab Module J -- Data-Directed Programming

CISC 280 Program Development Techniques

Reading: SICP (Structure and Interpretation of Computer Programs), Section 3.4

The necessary code for this homework is at
http://udel.edu/~saunders/280/labJ_code.scm

Homework exercise (Due at start of lab on 29 Apr 02): i

Exercise 1
Implement a ticket selling program with multiple ticket sellers. There will be multiple threads of execution that are independently selling the tickets to a new movie. There is one shared database variable that represents the tickets available to sell. Assume all tickets are equal and the only real issue is that you do not sell more tickets than there are seats. Each thread of execution will take a random amount of time to sell a seat, thus ensuring a random order of the threads of execution. For simplicity we will limit each sale to one ticket. While a particular seller is selling, all other sellers must be excluded from access to the shared database variable. When all of the tickets are gone, each seller should terminate with a message indicating how many tickets were sold by that seller. Obviously these messages should add up to no more than the total number of available tickets.

You should concentrate only on rewriting make-ticket-seller.
Do not change customer-order or print-tickets-sold.
You must preserve the order of selling in your modifications. This means that customer-order works with the number of seats available before the customer finally decides to order. The number of available seats is reduced only after the customer asks for a ticket.
The function (test) will run 10 ticket seller threads.
Not only do you have to be careful about SELLING the tickets, you must also be careful about PRINTING THE FINAL MESSAGE.
ONLY serialize the parts that access and modify the mutually shared variables. Parts that do not access the shared variables should not be serialized.

Thus your task is to modify this make-ticket-seller so that the key parts are serialized.

(define (make-ticket-seller name)
    (let ((total-tickets-sold 0))
	 (define (sell)
	    (sleep (random 2)) ;sleep up to two seconds until customer arrives
	    (if (customer-order *SEATS-LEFT*)
		(begin (set! *SEATS-LEFT* (- *SEATS-LEFT* 1)) ;record the sale globally
		       (set! total-tickets-sold (+ total-tickets-sold 1)) ; local record
		       (sell)) ;continue selling
		(print-tickets-sold name total-tickets-sold)
  	    ))
	 sell))

Before you modify make-ticket-seller you should load the downloaded code and run (test) several times to verify that the total tickets sold generally exceeds 100 tickets. Then make sure that your code solves the problem, while serializing as little of the code as you can.