Notes: 35 Check Please

I grabbed the menu from a local chinese restaraunt:

https://www.hongkonggardenplymouth.com/menu/menu-1.php

At least for page 1 of the menu, every item has a code. That means we can (almost) describe an order with a list of codes.

• Problem: Some items say “Chicken or Beef”; we’ll igore that.
• Problem: Duplicates; we can just have duplicates in our list.

Let’s build the worst point of sale system for this restaraunt:

• Interactive terminal program.
• User types in an order (list of codes)
• Program prints a bill

A bill is:

• A list of: item, price each, quantity, item total
• A subtotal
• A total with 6.25% meal tax included

We want our program to be efficient so:

• We’ll read in the menu data at startup and process it in O(n) time in the size of the menu.
• We’ll make sure that we can generate our bill in O(n) time in the size of the order.

I’ve pre-processed the menu into a tab-separated text file, tossed that in our resource directory, and added some code to read it:

    Stream<String> readMenuLines() {
        InputStream txt = App.class
            .getResourceAsStream("/menu.tsv");
        InputStreamReader rdr = new InputStreamReader(txt);
        BufferedReader buf = new BufferedReader(rdr);
        return buf.lines();
    }

And here’s some hints for console read / write:

import java.io.Console;

    Console con = System.console();
    var line = con.readLine("order> ");
    con.printf("Your order: [%s]\n", line);

But from there, we’re going to build this using standard library tools.

Hints:

• Split lines with String#split
  • Talk about regular expressions
  • Split on literal tab
• For O(1) lookup, we want HashMap
  • get
  • put
• We want to store the whole menu line, so we need a MenuItem record.
• So setup is:
  • Read the menu data, build MenuItems, stick in HashMap keyed by code.
• For each order:
  • Split on non-word
  • Easy to get list of MenuItem
  • To handle multiples, we need BillRow record with quantity.
  • To do that in O(n) time, we need another HashTable
  • Then we can print out order.
• Tradeoff: Print bill in alphabetical order? Requries sorting or treemap, which would make this take O(n log n) time.