Exercise 03: Comparing Function Outputs

For this exercise, make sure you run all commands from the directory:

  /path/to/cbat_tools/docs/exercises/03

The first sample binary program

There is a program at binary/main_1. The source code is at binary/main_1.c.

You may imagine that this is a program used by a distributor of a particular product. The program takes as its argument the number of units ordered, and returns the price for that many units. There is a function calculate_price that actually computes the price for the order.

Built in to the calculate_price function is an algorithm for computing a discount, which can be summarized as follows:

• The price-per-unit is heavily discounted when 1000 or more units are ordered.
• The price-per-unit is less discounted if 100 to 999 units are ordered.
• The price-per-unit is not discounted if 20 to 99 units are ordered.

There are some requirements on orders:

• 20 units is the minimum number that can be ordered.
• If 20 to 100 units are ordered, units must be ordered in multiples of 10.
• If more than 100 units are ordered, units must be ordered in multiples of 100.

Try it out with various different numbers:

    $ ./binary/main_1 50
    $ ./binary/main_1 52
    $ ./binary/main_1 500
    $ ./binary/main_1 3400

The second sample binary program

Some time after the first program was written, an engineer decided to move the logic for smaller orders higher up in the calculate_price function. Why this was done is now long forgotten. Whatever the case, the modified version of the program lives at binary/main_2, and the source code lives at binary/main_2.c.

You can try this version of the program too:

    $ ./binary/main_2 50
    $ ./binary/main_2 52
    $ ./binary/main_2 500
    $ ./binary/main_2 3400

Your task

Your task is to figure out if the modified version of the program is functionally equivalent to the first version. Does the modified version of calculate_price produce the same output as the original version, given the same arguments?

To answer that, use wp to complete the following tasks:

1. Compare the output in RAX of the function calculate_price in the older program (binary/main_1) and the newer program (binary/main_2). Can binary/main_2 produce different output than main_1, when given the same argument? (Note: wp might take a minute or two to complete its analysis on this one.)
2. If so, what is an example of an argument that makes calculate_price behave differently than the original version?

Solution

To see the solution, click here.