PrimeTable

Calculation of multiplication of the N prime numbers in a table output.

Solution

This is a Visual Studio 2015 Solution. The solution contains three projects:

• PrimeTable.CLI: The Command Line Interface application. This project is the StartUp project. When run, it opens a console application that displays the multiplication table of the 10 first numbers in a text-based format.

• PrimeTable.Lib: The business objects and interfaces. This project contains the generators and validators of prime number arrays and multiplication tables.

• PrimeTable.Tests: The unit tests for the other two projects. The unit tests are written with the nunit.org framework.

How to run it

1. Open the Solution in Visual Studio 2015.

• Note that the application is written using C# 6.0 language features. Visual Studio 2015 ships with a compiler that can make use of these features. Earlier versions of the compiler may throw exceptions.

2. Make sure that the PrimeTable.CLI project is marked as the "StartUp Project".
3. In the Properties editor, in the Debug section, set the Command Line Arguments value to an integer between 1 and 10. The default value is 10.

• The application only generates tables for the first 10 prime numbers as per the specification. It can be modified to print larger tables if needed, but they won't look good in a console application.

4. Press the Start button in Visual Studio. The application will run in a console, print the appropriate table, and wait for input. Any input will close the application.

You may also compile the application and run the generated executable directly, passing an integer between 1 and 10 as a parameter.

NuGet dependencies

Note the following NuGet package dependencies:

• Moq.4.2.1510.2205
• NUnit.2.6.4

The packages folder has been added to GIT for safety, but when running on Visual Studio, make sure that the option Allow NuGet to download missing packages is enabled.

What I am pleased with

• Testability: Almost every class has been abstracted to an appropriate interface in order to enhance testability and enable mocking.
• For example, the Console is not used directly, but it is enveloped in an implementation of the IOutputWriter interface. This allows tests to mock the interface and retrieve the output, therefore allowing for the testability of that output.
• SOLID: The application doesn't run directly on the Program class as most small console applications do. Its logic is mostly transported to the PrimeNumbersAppController.
• Although this is not abstracted, an interface IAppController can easily be extracted and even a base class could be generated. This would enable the application to be extended with new commands to generate a more versatile calculator. A factory would have to be implemented if that was the case to wire the correct command to the correct input parameters in the Program class.
• This also enhances testability and scalability, as each IAppController could be tested independently.
• Test cases: Instead of creating a new unit test for every different scenario in each tested method, where possible, the TestCaseAttribute has been used. In NUnit this allows for a single implementation of the test (with parameters and/or return value) to test several scenarios (like edge scenarios, or situations where an exception is expected).
• Extending these test cases is simple, as it only involves adding a new TestCaseAttribute with the new parameters and expected behavior. This makes it easier (and cheaper in time-effort) to enhance already existing functionality in the future when the system needs to be extended.
• Elastic output: Notice that squares in the array printed are calculated to have the same width. It is not just printing what is in the result, but it is aligned to the right too. That's just to present the information in a readable, user-friendly manner.
• To see the difference, run the application with parameters 3, 4, and 10. Notice how when the numbers in the array are single-digit, the cells' width is smaller than when the numbers have two or three digits. Note also that in these cases, all numbers are aligned to the right.
• Error messaging: If you try to run the application with a bad input, the application will run but show error messages in red. The messages are humanly readable so that the user can know what is wrong with the input.
• Yield enumeration: Note the use of the yield C# keyword in the PrimeNumberGenerator.Generate method. This was added as a late revision on the code, to improve the performance for the generation of very large arrays of prime numbers. With this, only the requested elements in the enumerable will be generated, saving execution time when the number of elements is very large.

What I would do if I had more time

• Edge cases: One of my last additions to the code was to improve the enumeration of prime numbers with a yield keyword to improve performance of generations of very large arrays. I would like to investigate more on caching the numbers so that multiple generations in the same thread (for example) would only compute once.

• Web interface: I would have liked to provide a web interface too, and consume the array via asynchronous JavaScript through an API.

• More generalization: The IPrimeNumberGenerator and IPrimeTableGenerator interfaces have a similar method Generate. There is a possible generalization there for an IGenerator{TIn, TOut} interface and possible base classes. This would allow for a generator of other than prime numbers (say "Fibonacci", "Triangular", "Even", "Odd" or the simplest "Natural" number generators).

• The same goes for the PrimeNumbersAppController. An IAppController can be extracted and a base class generated. Then an AppControllerFactory could bring up and execute the correct implementation for the commands in the input and have several tools in the same application.

• Another generalization can be done in the operation that the PrimeTableGenerator uses. If a more generic TableGenerator is created instead, it can be injected an operation to perform with the operands of the table (say "Multiply", "Add", "Subtract", "Divide", "Power", etc.).

• Dependency Injection (IoC): The classes in the application already have the constructors ready for dependency injection. A framework for DI can be used here, particularly if the AppControllerFactory is created, to reduce coupling.