Writing the implementation code

The purpose of this phase is to write code that will make the last test pass. Do not try to make it perfect, nor try to spend too much time with it. If it's not well-written or is not optimum, that is still okay. It'll become better later on. What we're really trying to do is to create a safety net in the form of tests that are confirmed to pass. Do not try to introduce any functionality that was not described in the last test. To do that, we are required to go back to the first step and start with a new test. However, we should not write new tests until all the existing ones are passing.

In this phase, we are still in the red stage. While the code that was written would probably pass all the tests, that assumption is not yet confirmed.