This is my solution to the Simple Database Challenge.

I created a test file early on in the project in order to test it with every meaningfull change, and check that nothing broke.

I think that the solution up to the point of the transactions is quite easy and self-explanatory so I will jump right into how I solved the transactions part.

##TRANSACTIONS SOLUTION: My first thoughts were to create some kind of Command history to reverse it when a rollback happened. But I rapidly discarded this solution as this kind actions cannot really be reversed.

Then I thought about using a list of states, without altering the db_state until the COMMIT. Everytime the user issued the BEGIN command, a new object would be added to the list and all the new changes would be stored in that object. If a ROLLBACK is issued, the last state is discarded; if a COMMIT is issued, all the changes are merged. This presented the problem that, to get a value, I would need to loop through the list of states until I found a record for that value. This solution would perform poorly with a high number of transactions, so it was also discarded.

A fast improvement to the previous solution could have been to copy all the state of the last transaction into the new one, but that would waste a lot of memory, which is also an important concern in the problem proposition.

Finally, I saw that the best solution would be to create a new object with every BEGIN, as before, but to make the changes directly on the current db_state. If the record was already in the database, I would save it's old value to the newly created object in order to recover it if a ROLLBACK was executed. A COMMIT would delete all the transaction objects.

##EXECUTION: The solution has been developed for Python 2.7

To execute it interactively cd into the project folder and do:

python DB.py

To pass a file of commands to standard input do:

python DB.py < /path/to/your/file