Looks like I need to parse the file, process data while parsing and prepare it for fast reading.

Let's start with reading the data first.

One consideration, from the task details, I am not sure should I create a service for this,
but since it says that other components need to read from this one, looks like web service is a valid option.
I will create micro micro-service.

I will use standard web service code structure (controller, service, model).
It is not necessary in this case, but if project grow up just a bit,
it would be helpful to have clean design.

I will be using Lombok to avoid boilerplate code in controller, service and model classes.

Introducing ActivityCsvParser to read CSV file.
Having enhancement so it is not only reading CSV, but collecting activities by Case ID.
I am doing this to avoid multiple iteration through the list of parsed items.

I did intuitive way implementation with following steps:
reading activities, grouping, finding all variants and finding top variants,
but it is not performing well (takes around 150ms to finish).

After getting proper inner analysis, I noticed that ObjectMapper was taking more time than Gson
for translating object to Json string, so I replaced it.

Also, I noticed that first run is taking much more time than any following run.
That makes sense since in first run, Java is initializing all the necessary classes.
Any following run is dramatically faster. And real case scenario would be
this service to be running and other services to call it.
So I created performance test "ProcessIntelligencePTest"
where I am calling function 100 times and calculate the average time.

While looking for the best solution, I had idea to cache the response in the very beginning.
That idea make service works much better in non cached scenario,
since Java initialized all the classes necessary while running first caching method.
I will leave it, just to make performance better.

Later on, I had to remove the feature of mapping cases while reading CSV file,
since I am reading CSV file only once and use it as a database.
I am doing this to make solution according to task description.

On second thinking about solution, I would say that "component"
that would be interacting with this service might be another class,
so I am removing micro-service approach and making this solution
as a class ("ProcessIntelligence.java") to avoid over-engineering.
It is very easy to create micro-service if necessary.

I copied provided CSV file into standard Java resource folder,
where I believe it belongs, so if you want to change input data,
please use resource folder for that. 

### How to run
You can run solution once, by executing command:
`gradlew run` on Windows or `./gradlew run` on Linux

### Notes
If you are changing any code, and you want to build the project (`gradlew clean build`),
please run command `gradlew spotlessApply` before building
since I am using plugin to avoid non formatted code to be pushed.