Running

To run the application use ./sbt run.

To run tests use ./sbt test.

The configuration file at src/main/resources/application.conf contains the definition of the readers along with other settings that can be adjusted.

Overview

In the abscence of specific requirements regarding frameworks used, I decided to go with Akka streams. Since asynchronous producers are involved, streaming seems to fit the task naturally.

The idea is to adapt CharacterReader interface to become an Akka Source and build a stream to perform the given task.

I only added short comments to indicate classes purpose, otherwise I let signatures speak for themselves.

To finish in a reasonable amount of time, some functionality has been intentionally omitted:

• there is no logging and no error reporting
• support for special characters is limited
• ActorSystem is slow to shutdown after hitting ctrl-c
• the reported results are eventual in nature due to Akka operators' internal buffers, but it is close enough

I followed TDD process closely. The only exception is App which I consider to be outside of the solution boundary. It would take significant effort to test it, and it only contains straightforward wiring code.

The code is formatted according to the official style guide with rare exceptions where it makes sense.

Implementation notes

CharacterReaderInputStream

CharacterReaderInputStream is used to ingest elements from CharacterReader into an Akka stream.

Since Akka already contains InputStream to Source adapter and CharacterReader resembles InputStream closely, I decided to make CharacterReader to InputStream adapter.

The whole conversion scheme looks like this:

CharacterReader -> CharacterReaderInputStream extends InputStream -> InputStreamSource extends Source

WordCount

WordCount contains stream building blocks that form the solution.

.source

Two .source methods allow to get an Akka source given one or more CharacterReader. Resulting source emits whole words. It is essential to combine characters into words before merging readers to avoid mingling characters from different readers.

Setting chunkSize to 1 is important. By default InputStreamSource tries to read 8K elements before emitting anything.

.flow

Contains the heart of the solution. .scan operator is used to fold over the stream and aggregate words into a word-to-count Map.

.conflate operator is used to prevent slow downstream of backpressuring the upstream. It does it by dropping old word-to-count Maps and keeping just the latest one.

.extrapolate operator repeats the latest word-to-count Map indefenitely to make it always available for consuming.

Lastly, the flow is zipped with a tick source to only emit when a tick is received.

.sink

.sink formats the word-to-count Map into a String and prints it. The formatting part is extracted outside of the .flow to ease testing.