Implement a PoC based on user collaborative filtering that can enhance the 

NoopResultRanker.

The ranking system based on recommendation is described here:

https://www.toptal.com/algorithms/predicting-likes-inside-a-simple-recommendation-engine

Note: the recommendation engine above can be simplified by ignoring the "dislike" feature.

Implementation examples can be found here:

• https://github.com/sumit1202/User-based-Collaborative-Filtering/blob/master/Recommender.java
• https://github.com/guoguibing/librec/tree/3.0.0/core/src/main/java/net/librec/recommender

Implementation details:

  Focus on simplicity

  Prioritise search results that have been previously selected by the active user.

  Use an in memory storage system