We had a discussion recently in our office on one of the famous streaming algorithms known as lossy count. I decided to write down my understanding of it focusing on the space complexity discussion and proving why it works in logarithmic space. This is the original paper introducing this algorithm and most of the information discussed in this blog is (hopefully) a simplified version of this paper relating to lossy count. Problem Statement There are use cases where you may be receiving a continuous stream of data and you want to know which items in that data stream exceed a certain threshold. The data stream could be really big so you can't store it in memory. Moreover you want to answer queries in real time. So here are the challenges this problem imposes: 1. Need to do it in single pass (time constraint) 2. Limited memory 3. Large volume of data in real-time The above issues warrant the need of a smart counting algorithm. Data stream mining to identify events & patterns can