At my last job, the coolest project i did was to create an image duplicate detection system (so that it would tell, given an image, that the image is new, or it

At my last job, the coolest project i did was to create an image duplicate detection system (so that it would tell, given an image, that the image is new, or it's a version of what was uploaded before, or the image contains fragments of some known image, etc). It used opencv for feature points detection, so i had to maintain an index to search for those points (there were around 7M images, and load was up to 50 rps). I came up with a rather clever algorithm, which as i later discovered (thanks, wikipedia) is a kind of implicid k-d tree. That task was the most algorithm-intense task I ever had, and that algorithmical part took 20% of the time. 80% was straight coding, glueing things together, testing and putting all that in production system.