Understanding AMCL, gmapping and hector_slam codebase and algorithms

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I've never come across ay "advanced" tutorials for amcl, gmapping, or hector_slam. However, I would strongly recommend reading Probabilistic Robotics. It covers the core components of amcl, and if your are not familiar with any of the subject matter then it should give you the prerequisite knowledge to understand the concepts behind gmapping, hector_slam, and many others. You could also look at the published papers for mapping, which is listed on the OpenSLAM site here and the hector_slam paper which is listed on that package's wiki here.

As for contributing and learning the codebases, amcl and mapping are quite old, so there doesn't seem to much for active development currently. hector_slam is newer, so they may still be some development going on for that. My recommendation would be to study the theory, review the code to understand it, and follow any current issues on each packages github repo and see if you could contribute that way.