It was a system developed by Szalay, et al., (https://arxiv.org/pdf/cs/0701164.pdf)
and was first used as part of the Sloan Digital Sky Survey.(http://www.skyserver.org/htm/)
There have been a couple of implementations of HTM algorithms and functions by the authors over time. Including C, Java, and the most update current implementation C#.
Structure of the code
The package source lives in 'modules'. It is split up into two folders, 'core' and 'utils'
Code in utils is meant to be tools or functions that utilize HTM functionality in some way but were not originally part of HTM core functionality.
For more details on what HTM is and what it is/can be used for, please see the publication by the authors ((https://arxiv.org/pdf/cs/0701164.pdf)) and its website (http://www.skyserver.org/htm/)