APFS technical documentation Is any technical documentation available for Apple's new File System. By technical, I mean File System's internal structures, File allocation block etc.
If Apple has not released any documentation, did someone tried to reverse engineer it?
Apple File System Developer Documentation
  
  Apple File System is a new, modern file system for iOS, macOS, tvOS, and watchOS. It is optimized for Flash/SSD storage and features strong encryption, copy-on-write metadata, space sharing, cloning for files and directories, snapshots, fast directory sizing, atomic safe-save primitives, and improved file system fundamentals.


See also Apple's File System Programming Guide.

Additionally, this blog APFS – A Backup Software Developer’s Perspective  makes mention of APFS already being reverse engineered.