Very, very cool exercise. For anyone who hasn't been following APFS much though, worth knowing that Apple has stated in their AFPS FAQ [1] that they do intend to directly "document and publish the APFS volume format specification" themselves later this year. Presumably (as they do with other stuff they develop) they want to be able to more easily make breaking changes during the shakedown period of initial public deployment (first on iOS right now, later on Macs, though maybe not a default option until 10.13 this Summer/Fall). That'd be the most helpful in getting read/write into FUSE and other platforms should anyone choose to do so.
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1. https://developer.apple.com/library/content/documentation/Fi...