Report on contents of AppleSingle

[alexgreenDP]

related to https://groups.google.com/forum/#!topic/droid-list/G35r2kwtPCY

AppleSingle is a 'container' (in the archival container sense, rather than DROID container signature sense)type format - essentially a wrapper with Apple metadata at the top, followed by actual file as a payload(e.g. pdf). The relationship is always 1:1 and the contained format is uncompressed.

Similar to .zip, .gz etc. DROID should report the content of AppleSingle (AppleSingle will be added in September PRONOM release so has yet to be assigned a PUID)

[thorsted]

This is worth discussing, and I would add MacBinary, not in PRONOM yet, in the same thought. Some formats should have better identification in these containers as they container the Type / Creator codes Macintosh uses to identify files and Applications used to create the files.

[thorsted]

In theory, the data fork should be identifiable within a AppleSingle file if a PRONOM signature exists. We would just need to add a container parser for the AppleSingle format. KaiTai?
(https://web.archive.org/web/20151028084447/http://kaiser-edv.de/documents/AppleSingle_AppleDouble.pdf)
The header for an AppleSingle file can be parsed to locate the Entry ID for the DataFork "00000001" with the next 4 bytes identifying the offset for the where the data fork begins from the BOF.

For example, attached is a PDF and a PDF encoded as AppleSingle.
AppleSinglePDF.zip

Offset 74 has the 4 bytes for the data fork Entry ID. "00000001" followed by the offset "00000098" which converts to decimal "152" which is the offset for where the PDF header begins.

Then the regular signature could take over and identify the PDF version "1.6" fmt/20.

This way an early Macintosh file with a resource fork could be preserved in a modern preservation repository and be renderable back in a MacOS environment when needed.