NASA’s Magellan spacecraft and Perl’s pack function

I recently found this description of the origin of a number of the pack format specifiers in Perl’s pack function (which takes a list of values and converts it into a string using a specified rule template). Larry Wall recalls that they were added for processing data from NASA’s Magellan spacecraft (launched in 1989, also known as the Venus Radar Mapper):

+=begin comment
+Larry recalls that the hex and bit string formats (H, h, B, b) were added
+to pack for processing data from NASA's Magellan probe. Magellan was in
+an elliptical orbit, using the antenna for the radar mapping when close
+to Venus and for communicating data back to Earth for the rest of the
+orbit. There were two transmission units, but one of these failed, and
+then the other developed a fault whereby it would randomly flip the sense
+of all the bits. It was easy to automatically detect complete records
+with the correct sense, and complete records with all the bits flipped.
+However, this didn't recover the records where the sense flipped midway.
+A colleague of Larry's was able to pretty much eyeball where the records
+flipped, so they wrote an editor named kybble (a pun on the dog food
+Kibbles 'n' Bits) to enable him to manually correct the records and
+recover the data. For this purpose pack gained the hex and bit string
+format specifiers.
+git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
+27e2fb84680b9cc1), but the patch description makes no mention of their
+addition, let alone the story behind them.
+=end comment

N.B. I’m a big fan of Perl — this kind of ad hockery perfectly encapsulates why!

2 thoughts

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