23 Sep
2022
23 Sep
'22
6:49 p.m.
On 23/09/22 11:21, Paul Koning via cctalk wrote:
Cray did? I didn't know that. It first appeared in the CDC 6600, and yes, according
to rumor at the request of NSA. I can imagine it being used for statistical analysis of
character patterns
Yep. Pop count is essentially the Hamming Weight, which is a measure of
information content and for a hunk of text can serve as a hash to drive
further analysis.
A non-classified example of its use is in the PLATO
system for "fuzzy string matching". PLATO needed to be able to recognize
student answers that were correct but misspelled; it would do that, roughly speaking, by
taking the difference of the expected string and the actual input and running population
count on that. A difference of less than n bits would be defined as a misspelled match.
This is a use of Hamming Distance (or a riff on it). XOR two identical
length binary strings, a pop count on the result tells you how "far
apart" they are.
--
Christian Kennedy, Ph.D.
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