[cctalk] Re: Minicomputer front panel.

[Paul Koning via cctalk]

> On Sep 23, 2022, at 4:47 PM, Fred Cisin via cctalk <cctalk@classiccmp.org> 
> wrote:
> 
> On Fri, 23 Sep 2022, Randy Dawson via cctalk wrote:
>> On the top secret number cruching....
>> The Cray had an instruction called 'population count'
>> asked for by the NSA.
>> The number of bits on in a word, not sure what this was used for.
> 
> Interesting.
> 
> A friend of my ex was asked to code that (in C) as a test in a job interview.
> 
> It was useful as a simple test of whether an applicant had any bit-twiddling 
> experience.  But, I couldn't think of practical application.

As was mentioned, it gives you the Hamming distance between two 60-bit values:

        BX1     X2-X3
        CX1     X1

> Perhaps, some measures of central tendency of values of that in a large body 
> of data could be useful for testing randomness in cryptography, such as 
> checking for steganography?

A typical test in cryptography would be entropy; another would be Friedman's 
"Index of coincidence".  Both use histograms, so there a population count would 
not come into play.

        paul