If you want to understand the basics of the Walsh Hadamard transform and some useful applications for machine learning you can look here:
I actually have a second article in with kdnuggets called “The Weighted Sum a Second Glance.”
Maybe they will accept, maybe they will reject.
It is really the math to construct general associative memory using the weighted sum. However, I did not say that directly, out of politeness.
There was active research into associative memory in the 1960s, even using very advanced hardware for the time: