>The equations that do autocorrelation are computationally exhaustive: for every one point of autocorrelation (each line on the chart above, right), it might’ve been necessary for Hildebrand to do something like 500 summations of multiply-adds... >Hildebrand realized he was limited by the technology, and instead of giving up, he found a way to work within it using math. “I realized that most of the arithmetic was redundant, and could be simplified,” he says. “My simplification changed a million multiply adds into just four. It was a trick — a mathematical trick.”

I think this is the article's way of dramatizing the standard way of calculating the autocorrelation using the convolution theorem: https://en.wikipedia.org/wiki/Autocorrelation#Efficient_comp...