the "units of measure" crate uom would be a nice fit also, but it's not available here

Damn, I completely forgot about bitwise operators! Nice solution bro.

Nice code

oh sorry, will do! thanks

Please use spoiler flag next time. The comments to solutions are visible from the dashboard.

Yes sometimes using memory in the right way (when input data allow it) can greatly optimize a program like tables of sinus in ancient architechtures which hasn't got any mathematical processor.

who voted best practice on this xD
Well i guess in some circumstances this would be the safest bet (system with ROM?)...

for testing i used this kind of method as well, i admit it

wow, this is something... took me like 3 times as long to understand this code than it took to write my own solution :D
Are people really writing this kind of code for production?

You could use a match on the comparison, this way you don't have "i if ..." (that would be clearer to read). But just a suggestion :)

use std::cmp::Ordering;

// ...

match i.cmp(&3) {
      Ordering::Less => arr.push(signature[i]),
      _ => arr.push(arr[i - 3] + arr[i - 2] + arr[i - 1]),
  }

Alternatively you could just use a simple if {} else {} of course, as you just want to distinguish exactly two cases.