[Codewars] Tribonacci Sequence

Q. Well met with Fibonacci bigger brother, AKA Tribonacci.

 

As the name may already reveal, it works basically like a Fibonacci, but summing the last 3 (instead of 2) numbers of the sequence to generate the next. And, worse part of it, regrettably I won't get to hear non-native Italian speakers trying to pronounce it :(

 

So, if we are to start our Tribonacci sequence with [1, 1, 1] as a starting input (AKA signature), we have this sequence:

[1, 1 ,1, 3, 5, 9, 17, 31, ...]

 

But what if we started with [0, 0, 1] as a signature? As starting with [0, 1] instead of [1, 1] basically shifts the common Fibonacci sequence by once place, you may be tempted to think that we would get the same sequence shifted by 2 places, but that is not the case and we would get:

[0, 0, 1, 1, 2, 4, 7, 13, 24, ...]

 

Well, you may have guessed it by now, but to be clear: you need to create a fibonacci function that given a signature array/list, returns the first n elements - signature included of the so seeded sequence.

 

Signature will always contain 3 numbers; n will always be a non-negative number; if n == 0, then return an empty array (except in C return NULL) and be ready for anything else which is not clearly specified ;)

 

S. 

List<num> tribonacci(List<num> signature, int n) {
  List<num> result = [];
  for (int i = 0; i < n; i++) {
    if (i < 3) {
      result.add(signature[i]);
    } else {
      result.add(result.sublist(i - 3, i).fold(0.0, (a, b) => a + b));
    }
  }
  return result;
}