Calculate Fibonacci sequences in Kotlin as well as check if a number is a Fibonacci number. see: https://en.wikipedia.org/wiki/Fibonacci_number

Fibonacci.kt

class Fibonacci(number: Int = 100) {

    val sequence = mutableListOf(0, 1, 1)
    val isPartOfSequence by lazy { sequence.contains(number) }

    init {
        require(number <= 0) { "Only positive integers allowed in Fibonacci sequence." }
        if (number > 1) {
            var old = 1; var current = 1; var next: Int
            while (current + (current / 2) <= number) {
                next = old + current
                sequence.add(next)
                old = current
                current = next
            }
        }
    }
}

Fibonacci.md

In mathematics, the Fibonacci numbers, commonly denoted Fn, form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is,

and

for

calculation example (where each result is representing a fibonacci number)

0+1=1
  ↓ ↓
  1+1=2 
    ↓ ↓
    1+2=3
      ↓ ↓
      2+3=5
        ↓ ↓
        3+5=8
          ↓ ↓
          5+8=13

FibonacciTest.kt

import org.junit.jupiter.api.Test
import strikt.api.expectThat
import strikt.api.expectThrows
import strikt.assertions.containsExactly
import strikt.assertions.containsExactlyInAnyOrder
import strikt.assertions.isFalse
import strikt.assertions.isTrue


class FibonacciTest {

    @Test
    fun `get fibonacci numbers with maximum 1`() {
        val result = Fibonacci(1).sequence
        expectThat(result).containsExactly(0, 1, 1)
    }

    @Test
    fun `get fibonacci numbers with maximum 2`() {
        val result = Fibonacci(2).sequence
        expectThat(result).containsExactly(0, 1, 1, 2)
    }

    @Test
    fun `get fibonacci numbers with maximum 10`() {
        val result = Fibonacci(10).sequence
        expectThat(result).containsExactly(0, 1, 1, 2, 3, 5, 8)
    }

    @Test
    fun `get fibonacci numbers with maximum 100 by default`() {
        val result = Fibonacci().sequence
        expectThat(result).containsExactly(0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89)
    }

    @Test
    fun `get fibonacci numbers with maximum of actual fibonacci number`() {
        val result = Fibonacci(89).sequence
        expectThat(result).containsExactly(0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89)
    }

    @Test
    fun `get fibonacci numbers with maximum of negative integer`() {
        expectThrows<IllegalArgumentException> {
            Fibonacci(-1)
        }
    }

    @Test
    fun `can check if number IS NOT in fibonacci sequence`() {
        val result = Fibonacci(50).isPartOfSequence
        expectThat(result).isFalse()
    }

    @Test
    fun `can check if number IS in fibonacci sequence`() {
        val result = Fibonacci(89).isPartOfSequence
        expectThat(result).isTrue()
    }
}