A array except the indexes are fibonacci numbers!

fibArray.rb

class FibArray
  attr_accessor :array, :cache

  def initialize(array =  [])
    @array = array
    @cache = [1, 2] # put precomputed results as needed    
    generate_next_fib(array.size - cache.size)
  end

  def get(i)
    check_index_error(i)
    array[reverse_fib(i)] 
  end

  def set(i, element)
    check_index_error(i)
    array[reverse_fib(i)] = element
    element
  end

  def push(element)
   generate_next_fib
   array << element
  end

  def pop
    cache.pop
    array.pop
  end

  def remove(i)
    check_index_error(i)
    cache.pop
    array.delete_at(reverse_fib(i))
  end

  def check_index_error(i)
    fib_index = reverse_fib(i)
    raise IndexError if fib_index.nil?
  end

  private
    def fib(fib_i)
      if fib_i >= cache.size 
        cache[fib_i] = fib(fib_i - 1) + fib(fib_i - 2)
      end

      cache[fib_i]
    end

    def generate_next_fib(n = 1)
      n.times { fib(cache.size) }
    end

    def reverse_fib(number)
      cache.index(number)
    end
end

To test it out I ran it on the console using pry

> farray = FibArray.new [:a, :b, :c, :d, :e]
=> #<FibArray:0x000055f0330ebea0 @array=[:a, :b, :c, :d, :e], @cache=[1, 2, 3, 5, 8]>
> farray.get(5)
=> :d
> farray.set(3, :z)
=> :z
> farray.get(3)
=> :z
> farray.array
=> [:a, :b, :z, :d, :e]
> farray.push :f
=> 1
> farray.get(8)
=> :e
> farray.cache # Cached fibonacci results
=> [1, 2, 3, 5, 8, 13]
> farray.pop
=> :e
> farray.remove(1)
=> :a
> farray.get(7) # A fibonacci number that doesn't exist
IndexError: IndexError
from fibArray.rb:29:in `check_index_error'

https://www.reddit.com/r/ruby/comments/9xojgm/recursive_and_nonrecursive_fibonacci/e9uesl0/ if you want to see alternative Fibonacci implementations in ruby that can replace fib in the source code.

MIT License