Large fibonacci check

large_fibonacci.rb

def is_fibonacci?(i)
  require "bigdecimal"
  require "bigdecimal/math"
  include BigMath
  
  plusfour =  5 * i**2 + 4
  minusfour = 5 * i**2 - 4
  
  #second argument is number of decimels for all big num arguments
  plusfour = BigDecimal.new(plusfour, 5)
  minusfour = BigDecimal.new(minusfour, 5)
  
  plusfour = BigMath.sqrt(plusfour, 5) 
  minusfour = BigMath.sqrt(minusfour, 5)
  
  plusremainder  = plusfour % 1 
  minusremainder = minusfour % 1
  
  if plusremainder == 0.0 && minusremainder == 0.0
    return false
  elsif plusremainder == 0.0
    return true
  elsif minusremainder == 0.0
    return true
  else
    return false
  end

end

Big num's are needed so as not to truncate bad remainders on very large fibonacci checks… implemented on all numbers for simplicity