max multiplication

max_multiplication.rb

# negative numbers are also taken into code
def max_multiplication(str)
  return nil if !str.is_a?(String)
  multipliers = CircularBuffer.new(4)
  negative_factor = 1
  max_multiplication = nil

  str.each_char do |char|

    if char == '-' && negative_factor == 1
      negative_factor = -1
      next
    end

    if char == '-' && negative_factor == -1
      multipliers.clear
      next
    end

    if numeric?(char)
      multipliers.write(char.to_i * negative_factor)
      if multipliers.full?
        current_multiplication = multipliers.to_a.reduce(&:*)
        max_multiplication = current_multiplication if max_multiplication.nil? || max_multiplication < current_multiplication
      end
    else
      multipliers.clear
    end

    negative_factor = 1
  end
  max_multiplication
end

def numeric?(s)
  Integer(s) != nil rescue false
end

class CircularBuffer

  def initialize(max_size)
    @max_size = max_size
    @buffer = []
  end

  def write(val)
    @buffer.shift if full?
    @buffer.push(val)
  end

  def clear
    @buffer.clear
  end

  def full?
    @buffer.size >= @max_size
  end

  def to_a
    @buffer
  end
end