min_natural_numbers_spec.rb

require 'rspec'

class MinNaturalNumber
  def initialize(*args)
    @numbers = args
  end

  attr_reader :numbers

  def solution
    array = Array.new(numbers.count)
    numbers.each do |n|
      array[n] = true if n < array.size
    end

    array.each.with_index do |present, index|
      return index if !present
    end

    numbers.count
  end
end

describe MinNaturalNumber do
  it "takes a list of numbers" do
    solver = MinNaturalNumber.new(3, 2, 1)
    expect(solver.numbers).to eql [3, 2, 1]
  end
  context "can find the smallest natural (postive) number not in the set" do
    it "finds 0 for [3, 2, 1]" do
      solver = MinNaturalNumber.new(3, 2, 1)
      expect(solver.solution).to eql 0
    end

    it "finds 4 for [3, 2, 0, 1]" do
      solver = MinNaturalNumber.new(3, 2, 0, 1)
      expect(solver.solution).to eql 4
    end

    it "finds 15 for [8, 23, 9, 0, 12, 11, 1, 10, 13, 7, 41, 4, 14, 21, 5, 17, 3, 19, 2, 6]" do
      solver = MinNaturalNumber.new(8, 23, 9, 0, 12, 11, 1, 10, 13, 7, 41, 4, 14, 21, 5, 17, 3, 19, 2, 6)

      expect(solver.solution).to eql 15
    end

  end
end