percentage_test.rb

class Rotator
  attr_accessor :id, :values
  BASE = 10

  def initialize(id, values)
    @id = id
    @values = values
  end

  def rotate
    decisive_id = analyse_visiting_id(id)
    rotating_url(decisive_id, bifurcation_scale)
  end

  private

  def analyse_visiting_id(id)
    id % BASE
  end

  def rotating_url(decisive_id, bifurcation_scale)
    bifurcation_scale.each_index.select{ |index| bifurcation_scale[index].include? decisive_id }
  end

  def bifurcation_scale
    total = values.sum
    percentages = values.map! { |v| (v/total.to_f).round(2) }
    pre_indexes = percentages.each_with_index.map do |per, index|
      if percentages.uniq.length == 1
        index.even? ? (per * BASE).round : (per * BASE).to_i
      else
        (per * BASE).round
      end
    end

    pre_indexes = pre_indexes.each.with_index.map do |pi, index|
      if index == 0
        pre_indexes[index]
      else
        pre_indexes[index] += pre_indexes[index-1]
      end
    end

    indexes = []
    pre_indexes.each.with_index do |pi, index|
      if index == 0
        indexes << [*(0...pre_indexes[index])]
      else
        indexes << [*(pre_indexes[index-1]...pre_indexes[index])]
      end
    end
    indexes
  end
end

require 'test/unit'
class MyTestCase < Test::Unit::TestCase
  def test_1
    ids = [*(1..100)]
    ans = []
    ids.each do |id|
      ans << Rotator.new(id, [50, 50]).rotate
    end
    assert_equal(ans.flatten.count(0), 50)
    assert_equal(ans.flatten.count(0), 50)
  end

  def test_2
    ids = [*(1..100)]
    ans = []
    ids.each do |id|
      ans << Rotator.new(id, [20, 40]).rotate
    end
    assert_equal(ans.flatten.count(0), 30)
    assert_equal(ans.flatten.count(1), 70)
  end

  def test_3
    ids = [*(1..100)]
    ans = []
    ids.each do |id|
      ans << Rotator.new(id, [25, 25, 25, 25]).rotate
    end
    assert_equal(ans.flatten.count(0), 30)
    assert_equal(ans.flatten.count(1), 20)
    assert_equal(ans.flatten.count(2), 30)
    assert_equal(ans.flatten.count(3), 20)
  end

  def test_4
    ids = [*(1..100)]
    ans = []
    ids.each do |id|
      ans << Rotator.new(id, [25, 30, 35]).rotate
    end
    assert_equal(ans.flatten.count(0), 30)
    assert_equal(ans.flatten.count(1), 30)
    assert_equal(ans.flatten.count(2), 40)
  end
end