enumerable.rb

module Enumerable
  def exactly?(percentage, &block)
    matching(percentage, &block) == percentage.to_f
  end

  def more?(percentage, &block)
    matching(percentage, &block) >= percentage.to_f
  end

  def less?(percentage, &block)
    matching(percentage, &block) <= percentage.to_f
  end

  def fuzzy?(percentage, delta=0.0, &block)
    matching = matching(percentage, &block)
    range = ((percentage - delta)..(percentage + delta))
    range.include?(matching)
  end

  def matching(percentage, &block)
    (select(&block).size.to_f / self.size * 100)
  end
end

enumerable_spec.rb

require 'rspec'
require_relative '../../lib/enumerable'

describe Enumerable do
  let(:array) { [1, 2, 3, 4, 5] }

  describe "#exactly?" do
    it "is true when the predicate matches the given percent" do
      array.should be_exactly(20) {|x| x <= 1}
    end

    it "is false when the predicate does not match the given percent" do
      array.should_not be_exactly(30) {|x| x <= 1}
    end
  end

  describe "#more?" do
    it "is true when the predicate matches the given percent" do
      array.should be_more(10) {|x| x <= 1}
    end

    it "is false when the predicate does not match the given percent" do
      array.should_not be_more(30) {|x| x <= 1}
    end
  end

  describe "#less?" do
    it "is true when the predicate matches the given percent" do
      array.should be_less(30) {|x| x <= 1}
    end

    it "is false when the predicate does not match the given percent" do
      array.should_not be_less(10) {|x| x <= 1}
    end
  end

  describe "#fuzzy?" do
    it "is true when the predicate matches the given percentage range" do
      array.should be_fuzzy(17, 5) {|x| x <= 1}
    end

    it "is false when the predicate does not match the given percentage range" do
      array.should_not be_fuzzy(10, 1) {|x| x <= 1}
    end
  end
end