create array of ranges from array of individual values

to_range.rb

class Array
  def to_range
    highs = self.select { |x| !self.include?(x+1) }.sort_by { |x| x }
    lows = self.select  { |x| !self.include?(x-1) }.sort_by { |x| x }
    lows.zip(highs).map { |a,b| a..b } 
  end
end

# Running tests:

#................

#Finished tests in 0.004272s, 3745.3184 tests/s, 9363.2959 assertions/s.

#16 tests, 40 assertions, 0 failures, 0 errors, 0 skips

to_range_spec.rb

require 'minitest/spec'
require 'minitest/autorun'
require_relative 'to_range'

describe '#to_range' do
  describe "empty set" do
    it "should return an empty set" do
      [].to_range.count.must_equal 0
    end
  end

  describe "one item" do
    [1, 2, 1001].each do |value|
      it "should return one range" do
        result = [value].to_range
        result.count.must_equal 1
        result[0].must_equal value..value
      end
    end
  end

  describe "two consecutive items" do
    [[1,2], [3,4], [10000, 10001]].each do |array|
      it "should return one range" do
        result = [array[0],array[1]].to_range
        result.count.must_equal 1
        result[0].must_equal array[0]..array[1]
      end
    end
  end

  describe "two nonconsecutive items" do
    [[1,3], [3, 5], [566, 1001]].each do |array|
      it "should return two ranges for each item" do
        result = array.to_range
        result.count.must_equal 2
        result[0].must_equal array[0]..array[0]
        result[1].must_equal array[1]..array[1]
      end
    end
  end

  describe "three consecutive items" do
    [[1,2,3], [2,3,4], [100,101,102]].each do |array|
      it "should return one range" do
        result = array.to_range
        result.count.must_equal 1
        result[0].must_equal array.first..array.last
      end
    end
  end

  describe "joe's example" do
    it "should return one range" do
      result = [1,2,3,4,5,100,101,102,400,401,402,403,404,405,406].to_range
      result.count.must_equal 3
      result[0].must_equal 1..5
      result[1].must_equal 100..102
      result[2].must_equal 400..406
    end
  end

  describe "unordered example" do
    it "should return one range" do
      result = [4,3,100,102,1000,1,2,101].to_range
      result.count.must_equal 3
      result[0].must_equal 1..4
      result[1].must_equal 100..102
      result[2].must_equal 1000..1000
    end
  end

  describe "with dates" do
    it "should return ranges just like with integers" do
      result = ['2012-11-26', '2012-11-27', '2012-11-28', '2013-01-01', '2013-01-02', '2013-01-04'].
        map { |x| Date.parse(x) }.to_range
      result.count.must_equal 3
      result[0].must_equal Date.parse('2012-11-26')..Date.parse('2012-11-28')
      result[1].must_equal Date.parse('2013-01-01')..Date.parse('2013-01-02')
      result[2].must_equal Date.parse('2013-01-04')..Date.parse('2013-01-04')
    end
  end
end