employment_spec.rb

require 'rspec'
require 'date'

class EmploymentEvent < Struct.new(:date, :status)
  def <=> (b)
    date <=> b.date
  end

  def as_start
    date + 1
  end

  def as_end
    status != :end ? date - 1 : date
  end
end

class EmploymentPeriodService
  def periods_with_no_employment_changes(employment_periods)
    events = inject_events(employment_periods)
    events = remove_duplicates(events)
    find_ranges(events)
  end

private

  def inject_events(eps)
    [].tap do |events|
      eps.each do |ep|
        events << EmploymentEvent.new(ep[:start_date], :start)
        events << EmploymentEvent.new(ep[:end_date], :end)
      end
    end
  end

  def remove_duplicates(events)
    events.sort!
    [].tap do |sifted_events|
      current_event = events.shift
      while (!current_event.nil?)
        next_event = events.shift
        if next_event && next_event.date == current_event.date
          if next_event.status != current_event.status
            current_event.status = :both
          end
        else
          sifted_events << current_event
          current_event = next_event
        end
      end
    end
  end

  def find_ranges(events)
    [].tap do |no_changes_ranges|
      events.each.with_index do |e, i|
        break if i+1 == events.size
        no_changes_ranges << {
          start_date: e.as_start,
          end_date: events[i+1].as_end
        }
      end
    end
  end
end

describe EmploymentPeriodService do

  let(:service) { EmploymentPeriodService.new }

  it "should return the day after the only start date to the end date given one employee" do
    ranges = [
      { start_date: Date.parse('2016-02-02'), end_date: Date.parse('2016-04-09') }
    ]
    expect(service.periods_with_no_employment_changes(ranges)).to eql([{
      start_date: Date.parse('2016-02-03'),
      end_date: Date.parse('2016-04-09')
    }])
  end

  it "should return proper ranges for multiple employees" do
    ranges = [
      { start_date: Date.parse('2016-02-02'), end_date: Date.parse('2016-04-09') },
      { start_date: Date.parse('2016-03-01'), end_date: Date.parse('2016-05-01') }
    ]
    expect(service.periods_with_no_employment_changes(ranges)).to eql([{
      start_date: Date.parse('2016-02-03'),
      end_date: Date.parse('2016-02-29')
    }, {
      start_date: Date.parse('2016-03-02'),
      end_date: Date.parse('2016-04-09')
    }, {
      start_date: Date.parse('2016-04-10'),
      end_date: Date.parse('2016-05-01')
    }])
  end

  it "should account for multiple people starting the same day" do
    ranges = [
      { start_date: Date.parse('2016-02-02'), end_date: Date.parse('2016-04-09') },
      { start_date: Date.parse('2016-02-02'), end_date: Date.parse('2016-05-01') }
    ]
    expect(service.periods_with_no_employment_changes(ranges)).to eql([{
      start_date: Date.parse('2016-02-03'),
      end_date: Date.parse('2016-04-09')
    }, {
      start_date: Date.parse('2016-04-10'),
      end_date: Date.parse('2016-05-01')
    }])
  end

  it "should account for multiple people starting and ending the same day" do
    ranges = [
      { start_date: Date.parse('2016-02-02'), end_date: Date.parse('2016-04-09') },
      { start_date: Date.parse('2016-04-09'), end_date: Date.parse('2016-05-01') },
      { start_date: Date.parse('2016-05-01'), end_date: Date.parse('2016-06-01') },
      { start_date: Date.parse('2016-06-01'), end_date: Date.parse('2016-07-01') }
    ]
    expect(service.periods_with_no_employment_changes(ranges)).to eql([{
      start_date: Date.parse('2016-02-03'),
      end_date: Date.parse('2016-04-08')
    }, {
      start_date: Date.parse('2016-04-10'),
      end_date: Date.parse('2016-04-30')
    }, {
      start_date: Date.parse('2016-05-02'),
      end_date: Date.parse('2016-05-31')
    }, {
      start_date: Date.parse('2016-06-02'),
      end_date: Date.parse('2016-07-01')
    } ])
  end

end