date_ranges.rb

require 'active_support/core_ext'

def dates_with_no_changes(employment_dates)
  dates = []
  employment_dates.each do |date|
    dates << date[:start_at].to_date
    if date[:end_at].nil?
      dates << Date.current.next_year
    else
      dates << date[:end_at].to_date
    end
  end
  dates.sort!
  ranges_from_dates(dates)
end

def ranges_from_dates(dates)
  ranges = []
  dates.each_with_index do |date, i|
    next_date = dates[i+1]
    break unless next_date
    next if date == next_date
    range = {
      start: date,
      end: next_date
    }
    ranges << range
  end
  ranges
end

dates = [
  { name: 'Chris',
    company: 'Krux',
    start_at: 3.years.ago,
    end_at: nil
  },
  { name: 'Bob',
    company: 'Homerun',
    start_at: 10.months.ago,
    end_at: nil
  },
  { name: 'Kevin',
    company: 'self employed',
    start_at: 1.year.ago,
    end_at: 3.months.ago
  },
  { name: 'James',
    company: 'Homerun',
    start_at: 1.year.ago,
    end_at: 3.months.ago
  }
]

# 3yrs -> 1 yr ago
# 1 yr -> 10 months
# 10 months -> 3 months
# 3 months -> unknown end date

p dates_with_no_changes(dates)
p dates_with_no_changes(dates).length == 4

sort! does not work with nil elements in the dates array.