StringCompressor.rb

def compress(string)
  compressed = string.chars.inject([]) do |accu, char|
    if accu.empty? || accu.last.first != char
      accu << [char, 1]
    else
      accu[0...-1] << [char, accu.last.last + 1]
    end
  end.map(&:join).join

  [string, compressed].min_by(&:length)
end

if __FILE__ == $0
  for test in %w[a aaa aaaaaaaaaaac abc aabcccccaa] + [""]
    puts "compress('#{test}'): '#{compress(test)}'"
  end
end
# >> compress('a'): 'a'
# >> compress('aaa'): 'a3'
# >> compress('aaaaaaaaaaac'): 'a11c1'
# >> compress('abc'): 'abc'
# >> compress('aabcccccaa'): 'a2b1c5a2'
# >> compress(''): ''