array_compressor.rb

class ArrayCompressor

  def initialize(arr:)
    @arr = arr.dup
  end

  def compressed
    compressed_array = []

    while !@arr.empty? do
      first = @arr.first
      first_elements = @arr.take_while {|e| e == first }

      compressed_array << if first_elements.size > 2
        "#{first} (#{first_elements.size})"
      else
        first_elements
      end
      @arr.shift(first_elements.size)
    end
    compressed_array.flatten
  end
end

class ArrayCompressorTest
  class << self
    def passed?
      match(['cat', 'cat', 'cat', 'dog', 'dog', 'dog'], ['cat (3)', 'dog (3)']) &&
      match(['cat', 'cat', 'dog', 'dog', 'dog'], ['cat', 'cat', 'dog (3)']) &&
      match(['cat', 'dog', 'dog', 'dog'], ['cat', 'dog (3)']) &&
      match(['cat', 'dog', 'dog'], ['cat', 'dog', 'dog']) &&
      match(['cat', 'dog'], ['cat', 'dog']) &&
      match(['cat'], ['cat']) &&
      match([], [])
    end

    private

    def match(set, result)
      compressed = ArrayCompressor.new(arr: set).compressed
      puts "Array '#{set}' should be compressed to the '#{result}'"
      compressed == result
    end
  end
end

puts (ArrayCompressorTest.passed? ? 'Tests passed successfully' : 'Tests passed with failure')

object_compressor.rb

load 'string_compressor.rb'
load 'array_compressor.rb'

class ObjectCompressor
  def initialize(obj)
    @obj = obj
  end

  def compressed
    case @obj.class.to_s
      when 'String' then StringCompressor.new(str: @obj).compressed
      when 'Array' then ArrayCompressor.new(arr: @obj).compressed
      else 'Incorrect input format. The only String and Array are acceptable.'
    end
  end
end

class ObjectCompressorTest
  class << self
    def passed?
      match('abcdab987612', 'a-dab9-612') &&
      match(['cat', 'cat', 'dog', 'dog', 'dog'], ['cat', 'cat', 'dog (3)']) &&
      match(1, 'Incorrect input format. The only String and Array are acceptable.')
    end

    private

    def match(set, result)
      compressed = ObjectCompressor.new(set).compressed
      puts "Object '#{set}' should be compressed to the '#{result}'"
      compressed == result
    end
  end
end

puts (ObjectCompressorTest.passed? ? 'Tests passed successfully' : 'Tests passed with failure')

string_compressor.rb

class StringCompressor

  BORDER = 0

  def initialize(str:)
    @str = str
  end

  def compressed
    splited_lexemes.map do |lexem|
      if lexem.size == 1
        lexem[0].first
      elsif lexem.size == 2
        "#{lexem[0].first}#{lexem[-1].first}"
      elsif lexem.size > 2
        "#{lexem[0].first}-#{lexem[-1].first}"
      end
    end.join
  end

  private

  def zippped_neighbour_differences
    numbers = @str.chars.map(&:ord)
    differences = [0] + numbers[0..-2].each_with_index.map{|num, i| num - numbers[i+1] }
    differences.map! {|code| code.abs == 1 ? code : 0 }
    numbers.map(&:chr).zip(differences)
  end

  def splited_lexemes
    lexemes = []
    marked_chars = zippped_neighbour_differences

    while !marked_chars.empty? do
      lexeme = [marked_chars.shift]

      delta = marked_chars.first
      if delta && delta[1].abs == 1
        while delta && delta[1].abs != BORDER do
          lexeme << marked_chars.shift
          delta = marked_chars.first
        end
        lexemes << lexeme
      else
        lexemes << lexeme
      end
    end
    lexemes
  end
end

class StringCompressorTest
  class << self
    def passed?
      match('abcdab987612', 'a-dab9-612') &&
      match('abbdab987612', 'abbdab9-612') &&
      match('cbaddd9712345', 'c-addd971-5') &&
      match('abcd', 'a-d') &&
      match('a', 'a') &&
      match('', '')
    end

    private

    def match(set, result)
      compressed = StringCompressor.new(str: set).compressed
      puts "String '#{set}' should be compressed to the '#{result}'"
      compressed == result
    end
  end
end

puts (StringCompressorTest.passed? ? 'Tests passed successfully' : 'Tests passed with failure')