Mateus Jabour


Understanding how computation works part 6-2

Computation Logo

On this post, I will try to make you understand more about computer science. In this part of the post we are going to see two more universal system: Tag System, Cyclic Tag System.

Tag system

Tag system works like a Turing machine, but, instead of moving to the sides, Tag system oparates on a string by repeatedly adding new characters to the end of the string and removing some characters from the beginning, it oparates just on the edges of the string.

We can split the Tag System description in two parts, first is the collections of rules, each rule specifies which characters to append to the string when a particular character appears at the beginning of the string. Let’s see an example of tag system:

  • When the string begins with a, append the character bc.
  • When the string begins with b, append the character caad.
  • When the string begins with c, append the character ccd.
  • After applying any rule, delete three character from the beginning of the string(deletion number is 3).

Performing a tag system is simply following the rules one after other. Have a look at the execution of this tag system:


Tag Systems only operate directly on string, but we can fet them to perform operations on other kinds of values, like numbers, just by encoding numbers. Let’s try to do it using a encoding system that works like this: represent the number n as the string aa followed by n repetitions of the string bb.

Now that we have a encoding scheme, we can manipulate those numbers and make operations with them. One operation is to double the input, like this:

  • When the string begins with a, append the character aa.
  • When the string begins with b, append the character bbbb.
  • After applying any rule, delete two character from the beginning of the string(deletion number is 2).

Watch how this tag system works with the string aabbbb(number 2):

aabbbb bbbbaa bbaabbbb aabbbbbbbb(number 4) bbbbbbbbaa bbbbbbaabbbb bbbbaabbbbbbbb bbaabbbbbbbbbbbb aabbbbbbbbbbbbbbbb(number 8)

We can see that the doubling is happening, but, it’s a infinite loop, what isn’t really what we had in mind. We could append new characters instead of appending a character that has a rule to proceed. So, when the string starts with a, let’s put cc, and when the string starts with b, let’s put dddd, let’s do this:

aabbbb bbbbcc bbccdddd ccdddddddd

Now, when we reach the doubled value, the tag system stops, like we wanted. We can simulate a tag system in Ruby, for this, we need to implement an object for the rules, one for the collection of rules and the tag system:

class TagRule <, :append_characters) def applies_to?(string) string.chars.first == first_character end
def follow(string) string + append_characters end end
class TagRulebook <, :rules) def next_string(string) rule_for(string).follow(string).slice(deletion_number..-1) end
def rule_for(string) rules.detect { |r| r.applies_to?(string)} end end
class TagSystem <, :rulebook) def step self.current_string = rulebook.next_string(current_string) end end

With those objects, we can simulate a tag system, let’s try to simulate the tag system that doubles the input:

rulebook =, ['a', 'aa'),'b', 'bbbb') ] ) => #<struct TagRulebook ...> system ='aabbbbbb', rulebook) => #<struct TagSystem ...> 4.times do puts system.current_string system.step end; puts system.current_string aabbbbbb bbbbbbaa bbbbaabbbb bbaabbbbbbbb aabbbbbbbbbbbb => nil

We made just 4 steps in here because it would loop forever if we had done one while or until. We can avoid those infinite loops, just by adding some code on those objects:

class TagRulebook def applies_to?(string) !rule_for(string).nil? && string.length >= deletion_number end end
class TagSystem def run while rulebook.applies_to?(current_string) puts current_string step end

puts current_string   end end </code>

Now, we can just call #run and let it stop naturally:

rulebook = 2, ['a', 'cc'),'b', 'dddd') ] ) => #<struct TagRulebook ...> system ='aabbbbbb', rulebook) => #<struct TagSystem ...> aabbbbbb bbbbbbcc bbbbccdddd bbccdddddddd ccdddddddddddd => nil

With this implementation, we can explore more about tags using this encoding scheme. Let’s desing a tag system that halves a number:

rulebook = 2, ['a', 'cc'),'b', 'd') ] ) => #<struct TagRulebook ...> system ='aabbbbbb', rulebook) => #<struct TagSystem ...> aabbbbbb bbbbbbcc bbbbccd bbccdd ccddd => nil

We can even design one that increments number:

rulebook = 2, ['a', 'ccdd'),'b', 'dd') ] ) => #<struct TagRulebook ...> system ='aabbbb', rulebook) => #<struct TagSystem ...> aabbbb bbbbccdd bbccdddd ccdddddd => nil

We can also design one tag system that combines those two, just by making the output of the first task matching the encoding of the second. We will see it by designing a system that combines incrementing with doubling, have a look:

rulebook =, ['a', 'cc'),'b', 'dddd'),'c', 'eeff'),'d', 'ff') ] ) => #<struct TagRulebook ...> system ='aabbbb', rulebook) => #<struct TagSystem ...> aabbbb (number 2) bbbbcc bbccdddd ccdddddddd (number 4) ddddddddeeff ddddddeeffff ddddeeffffff ddeeffffffff eeffffffffff (number 5) => nil

You can see that the first system just turns the number 2 to the number 4 enconded with the characters c and d, and then, the second system takes this doubled number and increment it, turning it to 5 enconded with the characters e and f.

We can use tag system to check mathematical properties from numbers, we can design a system that says if the number is an even or an odd:

rulebook =, ['a', 'cc'),'b', 'd'),'c', 'eo'),'d', ''),'e', 'e') ] ) => #<struct TagRulebook ...> system ='aabbbbbbbb', rulebook) => #<struct TagSystem ...> aabbbbbbbb bbbbbbbbcc bbbbbbccd bbbbccdd bbccddd ccdddd ddddeo ddeo eo e => nil

Now you wondering, how it works? Easy:

  • First the number is halved, using the encoding c/d.
  • The rule ‘c’ takes off ‘cc’ and adds the characters ‘eo’, this will give us the result.
  • The rule ‘d’ takes off all ‘dd’ pairs, leaving only ‘eo’ if it’s an even and leaving ‘deo’ if it’s an odd.
  • In the even’s case: the ‘e’ rule will take off ‘eo’ and will add an ‘e’. In the odd’s case: the ‘d’ rule will take off the ‘de’ and will let the ‘o’ there.

This is how this system works, let’s see an example of an odd:

system ='aabbbbbbbbbb', rulebook) => #<struct TagSystem ...> aabbbbbbbbbb bbbbbbbbbbcc bbbbbbbbccd bbbbbbccdd bbbbccddd bbccdddd ccddddd dddddeo dddeo deo o => nil

We can use those techniques to simulate a Turing machine. As we saw, tah system is very simple, so, there’re a lot of details involved to make this simulation, let’s list the steps:

  • We’ll use a Turing machine that uses only 0’s and 1’s, with 0 acting as the blank.
  • Let’s split the tape into two pieces: left part(all the left characters and the head), and the right part(all right characters).
  • Treat the left part as a binary number, for example, if we have a tape: 0001101(0)0011000, the left part is 11010, which is 26.
  • Treat the right part as a binary number written backward, on the last example, our right part is 1100, 12 on decimal.
  • Now, we need to encode those numbers, we can use ‘aa’ with 26 copies of ‘bb’ and ‘cc’ with 12 copies of ‘dd’.
  • In our example, one way of moving the head to the right is by doubling the left part and halving the right part: 26 becomes 52, that in binary is 110100, and 12 becomes 6, 110 in binary, so, our new tape is 011010(0)011000. To read from the tape, we could just check if the number on the left is an even or odd, and writing 1 or 0 to the tape means incrementing or decremeting that number.
  • We can represent the current state with the encode characters used on the left and right tape numbers, for example, we can use a, b, c and d on state 1, on state 2 we can use e, f, g and h, and so on.
  • Each rule for the machine is a tag system that rewrites the current string in the appropriate way.
  • Combine all those rules to make a large system to simulate the whole machine.

Tag system being capable of simulate a Turing machine give the universal aspect to it.

Cyclic Tag System

The cyclic tag system is simpler, but, this is because some extra constraints are used to implement it:

  • A cyclic tag system’s string can contain only 1’s and 0’s.
  • The rule can only apply when the current string begins with 1.
  • It’s deletion number is always 1.

Those constraints are very severe, but, to compensate it, there’s an aspect on this system, the cyclic aspect. The cyclic aspect consists on the following execution: the first rule is the current rule when the execution begins, but, after each step, the next rule becomes the current rule, this way, the first rule will be called again when the rulebook’s end is reached. Let’s see an example with three rules: the character 1, 0010 and 10, respectively, are appended. Let’s start with the string 11:

11 11 10010 001010 001010 01010 1010 01010 1010 0100010 100010

So, you can see that is very simple, but, we never know what will happens next, isn’t obvious. The question is, it will get longer and longer, or it will settle into a repeating pattern of contradictions? We would need to keep running the system to find out.

Let’s implement it from the normal tag system, so we don’t have a lot of extra work. Let’s start by the rules, we just need to hardcode the first character using 1:

class CyclicTagRule < TagRule FIRST_CHARACTER = '1'
def initialize(append_characters) super(FIRST_CHARACTER, append_characters) end
def inspect "#<CyclicTagRule #{append_characters.inspect}>" end end

However, the rulebook is different, we’ll implement it providing a new #applies_to? and #next_string:

class CyclicTagRulebook < DELETION_NUMBER = 1

def initialize(rules) super(rules.cycle) end
def applies_to?(string) string.length >= DELETION_NUMBER end

def next_string(string) follow_next_rules(string).slice(DELETION_NUMBER..-1) end
def follow_next_rules(string) rule =
if rule.applies_to?(string) rule.follow(string) else string end end end </code>

Different from the normal rulebook, this one always applies to any nonempty string, even if the current rule doesn’t.

Now we can create a rulebook full of rules from the cyclic tag system, using the normal tag system object:

rulebook =['1'),'0010'),'10') ]) => #<struct CyclicTagRulebook ...> system ='11', rulebook) => #<struct TagSystem ...> 16.times do puts system.current_string system.step end; puts system.current_string 11 11 10010 001010 01010 1010 01010 1010 0100010 100010 000101 00101 0101 101 010010 10010 00101 => nil

We can notice the same behavior we saw on the execution by hand, now, let’s go further on it:

20.times do puts system.current_string system.step end; puts system.current_string 00101 0101 101 011 11 110 101 010010 10010 00101 0101 101 011 11 110 101 010010 10010 00101 0101 101 => nil

So, it turns out that this sytem sttle down into a repetitive behavior. Cyclic tag systems are very limited, but, it’s possible to use them to simulate any tag system. We can do it following those steps:

  • Determine a alphabet(set of characters used).
  • Design an encoding scheme that associates each character with a unique string suitable for use in a cyclic tag system.
  • Convert each of the original system’s rule into a cyclic tag system rule by encoding the characters it appends.
  • Fill the cyclic tag system’s rulebook with empty rules to simulate deletion.
  • Encode the orignal tag system’s input string and use it as input to the cyclic tag system.

Let’s start by adding the alphabet on the tag system implemetation:

class TagRule def alphabet ([first_character] + append_characters.chars.entries).uniq end end
class TagRulebook def alphabet rules.flat_map(&:alphabet).uniq end end
class TagSystem def alphabet (rulebook.alphabet + current_string.chars.entries).uniq.sort end end

Let’s test it:

rulebook =, ['a', 'ccdd'),'b', 'dd') ] ) => #<struct TagRulebook ...> system ='aabbbbbb', rulebook) => #<struct TagSystem ...> system.alphabet => ["a", "b", "c", "d"]

Now, we need to figure out how to encode each character as a string that the cyclic tag system can use. We can use the scheme that each character is represented as a string of 0s with the same length as the alphabet, with a single 1 character in a position that reflects that character’s position in the alphabet. So, for example, 1 would be position 0, we could encode it with 1000, and so on.

To implement this encoding scheme, we’ll need to create a object called CyclicTagEncoder that gets constructed with a specific alphabet and then asked to encode strings of characters from that alphabet:

class CyclicTagEncoder < def encode_string(string) { |character| encode_character(character) }.join end
def encode_character(character) character_position = alphabet.index(character) (0...alphabet.length).map { |n| n == character_position ? "1" : "0" }.join end end
class TagSystem def encoder end end

Now we can encode any string mapde up of a,b, c and d:

encoder = system.encoder => #<struct CyclicTagEncoder alphabet=["a", "b", "c", "d"]> encoder.encode_character('c') => "0010" encoder.encode_string('cab') => "001010000100"

The encoder give us a way to convert each tag system rule into a cyclic tag system rule. We just need to encode the append_characters of a TagRule ans use the resulting string to build a CyclicTagRule:

class TagRule def to_cyclic(encoder) end end

Let’s see it working on one rule:

rule = system.rulebook.rules.first => #<struct TagRule first_character="a", append_characters="ccdd"> rule.to_cyclic(encoder) => #<CyclicTagRule "0010001000010001">

Now that we converted the append_characters, we lost the first_character information, that triggers the rule, reminding that in cyclic tag system, just the character 1 triggers the rule.

For the conversion, that information will be communicated by the order of the rule in the cyclic tag system: the first rule is for the first character and so on. Any character without a correspoding rule will get a blank rule. Let’s implement a #cyclic_rules for the normal rulebook.

class TagRulebook def cyclic_rules(encoder) { |character| cyclic_rule_for(character, encoder) } end
def cyclic_rule_for(character, encoder) rule = rule_for(character)
if rule.nil?'') else rule.to_cyclic(encoder) end end end

And this is what #cyclic_rules produces:

system.rulebook.cyclic_rules(encoder) => [ #<CyclicTagRule "0010001000010001">, #<CyclicTagRule "00010001">, #<CyclicTagRule "">, #<CyclicTagRule ""> ]

We converted the rules for a and b, that appears first, and the other two blank rules for c and d.

Now, we need to simulate the deletion part, and that’s easy, just inserting extra empty rules into cyclic tag system’s rulebook so that the right number of characters get deleted after a single encoded character has been successfully processed. Let’s implement it:

class TagRulebook def cyclic_padding_rules(encoder),'')) * (deletion_number - 1) end end

Our tag system has 4 characters and deletion number of 2, so we need to add four empty rules to delete one simulated character in addition to the one that already gets deleted by the converted rules:

class TagRulebook def to_cyclic(encoder) + cyclic_padding_rules(encoder)) end end
class TagSystem def to_cyclic, rulebook.to_cyclic(encoder)) end end

Let’s see what happens qhen we convert our number-incremeting tag system and run it:

conversionExample.rb(main):034:0* cyclic_system = system.to_cyclic => #<struct TagSystem current_string="10001000010001000100010001000100", rulebook=#<struct CyclicTagRulebook rules=#<Enumerator: [#<CyclicTagRule "0010001000010001">, #<CyclicTagRule "00010001">, #<CyclicTagRule "">, #<CyclicTagRule "">, #<CyclicTagRule "">, #<CyclicTagRule "">, #<CyclicTagRule "">, #<CyclicTagRule "">]:cycle>>> conversionExample.rb(main):035:0> 10001000010001000100010001000100 00010000100010001000100010001000010001000010001 0010000100010001000100010001000010001000010001 010000100010001000100010001000010001000010001 10000100010001000100010001000010001000010001 0000100010001000100010001000010001000010001 000100010001000100010001000010001000010001 00100010001000100010001000010001000010001 0100010001000100010001000010001000010001 100010001000100010001000010001000010001 0001000100010001000100001000100001000100010001 001000100010001000100001000100001000100010001 01000100010001000100001000100001000100010001 1000100010001000100001000100001000100010001 000100010001000100001000100001000100010001 00100010001000100001000100001000100010001 0100010001000100001000100001000100010001 100010001000100001000100001000100010001 0001000100010000100010000100010001000100010001 001000100010000100010000100010001000100010001 01000100010000100010000100010001000100010001 1000100010000100010000100010001000100010001 000100010000100010000100010001000100010001 00100010000100010000100010001000100010001 0100010000100010000100010001000100010001 100010000100010000100010001000100010001 0001000010001000010001000100010001000100010001 001000010001000010001000100010001000100010001 01000010001000010001000100010001000100010001 1000010001000010001000100010001000100010001 000010001000010001000100010001000100010001 00010001000010001000100010001000100010001 0010001000010001000100010001000100010001 010001000010001000100010001000100010001 10001000010001000100010001000100010001 0001000010001000100010001000100010001 001000010001000100010001000100010001 01000010001000100010001000100010001 1000010001000100010001000100010001 000010001000100010001000100010001 00010001000100010001000100010001 0010001000100010001000100010001 010001000100010001000100010001 10001000100010001000100010001 0001000100010001000100010001 001000100010001000100010001 01000100010001000100010001 1000100010001000100010001 000100010001000100010001 00100010001000100010001 0100010001000100010001 100010001000100010001 00010001000100010001 0010001000100010001 010001000100010001 10001000100010001 0001000100010001 001000100010001 01000100010001 1000100010001 000100010001 00100010001 0100010001 100010001 00010001 0010001 010001 10001 0001 001 01 1 => nil

We can see that:

  • The encoded version of the tag system’s a rule kicks in here.
  • The first full character of the simulated string has been processed, so the following four steps use blank rules to delete the next simulated character.
  • After eight steps of the cyclic tag system, one full step of the simulated tag system is completed.

In the next post, we’re going to see more universal systems. If you have any doubts about this post, talk with me on Facebook or Twitter