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*What is a Turing machine?*Depending on who you ask, it's either an abstract model of an algorithmic machine or an esoteric programming language. It's named after its creator, Alan Turing– an English cryptologist.

The machine consists of:

- an infinitely long (in real life: long enough to do all practical programs) tape with
**symbols**that can be moved left and right - a "head" over one of the symbols that can read it and overwrite it with a new one,
- some registers containing the
**state**of the machine, - a table linking each combination of state and symbol to an instruction what to do next.

…and the working cycle of the machine looks like this:

- read the symbol from the tape
- based on the symbol and the state, see what to do next in the table
- based on the instruction, go to a new state and print a new symbol in place of the one that has been read
- also based on the instruction, move the tape 1 symbol left or right or exit the program

The cycle continues until the machine stops. This way it can execute any computer algorithm.

*About the model*The model has 4 (2²) possible symbols and 8 (2³) possible states, so in total 32 possible symbol-state combinations. Each instruction has 7 bits (3 for the state, 2 for the symbol, 1 for moving left/right and 1 for stopping), so the "source code" takes 7*32 = 224 bits or 14 bytes. That means you can make 2^224 ≈

**2.69*10⁶⁷ programs**!**(most of them will be useless though).**The mechanism is also quiet complex for a Lego set. It connects all the functions to 1 input, so that the model requires

**no**electric motor. You can see how it works in the video.

*Won't there be a problem with Intellectual Property?*No, the Turing machine is a mathematical model and has no Intellectual Property :)

*Isn't it too big?*The biggest disadvantage of the model is that it has ~2900 parts and barely fits of Lego IDEAS' limit of 3000 parts. However, a lot of them are functional (i.e. part of the mechanism) or used to make the framework more sturdy because the mechanism is very precise and needs good support.

*Why would it be a good set?*Because:

- You can play with it,
- You can make your own programs,
- It has an interesting mechanism inside,
- Its an implementation of a popular theoretical model.