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Finite state machine
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A finite state machine (FSM) or finite state automaton (plural: automata) or simply a state machine, is a model of behavior composed of a finite number of states, transitions between those states, and actions. A finite state machine is an abstract model of a machine with a primitive internal memory.
rrent state is determined by past states of the system.
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Encyclopedia
A finite state machine (FSM) or finite state automaton (plural: automata) or simply a state machine, is a model of behavior composed of a finite number of states, transitions between those states, and actions. A finite state machine is an abstract model of a machine with a primitive internal memory.
Concepts and vocabulary
A current state is determined by past states of the system. As such, it can be said to record information about the past, i.e. it reflects the input changes from the system start to the present moment. A transition indicates a state change and is described by a condition that would need to be fulfilled to enable the transition. An action is a description of an activity that is to be performed at a given moment. There are several action types:
Entry action: which is performed when entering the state
Exit action: which is performed when exiting the state
Input action: which is performed depending on present state and input conditions
Transition action: which is performed when performing a certain transition
A FSM can be represented using a state diagram (or state transition diagram) as in figure 1 above. Besides this, several state transition table types are used. The most common representation is shown below: the combination of current state (B) and condition (Y) shows the next state (C). The complete actions information can be added only using footnotes. An FSM definition including the full actions information is possible using state tables (see also VFSM).
In addition to their use in modeling reactive systems presented here, finite state automata are significant in many different areas, including electrical engineering, linguistics, computer science, philosophy, biology, mathematics, and logic. A complete survey of their applications is outside the scope of this article. Finite state machines are a class of automata studied in automata theory and the theory of computation.
In computer science, finite state machines are widely used in modeling of application behavior, design of hardware digital systems, software engineering, compilers, network protocols, and the study of computation and languages.
Classification
There are two different groups: Acceptors/Recognizers and Transducers.
Acceptors and recognizers
Acceptors and recognizers (also sequence detectors) produce a binary output, saying either yes or no to answer whether the input is accepted by the machine or not. All states of the FSM are said to be either accepting or not accepting. At the time when all input is processed, if the current state is an accepting state, the input is accepted; otherwise it is rejected. As a rule the input are symbols (characters); actions are not used. The example in figure 2 shows a finite state machine which accepts the word "nice". In this FSM the only accepting state is number 7.
The machine can also be described as defining a language, which would contain every word accepted by the machine but none of the rejected ones; we say then that the language is accepted by the machine. By definition, the languages accepted by FSMs are the regular languages - that is, a language is regular if there is some FSM that accepts it.
Start state
The start state is usually shown drawn with an arrow "pointing at it from nowhere" (Sipser (2006) p.34).
Accept state
An accept state (sometimes referred to as an accepting state) is a state at which the machine has successfully performed its procedure. It is usually represented by a double circle.
An example of an accepting state appears on the right in this diagram of a deterministic finite automaton (DFA) which determines if the binary input contains an even number of 0s.
S1 (which is also the start state) indicates the state at which an even number of 0s has been input and is therefore defined as an accepting state. This machine will give a correct end state if the binary number contains an even number of zeros including a string with no zeros. Examples of strings accepted by this DFA are epsilon (the empty string), 1, 11, 11..., 00, 010, 1010, 10110 and so on.
Transducers
Transducers generate output based on a given input and/or a state using actions. They are used for control applications and in the field of computational linguistics. Here two types are distinguished:
Moore machine: The FSM uses only entry actions, i.e. output depends only on the state. The advantage of the Moore model is a simplification of the behaviour. Consider an elevator door. The state machine recognizes two commands: "command_open" and "command_close" which trigger state changes. The entry action (E:) in state "Opening" starts a motor opening the door, the entry action in state "Closing" starts a motor in the other direction closing the door. States "Opened" and "Closed" don't perform any actions. They signal to the outside world (e.g. to other state machines) the situation: "door is open" or "door is closed".
Mealy machine: The FSM uses only input actions, i.e. output depends on input and state. The use of a Mealy FSM leads often to a reduction of the number of states. The example in figure 4 shows a Mealy FSM implementing the same behaviour as in the Moore example (the behaviour depends on the implemented FSM execution model and will work e.g. for virtual FSM but not for event driven FSM). There are two input actions (I:): "start motor to close the door if command_close arrives" and "start motor in the other direction to open the door if command_open arrives". The "opening" and "closing" intermediate states are not shown.
In practice mixed models are often used.
More details about the differences and usage of Moore and Mealy models, including an executable example, can be found in the external technical note
A further distinction is between deterministic (DFA) and non-deterministic (NDFA, GNFA) automata. In deterministic automata, for each state there is exactly one transition for each possible input. In non-deterministic automata, there can be none, one, or more than one transition from a given state for a given possible input. This distinction is relevant in practice, but not in theory, as there exists an algorithm which can transform any NDFA into an equivalent DFA, although this transformation typically significantly increases the complexity of the automaton.
The FSM with only one state is called a combinatorial FSM and uses only input actions. This concept is useful in cases where a number of FSM are required to work together, and where it is convenient to consider a purely combinatorial part as a form of FSM to suit the design tools.edited by eng.mahmoud
FSM logic
The next state and output of an FSM is a function of the input and of the current state. The FSM logic is shown in Figure 5.
Mathematical model
Depending on the type, there are several definitions:
- A deterministic finite state machine is a quintuple , where:
- is the input alphabet (a finite, non-empty set of symbols).
- is a finite, non-empty set of states.
- is an initial state, an element of .
- is the state-transition function: . In a Nondeterministic finite state machine, , returns a set of states.
- is the set of final states, a (possibly empty) subset of .
- An acceptor finite-state machine is a quintuple , where:
- is the input alphabet (a finite, non-empty set of symbols).
- is a finite, non-empty set of states.
- is an initial state, an element of .
- is the state-transition function: .
- is the set of final states, a (possibly empty) subset of .
- A finite state transducer is a sextuple , where:
- is the input alphabet (a finite non empty set of symbols).
- is the output alphabet (a finite, non-empty set of symbols).
- is a finite, non-empty set of states.
- is the initial state, an element of . In a Nondeterministic finite state machine, is a set of initial states.
- is the state-transition function: .
- is the output function.
If the output function is a function of a state and input alphabet that definition corresponds to the Mealy model, and can be modelled as a Mealy machine. If the output function depends only on a state that definition corresponds to the Moore model, and can be modelled as a Moore machine. A finite-state machine with no output function at all is known as a semiautomaton or transition system.
Optimization
Optimizing an FSM means finding the machine with the minimum number of states that performs the same function. The fastest known algorithm doing this is the Hopcroft minimization algorithm. Other techniques include using an Implication table, or the Moore reduction procedure. Additionally, acyclic FSAs can be optimized using a simple bottom up algorithm.
Implementation
Hardware applications
In a digital circuit, an FSM may be built using a programmable logic device, a programmable logic controller, logic gates and flip flops or relays. More specifically, a hardware implementation requires a register to store state variables, a block of combinational logic which determines the state transition, and a second block of combinational logic that determines the output of an FSM. One of the classic hardware implementations is the Richards controller.
Software applications
The following concepts are commonly used to build software applications with finite state machines:
See also
Further reading
General
- Wagner, F., "Modeling Software with Finite State Machines: A Practical Approach", Auerbach Publications, 2006, ISBN 0-8493-8086-3.
- Samek, M., , CMP Books, 2002, ISBN 1-57820-110-1.
- Samek, M., , Newnes, 2008, ISBN 0-75068-706-1.
- Gardner, T., , 2007
- Cassandras, C., Lafortune, S., "Introduction to Discrete Event Systems". Kluwer, 1999, ISBN 0-7923-8609-4.
- Timothy Kam, Synthesis of Finite State Machines: Functional Optimization. Kluwer Academic Publishers, Boston 1997, ISBN 0-7923-9842-4
- Tiziano Villa, Synthesis of Finite State Machines: Logic Optimization. Kluwer Academic Publishers, Boston 1997, ISBN 0-7923-9892-0
- Carroll, J., Long, D. , Theory of Finite Automata with an Introduction to Formal Languages. Prentice Hall, Englewood Cliffs, 1989.
- Kohavi, Z., Switching and Finite Automata Theory. McGraw-Hill, 1978.
- Gill, A., Introduction to the Theory of Finite-state Machines. McGraw-Hill, 1962.
- Ginsburg, S., An Introduction to Mathematical Machine Theory. Addison-Wesley, 1962.
Finite state machines (automata theory) in theoretical computer science
- Extensive, wide-ranging book meant for specialists, written for both theoretical computer scientists as well as electrical engineers. With detailed explanations of state minimization techniques, FSMs, Turing machines, Markov processes, and undecidability. Excellent treatment of Markov processes.
- Excellent. Has been in print in various editions and reprints since 1974 (1974, 1980, 1989, 1999).
- Approaches Church-Turing thesis from three angles: levels of finite automata as acceptors of formal languages, primitive and partial recursive theory, and power of bare-bones programming languages to implement algorithms, all in one slim volume.
- A difficult book centered around the issues of machine-interpretation of "languages", NP-Completeness, etc.
- Distinctly different and less intimidating than the first edition.
- Minsky spends pages 11-20 defining what a “state” is in context of FSMs. His state diagram convention is unconventional. Excellent, i.e. relatively readable, sometimes funny.
- Abstract algebra is at the core of the book, rendering it advanced and less accessible than other texts.
- cf Finite state machines (finite automata) in chapter 29.
Abstract state machines in theoretical computer science
- Yuri Gurevich (2000), Sequential Abstract State Machines Capture Sequential Algorithms, ACM Transactions on Computational Logic, vl. 1, no. 1 (July 2000), pages 77-111. http://research.microsoft.com/~gurevich/Opera/141.pdf
Machine learning using finite-state algorithms
- A broad brush but quite thorough and sometimes difficult, meant for computer scientists and engineers. Chapter 13 Reinforcement Learning deals with robot-learning involving state-machine-like algorithms.
Hardware engineering: state minimization and synthesis of sequential circuits
- Extensive, wide-ranging book meant for specialists, written for both theoretical computer scientists as well as electrical engineers. With detailed explanations of state minimization techniques, FSMs, Turing machines, Markov processes, and undecidability. Excellent treatment of Markov processes.
- Meant for electrical engineers. More focused, less demanding than his earlier book. His treatment of computers is out-dated. Interesting take on definition of ‘algorithm’.
- Meant for hardware electrical engineers. With detailed explanations of state minimization techniques and synthesis techniques for design of combinatory logic circuits.
- Meant for hardware electrical engineers. Excellent explanations of state minimization techniques and synthesis techniques for design of combinatory and sequential logic circuits.
Finite Markov chain processes
"We may think of a Markov chain as a process that moves successively through a set of states s1, s2, ..., sr. ... if it is in state si it moves on to the next stop to state sj with probability pij. These probabilities can be exhibited in the form of a transition matrix" (Kemeny (1959), p. 384)
Finite Markov-chain processes are also known as subshifts of finite type.
- Extensive, wide-ranging book meant for specialists, written for both theoretical computer scientists as well as electrical engineers. With detailed explanations of state minimization techniques, FSMs, Turing machines, Markov processes, and undecidability. Excellent treatment of Markov processes.
- Classical text . cf Chapter 6 ‘’Finite Markov Chains”.
External links
- NIST Dictionary of Algorithms and Data Structures
- - Cogs is a open source, small FSM and HSM ( hierarchical state machine) library for use with Flash / Flex Actionscript 3.0 projects. An introductory talk
- - FSMGenerator is a turn-key solution for FSM (Finite State Machine) automatic generation and integration within user's software
- - An open-source tool for the graphical development of FSMs and automatic Verilog code generation
- - StateWizard: a ClassWizard-like round-trip UML dynamic modeling/development framework and tool running in popular IDEs.
- - generates human readable c-code from state-charts especially targeting embedded real-time systems
- - An open-source C library for automata computations
- - A tool to model and compile statemachines for a number of languages; java, C# etc. As well as display them as graphically.
- - Provider of open source, state machine-based frameworks. Includes several examples and C/C++ code.
- - An open source implementation of FSM (Finite State Machine) programming framework in C.
- - An open source C++ finite state machine manipulation platform, consisting of a library and tools implemented on top of it.
- - An open source plugin for Eclipse for automata-based programming.
- - A simple but effective FSM implementation integrated seamlessly into the PureMVC Framework.
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