green-fsm

This is a small finite state machine library which I built mainly for my own use.

Installation

npm install green-fsm

Example

import { fsm } from 'green-fsm'

const a = fsm(
  ['a', 'b'],   // alphabet
  ['0', '1'],   // states
  '0',          // initial state
  ['1'],        // final states
  {0: {a: '1'}} // transition map
)

console.log(a.accepts([]))         // false
console.log(a.accepts(['a']))      // true
console.log(a.accepts(['b']))      // false
console.log(a.accepts(['a', 'a'])) // false
console.log(a.accepts(['c']))      // throws exception

API

green-fsm exposes the following properties and functions. These are a little up in the air right now.

fsm(alphabet, states, finals, map)

Build a finite state machine according to the supplied parameters. Symbols in the alphabet and states are used as keys in Objects, so they should be either Strings or Symbols. states[0] is the initial state.

map may be sparse. If a transition is missing from map, then it is assumed that this transition leads to an undocumented "oblivion state" which is not final. This oblivion state does not appear when the FSM is printed out.

The resulting object has some properties and methods on it.

alphabet

states

finals

map

These are just the properties which were originally passed in.

follow(state, symbol)

Use this for preference over just looking up transitions in the map - it handles sparse transitions and suchlike correctly.

hasFinalState(state)

Use this to determine whether a given state is final in this FSM.

accepts(input)

input should be an array of symbols chosen from this FSM's alphabet. Returns a Boolean indicating whether the input is accepted.

toString()

Pretty-prints this FSM's structure.

strings()

Returns an object conforming to the iterator protocol. This means it has a single property, next, which is a function which can be called repeatedly. At first, calling next will return results of the form {value, done: false} where value is an accepted input of the FSM i.e. an array of symbols. If the FSM is finite, eventally results will take the form {done: true}.

ANYTHING_ELSE

Ordinarily, you may only feed known alphabet symbols into the FSM. Any other symbol will result in an exception being thrown. However, if you add the special Symbol ANYTHING_ELSE to your alphabet, then any unrecognised symbol will be automatically converted into ANYTHING_ELSE before following whatever transition you have specified for this symbol.

crawl(alphabet, initial, final, follow)

Crawl what is assumed to be an FSM and return a new finite state machine object representing it. Starts at state initial. At any given state, crawl calls final(state) to determine whether it is final. Then, for each symbol in alphabet, it calls follow(state, symbol) to try to discover new states. Obviously this procedure could go on for ever if your implementation of follow is faulty.

OBLIVION_STATE

Your implementation of follow (above) may also return the special Symbol OBLIVION_STATE to indicate that you have reached an inescapable, non-final "oblivion state". This state and transitions to it will be omitted from the resulting FSM.

nothing(alphabet)

Returns an FSM over the supplied alphabet which accepts no inputs at all.

epsilon(alphabet)

Returns an FSM over the supplied alphabet which accepts only the empty input, [].

union(fsms)

Returns an FSM accepting all inputs accepted by any of the supplied input FSMs.

intersection(fsms)

Returns an FSM accepting all inputs accepted by all of the supplied input FSMs.

concatenate(fsms)

Returns an FSM accepting any input a·b·... where a is an input accepted by the first FSM, b is an input accepted by the second FSM, and so on.

multiply(fsm, multiplier)

Returns an FSM equivalent to the concatenation of multiplier instances of the original FSM.

star(fsm)

Kleene star closure. Turns an FSM accepting only ['a'] into one accepting any of [], ['a'], ['a', 'a'], ...