Deterministic finite-state acceptor

FSM.js

/**
 * Thanks for this FSM implementation to Zakhar Ovcharov (https://www.linkedin.com/in/zakhar-ovcharov-a657a4197/)
 * I did small adjustments to meet requirements of the specific task
 */

class FSM {
  /**
   * Each instance of this class will have a certain alphabet, a list of states and transitions, and could be used to test some value against these params
   * @param {String} alphabet
   * @param {String[]} states
   * @param {String} startState
   * @param {String[]} finiteStates
   * @param {Object} transitions
   */
  constructor(alphabet, states, startState, finiteStates, transitions) {
    this.alphabet = alphabet
    this.states = states
    this.startState = startState
    this.transitions = transitions
    this.finiteStates = finiteStates
    this.currentState = null
  }

  /**
   * Check if the symbol belongs to the specified alphabet
   * @param {String} symbol
   * @returns {Boolean}
   */
  _checkIsBelongAlphabet(symbol) {
    return this.alphabet.includes(symbol)
  }

  /**
   * Change the current state depending on the symbol. FSM should abort if there is no transition for the current symbol on the current state
   * @param {String} symbol
   */
  _changeState(symbol) {
    if (
      this.transitions[this.currentState] &&
      this.transitions[this.currentState][symbol]
    ) {
      this.currentState = this.transitions[this.currentState][symbol]
    } else {
      throw new Error(`No transition from ${this.currentState} by ${symbol}`)
    }
  }

  /**
   * Test some value against specified params. Accepts value if the finiteStates array includes the last current state and rejects otherwise
   * @param {String} value
   * @returns {String}
   */
  test(value) {
    this.currentState = this.startState

    for (let symbol of value) {
      if (this._checkIsBelongAlphabet(symbol)) {
        this._changeState(symbol)
      } else {
        return false
      }
    }

    return this.finiteStates.includes(this.currentState)
  }
}