Building a State Machine

I have been doing coding challenges every day and found interesting approach on how to solve Valid Number? coding challenge.

The challenge is to create function that will take string and return boolean

4.325 is a valid number.
1.1.1 is NOT a valid number.
222 is a valid number.
22. is NOT a valid number.
0.1 is a valid number.
22.22. is NOT a valid number.
1. is NOT a valid number.

You can try this for yourself and implement the function bellow.

function isNumberValid(string) {
  //TODO: Write - Your - Code
}

Hints

• use regex or
• implement state machine

…

Solution

For State Machine

To solve this problem with state machine we have an object or hash table that contains different states and a nextState function that would make transition from one state to the next.

const STATE = {
  START: 'START',
  INTEGER: 'INTEGER',
  DECIMAL: 'DECIMAL',
  AFTER_DECIMAL: 'AFTER_DECIMAL',
  UNKNOWN: 'UNKNOWN',
};

function nextState(state, chr) {
  const isInteger = chr >= '0' && chr <= '9';
  if (state === STATE.START) {
    if (isInteger) return STATE.INTEGER;
    return STATE.UNKNOWN;
  }

  if (state === STATE.INTEGER) {
    if (isInteger) return STATE.INTEGER;
    if (chr === '.') return STATE.DECIMAL;
  }

  if (state === STATE.DECIMAL) {
    if (isInteger) return STATE.AFTER_DECIMAL;
    return STATE.UNKNOWN;
  }

  if (state === STATE.AFTER_DECIMAL) {
    if (isInteger) return STATE.AFTER_DECIMAL;
    return STATE.UNKNOWN;
  }
}

function isNumberValid(string) {
  let currState = STATE.START;
  for (let chr of string) {
    currState = nextState(currState, chr);
    if (currState == STATE.UNKNOWN) return false;
  }
  if (currState === STATE.DECIMAL) return false;
  return true;
}