#!/usr/bin/env node

"use strict";

var level = 5;

// FUNCTIONS
Array.prototype.rand = function (){
    return this[Math.floor(Math.random() * this.length)];
};

function rand(min, max){
    return Math.floor(Math.random() * (max-min+1))+min;
}

function randOperator(exceptOne){
    var operators = ["+", "-", "*", "/"];
    if (exceptOne) {
        operators.splice(operators.indexOf(exceptOne), 1);
    }
    return operators.rand();
}

function getResult(arr){
    return eval(arr.slice(0, arr.length).join(''));
}

var operators = [],
    expressionMask = '',
    expressionMaskArray = [];

// GENERATE OPERATORS
for (var i = 0; i < level-1; i++) {
    var operator = randOperator();
    operators.push(operator);
}

// GENERATE BRACKETS
for (var i = 0; i < operators.length; i++) {
    var operator = operators[i],
        next = operators[i+1] || operator;

    if (operator != next) {
        expressionMask += '(';
    }

    expressionMask += operator;

    if (operator != next) {
        expressionMask += ')';
    }
}
expressionMask = expressionMask.replace(/\)\(/g, '');

// ADD PALCEHOLDERS FOR NUMBERS
if (expressionMask.substr(0, 1) != '(' && expressionMask.substr(0, 1) != ')') {
    expressionMask = '1'+expressionMask;
}
if (expressionMask.substr(-1) != '(' && expressionMask.substr(-1) != ')') {
    expressionMask += '1';
}

expressionMask = expressionMask.replace(/(\(|\))/g, function (bracket){
    return bracket == '('
        ? '(1'
        : '1)';
});

var doubleSymbolsMask = /[^0-9)(]{2}/g;
while (doubleSymbolsMask.test(expressionMask)) {
    expressionMask = expressionMask.replace(doubleSymbolsMask, function (doubleSymbols){
        return doubleSymbols.substr(0, 1)+'1'+doubleSymbols.substr(1);
    });
}

// REPLACE PLACEHOLDERS WITH RAND NUMBERS
expressionMaskArray = expressionMask.split('');
for (var i = 0; i < expressionMaskArray.length; i++) {
    if (expressionMaskArray[i] > 0) {
        expressionMaskArray[i] = rand(1, 100);
    }
}

// CHANGES WHILE RESULT IS NOT VALID
var result = getResult(expressionMaskArray),
    randChar,
    randIndex,
    replaceTo;
while (result <= 0 || result % 1 !== 0) {
    randIndex = rand(0, expressionMaskArray.length-1);
    randChar = expressionMaskArray[randIndex];

    if (['(', ')', ''].indexOf(randChar) != -1) continue;

    replaceTo = randChar > 0
        ? rand(1, 100)
        : randOperator(randChar);

    expressionMaskArray[randIndex] = replaceTo;

    result = getResult(expressionMaskArray);
}

console.log(expressionMaskArray.join(''), '=', eval(expressionMaskArray.join('')));