fun main(args: Array<String>) {
    doPart1()
    doPart2()
    doPart3()
    doPart4()
}

fun hasRequiredDigits(number: Int, digits: Int, canBeGreater: Boolean = false): Boolean {
    val numberLength = Math.abs(number).toString().length
    if(canBeGreater)
        if(numberLength >= digits) return true
        else {
            println("Number must be $digits-digit or greater.")
            return false
        }
    if(numberLength == digits) return true
    else {
        println("Number is not $digits-digit.")
        return false
    }
}

fun isNatural(number: Int): Boolean {
    if(number > 0) return true
    else {
        println("Number is not natural.")
        return false
    }
}

fun inputNumber(objectName: String,
                digitsNeeded:Int = 0,
                naturalNeeded: Boolean = false) :Int {
    var inputValue: Int
    while(true) {
        try{
            println("\nInput the value of $objectName, please.")
            inputValue = readLine()!!.toInt()
            if(digitsNeeded == 0 || hasRequiredDigits(inputValue, digitsNeeded))
                if(!naturalNeeded || isNatural(inputValue)) break
        }
        catch(e: NumberFormatException){
            println("An integer value is expected. ")
        }
    }
    return inputValue
}

fun doPart1 () {
    println("--Part 1, variant 4: get sum of all whole numbers from a to b.")
    var result = 0
    val a = inputNumber("a")
    var b: Int
    while(true) {
        b = inputNumber("b")
        if(b < a) println("b can NOT be less than a.")
        else break
    }
    for(i in a..b) result += i
    println("Sum of the numbers from a to b is $result.")
}

fun doPart2() {
    println("\n--Part 2, variant 4: check if digits of a two-digit number are equal.")
    val number = inputNumber("two-digit number",2)
    val numberInString = Math.abs(number).toString()
    if(numberInString[0]==numberInString[1]) println("Digits are equal.")
    else println("Digits are NOT equal.")
}

fun doPart3() {
    println("\n--Part 3, variant 4: check if sum of digits of a four-digit number is divisible by 3.")
    val number = Math.abs(inputNumber("four-digit number"))
    var numberInString = number.toString()
    var sum = 0
    for(i in numberInString) sum+= i.toInt()
    println("Sum of digits is divisible by 3: ${sum % 3 == 0}.")
}

fun doPart4() {
    println("\n--Part 4, variant 4: check if first digit is greater than the last one or vice versa.")
    val number = inputNumber("natural two-digit number",2,true)
    val firstDigit = number.toString()[0].toInt()
    val lastDigit = number % 10
    if(firstDigit > lastDigit) println("First digit is greater than the last one.")
    else if(firstDigit < lastDigit) println("Last digit is greater than the first one")
    else println("First and last digits of the number are equal.")
}