NumberConverter.kt

import java.math.BigDecimal
import java.math.RoundingMode
import kotlin.math.pow

const val EXIT = "/exit"
const val BACK = "/back"

const val BASE_10 = 10
const val MAX_LENGTH_FRACTIONAL_PART = 5
const val FRACTIONAL_PART_DELIMITER = '.'

val symbols = ('0'..'9').plus('a'..'z')

fun main() {
    val entryMessage =
        "Enter two numbers in format: {source base} {target base} (To quit type /exit): "
    print(entryMessage)
    var entryInput = readln()
    while (entryInput != EXIT) {
        val (sourceBase, targetBase) = entryInput.split(" ").map { it.toInt() }
        val inputNumberMessage =
            "Enter number in base $sourceBase to convert to base $targetBase (To go back type /back): "
        print(inputNumberMessage)
        var numberToConvert = readln()
        while (numberToConvert != BACK) {
            println(
                "Conversion result: ${
                    convertNumberFromBaseToBase(
                        numberToConvert,
                        sourceBase,
                        targetBase
                    )
                }"
            )
            println()
            print(inputNumberMessage)
            numberToConvert = readln()
        }
        println()
        print(entryMessage)
        entryInput = readln()
    }
}

fun convertNumberFromBaseToBase(numberToConvert: String, sourceBase: Int, targetBase: Int): String {
    val integerPartInBase10 = convertIntegerPartToBase10(numberToConvert, sourceBase)
    val fractionalPartInBase10 = convertFractionalPartToBase10(numberToConvert, sourceBase)
    val numberInBase10 = integerPartInBase10 + fractionalPartInBase10
    val integerPartInTargetBase =
        convertIntegerPartFromBase10ToTargetBase(numberInBase10, targetBase)
    val fractionalPartInTargetBase =
        convertFractionalPartFromBase10ToTargetBase(numberInBase10, targetBase)
    val convertedNumber = if (fractionalPartInTargetBase != "") {
        "$integerPartInTargetBase.$fractionalPartInTargetBase"
    } else {
        integerPartInTargetBase
    }
    return convertedNumber
}

fun convertIntegerPartToBase10(sourceNumber: String, sourceBase: Int): BigDecimal {
    val truncatedSourceNumber = sourceNumber.substringBefore(FRACTIONAL_PART_DELIMITER, sourceNumber)
    if (sourceBase != BASE_10) {
        var convertedIntegerPart = BigDecimal.ZERO
        val reversedIntegerPart = truncatedSourceNumber.reversed()
        for (i in reversedIntegerPart.indices) {
            convertedIntegerPart += BigDecimal(
                (mapSymbolToNumber(reversedIntegerPart[i]) * sourceBase.toDouble().pow(i)).toString()
            )
        }
        return convertedIntegerPart.setScale(0, RoundingMode.DOWN)
    }
    return BigDecimal(truncatedSourceNumber)
}

fun convertFractionalPartToBase10(sourceNumber: String, sourceBase: Int): BigDecimal {
    var convertedFractionalPart: BigDecimal
    if (!sourceNumber.contains(FRACTIONAL_PART_DELIMITER)) {
        convertedFractionalPart = BigDecimal.ZERO
    } else {
        if (sourceBase == BASE_10) {
            convertedFractionalPart = BigDecimal(sourceNumber).remainder(BigDecimal.ONE)
        } else {
            convertedFractionalPart = BigDecimal.ZERO
            val fractionalPartWithoutPoint = sourceNumber.substringAfter(FRACTIONAL_PART_DELIMITER, "0")
            for (i in fractionalPartWithoutPoint.indices) {
                convertedFractionalPart += BigDecimal(
                    (mapSymbolToNumber(fractionalPartWithoutPoint[i]) * sourceBase.toDouble().pow(-(i + 1))).toString()
                )
            }
        }
    }
    return convertedFractionalPart
}

fun convertIntegerPartFromBase10ToTargetBase(sourceNumber: BigDecimal, targetBase: Int): String {
    val truncatedSourceNumber = sourceNumber.setScale(0, RoundingMode.DOWN)
    if (targetBase != BASE_10) {
        val remainders = mutableListOf<Char>()
        var quotation = truncatedSourceNumber
        var remainder: BigDecimal
        while (quotation > BigDecimal.ZERO) {
            remainder = quotation % targetBase.toBigDecimal()
            remainders.add(0, mapNumberToSymbol(remainder.toInt()))
            quotation = quotation.divide(targetBase.toBigDecimal(), RoundingMode.DOWN)
        }
        return (
                if (truncatedSourceNumber == BigDecimal.ZERO) truncatedSourceNumber.toString()
                else remainders.joinToString("")
                )
    }
    return truncatedSourceNumber.toString()
}

fun convertFractionalPartFromBase10ToTargetBase(sourceNumber: BigDecimal, targetBase: Int): String {
    var fractionalPartFromBase10 = sourceNumber.remainder(BigDecimal.ONE)
    var fractionalPartTargetBase = ""
    if (fractionalPartFromBase10 != BigDecimal.ZERO) {
        if (targetBase == BASE_10) {
            fractionalPartTargetBase = fractionalPartFromBase10.toString().substringAfter(FRACTIONAL_PART_DELIMITER).take(MAX_LENGTH_FRACTIONAL_PART)
        } else {
            val symbolsConvertedNumber = mutableListOf<Char>()
            var lengthFractionalPart = 0
            while (fractionalPartFromBase10 > BigDecimal.ZERO && lengthFractionalPart < MAX_LENGTH_FRACTIONAL_PART) {
                val integerPart = (fractionalPartFromBase10 * targetBase.toBigDecimal()).setScale(0, RoundingMode.DOWN)
                symbolsConvertedNumber.add(mapNumberToSymbol(integerPart.toInt()))
                fractionalPartFromBase10 = fractionalPartFromBase10 * targetBase.toBigDecimal() - integerPart
                lengthFractionalPart++
            }
            fractionalPartTargetBase = symbolsConvertedNumber.joinToString("")
        }
        if (fractionalPartTargetBase.length < MAX_LENGTH_FRACTIONAL_PART) {
            fractionalPartTargetBase += "0".repeat(MAX_LENGTH_FRACTIONAL_PART - fractionalPartTargetBase.length)
        }
    }
    return fractionalPartTargetBase
}

fun mapSymbolToNumber(symbol: Char): Int {
    return symbols.indexOf(symbol)
}

fun mapNumberToSymbol(number: Int): Char {
    return symbols.elementAt(number)
}

/*
Test cases:

Enter two numbers in format: {source base} {target base} (To quit type /exit) > 10 7
Enter number in base 10 to convert to base 7 (To go back type /back) > 0.234
Conversion result: 0.14315

Enter number in base 10 to convert to base 7 (To go back type /back) > 10.234
Conversion result: 13.14315

Enter number in base 10 to convert to base 7 (To go back type /back) > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit) > 35 17
Enter number in base 35 to convert to base 17 (To go back type /back) > af.xy
Conversion result: 148.g88a8

Enter number in base 35 to convert to base 17 (To go back type /back) > aaaa.0
Conversion result: 54e36.00000

Enter number in base 35 to convert to base 17 (To go back type /back) > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit) > 21 10
Enter number in base 21 to convert to base 10 (To go back type /back) > 4242
Conversion result: 38012

Enter number in base 21 to convert to base 10 (To go back type /back) > 4242.13a
Conversion result: 38012.05550

Enter number in base 21 to convert to base 10 (To go back type /back) > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 5
Enter number in base 2 to convert to base 5 (To go back type /back): > 11101001.00000
Conversion result: 1413.00000

Enter number in base 2 to convert to base 5 (To go back type /back): > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 8
Enter number in base 2 to convert to base 8 (To go back type /back): > 1101111000.10000
Conversion result: 1570.40000

Enter number in base 2 to convert to base 8 (To go back type /back): > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 11
Enter number in base 2 to convert to base 11 (To go back type /back): > 111111.11000
Conversion result: 58.83838

Enter number in base 2 to convert to base 11 (To go back type /back): > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 14
Enter number in base 2 to convert to base 14 (To go back type /back): > 1000100.00000
Conversion result: 4c.00000

Enter number in base 2 to convert to base 14 (To go back type /back): > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 17
Enter number in base 2 to convert to base 17 (To go back type /back): > 1010011.00000
Conversion result: 4f.00000

Enter number in base 2 to convert to base 17 (To go back type /back): > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 20
Enter number in base 2 to convert to base 20 (To go back type /back): > 101010111.10000
Conversion result: h3.a0000

Enter number in base 2 to convert to base 20 (To go back type /back): > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 23
Enter number in base 2 to convert to base 23 (To go back type /back): > 1010010011.10000
Conversion result: 15f.bbbbb

Enter number in base 2 to convert to base 23 (To go back type /back): > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 26
Enter number in base 2 to convert to base 26 (To go back type /back): > 1000011101.01000
Conversion result: kl.6d000

Enter number in base 2 to convert to base 26 (To go back type /back): > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 29
Enter number in base 2 to convert to base 29 (To go back type /back): > 100011001.00000
Conversion result: 9k.00000

Enter number in base 2 to convert to base 29 (To go back type /back): > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 32
Enter number in base 2 to convert to base 32 (To go back type /back): > 1001100111.10000
Conversion result: j7.g0000

Enter number in base 2 to convert to base 32 (To go back type /back): > /back

Enter two numbers in format: {source base} {target base} (To quit type /exit): > 2 35
Enter number in base 2 to convert to base 35 (To go back type /back): > 100001110.01000
Conversion result: 7p.9q9q9
*/