NumberConverter.kt

package converter

import java.math.BigInteger

private const val MAXIMUM_OCTAL_SYSTEM = 10

private const val CAPITAL_LETTER_A = 64

class NumberConverter(
    private val sourceBase: BigInteger,
    private val targetBase: BigInteger
) {

    private fun convertFromDecimal(decimalNumber: String): String {
        var tempNumber = decimalNumber.toBigInteger()
        var result = ""
        do {
            val currentResult = tempNumber.divideAndRemainder(targetBase)[1]
            result += if (currentResult >= MAXIMUM_OCTAL_SYSTEM.toBigInteger()) {
                (CAPITAL_LETTER_A.toBigInteger().plus(currentResult - (MAXIMUM_OCTAL_SYSTEM.toBigInteger() - BigInteger.ONE))).toString()
            } else {
                currentResult.toString()
            }
            tempNumber = tempNumber.divide(targetBase)
        } while (tempNumber >= BigInteger.ONE)

        return result.reversed()
    }

    /**
     * This function converts the input string to decimal number
     * system
     */
    private fun convertToDecimal(numberInput: String): String {
        var result = BigInteger.ZERO
        for (i in numberInput.indices) {
            // leftIndex is the index of current symbol from
            // left to right.
            // EG:
            // In number 123456 if `i` is equal to zero
            // it will be 6
            val leftIndex = (i - numberInput.lastIndex) * -1
            val currentSymbol = numberInput[i]
            // \u0030 and \u0039 are the UTF codes of the numbers
            // 0 and 9
            result += if (currentSymbol in '\u0030'..'\u0039') {
                currentSymbol.toString().toBigInteger() * sourceBase.pow(leftIndex)
            } else {
                val number = 9 + (currentSymbol.code - CAPITAL_LETTER_A)
                number.toBigInteger() * sourceBase.pow(leftIndex)
            }
        }
        return result.toString()
    }

    fun convert(input: String): String {
        val decimalNumber = convertToDecimal(input)
        return convertFromDecimal(decimalNumber)
    }
}