convert id between number systems

NumberSystem.kt

package org.timeout.app

import java.math.BigInteger

class NumberSystem(val alphabet: String = "0123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ") {
    val base = alphabet.length.toBigInteger()
    fun encode(input: String): String {
        val number = BigInteger(1, input.toByteArray())
        val encoded = StringBuilder()
        var num = number
        while (num > BigInteger.ZERO) {
            val remainder = (num % base).toInt()
            encoded.append(alphabet[remainder])
            num /= base
        }

        return encoded.reverse().toString()
    }

    fun decode(input: String): String {
        var num = BigInteger.ZERO
        for (char in input) {
            num = num * base + alphabet.indexOf(char).toBigInteger()
        }

        return String(num.toByteArray())
    }
}

NumberSystem.php

final readonly class NumberSystem
{
    public function __construct(
        private string $alphabet = '0123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ',
    ) {
    }

    public function encode(int $num): string
    {
        $baseCount = \strlen($this->alphabet);
        $encoded = '';
        while ($num >= $baseCount) {
            $div = $num / $baseCount;
            $mod = ($num - ($baseCount * (int)$div));
            $encoded = $this->alphabet[$mod] . $encoded;
            $num = (int)$div;
        }

        if ($num) {
            $encoded = $this->alphabet[$num] . $encoded;
        }

        return $encoded;
    }

    public function decode(string $num): int
    {
        $decoded = 0;
        $multi = 1;
        while (\strlen($num) > 0) {
            $digit = $num[\strlen($num) - 1];
            $decoded += $multi * \strpos($this->alphabet, $digit);
            $multi = $multi * \strlen($this->alphabet);
            $num = \substr($num, 0, -1);
        }

        return $decoded;
    }
}

NumberSystemTest.kt

package app

import org.junit.jupiter.api.Tag
import org.junit.jupiter.params.ParameterizedTest
import org.junit.jupiter.params.provider.Arguments
import org.junit.jupiter.params.provider.MethodSource
import kotlin.test.assertEquals

class NumberSystemTest {
    @ParameterizedTest
    @MethodSource("provideStrings")
    @Tag("fast")
    fun testConvert(alphabet: String, input: String, expectedString: String) {
        val ns = NumberSystem(alphabet)
        val encoded = ns.encode(input)
        assertEquals(expectedString, ns.encode(input))
        assertEquals(input, ns.decode(encoded))
    }

    companion object {
        @JvmStatic
        fun provideStrings() = listOf(
            Arguments.of(
                "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz",
                "bla",
                "R40f",
            ),
            Arguments.of(
                "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz",
                "some text",
                "ev7YMQpyQie4",
            ),
            Arguments.of(
                "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz",
                "youtube",
                "2WY44DpVLR",
            ),
            Arguments.of(
                "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz",
                "very very very very very very long text",
                "2PRZUWZ8V84hvg31WAmSsIg8afccqQeGwVnRuXizEyfE4MgXinf52",
            ),
        )
    }
}