base_convert.php

<?php

declare(strict_types=1);

namespace Psl\Math;

use Psl\Str;
use Psl\Str\Byte;

use function bcadd;
use function bccomp;
use function bcdiv;
use function bcmod;
use function bcmul;
use function bcpow;

/**
 * Converts the given string in base `$from_base` to base `$to_base`, assuming
 * letters a-z are used for digits for bases greater than 10. The conversion is
 * done to arbitrary precision.
 *
 * @param non-empty-string $value
 * @param int<2, 36> $from_base
 * @param int<2, 36> $to_base
 *
 * @pure
 *
 * @throws Exception\InvalidArgumentException If the given value is invalid.
 */
function base_convert(string $value, int $from_base, int $to_base): string
{
    $from_alphabet = Byte\slice(Str\ALPHABET_ALPHANUMERIC, 0, $from_base);
    $result_decimal = '0';
    $place_value = bcpow((string) $from_base, (string) (Byte\length($value) - 1));
    foreach (Byte\chunk($value) as $digit) {
        $digit_numeric = Byte\search_ci($from_alphabet, $digit);
        if (null === $digit_numeric) {
            throw new Exception\InvalidArgumentException(Str\format('Invalid digit %s in base %d', $digit, $from_base));
        }

        $result_decimal = bcadd($result_decimal, bcmul((string) $digit_numeric, $place_value));
        $place_value = bcdiv($place_value, (string) $from_base);
    }

    if (10 === $to_base) {
        return $result_decimal;
    }

    $to_alphabet = Byte\slice(Str\ALPHABET_ALPHANUMERIC, 0, $to_base);
    $result = '';
    do {
        $result = $to_alphabet[(int) bcmod($result_decimal, (string) $to_base)] . $result;
        $result_decimal = bcdiv($result_decimal, (string) $to_base);
    } while (bccomp($result_decimal, '0') > 0);

    return $result;
}