Class which breaks hashids encoding

breaker.php

<?php
class Breaker
{
    const ONLY_PRINTABLE = 1;
    private $lowerLimit = 0;
    private $upperLimit = 256;

    public function __construct($flags = 0)
    {
        if ($flags & self::ONLY_PRINTABLE) {
            $this->lowerLimit = 32;
            $this->upperLimit = 127;
        }
    }

    private function swap(&$alphabet, $i, $j)
    {
        $temp = $alphabet[$j];
        $alphabet[$j] = $alphabet[$i];
        $alphabet[$i] = $temp;
    }

    private function findNextCharacters(array $salt, $outputIndex, $inputIndex)
    {
        $t = 0;
        foreach ($salt as $s) {
            $t = ($t + $s) % $outputIndex;
        }

        $return = [];

        for ($i = $this->lowerLimit; $i < $this->upperLimit; $i++) {
            $test = $t + $i+ $i + count($salt);
            if (($test % $outputIndex) === $inputIndex) {
                $return[] = $i;
            }
        }

        return $return;
    }

    private function checkSalt(array $salt, $outputIndex, $inputIndex, $length, $debug = false)
    {
        $t = 0;
        $saltIndex = 0;
        for ($i = 0; $i < $length; $i++) {
            $saltIndex = $i % count($salt);
            $t = ($t + $salt[$saltIndex]) % $outputIndex;
        }
        $t += $salt[$saltIndex] + $saltIndex;

        return (($t % $outputIndex) === $inputIndex);
    }

    public function findSalt($inputString, $outputString, $limit = 10)
    {
        $initialOutputIndex = strlen($inputString) - 1;
        $outputOffset = strlen($inputString) - strlen($outputString);

        $iterations = $initialOutputIndex - $outputOffset;
        $iterationLimit = $iterations - $limit + $outputOffset;

        $inputStringArray = str_split($inputString);

        $terminals = [];
        $stack = [[]];
        $length = 0;

        for ($outputIndex = $initialOutputIndex; $outputIndex > $iterationLimit; $outputIndex--) {
            $tStart = microtime(true);

            $target = array_search($outputString[$outputIndex-$outputOffset], $inputStringArray, true);
            $length++;
            $terminalsPassed = [];
            foreach ($terminals as $key => $test) {
                if($this->checkSalt($test, $outputIndex, $target, $length, ($key === 0))) {
                    $terminalsPassed[] = $test;
                }
            }

            $terminals = $terminalsPassed;


            $strings  = [];

            foreach ($stack as $salt) {
                $chars = $this->findNextCharacters($salt, $outputIndex, $target);
                foreach ($chars as $char) {
                    $string = $salt;
                    $string[] = $char;
                    $strings[] = $string;
                    $terminals[] = $string;
                }
            }

            $stack = $strings;

            $this->swap($inputStringArray, $target, $outputIndex);
            echo sprintf("Loop %d Took: %.2f \n", $length, (microtime(true) - $tStart));
            echo sprintf(
                "Found %d new candidate salts, confirmed %d previous salts as still possible\n",
                count($strings),
                count($terminalsPassed)
            );
         }
    }
}

$b = new Breaker(1);
$b->findSalt('abdegjklmnopqrvwxyzABDEGJKLMNOPQRVWXYZ1234567890',  'OXdwnqJxL41r7zAPpbayY290eNv6ZEBVGlgoQjRDMKW5', 20);