<?php

$str = "23456789ABCDEGHJKMNPQRSTUVXYZabcdeghjkmnpqrstuvxyz";
$excludes = ['cp','cb','ck','c6','c9','rn','rm','mm','co','do','cl','db','qp','qb','dp','ww'];
$length = 5;

$indexedExcludes = array_combine($excludes, $excludes);
$dictionaryArray = str_split($str);

function fastCaptcha(array $dictionary, array $excludedSyllables, int $length): string
{
    $captcha = '';
    $previousCharacter = '';
    $generatedLength = 0;

    while ($generatedLength < $length) {
        $randomCharacter = $dictionary[array_rand($dictionary)];

        if(array_key_exists($previousCharacter . $randomCharacter, $excludedSyllables)) {
            continue;
        }

        $captcha .= $randomCharacter;
        $previousCharacter = $randomCharacter;
        $generatedLength++;
    }

    return $captcha;
}

$excludesRegex = '/' . implode('|', $excludes) . '/';

function fuckedCaptcha(string $dictionary, string $excludedSyllables, int $length): string
{
    do {
        $code = substr(str_shuffle(str_repeat($dictionary, 3)), 0, $length);
    } while (preg_match($excludedSyllables, $code));

    return $code;
}

function fastFuckedCaptcha(array $dictionary, string $excludedSyllables, int $length): string
{
    $captcha = '';
    $previousCharacter = '';
    $generatedLength = 0;
    
    while ($generatedLength < $length) {
        $randomCharacter = $dictionary[array_rand($dictionary)];

        if(preg_match($excludedSyllables, $previousCharacter . $randomCharacter))
        	continue;

        $captcha .= $randomCharacter;
        $previousCharacter = $randomCharacter;
        $generatedLength++;
    }

    return $captcha;
}


$t0 = microtime(true);

for ($i = 0; $i < 100000; $i++)
    fuckedCaptcha($str, $excludesRegex, $length);

$t1 = microtime(true);

for ($i = 0; $i < 100000; $i++)
    fastCaptcha($dictionaryArray, $indexedExcludes, $length);

$t2 = microtime(true);

for ($i = 0; $i < 100000; $i++)
    fastFuckedCaptcha($dictionaryArray, $excludesRegex, $length);

$t3 = microtime(true);

echo "regexp     : " . ($t1 - $t0) . "\n";
echo "array index: " . ($t2 - $t1) . "\n";
echo "regexp again: " . ($t3 - $t2) . "\n";