Damerau–Levenshtein distance [PHP]

damerau–levenshtein-distance.php

<?php

/**
 * Find Damerau–Levenshtein distance between two string
 *
 * @param string $source
 * @param string $dest
 * @return int Damerau–Levenshtein distance
 */
function distance($source, $dest)
{
    if ($source == $dest) {
        return 0;
    }

    $slen = strlen($source);
    $dlen = strlen($dest);

    if ($slen == 0) {
        return $dlen;
    } else {
        if ($dlen == 0) {
            return $slen;
        }
    }

    $prevRow = range(0, $dlen);

    for ($i = 0; $i < $slen; $i++) {
        $thisRow = array($i + 1);
        $char = $source[$i];
        $prev_char = $source[$i - 1] ?? '';
        for ($j = 0; $j < $dlen; $j++) {
            $cost = $char == $dest[$j] ? 0 : 1;

            $thisRow[$j + 1] = min(
                $prevRow[$j + 1] + 1,   // deletion
                $thisRow[$j] + 1,      // insertion
                $prevRow[$j] + $cost    // substitution
            );

            if ($i > 0 && $j > 0 && $char == $dest[$j - 1] && $prev_char == $dest[$j]) {
                $thisRow[$j + 1] = min(
                    $thisRow[$j + 1],
                    $preprevRow[$j - 1] + $cost // transposition
                );
            }
        }
        $preprevRow = $prevRow;
        $prevRow = $thisRow;
    }
    return $prevRow[$j];
}

$source = 'Helol';
$dest = 'Hello';

preg_match('#([a-zA-Z])#u', $source, $matches);
if (!empty($matches)) {
    echo distance($source, $dest) / 2 . PHP_EOL . "<br>" . PHP_EOL;
    echo levenshtein($source, $dest) / 2; // built-in PHP function
} else {
    echo distance($source, $dest) . PHP_EOL . "<br>" . PHP_EOL;
    echo levenshtein($source, $dest); // built-in PHP function
}

PHP Notice: Uninitialized string offset: -1 in /home/jeff/dl-distance-gekh.php on line 27
Perhaps line 27 should be $prev_char = $i ? $source{$i-1} : '' ;

Thank you. I've updated this code.