Highlight string differences using `<del>` and `<ins>`

string-differences.php

<?php

/**
 * Generate two arrays:
 * Values array: a list of elements as they appear in the diff
 * Mask array: 0: unchanged, -1: removed, 1: added
 *
 * @param  string $from
 * @param  string $to
 * @return array
 */
function computeDiff($from, $to)
{
    $diffValues = [];
    $diffMask   = [];

    $dm = [];
    $n1 = count($from);
    $n2 = count($to);

    for ($j = -1; $j < $n2; $j++) {
        $dm[-1][$j] = 0;
    }

    for ($i = -1; $i < $n1; $i++) {
        $dm[$i][-1] = 0;
    }

    for ($i = 0; $i < $n1; $i++) {
        for ($j = 0; $j < $n2; $j++) {
            if ($from[$i] === $to[$j]) {
                $ad         = $dm[$i - 1][$j - 1];
                $dm[$i][$j] = $ad + 1;
            } else {
                $a1         = $dm[$i - 1][$j];
                $a2         = $dm[$i][$j - 1];
                $dm[$i][$j] = max($a1, $a2);
            }
        }
    }

    $i = $n1 - 1;
    $j = $n2 - 1;

    while (($i > -1) || ($j > -1)) {
        if ($j > -1) {
            if ($dm[$i][$j - 1] === $dm[$i][$j]) {
                $diffValues[] = $to[$j];
                $diffMask[]   = 1;
                $j--;

                continue;
            }
        }

        if ($i > -1) {
            if ($dm[$i - 1][$j] === $dm[$i][$j]) {
                $diffValues[] = $from[$i];
                $diffMask[]   = -1;
                $i--;

                continue;
            }
        }

        {
            $diffValues[] = $from[$i];
            $diffMask[]   = 0;
            $i--;
            $j--;
        }
    }

    $diffValues = array_reverse($diffValues);
    $diffMask   = array_reverse($diffMask);

    return [
        'values' => $diffValues,
        'mask'   => $diffMask,
    ];
}

/**
 * Highlight the string differences
 *
 * @param  string $line1
 * @param  string $line2
 * @return string
 */
function diff($line1, $line2)
{
    $diff     = computeDiff(str_split($line1), str_split($line2));
    $diffval  = $diff['values'];
    $diffmask = $diff['mask'];

    $n      = count($diffval);
    $pmc    = 0;
    $result = '';

    for ($i = 0; $i < $n; $i++) {
        $mc = $diffmask[$i];

        if ($mc != $pmc) {
            switch ($pmc) {
                case -1:
                    $result .= '</del>';

                    break;
                case 1:
                    $result .= '</ins>';

                    break;
            }

            switch ($mc) {
                case -1:
                    $result .= '<del>';

                    break;
                case 1:
                    $result .= '<ins>';

                    break;
            }
        }

        $result .= $diffval[$i];

        $pmc = $mc;
    }

    switch ($pmc) {
        case -1:
            $result .= '</del>';

            break;
        case 1:
            $result .= '</ins>';

            break;
    }

    return $result;
}

Confu<del>zi</del><ins>s</ins>ed

Confuzised