How to code a URL shortener?

How to code a URL shortener?

How to convert the ID to a shortened URL Think of an alphabet we want to use. In your case that's [a-zA-Z0-9]. It contains 62 letters. Take an auto-generated, unique numerical key (the auto-incremented id of a MySQL table for example). For this example I will use 12510 (125 with a base of 10). Now you have to convert 12510 to X62 (base 62). 12510 = 2×621 + 1×620 = [2,1] This requires use of integer division and modulo. A pseudo-code example: digits = [] while num > 0 remainder = modulo(num, 62) digits.push(remainder) num = divide(num, 62) digits = digits.reverse Now map the indices 2 and 1 to your alphabet. This is how your mapping (with an array for example) could look like: 0 → a 1 → b ... 25 → z ... 52 → 0 61 → 9 With 2 → c and 1 → b you will receive cb62 as the shortened URL. http://shor.ty/cb How to resolve a shortened URL to the initial ID The reverse is even easier. You just do a reverse lookup in your alphabet. e9a62 will be resolved to "4th, 61st, and 0th letter in alphabet". e9a62 = [4,61,0] = 4×622 + 61×621 + 0×620 = 1915810 Now find your database-record with WHERE id = 19158 and do the redirect.

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function base_encode($num, $alphabet) {
    $base_count = strlen($alphabet);
    $encoded = '';
    while ($num >= $base_count) {
        $div = $num / $base_count;
        $mod = ($num - ($base_count * intval($div)));
        $encoded = $alphabet[$mod].$encoded;
        $num = intval($div);
    }

    if ($num) $encoded = $alphabet[$num].$encoded;

    return $encoded;
}

function base_decode($num, $alphabet) {
    $decoded = 0;
    $multi = 1;
    while (strlen($num) > 0) {
        $digit = $num[strlen($num) - 1];
        $decoded += $multi * strpos($alphabet, $digit);
        $multi = $multi * strlen($alphabet);
        $num = substr($num, 0, -1);
    }

    return $decoded;
}