Beginner System Design Interview: Design Bitly w/ a Ex-Meta Staff Engineer

A step-by-step breakdown of everyones first system design interview question, Design Bit.ly.

Evan, a former Meta Staff Engineer and current co-founder of Hello Interview, walks through the problem from the perspective of an interviewer.
 

<?php

class Base62 {
    private static $charset = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
    private static $base = 62;

    // Encode a decimal number to Base62
    public static function encode($num) {
        if ($num == 0) return self::$charset[0];

        $result = '';
        while ($num > 0) {
            $result = self::$charset[$num % self::$base] . $result;
            $num = intdiv($num, self::$base);
        }

        return $result;
    }

    // Decode a Base62 string to decimal
    public static function decode($str) {
        $length = strlen($str);
        $num = 0;

        for ($i = 0; $i < $length; $i++) {
            $pos = strpos(self::$charset, $str[$i]);
            $num = $num * self::$base + $pos;
        }

        return $num;
    }
}

// Example usage
$original = 123456789;
$encoded = Base62::encode($original);
$decoded = Base62::decode($encoded);

echo "Original: $original\n";
echo "Encoded : $encoded\n";
echo "Decoded : $decoded\n";

Candidate expected to use https://excalidraw.com/