How to code URL shortener from scratch?


I am going to explain one of the common approaches used to create URL shortner.

  1. Think of an alphabet we want to use. In your case that’s [a-zA-Z0-9]. It contains 62 letters.
  2. 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).

  3. Now you have to convert 12510 to X62 (base 62).

    12510 = 2×621 + 1×620 = [2,1]

    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.

  1. e9a62 will be resolved to “4th, 61st, and 0th letter in alphabet”.

    e9a62 = [4,61,0] = 4×622 + 61×621 + 0×620 = 1915810

  2. Now find your database-record with WHERE id = 19158 and do the redirect.
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