# How to code a URL shortener?

Source: Internet
Author: User
Tags integer division

I want to create a URL shortener service where can write a long URL into an input field and the service shortens the U RL to " `http://www.example.org/abcdef` ". Instead of " `abcdef` " "there can is any and string with six characters containing `a-z, A-Z and 0-9` . That makes 56~57 billion possible strings.

My approach:

I have a database table with three columns:

1. ID, Integer, auto-increment
2. Long, string, the long URL the user entered
3. Short, string, the shortened URL (or just the six characters)

I would then insert the long URL into the table. Then I would select the Auto-increment value for " `id` " and build a hash of it. This hash is should then is inserted as " `short` ". But what sort of hash should I build? Hash algorithms like MD5 create too long strings. I don ' t use these algorithms, I think. A self-built algorithm would work, too.

My idea:

For " `http://www.google.de/` I get the auto-increment ID `239472` . Then I do the following steps:

``short = ‘‘;if divisible by 2, add "a"+the result to shortif divisible by 3, add "b"+the result to short... until I have divisors for a-z and A-Z.``

That could is repeated until the number isn ' t divisible any more. Does think is a good approach? Do you had a better idea?

Edit: Due to the ongoing interest-topic, I ' ve uploaded the code that I used to GitHub, with implementations for Java, PH P and JavaScript. ADD Your solutions If you like :)

I would continue your "convert number to string" approach. However you'll realize that your proposed algorithm fails if your ID is a prime and greater than.

Theoretical background

You need a bijective Function f. This is necessary so, can find a inverse functiong (' abc ') = 123 for your f (123) = ' abc ' function . This means:

• There must is no x1, X2 (with x1≠x2) that would make f (x1) = f (x2),
• And for every y must is able to find a x so , f (x) = y.
How to convert the ID to a shortened URL
1. Think of an alphabet we want. In your case that ' s `[a-zA-Z0-9]` . It contains letters.
2. Take an auto-generated, unique numerical key (the auto-incremented of `id` a MySQL table for example).

For this example I'll use the 12510 (with a base of 10).

3. Now there are to convert 12510 to X62 (base 62).

12510 = 2x621 + 1x620 =`[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 maps the indices 2 and 1 to your alphabet. This is what your mapping (with a array for example) could look like:

``0  → a1  → b...25 → z...52 → 061 → 9``

With 2→c and 1→b you'll 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 does a reverse lookup in your alphabet.

1. E9a62 'll be resolved to "4th, 61st, and 0th letter in Alphabet".

E9a62 = `[4,61,0]` = 4x622 + 61x621 + 0x620 = 1915810

2. Now find your database-record with and do the `WHERE id = 19158` redirect.
Some implementations (provided by commenters)
• Ruby
• Python
• Coffeescript
• C#

How to code a URL shortener?

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