Few months ago, I introduced a simple algorithmthat allow users to implement their own short URL into their system. Today, I have some spare time so I decided
To write the short URL algorithm 'simplementation in PHP.
At first, we define a function calledshorturl()
Thatreceives a URL as the input and returns anarray that contains 4 hashed values (each 6 characters ).
function shorturl($input) {
...
//
return array of results
}
Below is the original pseudo code:
...
loop2: from 1st 4 bytes to 4th 4 bytes of md5 result
cast the 4 bytes to an
integer
loop3: for shortCodeChar[0] to shortCodeChar[5]
use 1st 5 bits of the integer to find the value in codeMap
remove 5 bits from the integer
end loop3
save shortCodeChar as shortCode
...
// Database
checking for duplication
end loop2
...
The following code is written according to thealgorithm above excluding the database checking part for duplication:
function shorturl($input) { $base32 = array ( 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5' ); $hex = md5($input); $hexLen = strlen($hex); $subHexLen = $hexLen / 8; $output = array(); for ($i = 0; $i < $subHexLen; $i++) { $subHex = substr ($hex, $i * 8, 8); $int = 0x3FFFFFFF & (1 * ('0x'.$subHex)); $out = ''; for ($j = 0; $j < 6; $j++) { $val = 0x0000001F & $int; $out .= $base32[$val]; $int = $int >> 5; } $output[] = $out; } return $output;}
Sample code to test/use theabove function:
$input = 'http://www.snippetit.com/1';$output = shorturl($input); echo "Input : $input\n";echo "Output : {$output[0]}\n";echo " {$output[1]}\n";echo " {$output[2]}\n";echo " {$output[3]}\n";echo "\n"; $input = 'http://www.snippetit.com/2';$output = shorturl($input); echo "Input : $input\n";echo "Output : {$output[0]}\n";echo " {$output[1]}\n";echo " {$output[2]}\n";echo " {$output[3]}\n";echo "\n";
Output:
Input : http://www.snippetit.com/1Output : h0xg4r bdr3tw osk2d3 4azfqa Input : http://www.snippetit.com/2Output : tm5kxb ceoj2s yw3dvl nrmrxl
The function return an array of 4 elements, youcan use any one of them. The others can be used as alternative unique code for the input when youfound a duplicated code in your database (same code but different input -although it
is unlikely to happen but it will happen). Chances to get aduplicated code is about n/(32^6) or n/1,073,741,824 where n is the number of records in your database.
As you can see, the output results are quiterandom although you only have one character different in the input string. Theoutput is always consistent, for the same input you will always get the sameoutput.
To make the output more unpredictable by theothers, you can scramble the values in the $base32
array or/and add inyour ownprivate keyor/and XOR the value of $val
with a value from range0 to 31.
For example toscramble the values in the $base32
array, you canchange the positionof the values or/and replace the value with another (make sure the replacedvalue is URL safe character).
For example to add in private key, you can add in additional stringwhen calling the md5()
function, e.g.:
$hex = md5('my-secret-key'.$input.'my-another-secret-key');
For example to XOR the value of $val
with valueof 18:
$out .= $base32[$val ^ 18];