学步园

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 called shorturl() 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 pseudocode:

...

  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/1
Output : h0xg4r
         bdr3tw
         osk2d3
         4azfqa
 
Input  : http://www.snippetit.com/2
Output : 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];