Generate Unique Number Based On String Input In Javascript

In the past I have made a function that generates an unique id (number) from a string. Today I discover that it is not as unique as should be. Never saw a problem before with it. T

Solution 1:

All right, did allot of testing and come to this. A relative short unique id generated by the following:

o.lz = function(i,c)
{
  if( typeof c != 'number' || c <= 0 || (typeof i != 'number' && typeof i != 'string') )
   { return i; }
  i+='';

  while( i.length < c )
   { i='0'+i; }
  return i;  
}

o.getHashCode = function(s)
{
 var hash=0,c=(typeof s == 'string')?s.length:0,i=0;
 while(i<c) 
 {
   hash = ((hash<<5)-hash)+s.charCodeAt(i++);
   //hash = hash & hash; // Convert to 32bit integer
 }

 return ( hash < 0 )?((hash*-1)+0xFFFFFFFF):hash; // convert to unsigned
}; 

o.uniqueId = function( s, bres )
{ 
  if( s == undefined || typeof s != 'string' )
  { 
     if( !o.___uqidc )
      { o.___uqidc=0; }
     else { ++o.___uqidc; } 
     var od = newDate(),
         i = s = od.getTime()+''+o.___uqidc; 
  }
  else { var i = o.getHashCode( s ); }
  return ((bres)?'res:':'')+i.toString(32)+'-'+o.lz((s.length*4).toString(16),3);  
};

Examples:

o.uniqueId( 'M:/Mijn Muziek/Various Artists/Revs & ElBee - Tell It To My Heart.mp3' );
o.uniqueId( 'M:/Mijn Muziek/Various Artists/Dwight Yoakam - The Back Of Your Hand.Mp3');

Will produce the following id's:

dh8qi9t-114
je38ugg-120

For my purpose it seems to be unique enough, also the extra length adds some more uniqueness. Test it on filesystem with approx 40.000 mp3 files and did not found any collision.

If you think this is not the way to go, please let me know.

Solution 2:

You should increase the number of bits created by your hash function. Assuming that your hash function approximately uniform over the space, you can mathematically derive the probability of observing a collision.

This is strongly related to the birthday paradox. In the case of CRC16, where the hash value is 17 bits (though your implementation may have a mistake; I don't see how you obtained 224094 as that is greater than 2^17), you will have a collision probability above 50% when you store more than approximately 2^8 items. In addition, CRC is not really a great hashing function because it's meant for error detection, not uniform hashing.

This table shows mathematical probabilities of collision based on hash length. For example, if you have a 128-bit hash key, you can store up to 10^31 elements before the collision probability increases beyond 10^-15. As a comparison, this probability is lower than that of your hard drive failing, or maybe your computer being zapped by lightning, so a safe number to use.

Just increase your hash length based on the number of strings you are planning to identify, and a pick a collision probability that is acceptable to you.