algorithms: Implementing custom hash table based dict - actionscript-3

Question

I am learning programming abstract data types. Trying to build custom hash table based dict.
SO far I've created a class place holder.
public class HashMapDict implements IDict
{
private var _map:Array;
public function HashMapDict()
{
_map = new Array();
//TODO: implement function
}
public function set(keys:Array):Boolean
{
// 1. For each key in array of keys
// 2. Pass Key.key to hash function
// 3. Write Key to _map[hash(Key.key)]
return true;
}
}
I see the main method set doing the following
// 1. For each key in array of keys
// 2. Pass Key.key to hash function
// 3. Write Key to _map[hash(Key.key)]
What I am thinking about is to use cryptography libs for hash generation. But I am a bit confused with how it should work. e.g. Tried to look on several libs like as3crypto (http://crypto.hurlant.com/demo/) and it seems to produce hash in a way I don't really think can be used for indexes in arrays.
E.g.
http://screencast.com/t/bE1lYQEqp4D
Can you advise which lib can I use to generate usable hashes? and how should they look like

Answer 1

Just as a heads up -- I can almost guarantee that you will not be able to make something better than Dictionary or even Object at this. Your proposed plan could work, but it would offer no benefit over these. I also feel compelled to suggest Vector over Array as Vectors are faster and more powerful.
The problem with Hash libs is that they generally result in very, very large numbers. MD5, for example, will produce a hex string which represents far more than what can fit even into a uint (uint in as can fit 2 ^ 32, MD5 is 2^128). This also happens to be the maximum size of an Array/Vector in AS.
This isn't to say that they can't fit into Number (which can hold about 1.79*10^308), but it does mean that you'll lose the benefit of numeric indexing and you certainly won't get much benefit from Vectors at that point. You'll basically be falling back on Object.
To be honest, it really does look like you have one of two options. Either you can implement a direct lookup using a second Array/Vector. This has the problem of being O(n) lookup time while the lookup time of a Hash table would be O(1).
It seems, at least to me, that you'll need to use Dictionary or Object no matter what to get this done.

Answer 2

For implementation of a hash table, a cryptographic hash function is overkill.
Use this only if you are concerned with an attack of someone who tries to feed you bad data (e.g. keys with lots of hash collisions) to make the hash table slow.
For a hash table use, a hash function like the following one is enough (pseudocode, as I don't know the right syntax):
hash = 0
for c in string:
hash = hash * 13 + c;
return hash
But as other answers said, there is already a hash table built in, and you don't really need to reimplement it.

Answer 3

I might be missing something, but I think you should look at flash.utils::Dictionary.
It makes hashing obsolete. If you must have some sort of primitive key, I suggest using the following:
class UIDUtil {
static private var map:Dictionary = new Dictionary(true);
static private var counter:int = 0;
static public function getUID(value:*):int {
return map[value] ||= counter++;
}
}
But your class I would implement as:
public class HashMapDict implements IDict {
private var _map:Dictionary = new Dictionary();
public function set(keys:Array):Boolean {
for each (var key:* in keys) _map[key] = key;
return true;
}
}
I am not sure of its purpose though ;)