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From "Shapira, Yoav" <Yoav.Shap...@mpi.com>
Subject RE: [collections] Faster than HashMap?
Date Mon, 10 Mar 2003 14:07:35 GMT

Howdy,
I must be completely missing something here, but doesn't that require
your do not all the hashcodes of the items in the map at compile time?

Yoav Shapira
Millennium ChemInformatics


>-----Original Message-----
>From: Stephen Colebourne [mailto:scolebourne@btopenworld.com]
>Sent: Saturday, March 08, 2003 12:43 PM
>To: Jakarta Commons Developers List
>Subject: [collections] Faster than HashMap?
>
>I'd had an idea for a way to create a map thats faster to read than a
>HashMap...
>
>The idea was to bytecode generate the mapping from the hashcode to the
>index:
>public int lookupIndex(Object key) {
>  switch (key.hashCode()) {
>    case -1682:  // hashcode
>     return 2; // index of Map.Entry
>    case 86929  // hashcode
>     return 0; // index of Map.Entry
>    case 1596969:  // hashcode
>     return 1; // index of Map.Entry
>  }
>  return -1;
>}
>
>Simple? No hashcode collisions. Should go faster....
>
>Attached are the java files. My initial testing was with a size 3 map,
and
>that goes about 40% faster. Unfortunately, the bytecode map's
performance
>degenerates with size. Depending on exact data size 50 entries or more
may
>be slower, gradually getting slower.
>
>Of course now I realise that the switch statement in bytecode isn't
magical
>and is just doing a binary search, so was always going to suffer this
>problem. What an idiot I am.
>
>If anyone else wants to take a look and see if I've missed something,
or
>has
>another bright idea triggered by this then feel free to take a look at
the
>code. Otherwise, its not worth adding to [collections]. :-((((
>
>Stephen




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