Re: Hash Function for "switch statement"

["Unruh, Erwin" <erwin dot unruh at ts dot fujitsu dot com>]

Hi,

the discussion so far did omit one specific aspect. When comparing two implementations for a switch, you have to compare the full code. For the hash you have to include the code to calculate the hash function. This might be more code than a simple tree lookup.
The example function:

>public int hash32shift(int key)
>{
>  key = ~key + (key << 15); // key = (key << 15) - key - 1;
>  key = key ^ (key >>> 12);
>  key = key + (key << 2);
>  key = key ^ (key >>> 4);
>  key = key * 2057; // key = (key + (key << 3)) + (key << 11);
>  key = key ^ (key >>> 16);
>  return key;
>}

has 12 operations. Add a table and verification you get to about 18. That is worse than a tree search with 9 levels. So for all switches with less than 512 elements, the hash is not faster.

	Erwin