On Tue, Nov 24, 2009 at 1:18 PM, Rikard Bosnjakovic
<rikard.bosnjakovic@gmail.com> wrote:
> On Tue, Nov 24, 2009 at 16:47, Nathaniel Trellice <naptrel@yahoo.co.uk> wrote:
>
> [...]
>> If not, can anyone recommend the most efficient way within postgres to implement the kind of bit-counting tricks
foundat:
>
> Perhaps something like this:
>
> CREATE OR REPLACE FUNCTION bitcount(i integer) RETURNS integer AS $$
> DECLARE n integer;
> DECLARE amount integer;
> BEGIN
> amount := 0;
> FOR n IN 1..16 LOOP
> amount := amount + ((i >> (n-1)) & 1);
> END LOOP;
> RETURN amount;
> END
> $$ LANGUAGE plpgsql;
>
[snip]
Clever! Here's a pure SQL version I whipped up.. perhaps there's a
more efficient pure SQL way that can avoid the string operations I'm
using :-)
# SELECT num, char_length(replace(num::bit(16)::text, '0', '')) AS
num_set_bits FROM numbers;
num | num_set_bits
------+--------------
6 | 2
7 | 3
4711 | 7
1024 | 1
Josh