Re: Bit count

On Tue, Nov 24, 2009 at 1:18 PM, Rikard Bosnjakovic
<rikard(dot)bosnjakovic(at)gmail(dot)com> wrote:
> On Tue, Nov 24, 2009 at 16:47, Nathaniel Trellice <naptrel(at)yahoo(dot)co(dot)uk> wrote:
>
> [...]
>> If not, can anyone recommend the most efficient way within postgres to implement the kind of bit-counting tricks found at:
>
> 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