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up vote 0 down vote favorite

The goal is to fast extract bit at position N; So far i've found only this way to do that:

CREATE OR REPLACE FUNCTION test(x int, size int)
RETURNS int AS
$BODY$
DECLARE
y int;

BEGIN
    y = get_bit(x::bit(size)>>size-1,size-1);


    return y;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

But this says because of bit(size)

invalid input syntax for integer: "size"

So i'm forsed to write size as a constant, for example bit(3)

Is there any way to set bit size dynamically? Alternatively, maybe there are some other ways to extract specific bit of int/text?

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1 Answer 1

up vote 1 down vote accepted

You don't need bitfield types for this. To test bit 6 of the value 200:

SELECT 200 & (1<<6) != 0

you binary-AND a value with the 6th bit set, by upshifting 1 by 6 base-2 places, then test to see if the result is nonzero.

Alternately, cast to a suitable bit size based on the integer and use position, though I think there's no reason whatsoever to do that when you can use extremely fast bitwise AND and shifts like above. Because get_bit numbers from the left, and uses a 1-offset instead of 0-offset, you have to take the compliment of 31 (for a 32-bit int) to get the position from the right:

SELECT get_bit(200::bit(32), 31 - 6);

so it's way simpler to just use bitwise operations like the first example.

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