/******************************************************************************
### TileBBox ###

Given a Web Mercator tile ID as (z, x, y), returns a bounding-box
geometry of the area covered by that tile.

__Parameters:__

- `integer` z - A tile zoom level.
- `integer` x - A tile x-position.
- `integer` y - A tile y-position.
- `integer` srid - SRID of the desired target projection of the bounding
  box. Defaults to 3857 (Web Mercator).

__Returns:__ `geometry(polygon)`
******************************************************************************/
create or replace function TileBBox(z int, x int, y int, srid int = 3857)
    returns geometry
    language plpgsql immutable as
$func$
declare
    max numeric := 20037508.34;
    res numeric := (max*2)/(2^z);
    bbox geometry;
begin
    bbox := ST_MakeEnvelope(
        -max + (x * res),
        max - (y * res),
        -max + (x * res) + res,
        max - (y * res) - res,
        3857
    );
    if srid = 3857 then
        return bbox;
    else
        return ST_Transform(bbox, srid);
    end if;
end;
$func$;