-- Začátek aktuálního (+offset) 15min slotu v Europe/Prague jako timestamptz (UTC instants).
-- Volitelné p_at (např. job po uzavření slotu); null = now().

drop function if exists ems.fn_planning_slot_boundary_prague(int);

create or replace function ems.fn_planning_slot_boundary_prague(
  p_offset_slots int default 0,
  p_at timestamptz default null
)
returns timestamptz
language sql
stable
as $fn$
  select (
    (date_trunc('day', loc.ts)
     + make_interval(
         hours => extract(hour from loc.ts)::int,
         mins => (floor(extract(minute from loc.ts) / 15) * 15)::int
       )
    )::timestamp at time zone 'Europe/Prague'
  ) + make_interval(mins => coalesce(p_offset_slots, 0) * 15)
  from (
    select coalesce(p_at, now()) at time zone 'Europe/Prague' as ts
  ) loc;
$fn$;

comment on function ems.fn_planning_slot_boundary_prague(int, timestamptz) is
'Začátek 15min slotu (Europe/Prague floor); offset v násobcích 15 min; p_at volitelně místo now().';