create or replace function ems.fn_ev_session_transition(
  p_site_id int,
  p_charger_id int,
  p_prev_status text,
  p_new_status text,
  p_measured_at timestamptz
)
returns jsonb
language plpgsql
as $fn$
declare
  v_vehicle_id int;
begin
  if p_prev_status is not distinct from p_new_status then
    return jsonb_build_object('action', 'none');
  end if;

  if p_prev_status = 'available' and p_new_status is distinct from 'available' then
    select av.id
    into v_vehicle_id
    from ems.asset_vehicle av
    where av.site_id = p_site_id
      and av.default_charger_id = p_charger_id
      and av.active = true
    order by av.id
    limit 1;

    perform ems.fn_update_ev_arrival_stats(
      p_site_id,
      p_charger_id,
      v_vehicle_id,
      p_measured_at
    );

    insert into ems.ev_session (
      site_id,
      charger_id,
      vehicle_id,
      session_start,
      target_soc_pct,
      target_deadline
    )
    select
      ac.site_id,
      ac.id,
      av.id,
      now(),
      -- kaskáda fn_ev_session_defaults: týdenní požadavek (ev_weekly_requirement)
      -- → forecast (ev_usage_stats) → defaulty vozidla; ruční přepis přes
      -- fn_ev_session_apply_patch vždy vyhrává.
      (d.defaults ->> 'target_soc_pct')::double precision,
      (d.defaults ->> 'deadline')::timestamptz
    from ems.asset_ev_charger ac
    left join lateral (
      select v.id
      from ems.asset_vehicle v
      where v.default_charger_id = ac.id
        and v.site_id = ac.site_id
        and v.active = true
      order by v.id
      limit 1
    ) av on true
    cross join lateral (
      select ems.fn_ev_session_defaults(av.id, now()) as defaults
    ) d
    where ac.id = p_charger_id
      and ac.site_id = p_site_id
    on conflict (charger_id) where session_end is null do nothing;

    return jsonb_build_object('action', 'arrival');
  end if;

  if p_prev_status is distinct from 'available' and p_new_status = 'available' then
    update ems.ev_session es
    set session_end = now()
    where es.charger_id = p_charger_id
      and es.session_end is null;

    return jsonb_build_object('action', 'departure');
  end if;

  return jsonb_build_object('action', 'none');
end;
$fn$;

comment on function ems.fn_ev_session_transition is
  'Detekce příjezdu/odjezdu EV po změně statusu nabíječky (telemetry_collector); defaulty nové session z ems.fn_ev_session_defaults.';