-- Vacation is for full calendar year students only — spec section 18, refined.
--
-- The seeded policy grants four weeks to year-round full-time students and to
-- nobody else. That is deliberate: school-year students and part-time
-- year-round students carry no vacation entitlement.
--
-- Nothing enforced it, though. resolve_vacation_policy() simply returns no row
-- for those students, recalc_vacation_balance() then writes weeks_allotted = 0,
-- and an approved request still counted against it — driving weeks_remaining
-- negative and reporting a balance the student never had. This adds the missing
-- guard at the only point that matters: the moment a request becomes approved.
--
-- The escape hatch is vacation_balances.override_weeks. An administrator who
-- genuinely wants to grant vacation to a student outside the policy sets an
-- override on that student's balance row and the approval then succeeds, which
-- keeps the exception deliberate, attributable and visible in reporting rather
-- than silent.

-- ============================================================================
-- 1. ENTITLEMENT
-- ============================================================================

-- The weeks a student is actually entitled to for a policy year: an explicit
-- override if one has been set, otherwise the resolved policy, otherwise none.
-- This mirrors the COALESCE that recalc_vacation_balance() already applies, so
-- the guard and the balance can never disagree about what a student is owed.
CREATE OR REPLACE FUNCTION public.vacation_entitlement_weeks(_student UUID, _year INTEGER)
RETURNS NUMERIC
LANGUAGE plpgsql STABLE SECURITY DEFINER SET search_path = public AS $$
DECLARE
  p        public.vacation_policies%ROWTYPE;
  override NUMERIC;
BEGIN
  SELECT vb.override_weeks INTO override
  FROM public.vacation_balances vb
  WHERE vb.student_id = _student AND vb.policy_year = _year;

  IF override IS NOT NULL THEN
    RETURN override;
  END IF;

  SELECT * INTO p FROM public.resolve_vacation_policy(_student);
  RETURN COALESCE(p.weeks_per_year, 0);
END;
$$;

GRANT EXECUTE ON FUNCTION public.vacation_entitlement_weeks(UUID, INTEGER) TO authenticated;

-- ============================================================================
-- 2. THE GUARD
-- ============================================================================

-- Fires only on the transition into 'approved'. Requests may still be created,
-- edited, rejected and cancelled freely for any student — it is consuming
-- entitlement that requires having some.
CREATE OR REPLACE FUNCTION public.enforce_vacation_entitlement()
RETURNS TRIGGER LANGUAGE plpgsql SECURITY DEFINER SET search_path = public AS $$
DECLARE
  yr      INTEGER;
  weeks   NUMERIC;
  student TEXT;
BEGIN
  IF NEW.status <> 'approved' THEN
    RETURN NEW;
  END IF;

  -- Only the moment of approval is checked, so re-saving an already-approved
  -- request (or a later edit to it) does not fail after the fact.
  IF TG_OP = 'UPDATE' AND OLD.status = 'approved' THEN
    RETURN NEW;
  END IF;

  yr := COALESCE(NEW.policy_year, public.vacation_year_for(NEW.student_id, NEW.start_date));
  weeks := public.vacation_entitlement_weeks(NEW.student_id, yr);

  IF weeks > 0 THEN
    RETURN NEW;
  END IF;

  SELECT s.first_name || ' ' || s.last_name INTO student
  FROM public.students s WHERE s.id = NEW.student_id;

  RAISE EXCEPTION
    'Vacation cannot be approved for % — no vacation entitlement for % (vacation applies to full calendar year, full-time students). To grant an exception, set override_weeks on that student''s % vacation balance first.',
    COALESCE(student, NEW.student_id::text), yr, yr
    USING ERRCODE = 'check_violation';
END;
$$;

-- Named to sort after trg_vr_defaults, which populates policy_year on insert:
-- triggers sharing a timing and event fire in alphabetical order, and this one
-- reads the value that one sets.
DROP TRIGGER IF EXISTS trg_vr_entitlement ON public.vacation_requests;
CREATE TRIGGER trg_vr_entitlement BEFORE INSERT OR UPDATE ON public.vacation_requests
  FOR EACH ROW EXECUTE FUNCTION public.enforce_vacation_entitlement();