LL1Analyzer.java 文件源码

/**
     * Calculates the SLL(1) expected lookahead set for each outgoing transition
     * of an {@link ATNState}. The returned array has one element for each
     * outgoing transition in {@code s}. If the closure from transition
     * <em>i</em> leads to a semantic predicate before matching a symbol, the
     * element at index <em>i</em> of the result will be {@code null}.
     *
     * @param s the ATN state
     * @return the expected symbols for each outgoing transition of {@code s}.
     */
    @Nullable
    public IntervalSet[] getDecisionLookahead(@Nullable ATNState s) {
//      System.out.println("LOOK("+s.stateNumber+")");
        if ( s==null ) {
            return null;
        }

        IntervalSet[] look = new IntervalSet[s.getNumberOfTransitions()];
        for (int alt = 0; alt < s.getNumberOfTransitions(); alt++) {
            look[alt] = new IntervalSet();
            Set<ATNConfig> lookBusy = new HashSet<ATNConfig>();
            boolean seeThruPreds = false; // fail to get lookahead upon pred
            _LOOK(s.transition(alt).target, null, PredictionContext.EMPTY,
                  look[alt], lookBusy, new BitSet(), seeThruPreds, false);
            // Wipe out lookahead for this alternative if we found nothing
            // or we had a predicate when we !seeThruPreds
            if ( look[alt].size()==0 || look[alt].contains(HIT_PRED) ) {
                look[alt] = null;
            }
        }
        return look;
    }