public static Map<ATNState, BitSet> getStateToAltMap(ATNConfigSet configs) {
		Map<ATNState, BitSet> m = new HashMap<ATNState, BitSet>();
		for (ATNConfig c : configs) {
			BitSet alts = m.get(c.state);
			if ( alts==null ) {
				alts = new BitSet();
				m.put(c.state, alts);
			}
			alts.set(c.alt);
		}
		return m;
	}
Get a map from state to alt subset from a configuration set. For each
configuration {@code c} in {@code configs}:

<pre>
map[c.{@link ATNConfig#state state}] U= c.{@link ATNConfig#alt alt}
</pre>
runtime/Java/src/org/antlr/v4/runtime/atn/PredictionMode.java