LL_EXACT_AMBIG_DETECTION - Interpetation - antlr4

Question

When using PredictionMode::LL_EXACT_AMBIG_DETECTION I get the following error messages:
line 186:7 reportAttemptingFullContext d=30, input='ON REPORT HEAD
How am I to interpret the d attribute. Does it reference a rule in my grammar and how can I find out which?
According to the code:
#Override
public void reportAttemptingFullContext(#NotNull Parser recognizer,
#NotNull DFA dfa,
int startIndex, int stopIndex,
#NotNull ATNConfigSet configs)
{
recognizer.notifyErrorListeners("reportAttemptingFullContext d=" +
dfa.decision + ", input='" +
recognizer.getTokenStream().getText(Interval.of(startIndex, stopIndex)) + "'");
}
the attribute d is a decision in DFA. But I have not found out how the use the information back to the rule in the grammar.
Thank for your help.
Kind regards,
Wolfgang Hämmer

Answer 1

The following helper methods can convert a decision number to a rule name. You can create your own error listener implementation based on DiagnosticErrorListener and use these methods to include the name of the rule in each message.
If a rule has more than one decision, then you can pass the -atn flag to ANTLR when you generate code for your grammar. Once you have the name of the rule, look at the graph produced by ruleName.dot (where ruleName is the rule), and you'll see a node in the graph labeled d=decisionNumber (where decisionNumber is the number you're currently seeing). That will point you to the exact location where the problem is occurring.
Keep in mind that rule and decision numbers change when you change your grammar, so when you open ruleName.dot you'll want to verify the actual decision number each you regenerate the code for your grammar.
public static int getDecisionRule(Recognizer<?, ?> recognizer, int decision) {
if (recognizer == null || decision < 0) {
return -1;
}
if (decision >= recognizer.getATN().decisionToState.size()) {
return -1;
}
return recognizer.getATN().decisionToState.get(decision).ruleIndex;
}
public static String getRuleDisplayName(Recognizer<?, ?> recognizer, int ruleIndex) {
if (recognizer == null || ruleIndex < 0) {
return Integer.toString(ruleIndex);
}
String[] ruleNames = recognizer.getRuleNames();
if (ruleIndex < 0 || ruleIndex >= ruleNames.length) {
return Integer.toString(ruleIndex);
}
return ruleNames[ruleIndex];
}