org.antlr.v4.runtime.atn

Class ParserATNSimulator

java.lang.Object

org.antlr.v4.runtime.atn.ATNSimulator

org.antlr.v4.runtime.atn.ParserATNSimulator

public class ParserATNSimulator
extends ATNSimulator

The embodiment of the adaptive LL(*), ALL(*), parsing strategy. The basic complexity of the adaptive strategy makes it harder to understand. We begin with ATN simulation to build paths in a DFA. Subsequent prediction requests go through the DFA first. If they reach a state without an edge for the current symbol, the algorithm fails over to the ATN simulation to complete the DFA path for the current input (until it finds a conflict state or uniquely predicting state). All of that is done without using the outer context because we want to create a DFA that is not dependent upon the rule invocation stack when we do a prediction. One DFA works in all contexts. We avoid using context not necessarily because it's slower, although it can be, but because of the DFA caching problem. The closure routine only considers the rule invocation stack created during prediction beginning in the decision rule. For example, if prediction occurs without invoking another rule's ATN, there are no context stacks in the configurations. When lack of context leads to a conflict, we don't know if it's an ambiguity or a weakness in the strong LL(*) parsing strategy (versus full LL(*)). When SLL yields a configuration set with conflict, we rewind the input and retry the ATN simulation, this time using full outer context without adding to the DFA. Configuration context stacks will be the full invocation stacks from the start rule. If we get a conflict using full context, then we can definitively say we have a true ambiguity for that input sequence. If we don't get a conflict, it implies that the decision is sensitive to the outer context. (It is not context-sensitive in the sense of context-sensitive grammars.) The next time we reach this DFA state with an SLL conflict, through DFA simulation, we will again retry the ATN simulation using full context mode. This is slow because we can't save the results and have to "interpret" the ATN each time we get that input. CACHING FULL CONTEXT PREDICTIONS We could cache results from full context to predicted alternative easily and that saves a lot of time but doesn't work in presence of predicates. The set of visible predicates from the ATN start state changes depending on the context, because closure can fall off the end of a rule. I tried to cache tuples (stack context, semantic context, predicted alt) but it was slower than interpreting and much more complicated. Also required a huge amount of memory. The goal is not to create the world's fastest parser anyway. I'd like to keep this algorithm simple. By launching multiple threads, we can improve the speed of parsing across a large number of files. There is no strict ordering between the amount of input used by SLL vs LL, which makes it really hard to build a cache for full context. Let's say that we have input A B C that leads to an SLL conflict with full context X. That implies that using X we might only use A B but we could also use A B C D to resolve conflict. Input A B C D could predict alternative 1 in one position in the input and A B C E could predict alternative 2 in another position in input. The conflicting SLL configurations could still be non-unique in the full context prediction, which would lead us to requiring more input than the original A B C. To make a prediction cache work, we have to track the exact input used during the previous prediction. That amounts to a cache that maps X to a specific DFA for that context. Something should be done for left-recursive expression predictions. They are likely LL(1) + pred eval. Easier to do the whole SLL unless error and retry with full LL thing Sam does. AVOIDING FULL CONTEXT PREDICTION We avoid doing full context retry when the outer context is empty, we did not dip into the outer context by falling off the end of the decision state rule, or when we force SLL mode. As an example of the not dip into outer context case, consider as super constructor calls versus function calls. One grammar might look like this: ctorBody : '{' superCall? stat* '}' ; Or, you might see something like stat : superCall ';' | expression ';' | ... ; In both cases I believe that no closure operations will dip into the outer context. In the first case ctorBody in the worst case will stop at the '}'. In the 2nd case it should stop at the ';'. Both cases should stay within the entry rule and not dip into the outer context. PREDICATES Predicates are always evaluated if present in either SLL or LL both. SLL and LL simulation deals with predicates differently. SLL collects predicates as it performs closure operations like ANTLR v3 did. It delays predicate evaluation until it reaches and accept state. This allows us to cache the SLL ATN simulation whereas, if we had evaluated predicates on-the-fly during closure, the DFA state configuration sets would be different and we couldn't build up a suitable DFA. When building a DFA accept state during ATN simulation, we evaluate any predicates and return the sole semantically valid alternative. If there is more than 1 alternative, we report an ambiguity. If there are 0 alternatives, we throw an exception. Alternatives without predicates act like they have true predicates. The simple way to think about it is to strip away all alternatives with false predicates and choose the minimum alternative that remains. When we start in the DFA and reach an accept state that's predicated, we test those and return the minimum semantically viable alternative. If no alternatives are viable, we throw an exception. We don't report ambiguities in the DFA, but I'm not sure why anymore. During full LL ATN simulation, closure always evaluates predicates and on-the-fly. This is crucial to reducing the configuration set size during closure. It hits a landmine when parsing with the Java grammar, for example, without this on-the-fly evaluation. SHARING DFA All instances of the same parser share the same decision DFAs through a static field. Each instance gets its own ATN simulator but they share the same decisionToDFA field. They also share a PredictionContextCache object that makes sure that all PredictionContext objects are shared among the DFA states. This makes a big size difference. THREAD SAFETY The parser ATN simulator locks on the decisionDFA field when it adds a new DFA object to that array. addDFAEdge locks on the DFA for the current decision when setting the edges[] field. addDFAState locks on the DFA for the current decision when looking up a DFA state to see if it already exists. We must make sure that all requests to add DFA states that are equivalent result in the same shared DFA object. This is because lots of threads will be trying to update the DFA at once. The addDFAState method also locks inside the DFA lock but this time on the shared context cache when it rebuilds the configurations' PredictionContext objects using cached subgraphs/nodes. No other locking occurs, even during DFA simulation. This is safe as long as we can guarantee that all threads referencing s.edge[t] get the same physical target DFA state, or none. Once into the DFA, the DFA simulation does not reference the dfa.state map. It follows the edges[] field to new targets. The DFA simulator will either find dfa.edges to be null, to be non-null and dfa.edges[t] null, or dfa.edges[t] to be non-null. The addDFAEdge method could be racing to set the field but in either case the DFA simulator works; if null, and requests ATN simulation. It could also race trying to get dfa.edges[t], but either way it will work because it's not doing a test and set operation. Starting with SLL then failing to combined SLL/LL Sam pointed out that if SLL does not give a syntax error, then there is no point in doing full LL, which is slower. We only have to try LL if we get a syntax error. For maximum speed, Sam starts the parser with pure SLL mode: parser.getInterpreter().setSLL(true); and with the bail error strategy: parser.setErrorHandler(new BailErrorStrategy()); If it does not get a syntax error, then we're done. If it does get a syntax error, we need to retry with the combined SLL/LL strategy. The reason this works is as follows. If there are no SLL conflicts then the grammar is SLL for sure, at least for that input set. If there is an SLL conflict, the full LL analysis must yield a set of ambiguous alternatives that is no larger than the SLL set. If the LL set is a singleton, then the grammar is LL but not SLL. If the LL set is the same size as the SLL set, the decision is SLL. If the LL set has size > 1, then that decision is truly ambiguous on the current input. If the LL set is smaller, then the SLL conflict resolution might choose an alternative that the full LL would rule out as a possibility based upon better context information. If that's the case, then the SLL parse will definitely get an error because the full LL analysis says it's not viable. If SLL conflict resolution chooses an alternative within the LL set, them both SLL and LL would choose the same alternative because they both choose the minimum of multiple conflicting alternatives. Let's say we have a set of SLL conflicting alternatives {1, 2, 3} and a smaller LL set called s. If s is {2, 3}, then SLL parsing will get an error because SLL will pursue alternative 1. If s is {1, 2} or {1, 3} then both SLL and LL will choose the same alternative because alternative one is the minimum of either set. If s is {2} or {3} then SLL will get a syntax error. If s is {1} then SLL will succeed. Of course, if the input is invalid, then we will get an error for sure in both SLL and LL parsing. Erroneous input will therefore require 2 passes over the input.

Field Summary

Fields

Modifier and Type	Field and Description
protected TokenStream	_input
protected ParserRuleContext	_outerContext
protected int	_startIndex
static int	ATN_failover
static boolean	debug
static boolean	debug_list_atn_decisions
DFA[]	decisionToDFA
static boolean	dfa_debug
protected DoubleKeyMap<PredictionContext,PredictionContext,PredictionContext>	mergeCache Each prediction operation uses a cache for merge of prediction contexts.
protected PredictionMode	mode SLL, LL, or LL + exact ambig detection?
protected Parser	parser
static int	predict_calls
static boolean	retry_debug
static int	retry_with_context
static int	retry_with_context_from_dfa
static int	retry_with_context_indicates_no_conflict
static int	retry_with_context_predicts_same_alt

Fields inherited from class org.antlr.v4.runtime.atn.ATNSimulator

atn, ERROR, SERIALIZED_VERSION, sharedContextCache

Constructor Summary

Constructors

Constructor and Description
ParserATNSimulator(ATN atn, DFA[] decisionToDFA, PredictionContextCache sharedContextCache) Testing only!
ParserATNSimulator(Parser parser, ATN atn, DFA[] decisionToDFA, PredictionContextCache sharedContextCache)

Method Summary

Methods

Modifier and Type	Method and Description
ATNConfig	actionTransition(ATNConfig config, ActionTransition t)
int	adaptivePredict(TokenStream input, int decision, ParserRuleContext outerContext)
protected void	addDFAEdge(DFA dfa, DFAState from, int t, DFAState to)
protected DFAState	addDFAState(DFA dfa, DFAState D) Add D if not there and return D.
protected void	closure_(ATNConfig config, ATNConfigSet configs, Set<ATNConfig> closureBusy, boolean collectPredicates, boolean fullCtx, int depth) Do the actual work of walking epsilon edges
protected void	closure(ATNConfig config, ATNConfigSet configs, Set<ATNConfig> closureBusy, boolean collectPredicates, boolean fullCtx)
protected void	closureCheckingStopState(ATNConfig config, ATNConfigSet configs, Set<ATNConfig> closureBusy, boolean collectPredicates, boolean fullCtx, int depth)
protected ATNConfigSet	computeReachSet(ATNConfigSet closure, int t, boolean fullCtx)
ATNConfigSet	computeStartState(ATNState p, RuleContext ctx, boolean fullCtx)
void	dumpDeadEndConfigs(NoViableAltException nvae)
BitSet	evalSemanticContext(DFAState.PredPrediction[] predPredictions, ParserRuleContext outerContext, boolean complete) Look through a list of predicate/alt pairs, returning alts for the pairs that win.
int	execATN(DFA dfa, DFAState s0, TokenStream input, int startIndex, ParserRuleContext outerContext) Performs ATN simulation to compute a predicted alternative based upon the remaining input, but also updates the DFA cache to avoid having to traverse the ATN again for the same input sequence.
int	execATNWithFullContext(DFA dfa, DFAState D, ATNConfigSet s0, TokenStream input, int startIndex, ParserRuleContext outerContext, int SLL_min_alt)
int	execDFA(DFA dfa, DFAState s0, TokenStream input, int startIndex, ParserRuleContext outerContext)
int	getAltThatFinishedDecisionEntryRule(ATNConfigSet configs)
BitSet	getConflictingAlts(ATNConfigSet configs)
protected BitSet	getConflictingAltsOrUniqueAlt(ATNConfigSet configs) Sam pointed out a problem with the previous definition, v3, of ambiguous states.
ATNConfig	getEpsilonTarget(ATNConfig config, Transition t, boolean collectPredicates, boolean inContext, boolean fullCtx)
String	getLookaheadName(TokenStream input)
DFAState.PredPrediction[]	getPredicatePredictions(BitSet ambigAlts, SemanticContext[] altToPred)
PredictionMode	getPredictionMode()
SemanticContext[]	getPredsForAmbigAlts(BitSet ambigAlts, ATNConfigSet configs, int nalts)
ATNState	getReachableTarget(Transition trans, int ttype)
String	getRuleName(int index)
String	getTokenName(int t)
static int	getUniqueAlt(ATNConfigSet configs)
NoViableAltException	noViableAlt(TokenStream input, ParserRuleContext outerContext, ATNConfigSet configs, int startIndex)
protected void	predicateDFAState(DFAState dfaState, DecisionState decisionState)
int	predictATN(DFA dfa, TokenStream input, ParserRuleContext outerContext)
ATNConfig	predTransition(ATNConfig config, PredicateTransition pt, boolean collectPredicates, boolean inContext, boolean fullCtx)
void	reportAmbiguity(DFA dfa, DFAState D, int startIndex, int stopIndex, BitSet ambigAlts, ATNConfigSet configs) If context sensitive parsing, we know it's ambiguity not conflict
void	reportAttemptingFullContext(DFA dfa, ATNConfigSet configs, int startIndex, int stopIndex)
void	reportContextSensitivity(DFA dfa, ATNConfigSet configs, int startIndex, int stopIndex)
void	reset()
ATNConfig	ruleTransition(ATNConfig config, RuleTransition t)
void	setPredictionMode(PredictionMode mode)

Methods inherited from class org.antlr.v4.runtime.atn.ATNSimulator

deserialize, edgeFactory, getCachedContext, stateFactory, toInt

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

debug

public static final boolean debug

See Also:

Constant Field Values

debug_list_atn_decisions

public static final boolean debug_list_atn_decisions

See Also:

Constant Field Values

dfa_debug

public static final boolean dfa_debug

See Also:

Constant Field Values

retry_debug

public static final boolean retry_debug

See Also:

Constant Field Values

ATN_failover

public static int ATN_failover

predict_calls

public static int predict_calls

retry_with_context

public static int retry_with_context

retry_with_context_indicates_no_conflict

public static int retry_with_context_indicates_no_conflict

retry_with_context_predicts_same_alt

public static int retry_with_context_predicts_same_alt

retry_with_context_from_dfa

public static int retry_with_context_from_dfa

parser

@Nullable
protected final Parser parser

decisionToDFA

@NotNull
public final DFA[] decisionToDFA

mode

protected PredictionMode mode

SLL, LL, or LL + exact ambig detection?

mergeCache

protected DoubleKeyMap<PredictionContext,PredictionContext,PredictionContext> mergeCache

Each prediction operation uses a cache for merge of prediction contexts. Don't keep around as it wastes huge amounts of memory. DoubleKeyMap isn't synchronized but we're ok since two threads shouldn't reuse same parser/atnsim object because it can only handle one input at a time. This maps graphs a and b to merged result c. (a,b)->c. We can avoid the merge if we ever see a and b again. Note that (b,a)->c should also be examined during cache lookup.

_input

protected TokenStream _input

_startIndex

protected int _startIndex

_outerContext

protected ParserRuleContext _outerContext

Constructor Detail

ParserATNSimulator

public ParserATNSimulator(@NotNull
                  ATN atn,
                  @NotNull
                  DFA[] decisionToDFA,
                  @NotNull
                  PredictionContextCache sharedContextCache)

Testing only!

ParserATNSimulator

public ParserATNSimulator(@Nullable
                  Parser parser,
                  @NotNull
                  ATN atn,
                  @NotNull
                  DFA[] decisionToDFA,
                  @NotNull
                  PredictionContextCache sharedContextCache)

Method Detail

reset

public void reset()

Specified by:

reset in class ATNSimulator

adaptivePredict

public int adaptivePredict(@NotNull
                  TokenStream input,
                  int decision,
                  @Nullable
                  ParserRuleContext outerContext)

predictATN

public int predictATN(@NotNull
             DFA dfa,
             @NotNull
             TokenStream input,
             @Nullable
             ParserRuleContext outerContext)

execDFA

public int execDFA(@NotNull
          DFA dfa,
          @NotNull
          DFAState s0,
          @NotNull
          TokenStream input,
          int startIndex,
          @Nullable
          ParserRuleContext outerContext)

execATN

public int execATN(@NotNull
          DFA dfa,
          @NotNull
          DFAState s0,
          @NotNull
          TokenStream input,
          int startIndex,
          ParserRuleContext outerContext)

Performs ATN simulation to compute a predicted alternative based upon the remaining input, but also updates the DFA cache to avoid having to traverse the ATN again for the same input sequence. There are some key conditions we're looking for after computing a new set of ATN configs (proposed DFA state): if the set is empty, there is no viable alternative for current symbol does the state uniquely predict an alternative? does the state have a conflict that would prevent us from putting it on the work list? We also have some key operations to do: add an edge from previous DFA state to potentially new DFA state, D, upon current symbol but only if adding to work list, which means in all cases except no viable alternative (and possibly non-greedy decisions?) collecting predicates and adding semantic context to DFA accept states adding rule context to context-sensitive DFA accept states consuming an input symbol reporting a conflict reporting an ambiguity reporting a context sensitivity reporting insufficient predicates cover these cases: dead end single alt single alt + preds conflict conflict + preds

predicateDFAState

protected void predicateDFAState(DFAState dfaState,
                     DecisionState decisionState)

execATNWithFullContext

public int execATNWithFullContext(DFA dfa,
                         DFAState D,
                         @NotNull
                         ATNConfigSet s0,
                         @NotNull
                         TokenStream input,
                         int startIndex,
                         ParserRuleContext outerContext,
                         int SLL_min_alt)

computeReachSet

protected ATNConfigSet computeReachSet(ATNConfigSet closure,
                           int t,
                           boolean fullCtx)

computeStartState

@NotNull
public ATNConfigSet computeStartState(@NotNull
                                     ATNState p,
                                     @Nullable
                                     RuleContext ctx,
                                     boolean fullCtx)

getReachableTarget

@Nullable
public ATNState getReachableTarget(@NotNull
                                   Transition trans,
                                   int ttype)

getPredsForAmbigAlts

public SemanticContext[] getPredsForAmbigAlts(@NotNull
                                     BitSet ambigAlts,
                                     @NotNull
                                     ATNConfigSet configs,
                                     int nalts)

getPredicatePredictions

public DFAState.PredPrediction[] getPredicatePredictions(BitSet ambigAlts,
                                                SemanticContext[] altToPred)

getAltThatFinishedDecisionEntryRule

public int getAltThatFinishedDecisionEntryRule(ATNConfigSet configs)

evalSemanticContext

public BitSet evalSemanticContext(@NotNull
                         DFAState.PredPrediction[] predPredictions,
                         ParserRuleContext outerContext,
                         boolean complete)

Look through a list of predicate/alt pairs, returning alts for the pairs that win. A NONE predicate indicates an alt containing an unpredicated config which behaves as "always true." If !complete then we stop at the first predicate that evaluates to true. This includes pairs with null predicates.

closure

protected void closure(@NotNull
           ATNConfig config,
           @NotNull
           ATNConfigSet configs,
           @NotNull
           Set<ATNConfig> closureBusy,
           boolean collectPredicates,
           boolean fullCtx)

closureCheckingStopState

protected void closureCheckingStopState(@NotNull
                            ATNConfig config,
                            @NotNull
                            ATNConfigSet configs,
                            @NotNull
                            Set<ATNConfig> closureBusy,
                            boolean collectPredicates,
                            boolean fullCtx,
                            int depth)

closure_

protected void closure_(@NotNull
            ATNConfig config,
            @NotNull
            ATNConfigSet configs,
            @NotNull
            Set<ATNConfig> closureBusy,
            boolean collectPredicates,
            boolean fullCtx,
            int depth)

Do the actual work of walking epsilon edges

getRuleName

@NotNull
public String getRuleName(int index)

getEpsilonTarget

@Nullable
public ATNConfig getEpsilonTarget(@NotNull
                                  ATNConfig config,
                                  @NotNull
                                  Transition t,
                                  boolean collectPredicates,
                                  boolean inContext,
                                  boolean fullCtx)

actionTransition

@NotNull
public ATNConfig actionTransition(@NotNull
                                 ATNConfig config,
                                 @NotNull
                                 ActionTransition t)

predTransition

@Nullable
public ATNConfig predTransition(@NotNull
                                ATNConfig config,
                                @NotNull
                                PredicateTransition pt,
                                boolean collectPredicates,
                                boolean inContext,
                                boolean fullCtx)

ruleTransition

@NotNull
public ATNConfig ruleTransition(@NotNull
                               ATNConfig config,
                               @NotNull
                               RuleTransition t)

getConflictingAlts

public BitSet getConflictingAlts(ATNConfigSet configs)

getConflictingAltsOrUniqueAlt

protected BitSet getConflictingAltsOrUniqueAlt(ATNConfigSet configs)

Sam pointed out a problem with the previous definition, v3, of ambiguous states. If we have another state associated with conflicting alternatives, we should keep going. For example, the following grammar s : (ID | ID ID?) ';' ; When the ATN simulation reaches the state before ';', it has a DFA state that looks like: [12|1|[], 6|2|[], 12|2|[]]. Naturally 12|1|[] and 12|2|[] conflict, but we cannot stop processing this node because alternative to has another way to continue, via [6|2|[]]. The key is that we have a single state that has config's only associated with a single alternative, 2, and crucially the state transitions among the configurations are all non-epsilon transitions. That means we don't consider any conflicts that include alternative 2. So, we ignore the conflict between alts 1 and 2. We ignore a set of conflicting alts when there is an intersection with an alternative associated with a single alt state in the state->config-list map. It's also the case that we might have two conflicting configurations but also a 3rd nonconflicting configuration for a different alternative: [1|1|[], 1|2|[], 8|3|[]]. This can come about from grammar: a : A | A | A B ; After matching input A, we reach the stop state for rule A, state 1. State 8 is the state right before B. Clearly alternatives 1 and 2 conflict and no amount of further lookahead will separate the two. However, alternative 3 will be able to continue and so we do not stop working on this state. In the previous example, we're concerned with states associated with the conflicting alternatives. Here alt 3 is not associated with the conflicting configs, but since we can continue looking for input reasonably, I don't declare the state done. We ignore a set of conflicting alts when we have an alternative that we still need to pursue.

getTokenName

@NotNull
public String getTokenName(int t)

getLookaheadName

public String getLookaheadName(TokenStream input)

dumpDeadEndConfigs

public void dumpDeadEndConfigs(@NotNull
                      NoViableAltException nvae)

noViableAlt

@NotNull
public NoViableAltException noViableAlt(@NotNull
                                       TokenStream input,
                                       @NotNull
                                       ParserRuleContext outerContext,
                                       @NotNull
                                       ATNConfigSet configs,
                                       int startIndex)

getUniqueAlt

public static int getUniqueAlt(@NotNull
               ATNConfigSet configs)

addDFAEdge

protected void addDFAEdge(@NotNull
              DFA dfa,
              @Nullable
              DFAState from,
              int t,
              @Nullable
              DFAState to)

addDFAState

protected DFAState addDFAState(@NotNull
                   DFA dfa,
                   @NotNull
                   DFAState D)

Add D if not there and return D. Return previous if already present.

reportAttemptingFullContext

public void reportAttemptingFullContext(DFA dfa,
                               ATNConfigSet configs,
                               int startIndex,
                               int stopIndex)

reportContextSensitivity

public void reportContextSensitivity(DFA dfa,
                            ATNConfigSet configs,
                            int startIndex,
                            int stopIndex)

reportAmbiguity

public void reportAmbiguity(@NotNull
                   DFA dfa,
                   DFAState D,
                   int startIndex,
                   int stopIndex,
                   @NotNull
                   BitSet ambigAlts,
                   @NotNull
                   ATNConfigSet configs)

If context sensitive parsing, we know it's ambiguity not conflict

setPredictionMode

public void setPredictionMode(PredictionMode mode)

getPredictionMode

public PredictionMode getPredictionMode()