org.antlr.v4.runtime.atn

Enum PredictionMode

java.lang.Object

java.lang.Enum<PredictionMode>

org.antlr.v4.runtime.atn.PredictionMode

All Implemented Interfaces:

Serializable, Comparable<PredictionMode>

public enum PredictionMode
extends Enum<PredictionMode>

Enum Constant Summary

Enum Constants

Enum Constant and Description
LL Full LL(*) that always gets right answer.
LL_EXACT_AMBIG_DETECTION Tell the full LL prediction algorithm to pursue lookahead until it has uniquely predicted an alternative without conflict or it's certain that it's found an ambiguous input sequence.
SLL Do only local context prediction (SLL style) and using heuristic which almost always works but is much faster than precise answer.

Method Summary

Methods

Modifier and Type	Method and Description
static boolean	allConfigsInRuleStopStates(ATNConfigSet configs) Checks if all configurations in configs are in a RuleStopState.
static boolean	allSubsetsConflict(Collection<BitSet> altsets) Determines if every alternative subset in altsets contains more than one alternative.
static boolean	allSubsetsEqual(Collection<BitSet> altsets) Determines if every alternative subset in altsets is equivalent.
static BitSet	getAlts(Collection<BitSet> altsets) Gets the complete set of represented alternatives for a collection of alternative subsets.
static Collection<BitSet>	getConflictingAltSubsets(ATNConfigSet configs) This function gets the conflicting alt subsets from a configuration set.
static int	getSingleViableAlt(Collection<BitSet> altsets)
static Map<ATNState,BitSet>	getStateToAltMap(ATNConfigSet configs) Get a map from state to alt subset from a configuration set.
static int	getUniqueAlt(Collection<BitSet> altsets) Returns the unique alternative predicted by all alternative subsets in altsets.
static boolean	hasConfigInRuleStopState(ATNConfigSet configs) Checks if any configuration in configs is in a RuleStopState.
static boolean	hasConflictingAltSet(Collection<BitSet> altsets) Determines if any single alternative subset in altsets contains more than one alternative.
static boolean	hasNonConflictingAltSet(Collection<BitSet> altsets) Determines if any single alternative subset in altsets contains exactly one alternative.
static boolean	hasSLLConflictTerminatingPrediction(PredictionMode mode, ATNConfigSet configs) Computes the SLL prediction termination condition.
static boolean	hasStateAssociatedWithOneAlt(ATNConfigSet configs)
static int	resolvesToJustOneViableAlt(Collection<BitSet> altsets) Full LL prediction termination.
static PredictionMode	valueOf(String name) Returns the enum constant of this type with the specified name.
static PredictionMode[]	values() Returns an array containing the constants of this enum type, in the order they are declared.

Methods inherited from class java.lang.Enum

clone, compareTo, equals, finalize, getDeclaringClass, hashCode, name, ordinal, toString, valueOf

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Enum Constant Detail

SLL

public static final PredictionMode SLL

Do only local context prediction (SLL style) and using heuristic which almost always works but is much faster than precise answer.

LL

public static final PredictionMode LL

Full LL(*) that always gets right answer. For speed reasons, we terminate the prediction process when we know for sure which alt to predict. We don't always know what the ambiguity is in this mode.

LL_EXACT_AMBIG_DETECTION

public static final PredictionMode LL_EXACT_AMBIG_DETECTION

Tell the full LL prediction algorithm to pursue lookahead until it has uniquely predicted an alternative without conflict or it's certain that it's found an ambiguous input sequence. when this variable is false. When true, the prediction process will continue looking for the exact ambiguous sequence even if it has already figured out which alternative to predict.

Method Detail

values

public static PredictionMode[] values()

Returns an array containing the constants of this enum type, in the order they are declared. This method may be used to iterate over the constants as follows:

for (PredictionMode c : PredictionMode.values())
    System.out.println(c);

Returns:

an array containing the constants of this enum type, in the order they are declared

valueOf

public static PredictionMode valueOf(String name)

Returns the enum constant of this type with the specified name. The string must match exactly an identifier used to declare an enum constant in this type. (Extraneous whitespace characters are not permitted.)

Parameters:

name - the name of the enum constant to be returned.

Returns:

the enum constant with the specified name

Throws:

IllegalArgumentException - if this enum type has no constant with the specified name

NullPointerException - if the argument is null

hasSLLConflictTerminatingPrediction

public static boolean hasSLLConflictTerminatingPrediction(PredictionMode mode,
                                          @NotNull
                                          ATNConfigSet configs)

Computes the SLL prediction termination condition.

This method computes the SLL prediction termination condition for both of the following cases.

• The usual SLL+LL fallback upon SLL conflict
• Pure SLL without LL fallback

COMBINED SLL+LL PARSING

When LL-fallback is enabled upon SLL conflict, correct predictions are ensured regardless of how the termination condition is computed by this method. Due to the substantially higher cost of LL prediction, the prediction should only fall back to LL when the additional lookahead cannot lead to a unique SLL prediction.

Assuming combined SLL+LL parsing, an SLL configuration set with only conflicting subsets should fall back to full LL, even if the configuration sets don't resolve to the same alternative (e.g. {1,2} and {3,4}. If there is at least one non-conflicting configuration, SLL could continue with the hopes that more lookahead will resolve via one of those non-conflicting configurations.

Here's the prediction termination rule them: SLL (for SLL+LL parsing) stops when it sees only conflicting configuration subsets. In contrast, full LL keeps going when there is uncertainty.

HEURISTIC

As a heuristic, we stop prediction when we see any conflicting subset unless we see a state that only has one alternative associated with it. The single-alt-state thing lets prediction continue upon rules like (otherwise, it would admit defeat too soon):

[12|1|[], 6|2|[], 12|2|[]]. 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|[]].

It also let's us continue for this rule:

[1|1|[], 1|2|[], 8|3|[]] 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, don't declare the state done.

PURE SLL PARSING

To handle pure SLL parsing, all we have to do is make sure that we combine stack contexts for configurations that differ only by semantic predicate. From there, we can do the usual SLL termination heuristic.

PREDICATES IN SLL+LL PARSING

SLL decisions don't evaluate predicates until after they reach DFA stop states because they need to create the DFA cache that works in all semantic situations. In contrast, full LL evaluates predicates collected during start state computation so it can ignore predicates thereafter. This means that SLL termination detection can totally ignore semantic predicates.

Implementation-wise, ATNConfigSet combines stack contexts but not semantic predicate contexts so we might see two configurations like the following.

(s, 1, x, {}), (s, 1, x', {p})

Before testing these configurations against others, we have to merge x and x' (without modifying the existing configurations). For example, we test (x+x')==x'' when looking for conflicts in the following configurations.

(s, 1, x, {}), (s, 1, x', {p}), (s, 2, x'', {})

If the configuration set has predicates (as indicated by ATNConfigSet.hasSemanticContext), this algorithm makes a copy of the configurations to strip out all of the predicates so that a standard ATNConfigSet will merge everything ignoring predicates.

hasConfigInRuleStopState

public static boolean hasConfigInRuleStopState(ATNConfigSet configs)

Checks if any configuration in configs is in a RuleStopState. Configurations meeting this condition have reached the end of the decision rule (local context) or end of start rule (full context).

Parameters:

configs - the configuration set to test

Returns:

true if any configuration in configs is in a RuleStopState, otherwise false

allConfigsInRuleStopStates

public static boolean allConfigsInRuleStopStates(@NotNull
                                 ATNConfigSet configs)

Checks if all configurations in configs are in a RuleStopState. Configurations meeting this condition have reached the end of the decision rule (local context) or end of start rule (full context).

Parameters:

configs - the configuration set to test

Returns:

true if all configurations in configs are in a RuleStopState, otherwise false

resolvesToJustOneViableAlt

public static int resolvesToJustOneViableAlt(@NotNull
                             Collection<BitSet> altsets)

Full LL prediction termination.

Can we stop looking ahead during ATN simulation or is there some uncertainty as to which alternative we will ultimately pick, after consuming more input? Even if there are partial conflicts, we might know that everything is going to resolve to the same minimum alternative. That means we can stop since no more lookahead will change that fact. On the other hand, there might be multiple conflicts that resolve to different minimums. That means we need more look ahead to decide which of those alternatives we should predict.

The basic idea is to split the set of configurations C, into conflicting subsets (s, _, ctx, _) and singleton subsets with non-conflicting configurations. Two configurations conflict if they have identical ATNConfig.state and ATNConfig.context values but different ATNConfig.alt value, e.g. (s, i, ctx, _) and (s, j, ctx, _) for i!=j.

Reduce these configuration subsets to the set of possible alternatives. You can compute the alternative subsets in one pass as follows:

A_s,ctx = {i | (s, i, ctx, _)} for each configuration in C holding s and ctx fixed.

Or in pseudo-code, for each configuration c in C:

 map[c] U= c.alt # map hash/equals uses s and x, not
 alt and not pred
 

The values in map are the set of A_s,ctx sets.

If |A_s,ctx|=1 then there is no conflict associated with s and ctx.

Reduce the subsets to singletons by choosing a minimum of each subset. If the union of these alternative subsets is a singleton, then no amount of more lookahead will help us. We will always pick that alternative. If, however, there is more than one alternative, then we are uncertain which alternative to predict and must continue looking for resolution. We may or may not discover an ambiguity in the future, even if there are no conflicting subsets this round.

The biggest sin is to terminate early because it means we've made a decision but were uncertain as to the eventual outcome. We haven't used enough lookahead. On the other hand, announcing a conflict too late is no big deal; you will still have the conflict. It's just inefficient. It might even look until the end of file.

No special consideration for semantic predicates is required because predicates are evaluated on-the-fly for full LL prediction, ensuring that no configuration contains a semantic context during the termination check.

CONFLICTING CONFIGS

Two configurations (s, i, x) and (s, j, x'), conflict when i!=j but x=x'. Because we merge all (s, i, _) configurations together, that means that there are at most n configurations associated with state s for n possible alternatives in the decision. The merged stacks complicate the comparison of configuration contexts x and x'. Sam checks to see if one is a subset of the other by calling merge and checking to see if the merged result is either x or x'. If the x associated with lowest alternative i is the superset, then i is the only possible prediction since the others resolve to min(i) as well. However, if x is associated with j>i then at least one stack configuration for j is not in conflict with alternative i. The algorithm should keep going, looking for more lookahead due to the uncertainty.

For simplicity, I'm doing a equality check between x and x' that lets the algorithm continue to consume lookahead longer than necessary. The reason I like the equality is of course the simplicity but also because that is the test you need to detect the alternatives that are actually in conflict.

CONTINUE/STOP RULE

Continue if union of resolved alternative sets from non-conflicting and conflicting alternative subsets has more than one alternative. We are uncertain about which alternative to predict.

The complete set of alternatives, [i for (_,i,_)], tells us which alternatives are still in the running for the amount of input we've consumed at this point. The conflicting sets let us to strip away configurations that won't lead to more states because we resolve conflicts to the configuration with a minimum alternate for the conflicting set.

CASES

• no conflicts and more than 1 alternative in set => continue
• (s, 1, x), (s, 2, x), (s, 3, z), (s', 1, y), (s', 2, y) yields non-conflicting set {3} U conflicting sets min({1,2}) U min({1,2}) = {1,3} => continue
• (s, 1, x), (s, 2, x), (s', 1, y), (s', 2, y), (s'', 1, z) yields non-conflicting set {1} U conflicting sets min({1,2}) U min({1,2}) = {1} => stop and predict 1
• (s, 1, x), (s, 2, x), (s', 1, y), (s', 2, y) yields conflicting, reduced sets {1} U {1} = {1} => stop and predict 1, can announce ambiguity {1,2}
• (s, 1, x), (s, 2, x), (s', 2, y), (s', 3, y) yields conflicting, reduced sets {1} U {2} = {1,2} => continue
• (s, 1, x), (s, 2, x), (s', 3, y), (s', 4, y) yields conflicting, reduced sets {1} U {3} = {1,3} => continue

EXACT AMBIGUITY DETECTION

If all states report the same conflicting set of alternatives, then we know we have the exact ambiguity set.

|A_i|>1 and A_i = A_j for all i, j.

In other words, we continue examining lookahead until all A_i have more than one alternative and all A_i are the same. If A={{1,2}, {1,3}}, then regular LL prediction would terminate because the resolved set is {1}. To determine what the real ambiguity is, we have to know whether the ambiguity is between one and two or one and three so we keep going. We can only stop prediction when we need exact ambiguity detection when the sets look like A={{1,2}} or {{1,2},{1,2}}, etc...

allSubsetsConflict

public static boolean allSubsetsConflict(@NotNull
                         Collection<BitSet> altsets)

Determines if every alternative subset in altsets contains more than one alternative.

Parameters:

altsets - a collection of alternative subsets

Returns:

true if every BitSet in altsets has cardinality > 1, otherwise false

hasNonConflictingAltSet

public static boolean hasNonConflictingAltSet(@NotNull
                              Collection<BitSet> altsets)

Determines if any single alternative subset in altsets contains exactly one alternative.

Parameters:

altsets - a collection of alternative subsets

Returns:

true if altsets contains a BitSet with cardinality 1, otherwise false

hasConflictingAltSet

public static boolean hasConflictingAltSet(@NotNull
                           Collection<BitSet> altsets)

Determines if any single alternative subset in altsets contains more than one alternative.

Parameters:

altsets - a collection of alternative subsets

Returns:

true if altsets contains a BitSet with cardinality > 1, otherwise false

allSubsetsEqual

public static boolean allSubsetsEqual(@NotNull
                      Collection<BitSet> altsets)

Determines if every alternative subset in altsets is equivalent.

Parameters:

altsets - a collection of alternative subsets

Returns:

true if every member of altsets is equal to the others, otherwise false

getUniqueAlt

public static int getUniqueAlt(@NotNull
               Collection<BitSet> altsets)

Returns the unique alternative predicted by all alternative subsets in altsets. If no such alternative exists, this method returns ATN.INVALID_ALT_NUMBER.

Parameters:

altsets - a collection of alternative subsets

getAlts

public static BitSet getAlts(@NotNull
             Collection<BitSet> altsets)

Gets the complete set of represented alternatives for a collection of alternative subsets. This method returns the union of each BitSet in altsets.

Parameters:

altsets - a collection of alternative subsets

Returns:

the set of represented alternatives in altsets

getConflictingAltSubsets

@NotNull
public static Collection<BitSet> getConflictingAltSubsets(ATNConfigSet configs)

This function gets the conflicting alt subsets from a configuration set. For each configuration c in configs:

 map[c] U= c.alt # map hash/equals uses s and x, not
 alt and not pred
 

getStateToAltMap

@NotNull
public static Map<ATNState,BitSet> getStateToAltMap(@NotNull
                                            ATNConfigSet configs)

Get a map from state to alt subset from a configuration set. For each configuration c in configs:

 map[c.state] U= c.alt
 

hasStateAssociatedWithOneAlt

public static boolean hasStateAssociatedWithOneAlt(@NotNull
                                   ATNConfigSet configs)

getSingleViableAlt

public static int getSingleViableAlt(@NotNull
                     Collection<BitSet> altsets)