Class DFAState

java.lang.Object

org.antlr.v4.runtime.dfa.DFAState

public class DFAState
extends Object

A DFA state represents a set of possible ATN configurations. As Aho, Sethi, Ullman p. 117 says "The DFA uses its state to keep track of all possible states the ATN can be in after reading each input symbol. That is to say, after reading input a1a2..an, the DFA is in a state that represents the subset T of the states of the ATN that are reachable from the ATN's start state along some path labeled a1a2..an." In conventional NFA→DFA conversion, therefore, the subset T would be a bitset representing the set of states the ATN could be in. We need to track the alt predicted by each state as well, however. More importantly, we need to maintain a stack of states, tracking the closure operations as they jump from rule to rule, emulating rule invocations (method calls). I have to add a stack to simulate the proper lookahead sequences for the underlying LL grammar from which the ATN was derived.

I use a set of ATNConfig objects not simple states. An ATNConfig is both a state (ala normal conversion) and a RuleContext describing the chain of rules (if any) followed to arrive at that state.

A DFA state may have multiple references to a particular state, but with different ATN contexts (with same or different alts) meaning that state was reached via a different set of rule invocations.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static class	DFAState.PredPrediction	Map a predicate to a predicted alternative.

Field Summary

Fields

Modifier and Type	Field	Description
ATNConfigSet	configs
DFAState[]	edges	edges[symbol] points to target of symbol.
boolean	isAcceptState
LexerActionExecutor	lexerActionExecutor
DFAState.PredPrediction[]	predicates	During SLL parsing, this is a list of predicates associated with the ATN configurations of the DFA state.
int	prediction	if accept state, what ttype do we match or alt do we predict? This is set to ATN.INVALID_ALT_NUMBER when predicates!=null or requiresFullContext.
boolean	requiresFullContext	Indicates that this state was created during SLL prediction that discovered a conflict between the configurations in the state.
int	stateNumber

Constructor Summary

Constructors

Constructor	Description
DFAState()
DFAState(int stateNumber)
DFAState(ATNConfigSet configs)

Method Summary

Modifier and Type	Method	Description
boolean	equals(Object o)	Two DFAState instances are equal if their ATN configuration sets are the same.
Set<Integer>	getAltSet()	Get the set of all alts mentioned by all ATN configurations in this DFA state.
int	hashCode()
String	toString()

Methods inherited from class Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Field Details

stateNumber

public int stateNumber

configs

public ATNConfigSet configs

edges

public DFAState[] edges

edges[symbol] points to target of symbol. Shift up by 1 so (-1) Token.EOF maps to edges[0].

isAcceptState

public boolean isAcceptState

prediction

public int prediction

if accept state, what ttype do we match or alt do we predict? This is set to ATN.INVALID_ALT_NUMBER when predicates!=null or requiresFullContext.

lexerActionExecutor

public LexerActionExecutor lexerActionExecutor

requiresFullContext

public boolean requiresFullContext

Indicates that this state was created during SLL prediction that discovered a conflict between the configurations in the state. Future ParserATNSimulator.execATN(DFA, DFAState, TokenStream, int, ParserRuleContext) invocations immediately jumped doing full context prediction if this field is true.

predicates

public DFAState.PredPrediction[] predicates

During SLL parsing, this is a list of predicates associated with the ATN configurations of the DFA state. When we have predicates, requiresFullContext is false since full context prediction evaluates predicates on-the-fly. If this is not null, then prediction is ATN.INVALID_ALT_NUMBER.

We only use these for non-requiresFullContext but conflicting states. That means we know from the context (it's $ or we don't dip into outer context) that it's an ambiguity not a conflict.

This list is computed by ParserATNSimulator.predicateDFAState(DFAState, DecisionState).

Constructor Details

DFAState

public DFAState()

DFAState

public DFAState(int stateNumber)

DFAState

public DFAState(ATNConfigSet configs)

Method Details

getAltSet

public Set<Integer> getAltSet()

Get the set of all alts mentioned by all ATN configurations in this DFA state.

hashCode

public int hashCode()

Overrides:

hashCode in class Object

equals

public boolean equals(Object o)

Two DFAState instances are equal if their ATN configuration sets are the same. This method is used to see if a state already exists.

Because the number of alternatives and number of ATN configurations are finite, there is a finite number of DFA states that can be processed. This is necessary to show that the algorithm terminates.

Cannot test the DFA state numbers here because in ParserATNSimulator.addDFAState(DFA, DFAState) we need to know if any other state exists that has this exact set of ATN configurations. The stateNumber is irrelevant.

Overrides:

equals in class Object

toString

public String toString()

Overrides:

toString in class Object