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With inspiration from other libraries
/*
* Copyright (c) 2012-2017 The ANTLR Project. All rights reserved.
* Use of this file is governed by the BSD 3-clause license that
* can be found in the LICENSE.txt file in the project root.
*/
package org.antlr.v4.runtime.dfa;
import org.antlr.v4.runtime.Token;
import org.antlr.v4.runtime.atn.ATN;
import org.antlr.v4.runtime.atn.ATNConfig;
import org.antlr.v4.runtime.atn.ATNConfigSet;
import org.antlr.v4.runtime.atn.LexerActionExecutor;
import org.antlr.v4.runtime.atn.ParserATNSimulator;
import org.antlr.v4.runtime.atn.SemanticContext;
import org.antlr.v4.runtime.misc.MurmurHash;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;
/** 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.
*/
public class DFAState {
public int stateNumber = -1;
public ATNConfigSet configs = new ATNConfigSet();
/** {@code edges[symbol]} points to target of symbol. Shift up by 1 so (-1)
* {@link Token#EOF} maps to {@code edges[0]}.
*/
public DFAState[] edges;
public boolean isAcceptState = false;
/** if accept state, what ttype do we match or alt do we predict?
* This is set to {@link ATN#INVALID_ALT_NUMBER} when {@link #predicates}{@code !=null} or
* {@link #requiresFullContext}.
*/
public int prediction;
public LexerActionExecutor lexerActionExecutor;
/**
* Indicates that this state was created during SLL prediction that
* discovered a conflict between the configurations in the state. Future
* {@link ParserATNSimulator#execATN} invocations immediately jumped doing
* full context prediction if this field is true.
*/
public boolean requiresFullContext;
/** During SLL parsing, this is a list of predicates associated with the
* ATN configurations of the DFA state. When we have predicates,
* {@link #requiresFullContext} is {@code false} since full context prediction evaluates predicates
* on-the-fly. If this is not null, then {@link #prediction} is
* {@link ATN#INVALID_ALT_NUMBER}.
*
* We only use these for non-{@link #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 {@link ParserATNSimulator#predicateDFAState}.
*/
public PredPrediction[] predicates;
/** Map a predicate to a predicted alternative. */
public static class PredPrediction {
public SemanticContext pred; // never null; at least SemanticContext.NONE
public int alt;
public PredPrediction(SemanticContext pred, int alt) {
this.alt = alt;
this.pred = pred;
}
@Override
public String toString() {
return "("+pred+", "+alt+ ")";
}
}
public DFAState() { }
public DFAState(int stateNumber) { this.stateNumber = stateNumber; }
public DFAState(ATNConfigSet configs) { this.configs = configs; }
/** Get the set of all alts mentioned by all ATN configurations in this
* DFA state.
*/
public Set getAltSet() {
Set alts = new HashSet();
if ( configs!=null ) {
for (ATNConfig c : configs) {
alts.add(c.alt);
}
}
if ( alts.isEmpty() ) return null;
return alts;
}
@Override
public int hashCode() {
int hash = MurmurHash.initialize(7);
hash = MurmurHash.update(hash, configs.hashCode());
hash = MurmurHash.finish(hash, 1);
return hash;
}
/**
* Two {@link 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
* {@link ParserATNSimulator#addDFAState} we need to know if any other state
* exists that has this exact set of ATN configurations. The
* {@link #stateNumber} is irrelevant.
*/
@Override
public boolean equals(Object o) {
// compare set of ATN configurations in this set with other
if ( this==o ) return true;
if (!(o instanceof DFAState)) {
return false;
}
DFAState other = (DFAState)o;
// TODO (sam): what to do when configs==null?
boolean sameSet = this.configs.equals(other.configs);
// System.out.println("DFAState.equals: "+configs+(sameSet?"==":"!=")+other.configs);
return sameSet;
}
@Override
public String toString() {
StringBuilder buf = new StringBuilder();
buf.append(stateNumber).append(":").append(configs);
if ( isAcceptState ) {
buf.append("=>");
if ( predicates!=null ) {
buf.append(Arrays.toString(predicates));
}
else {
buf.append(prediction);
}
}
return buf.toString();
}
}