/*
 * 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.
/** 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.
 *
 *  <p>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.</p>
 *
 *  <p>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.</p>
 */
public class DFAState {
	public int stateNumber = -1;


	public ATNConfigSet configs = new ATNConfigSet();

	
edges[symbol] points to target of symbol. Shift up by 1 so (-1) Token.EOF maps to edges[0]. /** {@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 ATN.INVALID_ALT_NUMBER when predicates!=null or requiresFullContext. /** 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 ParserATNSimulator.execATN invocations immediately jumped doing full context prediction if this field is true. /**
	 * 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, 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.
/** 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}.
	 *
	 *  <p>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.</p>
	 *
	 *  <p>This list is computed by {@link ParserATNSimulator#predicateDFAState}.</p>
	 */

	public PredPrediction[] predicates;

	
Map a predicate to a predicted alternative. /** 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.
/** Get the set of all alts mentioned by all ATN configurations in this
	 *  DFA state.
	 */
	public Set<Integer> getAltSet() {
		Set<Integer> alts = new HashSet<Integer>();
		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 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 we need to know if any other state exists that has this exact set of ATN configurations. The stateNumber is irrelevant.
/**
	 * 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.
	 *
	 * <p>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.</p>
	 *
	 * <p>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.</p>
	 */
	@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();
	}
}