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/*
 * 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();
	}
}