antlr4-runtime/src/antlr4/dfa/DFAState.js

JavaScript runtime for ANTLR4

/* Copyright (c) 2012-2022 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.
 */

import ATNConfigSet from '../atn/ATNConfigSet.js';
import HashCode from "../misc/HashCode.js";
import HashSet from "../misc/HashSet.js";


/**
 * 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>
 */
export default class DFAState {
	constructor(stateNumber, configs) {
		if (stateNumber === null) {
			stateNumber = -1;
		}
		if (configs === null) {
			configs = new ATNConfigSet();
		}
		this.stateNumber = stateNumber;
		this.configs = configs;
		/**
		 * {@code edges[symbol]} points to target of symbol. Shift up by 1 so (-1)
		 * {@link Token//EOF} maps to {@code edges[0]}.
		 */
		this.edges = null;
		this.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}.
		 */
		this.prediction = 0;
		this.lexerActionExecutor = null;
		/**
		 * 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.
		 */
		this.requiresFullContext = false;
		/**
		 * 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>
		 */
		this.predicates = null;
		return this;
	}

	/**
	 * Get the set of all alts mentioned by all ATN configurations in this
	 * DFA state.
	 */
	getAltSet() {
		const alts = new HashSet();
		if (this.configs !== null) {
			for (let i = 0; i < this.configs.length; i++) {
				const c = this.configs[i];
				alts.add(c.alt);
			}
		}
		if (alts.length === 0) {
			return null;
		} else {
			return alts;
		}
	}

	/**
	 * 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>
	 */
	equals(other) {
		// compare set of ATN configurations in this set with other
		return this === other ||
				(other instanceof DFAState &&
					this.configs.equals(other.configs));
	}

	toString() {
		let s = "" + this.stateNumber + ":" + this.configs;
		if(this.isAcceptState) {
			s = s + "=>";
			if (this.predicates !== null)
				s = s + this.predicates;
			else
				s = s + this.prediction;
		}
		return s;
	}

	hashCode() {
		const hash = new HashCode();
		hash.update(this.configs);
		return hash.finish();
	}
}