antlr4-runtime/src/antlr4/atn/ATNSimulator.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 DFAState from '../dfa/DFAState.js';
import ATNConfigSet from './ATNConfigSet.js';
import { getCachedPredictionContext } from '../context/PredictionContextUtils.js';
import HashMap from "../misc/HashMap.js";

export default class ATNSimulator {
    constructor(atn, sharedContextCache) {
        /**
         * The context cache maps all PredictionContext objects that are ==
         * to a single cached copy. This cache is shared across all contexts
         * in all ATNConfigs in all DFA states.  We rebuild each ATNConfigSet
         * to use only cached nodes/graphs in addDFAState(). We don't want to
         * fill this during closure() since there are lots of contexts that
         * pop up but are not used ever again. It also greatly slows down closure().
         *
         * <p>This cache makes a huge difference in memory and a little bit in speed.
         * For the Java grammar on java.*, it dropped the memory requirements
         * at the end from 25M to 16M. We don't store any of the full context
         * graphs in the DFA because they are limited to local context only,
         * but apparently there's a lot of repetition there as well. We optimize
         * the config contexts before storing the config set in the DFA states
         * by literally rebuilding them with cached subgraphs only.</p>
         *
         * <p>I tried a cache for use during closure operations, that was
         * whacked after each adaptivePredict(). It cost a little bit
         * more time I think and doesn't save on the overall footprint
         * so it's not worth the complexity.</p>
         */
        this.atn = atn;
        this.sharedContextCache = sharedContextCache;
        return this;
    }

    getCachedContext(context) {
        if (this.sharedContextCache ===null) {
            return context;
        }
        const visited = new HashMap();
        return getCachedPredictionContext(context, this.sharedContextCache, visited);
    }
}

// Must distinguish between missing edge and edge we know leads nowhere///
ATNSimulator.ERROR = new DFAState(0x7FFFFFFF, new ATNConfigSet());