@fauton/cfg/dist/removeUnreachableProduction.js

A package to work with context free grammars and LL1 parsers

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.removeUnreachableProduction = void 0;
const linked_list_1 = require("@datastructures-js/linked-list");
const populateCfg_1 = require("./utils/populateCfg");
const setOperations_1 = require("./utils/setOperations");
/**
 * Removes unreachable variables and production of a cfg
 * @param cfg Production rules, start variable and variables array of cfg
 * @returns A new production rule record and variables with unreachable variable and rules removed
 */
function removeUnreachableProduction(inputCfg) {
    var _a;
    const cfg = (0, populateCfg_1.populateCfg)(inputCfg);
    const { productionRules, startVariable, variables } = cfg;
    const variablesSet = new Set(variables);
    const unvisitedVariables = new linked_list_1.LinkedList();
    unvisitedVariables.insertFirst(startVariable);
    const visitedVariables = new Set();
    visitedVariables.add(startVariable);
    // While we have unvisited variables
    while (unvisitedVariables.count()) {
        const unvisitedVariable = unvisitedVariables.removeFirst();
        // For each of the unvisited variable, check which variables we can reach
        (_a = productionRules[unvisitedVariable.getValue()]) === null || _a === void 0 ? void 0 : _a.forEach((productionRuleSubstitution) => {
            const tokens = productionRuleSubstitution.split(' ');
            for (let tokenIndex = 0; tokenIndex < tokens.length; tokenIndex += 1) {
                const token = tokens[tokenIndex];
                // If the letter is a variable, and we haven't visited the variable yet
                if (variablesSet.has(token) && !visitedVariables.has(token)) {
                    visitedVariables.add(token);
                    unvisitedVariables.insertLast(token);
                }
            }
        });
    }
    // The difference between the variables set and visited variables set will return the variables which couldn't be reached
    const nonReachableVariables = Array.from((0, setOperations_1.setDifference)(variablesSet, visitedVariables));
    // Delete the production rules for each of the unreachable variables
    nonReachableVariables.forEach((nonReachableVariable) => {
        delete productionRules[nonReachableVariable];
    });
    // There is no need to update the production rules as unreachable variables are not referenced there
    return Array.from(visitedVariables);
}
exports.removeUnreachableProduction = removeUnreachableProduction;