@fauton/cfg/dist/removeUnitProduction.js

A package to work with context free grammars and LL1 parsers

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.removeUnitProduction = exports.findFirstUnitProductionRule = void 0;
const populateCfg_1 = require("./utils/populateCfg");
/**
 * Returns the first occurrence of unit production inside production rules
 * @param variables Array of variables
 * @param productionRules Production rules of CFG
 * @returns A tuple where the first item is the variable which produces unit production, and second item indicates the rule position
 */
function findFirstUnitProductionRule(variables, productionRules) {
    const variablesSet = new Set(variables);
    // Loop through all the variables
    for (let variableIndex = 0; variableIndex < variables.length; variableIndex += 1) {
        const variable = variables[variableIndex];
        if (productionRules[variable]) {
            // For each variable, loop through its production rules
            for (let productionRuleSubstitutionIndex = 0; productionRuleSubstitutionIndex < productionRules[variable].length; productionRuleSubstitutionIndex += 1) {
                const productionRuleSubstitution = productionRules[variable][productionRuleSubstitutionIndex];
                const tokens = productionRuleSubstitution.split(' ');
                // If the production rule substitution is of length one and its a variable
                if (tokens.length === 1 && variablesSet.has(productionRuleSubstitution)) {
                    return [variable, productionRuleSubstitutionIndex];
                }
            }
        }
    }
    return null;
}
exports.findFirstUnitProductionRule = findFirstUnitProductionRule;
/**
 * Modifies the production rules of a cfg to remove unit production rules
 * @param cfg Variable and production rules of a cfg
 */
function removeUnitProduction(inputCfg) {
    const cfg = (0, populateCfg_1.populateCfg)(inputCfg);
    let unitProductionRule = findFirstUnitProductionRule(cfg.variables, cfg.productionRules);
    // Only continue if there is any unit production rule
    while (unitProductionRule) {
        const [unitProductionVariable, productionRuleSubstitutionIndex] = unitProductionRule;
        // Get all the production rules of the unit production variable
        const unitProductionRules = cfg.productionRules[unitProductionVariable];
        const unitProducingVariableRules = cfg.productionRules[unitProductionRules[productionRuleSubstitutionIndex]];
        // Remove the rule that generates unit production
        unitProductionRules.splice(productionRuleSubstitutionIndex, 1);
        // Some variables might not be present in the production rules
        if (unitProducingVariableRules) {
            // Making sure there are no duplicate rules
            cfg.productionRules[unitProductionVariable] = Array.from(
            // Add all the rules of the unit production rule
            new Set(unitProductionRules.concat(unitProducingVariableRules)));
        }
        unitProductionRule = findFirstUnitProductionRule(cfg.variables, cfg.productionRules);
    }
}
exports.removeUnitProduction = removeUnitProduction;