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49 lines
1.3 KiB
JavaScript
49 lines
1.3 KiB
JavaScript
var arrays = require("../../utils/arrays"),
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GrammarError = require("../../grammar-error"),
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asts = require("../asts"),
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visitor = require("../visitor");
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/*
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* Reports left recursion in the grammar, which prevents infinite recursion in
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* the generated parser.
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*
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* Both direct and indirect recursion is detected. The pass also correctly
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* reports cases like this:
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*
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* start = "a"? start
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*
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* In general, if a rule reference can be reached without consuming any input,
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* it can lead to left recursion.
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*/
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function reportLeftRecursion(ast) {
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var check = visitor.build({
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rule: function(node, visitedRules) {
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check(node.expression, visitedRules.concat(node.name));
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},
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sequence: function(node, visitedRules) {
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arrays.every(node.elements, function(element) {
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if (element.type === "rule_ref") {
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check(element, visitedRules);
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}
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return asts.matchesEmpty(ast, element);
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});
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},
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rule_ref: function(node, visitedRules) {
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if (arrays.contains(visitedRules, node.name)) {
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throw new GrammarError(
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"Left recursion detected for rule \"" + node.name + "\"."
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);
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}
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check(asts.findRule(ast, node.name), visitedRules);
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}
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});
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check(ast, []);
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}
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module.exports = reportLeftRecursion;
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