Implement infinite loop detection

Fixes #26.

Makefile

@@ -18,6 +18,7 @@ MODULES = utils/arrays                          \
           compiler/passes/generate-javascript   \
           compiler/passes/remove-proxy-rules    \
           compiler/passes/report-left-recursion \
+          compiler/passes/report-infinite-loops \
           compiler/passes/report-missing-rules  \
           compiler                              \
           peg

lib/compiler.js

@@ -12,7 +12,8 @@ var compiler = {
   passes: {
     check: {
       reportMissingRules:  require("./compiler/passes/report-missing-rules"),
-      reportLeftRecursion: require("./compiler/passes/report-left-recursion")
+      reportLeftRecursion: require("./compiler/passes/report-left-recursion"),
+      reportInfiniteLoops: require("./compiler/passes/report-infinite-loops")
     },
     transform: {
       removeProxyRules:    require("./compiler/passes/remove-proxy-rules")

lib/compiler/passes/report-infinite-loops.js

@@ -0,0 +1,27 @@
+var GrammarError = require("../../grammar-error"),
+    asts         = require("../asts"),
+    visitor      = require("../visitor");
+
+/*
+ * Reports expressions that don't consume any input inside |*| or |+| in the
+ * grammar, which prevents infinite loops in the generated parser.
+ */
+function reportInfiniteLoops(ast) {
+  var check = visitor.build({
+    zero_or_more: function(node) {
+      if (asts.matchesEmpty(ast, node.expression)) {
+        throw new GrammarError("Infinite loop detected.");
+      }
+    },
+
+    one_or_more: function(node) {
+      if (asts.matchesEmpty(ast, node.expression)) {
+        throw new GrammarError("Infinite loop detected.");
+      }
+    }
+  });
+
+  check(ast);
+}
+
+module.exports = reportInfiniteLoops;

package.json

@@ -27,6 +27,7 @@
     "lib/compiler/passes/generate-javascript.js",
     "lib/compiler/passes/remove-proxy-rules.js",
     "lib/compiler/passes/report-left-recursion.js",
+    "lib/compiler/passes/report-infinite-loops.js",
     "lib/compiler/passes/report-missing-rules.js",
     "lib/grammar-error.js",
     "lib/parser.js",

spec/index.html

@@ -11,6 +11,7 @@
     <script src="unit/compiler/passes/helpers.js"></script>
     <script src="unit/compiler/passes/report-missing-rules.spec.js"></script>
     <script src="unit/compiler/passes/report-left-recursion.spec.js"></script>
+    <script src="unit/compiler/passes/report-infinite-loops.spec.js"></script>
     <script src="unit/compiler/passes/remove-proxy-rules.spec.js"></script>
     <script src="unit/compiler/passes/generate-bytecode.spec.js"></script>
     <script src="api/pegjs-api.spec.js"></script>

spec/unit/compiler/passes/report-infinite-loops.spec.js

@@ -0,0 +1,71 @@
+describe("compiler pass |reportLeftRecursion|", function() {
+  var pass = PEG.compiler.passes.check.reportInfiniteLoops;
+
+  it("reports infinite loops for zero_or_more", function() {
+    expect(pass).toReportError('start = ("")*', {
+      message: "Infinite loop detected."
+    });
+  });
+
+  it("reports infinite loops for one_or_more", function() {
+    expect(pass).toReportError('start = ("")+', {
+      message: "Infinite loop detected."
+    });
+  });
+
+  it("computes empty string matching correctly", function() {
+    expect(pass).toReportError('start = ("" / "a" / "b")*');
+    expect(pass).toReportError('start = ("a" / "" / "b")*');
+    expect(pass).toReportError('start = ("a" / "b" / "")*');
+    expect(pass).not.toReportError('start = ("a" / "b" / "c")*');
+
+    expect(pass).toReportError('start = ("" { })*');
+    expect(pass).not.toReportError('start = ("a" { })*');
+
+    expect(pass).toReportError('start = ("" "" "")*');
+    expect(pass).not.toReportError('start = ("a" "" "")*');
+    expect(pass).not.toReportError('start = ("" "a" "")*');
+    expect(pass).not.toReportError('start = ("" "" "a")*');
+
+    expect(pass).toReportError('start = (a:"")*');
+    expect(pass).not.toReportError('start = (a:"a")*');
+
+    expect(pass).toReportError('start = ($"")*');
+    expect(pass).not.toReportError('start = ($"a")*');
+
+    expect(pass).toReportError('start = (&"")*');
+    expect(pass).toReportError('start = (&"a")*');
+
+    expect(pass).toReportError('start = (!"")*');
+    expect(pass).toReportError('start = (!"a")*');
+
+    expect(pass).toReportError('start = (""?)*');
+    expect(pass).toReportError('start = ("a"?)*');
+
+    expect(pass).toReportError('start = (""*)*');
+    expect(pass).toReportError('start = ("a"*)*');
+
+    expect(pass).toReportError('start = (""+)*');
+    expect(pass).not.toReportError('start = ("a"+)*');
+
+    expect(pass).toReportError('start = (&{ })*');
+
+    expect(pass).toReportError('start = (!{ })*');
+
+    expect(pass).toReportError([
+      'start = a*',
+      'a = ""'
+    ].join('\n'));
+    expect(pass).not.toReportError([
+      'start = a*',
+      'a = "a"'
+    ].join('\n'));
+
+    expect(pass).toReportError('start = ""*');
+    expect(pass).not.toReportError('start = "a"*');
+
+    expect(pass).not.toReportError('start = [a-d]*');
+
+    expect(pass).not.toReportError('start = "."*');
+  });
+});