Extract the |matchesEmpty| visitor from the |reportLeftRecursion| pass

Beside the recursion detector, the visitor will also be used by infinite
loop detector.

Note the newly created |asts.matchesEmpty| function re-creates the
visitor each time it is called, which makes it slower than necessary.
This could have been worked around in various ways but I chose to defer
that optimization because real-world performance impact is small.

Makefile

@@ -10,8 +10,8 @@ MODULES = utils/arrays                          \
           utils/classes                         \
           grammar-error                         \
           parser                                \
-          compiler/asts                         \
           compiler/visitor                      \
+          compiler/asts                         \
           compiler/opcodes                      \
           compiler/javascript                   \
           compiler/passes/generate-bytecode     \

lib/compiler/asts.js

@@ -1,4 +1,5 @@
-var arrays = require("../utils/arrays");
+var arrays  = require("../utils/arrays"),
+    visitor = require("./visitor");
 
 /* AST utilities. */
 var asts = {
@@ -8,6 +9,52 @@ var asts = {
 
   indexOfRule: function(ast, name) {
     return arrays.indexOf(ast.rules, function(r) { return r.name === name; });
+  },
+
+  matchesEmpty: function(ast, node) {
+    function matchesTrue()  { return true;  }
+    function matchesFalse() { return false; }
+
+    function matchesExpression(node) {
+      return matches(node.expression);
+    }
+
+    var matches = visitor.build({
+      rule: matchesExpression,
+
+      choice: function(node) {
+        return arrays.some(node.alternatives, matches);
+      },
+
+      action: matchesExpression,
+
+      sequence: function(node) {
+        return arrays.every(node.elements, matches);
+      },
+
+      labeled:      matchesExpression,
+      text:         matchesExpression,
+      simple_and:   matchesTrue,
+      simple_not:   matchesTrue,
+      optional:     matchesTrue,
+      zero_or_more: matchesTrue,
+      one_or_more:  matchesExpression,
+      semantic_and: matchesTrue,
+      semantic_not: matchesTrue,
+
+      rule_ref: function(node) {
+        return matches(asts.findRule(ast, node.name));
+      },
+
+      literal: function(node) {
+        return node.value === "";
+      },
+
+      "class": matchesFalse,
+      any:     matchesFalse
+    });
+
+    return matches(node);
   }
 };
 

lib/compiler/passes/report-left-recursion.js

@@ -16,48 +16,6 @@ var arrays       = require("../../utils/arrays"),
  * it can lead to left recursion.
  */
 function reportLeftRecursion(ast) {
-  function matchesEmptyTrue()  { return true;  }
-  function matchesEmptyFalse() { return false; }
-
-  function matchesEmptyExpression(node) {
-    return matchesEmpty(node.expression);
-  }
-
-  var matchesEmpty = visitor.build({
-    rule: matchesEmptyExpression,
-
-    choice: function(node) {
-      return arrays.some(node.alternatives, matchesEmpty);
-    },
-
-    action: matchesEmptyExpression,
-
-    sequence: function(node) {
-      return arrays.every(node.elements, matchesEmpty);
-    },
-
-    labeled:      matchesEmptyExpression,
-    text:         matchesEmptyExpression,
-    simple_and:   matchesEmptyTrue,
-    simple_not:   matchesEmptyTrue,
-    optional:     matchesEmptyTrue,
-    zero_or_more: matchesEmptyTrue,
-    one_or_more:  matchesEmptyExpression,
-    semantic_and: matchesEmptyTrue,
-    semantic_not: matchesEmptyTrue,
-
-    rule_ref: function(node) {
-      return matchesEmpty(asts.findRule(ast, node.name));
-    },
-
-    literal: function(node) {
-      return node.value === "";
-    },
-
-    "class": matchesEmptyFalse,
-    any:     matchesEmptyFalse
-  });
-
   var check = visitor.build({
     rule: function(node, visitedRules) {
       check(node.expression, visitedRules.concat(node.name));
@@ -69,7 +27,7 @@ function reportLeftRecursion(ast) {
           check(element, visitedRules);
         }
 
-        return matchesEmpty(element);
+        return asts.matchesEmpty(ast, element);
       });
     },