Bug 1448563 - Part 3a: Add iterator to js::Fifo. r=luke

MozReview-Commit-ID: CoYdOBNnORg

js/src/ds/Fifo.h

@@ -85,6 +85,41 @@ class Fifo
         return front_.empty();
     }
 
+    // Iterator from oldest to yongest element.
+    struct ConstIterator
+    {
+        const Fifo& self_;
+        size_t idx_;
+
+        ConstIterator(const Fifo& self, size_t idx)
+            : self_(self), idx_(idx)
+        { }
+
+        ConstIterator& operator++() {
+            ++idx_;
+            return *this;
+        }
+
+        const T& operator*() const {
+            // Iterate front in reverse, then rear.
+            size_t split = self_.front_.length();
+            return (idx_ < split) ? self_.front_[(split - 1) - idx_]
+                                  : self_.rear_[idx_ - split];
+        }
+
+        bool operator!=(const ConstIterator& other) const {
+            return (&self_ != &other.self_) || (idx_ != other.idx_);
+        }
+    };
+
+    ConstIterator begin() const {
+        return ConstIterator(*this, 0);
+    }
+
+    ConstIterator end() const {
+        return ConstIterator(*this, length());
+    }
+
     // Push an element to the back of the queue. This method can take either a
     // |const T&| or a |T&&|.
     template <typename U>