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diff --git a/klm/util/proxy_iterator.hh b/klm/util/proxy_iterator.hh
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+#ifndef UTIL_PROXY_ITERATOR__
+#define UTIL_PROXY_ITERATOR__
+
+#include <cstddef>
+#include <iterator>
+
+/* This is a RandomAccessIterator that uses a proxy to access the underlying
+ * data.  Useful for packing data at bit offsets but still using STL
+ * algorithms.  
+ *
+ * Normally I would use boost::iterator_facade but some people are too lazy to
+ * install boost and still want to use my language model.  It's amazing how
+ * many operators an iterator has. 
+ *
+ * The Proxy needs to provide:
+ *   class InnerIterator;
+ *   InnerIterator &Inner();
+ *   const InnerIterator &Inner() const;
+ *
+ * InnerIterator has to implement:
+ *   operator==(InnerIterator)
+ *   operator<(InnerIterator)
+ *   operator+=(std::ptrdiff_t)
+ *   operator-(InnerIterator)
+ * and of course whatever Proxy needs to dereference it.  
+ *
+ * It's also a good idea to specialize std::swap for Proxy.  
+ */
+
+namespace util {
+template <class Proxy> class ProxyIterator {
+  private:
+    // Self.  
+    typedef ProxyIterator<Proxy> S;
+    typedef typename Proxy::InnerIterator InnerIterator;
+
+  public:
+    typedef std::random_access_iterator_tag iterator_category;
+    typedef typename Proxy::value_type value_type;
+    typedef std::ptrdiff_t difference_type;
+    typedef Proxy reference;
+    typedef Proxy * pointer;
+
+    ProxyIterator() {}
+
+    // For cast from non const to const.  
+    template <class AlternateProxy> ProxyIterator(const ProxyIterator<AlternateProxy> &in) : p_(*in) {}
+    explicit ProxyIterator(const Proxy &p) : p_(p) {}
+
+    // p_'s operator= does value copying, but here we want iterator copying.  
+    S &operator=(const S &other) {
+      I() = other.I();
+      return *this;
+    }
+
+    bool operator==(const S &other) const { return I() == other.I(); }
+    bool operator!=(const S &other) const { return !(*this == other); }
+    bool operator<(const S &other) const { return I() < other.I(); }
+    bool operator>(const S &other) const { return other < *this; }
+    bool operator<=(const S &other) const { return !(*this > other); }
+    bool operator>=(const S &other) const { return !(*this < other); }
+
+    S &operator++() { return *this += 1; }
+    S operator++(int) { S ret(*this); ++*this; return ret; }
+    S &operator+=(std::ptrdiff_t amount) { I() += amount; return *this; }
+    S operator+(std::ptrdiff_t amount) const { S ret(*this); ret += amount; return ret; }
+
+    S &operator--() { return *this -= 1; }
+    S operator--(int) { S ret(*this); --*this; return ret; }
+    S &operator-=(std::ptrdiff_t amount) { I() += (-amount); return *this; }
+    S operator-(std::ptrdiff_t amount) const { S ret(*this); ret -= amount; return ret; }
+
+    std::ptrdiff_t operator-(const S &other) const { return I() - other.I(); }
+
+    Proxy operator*() { return p_; }
+    const Proxy operator*() const { return p_; }
+    Proxy *operator->() { return &p_; }
+    const Proxy *operator->() const { return &p_; }
+    Proxy operator[](std::ptrdiff_t amount) const { return *(*this + amount); }
+
+  private:
+    InnerIterator &I() { return p_.Inner(); }
+    const InnerIterator &I() const { return p_.Inner(); }
+
+    Proxy p_;
+};
+
+template <class Proxy> ProxyIterator<Proxy> operator+(std::ptrdiff_t amount, const ProxyIterator<Proxy> &it) {
+  return it + amount;
+}
+
+} // namespace util
+
+#endif // UTIL_PROXY_ITERATOR__