Fix index iterator on large problem sizes

src/std-indices/STDIndicesStream.h

@@ -23,35 +23,47 @@
 // implementation doesn't target
 template <typename N>
 class ranged {
-  N from, to;
 public:
-  ranged(N from, N to ): from(from), to(to) {}
-    class iterator {
-      N num;
+  class iterator {
+    friend class ranged;
     public:
-      using difference_type = N;
-      using value_type = N;
-      using pointer = const N*;
-      using reference = const N&;
-      using iterator_category = std::random_access_iterator_tag;
-      explicit iterator(N _num = 0) : num(_num) {}
+    using difference_type = N;
+    using value_type = N;
+    using pointer = const N*;
+    using reference = const N&;
+    using iterator_category = std::random_access_iterator_tag;
 
-      iterator& operator++() { num++; return *this; }
-      iterator operator++(int) { iterator retval = *this; ++(*this); return retval; }
-      iterator operator+(const value_type v) const { return iterator(num + v); }
-      iterator operator+=(int x) { iterator retval = *this; this->num+=x; return retval; }
+    reference operator *() const { return i_; }
+    const iterator &operator ++() { ++i_; return *this; }
+    iterator operator ++(int) { iterator copy(*this); ++i_; return copy; }
 
-      bool operator==(iterator other) const { return num == other.num; }
-      bool operator!=(iterator other) const { return *this != other; }
-      bool operator<(iterator other) const { return num < other.num; }
+    const iterator &operator --() { --i_; return *this; }
+    iterator operator --(int) { iterator copy(*this); --i_; return copy; }
 
-      reference operator*() const { return num;}
-      difference_type operator-(const iterator &it) const { return num - it.num; }
-      value_type operator[](const difference_type &i) const { return num + i; }
+    const iterator &operator +=(N by) { i_+=by; return *this; }
 
-    };
-    iterator begin() { return iterator(from); }
-    iterator end() { return iterator(to >= from? to+1 : to-1); }
+    value_type operator[](const difference_type &i) const { return i_ + i; }
+
+    difference_type operator-(const iterator &it) const { return i_ - it.i_; }
+    iterator operator+(const value_type v) const { return iterator(i_ + v); }
+
+    bool operator ==(const iterator &other) const { return i_ == other.i_; }
+    bool operator !=(const iterator &other) const { return i_ != other.i_; }
+    bool operator < (const iterator &other) const { return i_ < other.i_; }
+
+    protected:
+      explicit iterator(N start) : i_ (start) {}
+
+    private:
+        N i_;
+  };
+
+  [[nodiscard]] iterator begin() const { return begin_; }
+  [[nodiscard]] iterator end() const { return end_; }
+  ranged(N begin, N end) : begin_(begin), end_(end) {}
+private:
+  iterator begin_;
+  iterator end_;
 };
 
 template <class T>