view: add increment and decrement operators

view.hpp

@@ -22,6 +22,9 @@
 #include <stdexcept>
 #include <iterator>
 #include <type_traits>
+#include <concepts>
+#include <iterator>
+
 
 namespace cruft {
     template <typename BeginT, typename EndT = BeginT>
@@ -729,6 +732,34 @@ namespace cruft {
     }
 
 
+    ///////////////////////////////////////////////////////////////////////////
+    template <std::bidirectional_iterator IteratorT>
+    cruft::view<IteratorT>&
+    operator-= (
+        cruft::view<IteratorT> &val,
+        typename std::iterator_traits<IteratorT>::difference_type offset
+    ) {
+        return val = {
+            val.begin () - offset,
+            val.end   () - offset
+        };
+    }
+
+
+    //-------------------------------------------------------------------------
+    template <std::incrementable IteratorT>
+    cruft::view<IteratorT>&
+    operator+= (
+        cruft::view<IteratorT> &val,
+        typename std::iterator_traits<IteratorT>::difference_type offset
+    ) {
+        return val = {
+            val.begin () + offset,
+            val.end   () + offset
+        };
+    }
+
+
     ///////////////////////////////////////////////////////////////////////////
     template <typename BeginT, typename EndT, typename FunctionT>
     view<BeginT, EndT>