add slicing for finite random access ranges

octa/range.h

@@ -125,6 +125,11 @@ namespace octa {
             return p_range[length() - i - 1];
         }
 
+        ReverseRange<T> slice(size_t start, size_t end) {
+            size_t len = p_range.length();
+            return ReverseRange<T>(p_range.slice(len - end, len - start));
+        }
+
     private:
         T p_range;
     };
@@ -182,6 +187,10 @@ namespace octa {
             return move(p_range[i]);
         }
 
+        MoveRange<T> slice(size_t start, size_t end) {
+            return MoveRange<T>(p_range.slice(start, end));
+        }
+
     private:
         T p_range;
     };

octa/vector.h

@@ -55,6 +55,10 @@ namespace octa {
         /* satisfy RandomAccessRange */
         size_t length() const { return p_end - p_beg; }
 
+        VectorRange slice(size_t start, size_t end) {
+            return VectorRange(p_beg + start, p_beg + end);
+        }
+
               T &operator[](size_t i)       { return p_beg[i]; }
         const T &operator[](size_t i) const { return p_beg[i]; }