make DSSP::iterator bidirectional

include/cif++/Secondary.hpp

@@ -202,7 +202,7 @@ class DSSP
 	class iterator
 	{
 	  public:
-		using iterator_category = std::input_iterator_tag;
+		using iterator_category = std::bidirectional_iterator_tag;
 		using value_type = ResidueInfo;
 		using difference_type = std::ptrdiff_t;
 		using pointer = value_type*;
@@ -223,6 +223,14 @@ class DSSP
 			return tmp;
 		}
 
+		iterator& operator--();
+		iterator operator--(int)
+		{
+			auto tmp(*this);
+			this->operator--();
+			return tmp;
+		}
+
 		bool operator==(const iterator& rhs) const		{ return mCurrent.mImpl == rhs.mCurrent.mImpl; }
 		bool operator!=(const iterator& rhs) const		{ return mCurrent.mImpl != rhs.mCurrent.mImpl; }
 

src/Secondary.cpp

@@ -1431,6 +1431,12 @@ DSSP::iterator &DSSP::iterator::operator++()
 	return *this;
 }
 
+DSSP::iterator &DSSP::iterator::operator--()
+{
+	--mCurrent.mImpl;
+	return *this;
+}
+
 // --------------------------------------------------------------------
 
 DSSP::DSSP(const Structure &s, int min_poly_proline_stretch, bool calculateSurfaceAccessibility)