feat: make iterators bidirectional

include/yaml-cpp/node/detail/iterator.h

@@ -41,7 +41,7 @@ class iterator_base {
   };
 
  public:
-  using iterator_category = std::forward_iterator_tag;
+  using iterator_category = std::bidirectional_iterator_tag;
   using value_type = V;
   using difference_type = std::ptrdiff_t;
   using pointer = V*;
@@ -68,6 +68,17 @@ class iterator_base {
     ++(*this);
     return iterator_pre;
   }
+
+  iterator_base<V>& operator--() {
+    --m_iterator;
+    return *this;
+  }
+
+  iterator_base<V> operator--(int) {
+    iterator_base<V> iterator_pre(*this);
+    --(*this);
+    return iterator_pre;
+  }
 
   template <typename W>
   bool operator==(const iterator_base<W>& rhs) const {

include/yaml-cpp/node/detail/node_iterator.h

@@ -69,7 +69,7 @@ class node_iterator_base {
   };
 
  public:
-  using iterator_category = std::forward_iterator_tag;
+  using iterator_category = std::bidirectional_iterator_tag;
   using value_type = node_iterator_value<V>;
   using difference_type = std::ptrdiff_t;
   using pointer = node_iterator_value<V>*;
@@ -140,12 +140,33 @@ class node_iterator_base {
     return *this;
   }
 
+  node_iterator_base<V>& operator--() {
+    switch (m_type) {
+      case iterator_type::NoneType:
+        break;
+      case iterator_type::Sequence:
+        --m_seqIt;
+        break;
+      case iterator_type::Map:
+        --m_mapIt;
+        m_mapIt = decrement_until_defined(m_mapIt);
+        break;
+    }
+    return *this;
+  }
+
   node_iterator_base<V> operator++(int) {
     node_iterator_base<V> iterator_pre(*this);
     ++(*this);
     return iterator_pre;
   }
 
+  node_iterator_base<V> operator--(int) {
+    node_iterator_base<V> iterator_pre(*this);
+    --(*this);
+    return iterator_pre;
+  }
+
   value_type operator*() const {
     switch (m_type) {
       case iterator_type::NoneType:
@@ -166,6 +187,12 @@ class node_iterator_base {
     return it;
   }
 
+  MapIter decrement_until_defined(MapIter it) {
+    while (!is_defined(it))
+      --it;
+    return it;
+  }
+
   bool is_defined(MapIter it) const {
     return it->first->is_defined() && it->second->is_defined();
   }

test/node/node_test.cpp

@@ -343,6 +343,17 @@ TEST(NodeTest, MapIteratorWithUndefinedValues) {
   EXPECT_EQ(1, count);
 }
 
+TEST(NodeTest, MapIteratorWithUndefinedValuesBackward) {
+  Node node;
+  node["key"] = "value";
+  node["undefined"];
+
+  std::size_t count = 0;
+  for (const_iterator it = node.end(); it != node.begin(); --it)
+    count++;
+  EXPECT_EQ(1, count);
+}
+
 TEST(NodeTest, DestroyedMapIterator) {
   Node node;
   node["key"] = "value";
@@ -395,6 +406,22 @@ TEST(NodeTest, InteratorOnSequence) {
   EXPECT_EQ(3, count);
 }
 
+TEST(NodeTest, InteratorOnSequenceBackward) {
+  Node node;
+  node[0] = "a";
+  node[1] = "b";
+  node[2] = "c";
+  EXPECT_TRUE(node.IsSequence());
+
+  std::size_t count = 0;
+  for (iterator it = node.end(); it != node.begin(); --it)
+  {
+    EXPECT_FALSE(prev(it)->IsNull());
+    count++;
+  }
+  EXPECT_EQ(3, count);
+}
+
 TEST(NodeTest, ConstInteratorOnSequence) {
   Node node;
   node[0] = "a";