From 67ae33d90882690a9149dc7a5e8fe5d8946c4085 Mon Sep 17 00:00:00 2001
From: Fredrik Appelros <fredrik.appelros@hypergene.se>
Date: Thu, 7 Jun 2018 17:37:53 +0200
Subject: [PATCH] Make nodes hashable

Implement a specialization of `std::hash<YAML::Node>` so that nodes can
be used in unordered associative containers.
---
 include/yaml-cpp/node/detail/node.h |  2 ++
 include/yaml-cpp/node/hash.h        | 23 +++++++++++++++++++++++
 include/yaml-cpp/node/node.h        |  1 +
 include/yaml-cpp/yaml.h             |  1 +
 test/node/node_test.cpp             | 12 ++++++++++++
 5 files changed, 39 insertions(+)
 create mode 100644 include/yaml-cpp/node/hash.h

diff --git a/include/yaml-cpp/node/detail/node.h b/include/yaml-cpp/node/detail/node.h
index a2cc52b..48fb571 100644
--- a/include/yaml-cpp/node/detail/node.h
+++ b/include/yaml-cpp/node/detail/node.h
@@ -18,6 +18,8 @@ namespace YAML {
 namespace detail {
 class node {
  public:
+  friend struct std::hash<node>;
+
   node() : m_pRef(new node_ref), m_dependencies{} {}
   node(const node&) = delete;
   node& operator=(const node&) = delete;
diff --git a/include/yaml-cpp/node/hash.h b/include/yaml-cpp/node/hash.h
new file mode 100644
index 0000000..8d5f9b0
--- /dev/null
+++ b/include/yaml-cpp/node/hash.h
@@ -0,0 +1,23 @@
+#ifndef NODE_HASH_H_62B23520_7C8E_11DE_8A39_0800200C9A66
+#define NODE_HASH_H_62B23520_7C8E_11DE_8A39_0800200C9A66
+
+#include "yaml-cpp/node/detail/node.h"
+#include "yaml-cpp/node/node.h"
+
+namespace std {
+template <>
+struct hash<YAML::detail::node> {
+  size_t operator()(const YAML::detail::node& key) const noexcept {
+    return hash<YAML::detail::shared_node_ref>()(key.m_pRef);
+  }
+};
+
+template <>
+struct hash<YAML::Node> {
+  size_t operator()(const YAML::Node& key) const noexcept {
+    return hash<YAML::detail::node>()(*key.m_pNode);
+  }
+};
+}  // namespace std
+
+#endif  // NODE_HASH_H_62B23520_7C8E_11DE_8A39_0800200C9A66
diff --git a/include/yaml-cpp/node/node.h b/include/yaml-cpp/node/node.h
index 49af58e..e813cf7 100644
--- a/include/yaml-cpp/node/node.h
+++ b/include/yaml-cpp/node/node.h
@@ -38,6 +38,7 @@ class YAML_CPP_API Node {
   friend class detail::iterator_base;
   template <typename T, typename S>
   friend struct as_if;
+  friend struct std::hash<Node>;
 
   typedef YAML::iterator iterator;
   typedef YAML::const_iterator const_iterator;
diff --git a/include/yaml-cpp/yaml.h b/include/yaml-cpp/yaml.h
index 7f515ef..6a754ec 100644
--- a/include/yaml-cpp/yaml.h
+++ b/include/yaml-cpp/yaml.h
@@ -20,5 +20,6 @@
 #include "yaml-cpp/node/detail/impl.h"
 #include "yaml-cpp/node/parse.h"
 #include "yaml-cpp/node/emit.h"
+#include "yaml-cpp/node/hash.h"
 
 #endif  // YAML_H_62B23520_7C8E_11DE_8A39_0800200C9A66
diff --git a/test/node/node_test.cpp b/test/node/node_test.cpp
index 18234db..47df6cf 100644
--- a/test/node/node_test.cpp
+++ b/test/node/node_test.cpp
@@ -3,6 +3,7 @@
 #include "yaml-cpp/node/convert.h"
 #include "yaml-cpp/node/detail/impl.h"
 #include "yaml-cpp/node/emit.h"
+#include "yaml-cpp/node/hash.h"
 #include "yaml-cpp/node/impl.h"
 #include "yaml-cpp/node/iterator.h"
 
@@ -437,6 +438,17 @@ TEST(NodeTest, AccessNonexistentKeyOnConstNode) {
   ASSERT_FALSE(other["5"]);
 }
 
+TEST(NodeTest, NodeIsHashable) {
+  YAML::Node node1(YAML::NodeType::value::Null);
+  YAML::Node node2(YAML::NodeType::value::Null);
+  YAML::Node node3 = node1;
+  std::hash<YAML::Node> hash_func;
+  auto node1_digest = hash_func(node1);
+  ASSERT_EQ(hash_func(node1), node1_digest);
+  ASSERT_NE(hash_func(node2), node1_digest);
+  ASSERT_EQ(hash_func(node3), node1_digest);
+}
+
 class NodeEmitterTest : public ::testing::Test {
  protected:
   void ExpectOutput(const std::string& output, const Node& node) {