From 6f91dc796285d2ec1a2e21960ff9f30c232eef2a Mon Sep 17 00:00:00 2001
From: greg <greg7mdp@gmail.com>
Date: Mon, 29 Apr 2019 17:12:20 -0400
Subject: [PATCH] support the boost hash_value() method for providing hash
 value.

---
 README.md                | 32 +++++++++++++++++++++++++++++++-
 parallel_hashmap/phmap.h | 31 ++++++++++++++++++++++++++++++-
 2 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 2b6784d..7761e55 100644
--- a/README.md
+++ b/README.md
@@ -141,7 +141,37 @@ The rules are the same as for std::unordered_map, and are valid for all the phma
 
 ## Example 2 - providing a hash function for a user-defined class
 
-In order to use a flat_hash_set or flat_hash_map, a hash function should be provided. Even though a the hash function can be provided via the HashFcn template parameter, we recommend injecting a specialization of `std::hash` for the class into the "std" namespace. We provide a convenient and small header `phmap_utils.h` which allows to easily add such specializations.
+In order to use a flat_hash_set or flat_hash_map, a hash function should be provided. This can be done with one of the following methods:
+
+- Provide a hash functor via the HashFcn template parameter
+
+- As with boost, you may add a `hash_value()` friend function in your class. 
+
+For example:
+ 
+```c++
+#include <parallel_hashmap/phmap_utils.h> // minimal header providing phmap::HashState()
+#include <string>
+using std::string;
+
+struct Person
+{
+    bool operator==(const Person &o) const
+    { 
+        return _first == o._first && _last == o._last && _age == o._age; 
+    }
+
+    friend size_t hash_value(const Person &p)
+    {
+        return phmap::HashState().combine(0, p._first, p._last, p._age);
+    }
+
+    string _first;
+    string _last;
+    int    _age;
+};
+
+- Inject a specialization of `std::hash` for the class into the "std" namespace. We provide a convenient and small header `phmap_utils.h` which allows to easily add such specializations.
 
 For example:
 
diff --git a/parallel_hashmap/phmap.h b/parallel_hashmap/phmap.h
index e80279f..d046ef4 100644
--- a/parallel_hashmap/phmap.h
+++ b/parallel_hashmap/phmap.h
@@ -135,17 +135,46 @@ struct fold_if_needed<8>
     }
 };
 
+// ---------------------------------------------------------------
+// see if class T has a hash_value() friend method
+// ---------------------------------------------------------------
+template<typename T>
+struct has_hash_value
+{
+private:
+    typedef std::true_type yes;
+    typedef std::false_type no;
 
+    template<typename U> static auto test(int) -> decltype(hash_value(std::declval<U&>()) == 1, yes());
+
+    template<typename> static no test(...);
+
+public:
+    static constexpr bool value = std::is_same<decltype(test<T>(0)), yes>::value;
+};
+ 
 // ---------------------------------------------------------------
 //               phmap::Hash
 // ---------------------------------------------------------------
 template <class T>
 struct Hash
 {
-    inline size_t operator()(const T& val) const
+    template <class U, typename std::enable_if<has_hash_value<U>::value, int>::type = 0>
+    size_t _hash(const T& val) const
+    {
+        return hash_value(val);
+    }
+ 
+    template <class U, typename std::enable_if<!has_hash_value<U>::value, int>::type = 0>
+    size_t _hash(const T& val) const
     {
         return std::hash<T>()(val);
     }
+ 
+    inline size_t operator()(const T& val) const
+    {
+        return _hash<T>(val);
+    }
 };
 
 template <class T>