From 079e167e7c62c857e271bda0588064dc4030e337 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 20 Apr 2016 14:19:07 +0200
Subject: Add combine_hash() utility

---
 butl/utility | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/butl/utility b/butl/utility
index 1cc45ef..efbb13b 100644
--- a/butl/utility
+++ b/butl/utility
@@ -5,6 +5,7 @@
 #ifndef BUTL_UTILITY
 #define BUTL_UTILITY
 
+#include <cstddef> // std::size_t
 #include <utility> // forward()
 #include <cstring> // strcmp
 
@@ -26,6 +27,23 @@ namespace butl
     bool operator() (const P& x, const P& y) const {return *x < *y;}
   };
 
+  // Combine one or more hash values.
+  //
+  inline std::size_t
+  combine_hash (std::size_t s, std::size_t h)
+  {
+    // Magic formula from boost::hash_combine().
+    //
+    return s ^ (h + 0x9e3779b9 + (s << 6) + (s >> 2));
+  }
+
+  template <typename... S>
+  inline std::size_t
+  combine_hash (std::size_t s, std::size_t h, S... hs)
+  {
+    return combine_hash (combine_hash (s, h), hs...);
+  }
+
   // Support for reverse iteration using range-based for-loop:
   //
   // for (... : reverse_iterate (x)) ...
-- 
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