From 7624eceff08867516a8d33376852fe3794c8d6df Mon Sep 17 00:00:00 2001
From: BlackMATov <blackmatov@gmail.com>
Date: Sun, 30 Dec 2018 12:04:53 +0700
Subject: [PATCH] hash_combine for entity and const_entity std::hash
 specializations

---
 ecs.hpp | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/ecs.hpp b/ecs.hpp
index d72c9f4..57c57fc 100644
--- a/ecs.hpp
+++ b/ecs.hpp
@@ -74,6 +74,14 @@ namespace ecs_hpp
             return t;
         }
 
+        //
+        // hash_combine
+        //
+
+        constexpr std::size_t hash_combine(std::size_t l, std::size_t r) noexcept {
+            return l ^ (r + 0x9e3779b9 + (l << 6) + (l >> 2));
+        }
+
         //
         // tuple_tail
         //
@@ -695,7 +703,9 @@ namespace std
         : std::unary_function<const ecs_hpp::entity&, std::size_t>
     {
         std::size_t operator()(const ecs_hpp::entity& ent) const noexcept {
-            return std::hash<ecs_hpp::entity_id>()(ent.id());
+            return ecs_hpp::detail::hash_combine(
+                std::hash<const ecs_hpp::registry*>()(&ent.owner()),
+                std::hash<ecs_hpp::entity_id>()(ent.id()));
         }
     };
 }
@@ -750,7 +760,9 @@ namespace std
         : std::unary_function<const ecs_hpp::const_entity&, std::size_t>
     {
         std::size_t operator()(const ecs_hpp::const_entity& ent) const noexcept {
-            return std::hash<ecs_hpp::entity_id>()(ent.id());
+            return ecs_hpp::detail::hash_combine(
+                std::hash<const ecs_hpp::registry*>()(&ent.owner()),
+                std::hash<ecs_hpp::entity_id>()(ent.id()));
         }
     };
 }