From 5458877f2c7787a75d7a00e7a799f5090f59b536 Mon Sep 17 00:00:00 2001
From: Marius Bancila <marius.bancila@visma.com>
Date: Thu, 23 Dec 2021 01:00:23 +0200
Subject: [PATCH] faster std::hash<uuid> implementation

---
 include/uuid.h     | 33 +++++++++++++++++++++++++++++++++
 test/test_uuid.cpp |  4 ++++
 2 files changed, 37 insertions(+)

diff --git a/include/uuid.h b/include/uuid.h
index 0515d47..b8c4a49 100644
--- a/include/uuid.h
+++ b/include/uuid.h
@@ -526,6 +526,8 @@ namespace uuids
 
       template<class CharT, class Traits, class Allocator>
       friend std::basic_string<CharT, Traits, Allocator> to_string(uuid const& id);
+
+      friend std::hash<uuid>;
    };
 
    // --------------------------------------------------------------------------------------------------------------------------
@@ -922,8 +924,39 @@ namespace std
 
       result_type operator()(argument_type const &uuid) const
       {
+#ifdef UUID_HASH_STRING_BASED
          std::hash<std::string> hasher;
          return static_cast<result_type>(hasher(uuids::to_string(uuid)));
+#else
+         uint64_t l = 
+            static_cast<uint64_t>(uuid.data[0]) << 56 | 
+            static_cast<uint64_t>(uuid.data[1]) << 48 | 
+            static_cast<uint64_t>(uuid.data[2]) << 40 | 
+            static_cast<uint64_t>(uuid.data[3]) << 32 | 
+            static_cast<uint64_t>(uuid.data[4]) << 24 | 
+            static_cast<uint64_t>(uuid.data[5]) << 16 | 
+            static_cast<uint64_t>(uuid.data[6]) <<  8 | 
+            static_cast<uint64_t>(uuid.data[7]);
+         uint64_t h = 
+            static_cast<uint64_t>(uuid.data[8])  << 56 | 
+            static_cast<uint64_t>(uuid.data[9])  << 48 | 
+            static_cast<uint64_t>(uuid.data[10]) << 40 | 
+            static_cast<uint64_t>(uuid.data[11]) << 32 | 
+            static_cast<uint64_t>(uuid.data[12]) << 24 | 
+            static_cast<uint64_t>(uuid.data[13]) << 16 | 
+            static_cast<uint64_t>(uuid.data[14]) <<  8 | 
+            static_cast<uint64_t>(uuid.data[15]);
+
+         if (sizeof(result_type) > 4)
+         {
+            return result_type(l ^ h);
+         }
+         else
+         {
+            uint64_t hash64 = l ^ h;
+            return result_type(uint32_t(hash64 >> 32) ^ uint32_t(hash64));
+         }
+#endif
       }
    };
 }
diff --git a/test/test_uuid.cpp b/test/test_uuid.cpp
index 001a6ae..ea98eb1 100644
--- a/test/test_uuid.cpp
+++ b/test/test_uuid.cpp
@@ -525,7 +525,11 @@ TEST_CASE("Test hashing", "[ops]")
 
    auto h1 = std::hash<std::string>{};
    auto h2 = std::hash<uuid>{};
+#ifdef UUID_HASH_STRING_BASED
    REQUIRE(h1(str) == h2(guid));
+#else
+   REQUIRE(h1(str) != h2(guid));
+#endif
 
    auto engine = uuids::uuid_random_generator::engine_type{};
    seed_rng(engine);