From 35ebf5c413db690896aa1774056b6862e141e92f Mon Sep 17 00:00:00 2001
From: Michele Caini <michele.caini@gmail.com>
Date: Thu, 9 May 2024 09:44:00 +0200
Subject: [PATCH] hashed_string: avoid unnecessary class specializations

---
 src/entt/core/hashed_string.hpp  | 37 ++++++++++++++++----------------
 test/entt/core/hashed_string.cpp |  4 ++--
 2 files changed, 21 insertions(+), 20 deletions(-)

diff --git a/src/entt/core/hashed_string.hpp b/src/entt/core/hashed_string.hpp
index 51768ce29..6f100db2d 100644
--- a/src/entt/core/hashed_string.hpp
+++ b/src/entt/core/hashed_string.hpp
@@ -3,6 +3,7 @@
 
 #include <cstddef>
 #include <cstdint>
+#include <type_traits>
 #include "fwd.hpp"
 
 namespace entt {
@@ -10,20 +11,21 @@ namespace entt {
 /*! @cond TURN_OFF_DOXYGEN */
 namespace internal {
 
-template<typename>
-struct fnv1a_traits;
-
-template<>
-struct fnv1a_traits<std::uint32_t> {
-    static constexpr std::uint32_t offset = 2166136261;
-    static constexpr std::uint32_t prime = 16777619;
-};
+[[nodiscard]] inline constexpr auto offset() noexcept {
+    if constexpr(std::is_same_v<id_type, std::uint32_t>) {
+        return 2166136261;
+    } else if constexpr(std::is_same_v<id_type, std::uint64_t>) {
+        return 14695981039346656037ull;
+    }
+}
 
-template<>
-struct fnv1a_traits<std::uint64_t> {
-    static constexpr std::uint64_t offset = 14695981039346656037ull;
-    static constexpr std::uint64_t prime = 1099511628211ull;
-};
+[[nodiscard]] inline constexpr auto prime() noexcept {
+    if constexpr(std::is_same_v<id_type, std::uint32_t>) {
+        return 16777619;
+    } else if constexpr(std::is_same_v<id_type, std::uint64_t>) {
+        return 1099511628211ull;
+    }
+}
 
 template<typename Char>
 struct basic_hashed_string {
@@ -57,7 +59,6 @@ struct basic_hashed_string {
 template<typename Char>
 class basic_hashed_string: internal::basic_hashed_string<Char> {
     using base_type = internal::basic_hashed_string<Char>;
-    using traits_type = internal::fnv1a_traits<id_type>;
 
     struct const_wrapper {
         // non-explicit constructor on purpose
@@ -69,10 +70,10 @@ class basic_hashed_string: internal::basic_hashed_string<Char> {
 
     // Fowler–Noll–Vo hash function v. 1a - the good
     [[nodiscard]] static constexpr auto helper(const Char *str) noexcept {
-        base_type base{str, 0u, traits_type::offset};
+        base_type base{str, 0u, internal::offset()};
 
         for(; str[base.length]; ++base.length) {
-            base.hash = (base.hash ^ static_cast<id_type>(str[base.length])) * traits_type::prime;
+            base.hash = (base.hash ^ static_cast<id_type>(str[base.length])) * internal::prime();
         }
 
         return base;
@@ -80,10 +81,10 @@ class basic_hashed_string: internal::basic_hashed_string<Char> {
 
     // Fowler–Noll–Vo hash function v. 1a - the good
     [[nodiscard]] static constexpr auto helper(const Char *str, const std::size_t len) noexcept {
-        base_type base{str, len, traits_type::offset};
+        base_type base{str, len, internal::offset()};
 
         for(size_type pos{}; pos < len; ++pos) {
-            base.hash = (base.hash ^ static_cast<id_type>(str[pos])) * traits_type::prime;
+            base.hash = (base.hash ^ static_cast<id_type>(str[pos])) * internal::prime();
         }
 
         return base;
diff --git a/test/entt/core/hashed_string.cpp b/test/entt/core/hashed_string.cpp
index 9f66a6720..b5a346ef6 100644
--- a/test/entt/core/hashed_string.cpp
+++ b/test/entt/core/hashed_string.cpp
@@ -68,7 +68,7 @@ TEST_F(HashedString, Empty) {
     const entt::hashed_string hs{};
 
     ASSERT_EQ(hs.size(), 0u);
-    ASSERT_EQ(static_cast<hash_type>(hs), entt::internal::fnv1a_traits<entt::id_type>::offset);
+    ASSERT_EQ(static_cast<hash_type>(hs), entt::internal::offset());
     ASSERT_EQ(static_cast<const char *>(hs), nullptr);
 }
 
@@ -162,7 +162,7 @@ TEST_F(HashedWString, Empty) {
     const entt::hashed_wstring hws{};
 
     ASSERT_EQ(hws.size(), 0u);
-    ASSERT_EQ(static_cast<hash_type>(hws), entt::internal::fnv1a_traits<entt::id_type>::offset);
+    ASSERT_EQ(static_cast<hash_type>(hws), entt::internal::offset());
     ASSERT_EQ(static_cast<const wchar_t *>(hws), nullptr);
 }