storage: reserved entities support

src/entt/entity/storage.hpp

@@ -1255,6 +1255,17 @@ public:
         return const_reverse_iterable{it, it + offset};
     }
 
+    /**
+     * @brief Sets the starting identifier for generation.
+     *
+     * The version is ignored, regardless of the value.
+     *
+     * @param hint A valid identifier.
+     */
+    void start_from(const entity_type hint) {
+        placeholder = static_cast<size_type>(traits_type::to_entity(hint));
+    }
+
 private:
     size_type placeholder{};
 };

test/entt/entity/storage_entity.cpp

@@ -172,6 +172,20 @@ TEST(StorageEntity, Generate) {
     ASSERT_EQ(entity[1u], entt::entity{8});
 }
 
+TEST(StorageEntity, GenerateFrom) {
+    using traits_type = entt::entt_traits<entt::entity>;
+
+    entt::storage<entt::entity> pool;
+    std::array entity{entt::entity{0}, entt::entity{1}, entt::entity{2}};
+
+    ASSERT_EQ(pool.generate(), entity[0u]);
+
+    pool.start_from(entity[2u]);
+
+    ASSERT_EQ(pool.generate(), entity[2u]);
+    ASSERT_FALSE(pool.contains(entity[1u]));
+}
+
 TEST(StorageEntity, GenerateInUse) {
     entt::storage<entt::entity> pool;
     std::array<entt::entity, 2u> entity{};