fix batch creation (close #305)

src/entt/entity/registry.hpp

@@ -606,7 +606,8 @@ public:
         });
 
         if constexpr(sizeof...(Component) > 0) {
-            return std::make_tuple(assure<Component>()->batch(*this, first, last)...);
+            // the reverse iterators guarantee the ordering between entities and components (hint: the pools return begin())
+            return std::make_tuple(assure<Component>()->batch(*this, std::make_reverse_iterator(last), std::make_reverse_iterator(first))...);
         }
     }
 

src/entt/entity/sparse_set.hpp

@@ -419,7 +419,7 @@ public:
      */
     template<typename It>
     void batch(It first, It last) {
-        std::for_each(std::make_reverse_iterator(last), std::make_reverse_iterator(first), [this, next = direct.size()](const auto entt) mutable {
+        std::for_each(first, last, [this, next = direct.size()](const auto entt) mutable {
             ENTT_ASSERT(!has(entt));
             auto [page, offset] = map(entt);
             assure(page);

test/entt/entity/sparse_set.cpp

@@ -135,9 +135,13 @@ TEST(SparseSet, BatchAdd) {
     ASSERT_FALSE(set.empty());
     ASSERT_EQ(set.size(), 4u);
     ASSERT_EQ(set.index(entt::entity{12}), 0u);
-    ASSERT_EQ(set.index(entities[0]), 2u);
-    ASSERT_EQ(set.index(entities[1]), 1u);
+    ASSERT_EQ(set.index(entities[0]), 1u);
+    ASSERT_EQ(set.index(entities[1]), 2u);
     ASSERT_EQ(set.index(entt::entity{24}), 3u);
+    ASSERT_EQ(set.data()[set.index(entt::entity{12})], entt::entity{12});
+    ASSERT_EQ(set.data()[set.index(entities[0])], entities[0]);
+    ASSERT_EQ(set.data()[set.index(entities[1])], entities[1]);
+    ASSERT_EQ(set.data()[set.index(entt::entity{24})], entt::entity{24});
 }
 
 TEST(SparseSet, Iterator) {

test/entt/entity/storage.cpp

@@ -113,8 +113,8 @@ TEST(Storage, BatchAdd) {
     it[0] = 1;
     it[1] = 2;
 
-    ASSERT_EQ(pool.get(entities[0]), 1);
-    ASSERT_EQ(pool.get(entities[1]), 2);
+    ASSERT_EQ(pool.get(entities[0]), 2);
+    ASSERT_EQ(pool.get(entities[1]), 1);
 }
 
 TEST(Storage, BatchAddByCopy) {
@@ -136,8 +136,8 @@ TEST(Storage, BatchAddByCopy) {
     it[0] = 1;
     it[1] = 2;
 
-    ASSERT_EQ(pool.get(entities[0]), 1);
-    ASSERT_EQ(pool.get(entities[1]), 2);
+    ASSERT_EQ(pool.get(entities[0]), 2);
+    ASSERT_EQ(pool.get(entities[1]), 1);
 }
 
 TEST(Storage, BatchAddEmptyType) {