Mint re-entrancy guard with optimizations. (#131)

* Added reentrancy guard to _mint

* Optimized _mint

* Added more optimizations

* Added missing isContract check in safeTransferFrom

* Moved updatedIndex++ for gas savings

* Convert while to do while

contracts/ERC721A.sol

@@ -324,7 +324,7 @@ contract ERC721A is Context, ERC165, IERC721, IERC721Metadata {
         bytes memory _data
     ) public virtual override {
         _transfer(from, to, tokenId);
-        if (!_checkOnERC721Received(from, to, tokenId, _data)) {
+        if (to.isContract() && !_checkContractOnERC721Received(from, to, tokenId, _data)) {
             revert TransferToNonERC721ReceiverImplementer();
         }
     }
@@ -396,15 +396,22 @@ contract ERC721A is Context, ERC165, IERC721, IERC721Metadata {
             _ownerships[startTokenId].startTimestamp = uint64(block.timestamp);
 
             uint256 updatedIndex = startTokenId;
+            uint256 end = updatedIndex + quantity;
 
-            for (uint256 i; i < quantity; i++) {
-                emit Transfer(address(0), to, updatedIndex);
-                if (safe && !_checkOnERC721Received(address(0), to, updatedIndex, _data)) {
-                    revert TransferToNonERC721ReceiverImplementer();
-                }
-                updatedIndex++;
+            if (safe && to.isContract()) {
+                do {
+                    emit Transfer(address(0), to, updatedIndex);
+                    if (!_checkContractOnERC721Received(address(0), to, updatedIndex++, _data)) {
+                        revert TransferToNonERC721ReceiverImplementer();
+                    }
+                } while (updatedIndex != end);
+                // Reentrancy protection
+                if (_currentIndex != startTokenId) revert();
+            } else {
+                do {
+                    emit Transfer(address(0), to, updatedIndex++);
+                } while (updatedIndex != end);
             }
-
             _currentIndex = updatedIndex;
         }
         _afterTokenTransfers(address(0), to, startTokenId, quantity);
@@ -534,35 +541,30 @@ contract ERC721A is Context, ERC165, IERC721, IERC721Metadata {
     }
 
     /**
-     * @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target address.
-     * The call is not executed if the target address is not a contract.
+     * @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target contract.
      *
      * @param from address representing the previous owner of the given token ID
      * @param to target address that will receive the tokens
      * @param tokenId uint256 ID of the token to be transferred
      * @param _data bytes optional data to send along with the call
      * @return bool whether the call correctly returned the expected magic value
      */
-    function _checkOnERC721Received(
+    function _checkContractOnERC721Received(
         address from,
         address to,
         uint256 tokenId,
         bytes memory _data
     ) private returns (bool) {
-        if (to.isContract()) {
-            try IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data) returns (bytes4 retval) {
-                return retval == IERC721Receiver(to).onERC721Received.selector;
-            } catch (bytes memory reason) {
-                if (reason.length == 0) {
-                    revert TransferToNonERC721ReceiverImplementer();
-                } else {
-                    assembly {
-                        revert(add(32, reason), mload(reason))
-                    }
+        try IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data) returns (bytes4 retval) {
+            return retval == IERC721Receiver(to).onERC721Received.selector;
+        } catch (bytes memory reason) {
+            if (reason.length == 0) {
+                revert TransferToNonERC721ReceiverImplementer();
+            } else {
+                assembly {
+                    revert(add(32, reason), mload(reason))
                 }
             }
-        } else {
-            return true;
         }
     }