Start Sequentially Minting at _startTokenId() instead of hardcoded 0 (chiru-labs#66)

* replaces currentIndex with _nextTokenId which starts at 1 instead of 0

* adapts test to new starting index of 1

* makes `ERC721AOwnersExplicit` test work with new starting index

* explicitly sets `nextOwnerToExplicitlySet` to 1 because it's initialised with zero

* remove vscode files

* implements @jpegditials suggestions

* adds tests for `tokenByIndex` and `tokenOfOwnerByIndex`

* prevents tokenId 0 to be an existing token

* adapts `tokenOfOwnerByIndex` for starting token index 1 and making tests for it more elaborate.

* renames variable to make it more clear, it's index by the tokenId

* adds unchecked scopes to save on gas as suggested by @Vectorized

* more gas optimizations by @Vectorized

* implements suggested `_startTokenId` function to override to set starting index

* minor adaptions to work with _startTokenId() properly

* brings back original tests

* creates copy of original test with startTokenId set to 1

* adapts comment to reflect flexibility of the starting tokenId

* adapts solidity version of tests

* Replaces revert messages with new contract errors

* fix merge hiccup

* optimisations to save on gas used during deployment

* Added chiru-labs#61

* Edited comments

* Edited README roadmap

* Removed burn test with one-index mock

* Changed _startTokenId to _currentIndex

* adapting test and mock related to index starting at 1 to make them work with recent changes on `main`

* add _totalMinted function

* add tests for _totalMinted

* removes uint128 cast, simplifies comment and lints file

* rename `ERC721AExplicitOwnershipMock` to `ERC721AOwnersExplicitMock` to be aligned with the naming of the contract

* extend from already mocked ERC721A versions to dry code.

* aligns file naming and contract naming

* creates new ERC721A mock which enables the user to specify the startTokenId dynamically

* converts ERC721A test to a parameterized test to work with other mocks as well.

* extracts Helper contract to it's own file

* parameterises ERC721A Explicit Owner Tests to also test a Version of the Contract with a different start token id.

* makes it more transparent where the startTokenId value is coming from

* creates a ERC721ABurnable Mock with custom startTokenId and uses it in a parameterised test suite testing the burnable extension.

* removes code used during development

* unifies code style across all tests

Co-authored-by: webby1111 <webby1111@hotmail.com>
Co-authored-by: Amirhossein Banavi <ahbanavi@gmail.com>

Author: 3 people

README.md

@@ -54,7 +54,6 @@ contract Azuki is ERC721A {
 
 ## Roadmap
 
-- [] Add flexibility for the first token id to not start at 0
 - [] Support ERC721 Upgradeable
 - [] Add more documentation on benefits of using ERC721A
 - [] Increase test coverage

contracts/ERC721A.sol

@@ -35,7 +35,7 @@ error URIQueryForNonexistentToken();
  * @dev Implementation of https://eips.ethereum.org/EIPS/eip-721[ERC721] Non-Fungible Token Standard, including
  * the Metadata extension. Built to optimize for lower gas during batch mints.
  *
- * Assumes serials are sequentially minted starting at 0 (e.g. 0, 1, 2, 3..).
+ * Assumes serials are sequentially minted starting at _startTokenId() (defaults to 0, e.g. 0, 1, 2, 3..).
  *
  * Assumes that an owner cannot have more than 2**64 - 1 (max value of uint64) of supply.
  *
@@ -64,7 +64,7 @@ contract ERC721A is Context, ERC165, IERC721, IERC721Metadata {
         // Keeps track of burn count with minimal overhead for tokenomics.
         uint64 numberBurned;
         // For miscellaneous variable(s) pertaining to the address
-        // (e.g. number of whitelist mint slots used). 
+        // (e.g. number of whitelist mint slots used).
         // If there are multiple variables, please pack them into a uint64.
         uint64 aux;
     }
@@ -97,16 +97,36 @@ contract ERC721A is Context, ERC165, IERC721, IERC721Metadata {
     constructor(string memory name_, string memory symbol_) {
         _name = name_;
         _symbol = symbol_;
+        _currentIndex = _startTokenId();
+    }
+
+    /**
+     * To change the starting tokenId, please override this function.
+     */
+    function _startTokenId() internal view virtual returns (uint256) {
+        return 0;
     }
 
     /**
      * @dev See {IERC721Enumerable-totalSupply}.
+     * @dev Burned tokens are calculated here, use _totalMinted() if you want to count just minted tokens.
      */
     function totalSupply() public view returns (uint256) {
         // Counter underflow is impossible as _burnCounter cannot be incremented
-        // more than _currentIndex times
+        // more than _currentIndex - _startTokenId() times
         unchecked {
-            return _currentIndex - _burnCounter;    
+            return _currentIndex - _burnCounter - _startTokenId();
+        }
+    }
+
+    /**
+     * Returns the total amount of tokens minted in the contract.
+     */
+    function _totalMinted() internal view returns (uint256) {
+        // Counter underflow is impossible as _currentIndex does not decrement,
+        // and it is initialized to _startTokenId()
+        unchecked {
+            return _currentIndex - _startTokenId();
         }
     }
 
@@ -169,14 +189,14 @@ contract ERC721A is Context, ERC165, IERC721, IERC721Metadata {
         uint256 curr = tokenId;
 
         unchecked {
-            if (curr < _currentIndex) {
+            if (_startTokenId() <= curr && curr < _currentIndex) {
                 TokenOwnership memory ownership = _ownerships[curr];
                 if (!ownership.burned) {
                     if (ownership.addr != address(0)) {
                         return ownership;
                     }
-                    // Invariant: 
-                    // There will always be an ownership that has an address and is not burned 
+                    // Invariant:
+                    // There will always be an ownership that has an address and is not burned
                     // before an ownership that does not have an address and is not burned.
                     // Hence, curr will not underflow.
                     while (true) {
@@ -317,7 +337,8 @@ contract ERC721A is Context, ERC165, IERC721, IERC721Metadata {
      * Tokens start existing when they are minted (`_mint`),
      */
     function _exists(uint256 tokenId) internal view returns (bool) {
-        return tokenId < _currentIndex && !_ownerships[tokenId].burned;
+        return _startTokenId() <= tokenId && tokenId < _currentIndex &&
+            !_ownerships[tokenId].burned;
     }
 
     function _safeMint(address to, uint256 quantity) internal {
@@ -493,7 +514,7 @@ contract ERC721A is Context, ERC165, IERC721, IERC721Metadata {
         _afterTokenTransfers(prevOwnership.addr, address(0), tokenId, 1);
 
         // Overflow not possible, as _burnCounter cannot be exceed _currentIndex times.
-        unchecked { 
+        unchecked {
             _burnCounter++;
         }
     }

contracts/extensions/ERC721AOwnersExplicit.sol

@@ -17,8 +17,11 @@ abstract contract ERC721AOwnersExplicit is ERC721A {
      */
     function _setOwnersExplicit(uint256 quantity) internal {
         if (quantity == 0) revert QuantityMustBeNonZero();
-        if (_currentIndex == 0) revert NoTokensMintedYet();
+        if (_currentIndex == _startTokenId()) revert NoTokensMintedYet();
         uint256 _nextOwnerToExplicitlySet = nextOwnerToExplicitlySet;
+        if (_nextOwnerToExplicitlySet == 0) {
+            _nextOwnerToExplicitlySet = _startTokenId();
+        }
         if (_nextOwnerToExplicitlySet >= _currentIndex) revert AllOwnershipsHaveBeenSet();
 
         // Index underflow is impossible.

contracts/mocks/ERC721ABurnableMock.sol

@@ -19,4 +19,8 @@ contract ERC721ABurnableMock is ERC721A, ERC721ABurnable {
     function getOwnershipAt(uint256 index) public view returns (TokenOwnership memory) {
         return _ownerships[index];
     }
+
+    function totalMinted() public view returns (uint256) {
+        return _totalMinted();
+    }
 }

contracts/mocks/ERC721ABurnableStartTokenIdMock.sol

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+// Creators: Chiru Labs
+
+pragma solidity ^0.8.4;
+
+import './ERC721ABurnableMock.sol';
+import './StartTokenIdHelper.sol';
+
+contract ERC721ABurnableStartTokenIdMock is StartTokenIdHelper, ERC721ABurnableMock {
+    constructor(
+        string memory name_,
+        string memory symbol_,
+        uint256 startTokenId_
+    ) StartTokenIdHelper(startTokenId_) ERC721ABurnableMock(name_, symbol_) {}
+
+    function _startTokenId() internal view override returns (uint256) {
+        return startTokenId;
+    }
+}

contracts/mocks/ERC721AMock.sol

@@ -12,6 +12,10 @@ contract ERC721AMock is ERC721A {
         return _numberMinted(owner);
     }
 
+    function totalMinted() public view returns (uint256) {
+        return _totalMinted();
+    }
+
     function getAux(address owner) public view returns (uint64) {
         return _getAux(owner);
     }

contracts/mocks/ERC721AExplicitOwnershipMock.sol contracts/mocks/ERC721AOwnersExplicitMock.sol

@@ -16,7 +16,7 @@ contract ERC721AOwnersExplicitMock is ERC721AOwnersExplicit {
         _setOwnersExplicit(quantity);
     }
 
-    function getOwnershipAt(uint256 index) public view returns (TokenOwnership memory) {
-        return _ownerships[index];
+    function getOwnershipAt(uint256 tokenId) public view returns (TokenOwnership memory) {
+        return _ownerships[tokenId];
     }
 }

contracts/mocks/ERC721AOwnersExplicitStartTokenIdMock.sol

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+// Creators: Chiru Labs
+
+pragma solidity ^0.8.4;
+
+import './ERC721AOwnersExplicitMock.sol';
+import './StartTokenIdHelper.sol';
+
+contract ERC721AOwnersExplicitStartTokenIdMock is StartTokenIdHelper, ERC721AOwnersExplicitMock {
+    constructor(
+        string memory name_,
+        string memory symbol_,
+        uint256 startTokenId_
+    ) StartTokenIdHelper(startTokenId_) ERC721AOwnersExplicitMock(name_, symbol_) {}
+
+    function _startTokenId() internal view override returns (uint256) {
+        return startTokenId;
+    }
+}

contracts/mocks/ERC721AStartTokenIdMock.sol

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+// Creators: Chiru Labs
+
+pragma solidity ^0.8.4;
+
+import './ERC721AMock.sol';
+import './StartTokenIdHelper.sol';
+
+contract ERC721AStartTokenIdMock is StartTokenIdHelper, ERC721AMock {
+    constructor(
+        string memory name_,
+        string memory symbol_,
+        uint256 startTokenId_
+    ) StartTokenIdHelper(startTokenId_) ERC721AMock(name_, symbol_) {}
+
+    function _startTokenId() internal view override returns (uint256) {
+        return startTokenId;
+    }
+}

contracts/mocks/StartTokenIdHelper.sol

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MIT
+// Creators: Chiru Labs
+
+pragma solidity ^0.8.4;
+
+/**
+ * This Helper is used to return a dynmamic value in the overriden _startTokenId() function.
+ * Extending this Helper before the ERC721A contract give us access to the herein set `startTokenId`
+ * to be returned by the overriden `_startTokenId()` function of ERC721A in the ERC721AStartTokenId mocks.
+ */
+contract StartTokenIdHelper {
+    uint256 public immutable startTokenId;
+
+    constructor(uint256 startTokenId_) {
+        startTokenId = startTokenId_;
+    }
+}