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packedOwnership as lengthless array #394

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packedOwnership as lengthless array #394

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112 changes: 76 additions & 36 deletions contracts/ERC721A.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -39,6 +39,11 @@ contract ERC721A is IERC721A {
address value;
}

// Reference type for packed ownership.
struct PackedOwnershipRef {
uint256 value;
}

// =============================================================
// CONSTANTS
// =============================================================
Expand Down Expand Up @@ -90,6 +95,11 @@ contract ERC721A is IERC721A {
bytes32 private constant _TRANSFER_EVENT_SIGNATURE =
0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef;

// Storage location for packedOwnerships.
// Equivalent to keccak256('com.erc721a.packedownership').
uint256 internal constant _PACKED_OWNERSHIP_STORAGE =
0x14d1d17051a21856adfcd8caa5ecfacc7e7d5e95e199d4bad2e93a5271be8b2d;

// =============================================================
// STORAGE
// =============================================================
Expand All @@ -110,13 +120,15 @@ contract ERC721A is IERC721A {
// An empty struct value does not necessarily mean the token is unowned.
// See {_packedOwnershipOf} implementation for details.
//
// Stored at _PACKED_OWNERSHIP_STORAGE and accessed via _packedOwnerships(uint256).
// Equivalent to mapping(uint256 => uint256) private _packedOwnerships;
//
// Bits Layout:
// - [0..159] `addr`
// - [160..223] `startTimestamp`
// - [224] `burned`
// - [225] `nextInitialized`
// - [232..255] `extraData`
mapping(uint256 => uint256) private _packedOwnerships;

// Mapping owner address to address data.
//
Expand Down Expand Up @@ -328,46 +340,53 @@ contract ERC721A is IERC721A {
* @dev Returns the unpacked `TokenOwnership` struct at `index`.
*/
function _ownershipAt(uint256 index) internal view virtual returns (TokenOwnership memory) {
return _unpackedOwnership(_packedOwnerships[index]);
return _unpackedOwnership(_packedOwnerships(index).value);
}

/**
* @dev Initializes the ownership slot minted at `index` for efficiency purposes.
*/
function _initializeOwnershipAt(uint256 index) internal virtual {
if (_packedOwnerships[index] == 0) {
_packedOwnerships[index] = _packedOwnershipOf(index);
PackedOwnershipRef storage packedOwnership = _packedOwnerships(index);
if (packedOwnership.value == 0) {
packedOwnership.value = _packedOwnershipOf(index);
}
}

/**
* Returns the packed ownership data of `tokenId`.
*/
function _packedOwnershipOf(uint256 tokenId) private view returns (uint256) {
uint256 curr = tokenId;

unchecked {
if (_startTokenId() <= curr)
if (curr < _currentIndex) {
uint256 packed = _packedOwnerships[curr];
// If not burned.
if (packed & _BITMASK_BURNED == 0) {
// Invariant:
// There will always be an initialized ownership slot
// (i.e. `ownership.addr != address(0) && ownership.burned == false`)
// before an unintialized ownership slot
// (i.e. `ownership.addr == address(0) && ownership.burned == false`)
// Hence, `curr` will not underflow.
//
// We can directly compare the packed value.
// If the address is zero, packed will be zero.
while (packed == 0) {
packed = _packedOwnerships[--curr];
}
return packed;
if (_startTokenId() <= tokenId) {
if (tokenId < _currentIndex) {
uint256 packed;

// Invariant:
// There will always be an initialized ownership slot
// (i.e. `ownership.addr != address(0) && ownership.burned == false`)
// before an unintialized ownership slot
// (i.e. `ownership.addr == address(0) && ownership.burned == false`)
// Hence, `slot` will not underflow.
//
// We can directly compare the packed value.
// If the address is zero, packed will be zero.
assembly {
for {
let slot := add(tokenId, _PACKED_OWNERSHIP_STORAGE)
packed := sload(slot)
slot := sub(slot, 1)
} iszero(packed) {
slot := sub(slot, 1)
} {
packed := sload(slot)
}
}

// If not burned.
if (packed & _BITMASK_BURNED == 0) return packed;
}
}

revert OwnerQueryForNonexistentToken();
}

Expand Down Expand Up @@ -404,6 +423,16 @@ contract ERC721A is IERC721A {
}
}

/**
* @dev Returns the packedOwnership storage for `tokenId`.
* `tokenId` is used as an indexer into lengthless array.
*/
function _packedOwnerships(uint256 tokenId) internal pure returns (PackedOwnershipRef storage po) {
assembly {
po.slot := add(tokenId, _PACKED_OWNERSHIP_STORAGE)
}
}

// =============================================================
// APPROVAL OPERATIONS
// =============================================================
Expand Down Expand Up @@ -485,7 +514,7 @@ contract ERC721A is IERC721A {
return
_startTokenId() <= tokenId &&
tokenId < _currentIndex && // If within bounds,
_packedOwnerships[tokenId] & _BITMASK_BURNED == 0; // and not burned.
_packedOwnerships(tokenId).value & _BITMASK_BURNED == 0; // and not burned.
}

/**
Expand Down Expand Up @@ -579,7 +608,8 @@ contract ERC721A is IERC721A {
// - `startTimestamp` to the timestamp of transfering.
// - `burned` to `false`.
// - `nextInitialized` to `true`.
_packedOwnerships[tokenId] = _packOwnershipData(
PackedOwnershipRef storage packedOwnership = _packedOwnerships(tokenId);
packedOwnership.value = _packOwnershipData(
to,
_BITMASK_NEXT_INITIALIZED | _nextExtraData(from, to, prevOwnershipPacked)
);
Expand All @@ -588,11 +618,14 @@ contract ERC721A is IERC721A {
if (prevOwnershipPacked & _BITMASK_NEXT_INITIALIZED == 0) {
uint256 nextTokenId = tokenId + 1;
// If the next slot's address is zero and not burned (i.e. packed value is zero).
if (_packedOwnerships[nextTokenId] == 0) {
assembly {
packedOwnership.slot := add(packedOwnership.slot, 1)
}
if (packedOwnership.value == 0) {
// If the next slot is within bounds.
if (nextTokenId != _currentIndex) {
// Initialize the next slot to maintain correctness for `ownerOf(tokenId + 1)`.
_packedOwnerships[nextTokenId] = prevOwnershipPacked;
packedOwnership.value = prevOwnershipPacked;
}
}
}
Expand Down Expand Up @@ -754,7 +787,8 @@ contract ERC721A is IERC721A {
// - `startTimestamp` to the timestamp of minting.
// - `burned` to `false`.
// - `nextInitialized` to `quantity == 1`.
_packedOwnerships[startTokenId] = _packOwnershipData(
PackedOwnershipRef storage packedOwnership = _packedOwnerships(startTokenId);
packedOwnership.value = _packOwnershipData(
to,
_nextInitializedFlag(quantity) | _nextExtraData(address(0), to, 0)
);
Expand Down Expand Up @@ -835,7 +869,8 @@ contract ERC721A is IERC721A {
// - `startTimestamp` to the timestamp of minting.
// - `burned` to `false`.
// - `nextInitialized` to `quantity == 1`.
_packedOwnerships[startTokenId] = _packOwnershipData(
PackedOwnershipRef storage packedOwnership = _packedOwnerships(startTokenId);
packedOwnership.value = _packOwnershipData(
to,
_nextInitializedFlag(quantity) | _nextExtraData(address(0), to, 0)
);
Expand Down Expand Up @@ -950,7 +985,8 @@ contract ERC721A is IERC721A {
// - `startTimestamp` to the timestamp of burning.
// - `burned` to `true`.
// - `nextInitialized` to `true`.
_packedOwnerships[tokenId] = _packOwnershipData(
PackedOwnershipRef storage packedOwnership = _packedOwnerships(tokenId);
packedOwnership.value = _packOwnershipData(
from,
(_BITMASK_BURNED | _BITMASK_NEXT_INITIALIZED) | _nextExtraData(from, address(0), prevOwnershipPacked)
);
Expand All @@ -959,11 +995,14 @@ contract ERC721A is IERC721A {
if (prevOwnershipPacked & _BITMASK_NEXT_INITIALIZED == 0) {
uint256 nextTokenId = tokenId + 1;
// If the next slot's address is zero and not burned (i.e. packed value is zero).
if (_packedOwnerships[nextTokenId] == 0) {
assembly {
packedOwnership.slot := add(1, packedOwnership.slot)
}
if (packedOwnership.value == 0) {
// If the next slot is within bounds.
if (nextTokenId != _currentIndex) {
// Initialize the next slot to maintain correctness for `ownerOf(tokenId + 1)`.
_packedOwnerships[nextTokenId] = prevOwnershipPacked;
packedOwnership.value = prevOwnershipPacked;
}
}
}
Expand All @@ -986,15 +1025,16 @@ contract ERC721A is IERC721A {
* @dev Directly sets the extra data for the ownership data `index`.
*/
function _setExtraDataAt(uint256 index, uint24 extraData) internal virtual {
uint256 packed = _packedOwnerships[index];
PackedOwnershipRef storage packedOwnership = _packedOwnerships(index);
uint256 packed = packedOwnership.value;
if (packed == 0) revert OwnershipNotInitializedForExtraData();
uint256 extraDataCasted;
// Cast `extraData` with assembly to avoid redundant masking.
assembly {
extraDataCasted := extraData
}
packed = (packed & _BITMASK_EXTRA_DATA_COMPLEMENT) | (extraDataCasted << _BITPOS_EXTRA_DATA);
_packedOwnerships[index] = packed;
packedOwnership.value = packed;
}

/**
Expand Down