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Solidity

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.6.0) (utils/structs/DoubleEndedQueue.sol)
pragma solidity ^0.8.4;
import "../math/SafeCast.sol";
/**
* @dev A sequence of items with the ability to efficiently push and pop items (i.e. insert and remove) on both ends of
* the sequence (called front and back). Among other access patterns, it can be used to implement efficient LIFO and
* FIFO queues. Storage use is optimized, and all operations are O(1) constant time. This includes {clear}, given that
* the existing queue contents are left in storage.
*
* The struct is called `Bytes32Deque`. Other types can be cast to and from `bytes32`. This data structure can only be
* used in storage, and not in memory.
* ```
* DoubleEndedQueue.Bytes32Deque queue;
* ```
*
* _Available since v4.6._
*/
library DoubleEndedQueue {
/**
* @dev An operation (e.g. {front}) couldn't be completed due to the queue being empty.
*/
error Empty();
/**
* @dev An operation (e.g. {at}) couldn't be completed due to an index being out of bounds.
*/
error OutOfBounds();
/**
* @dev Indices are signed integers because the queue can grow in any direction. They are 128 bits so begin and end
* are packed in a single storage slot for efficient access. Since the items are added one at a time we can safely
* assume that these 128-bit indices will not overflow, and use unchecked arithmetic.
*
* Struct members have an underscore prefix indicating that they are "private" and should not be read or written to
* directly. Use the functions provided below instead. Modifying the struct manually may violate assumptions and
* lead to unexpected behavior.
*
* Indices are in the range [begin, end) which means the first item is at data[begin] and the last item is at
* data[end - 1].
*/
struct Bytes32Deque {
int128 _begin;
int128 _end;
mapping(int128 => bytes32) _data;
}
/**
* @dev Inserts an item at the end of the queue.
*/
function pushBack(Bytes32Deque storage deque, bytes32 value) internal {
int128 backIndex = deque._end;
deque._data[backIndex] = value;
unchecked {
deque._end = backIndex + 1;
}
}
/**
* @dev Removes the item at the end of the queue and returns it.
*
* Reverts with `Empty` if the queue is empty.
*/
function popBack(Bytes32Deque storage deque) internal returns (bytes32 value) {
if (empty(deque)) revert Empty();
int128 backIndex;
unchecked {
backIndex = deque._end - 1;
}
value = deque._data[backIndex];
delete deque._data[backIndex];
deque._end = backIndex;
}
/**
* @dev Inserts an item at the beginning of the queue.
*/
function pushFront(Bytes32Deque storage deque, bytes32 value) internal {
int128 frontIndex;
unchecked {
frontIndex = deque._begin - 1;
}
deque._data[frontIndex] = value;
deque._begin = frontIndex;
}
/**
* @dev Removes the item at the beginning of the queue and returns it.
*
* Reverts with `Empty` if the queue is empty.
*/
function popFront(Bytes32Deque storage deque) internal returns (bytes32 value) {
if (empty(deque)) revert Empty();
int128 frontIndex = deque._begin;
value = deque._data[frontIndex];
delete deque._data[frontIndex];
unchecked {
deque._begin = frontIndex + 1;
}
}
/**
* @dev Returns the item at the beginning of the queue.
*
* Reverts with `Empty` if the queue is empty.
*/
function front(Bytes32Deque storage deque) internal view returns (bytes32 value) {
if (empty(deque)) revert Empty();
int128 frontIndex = deque._begin;
return deque._data[frontIndex];
}
/**
* @dev Returns the item at the end of the queue.
*
* Reverts with `Empty` if the queue is empty.
*/
function back(Bytes32Deque storage deque) internal view returns (bytes32 value) {
if (empty(deque)) revert Empty();
int128 backIndex;
unchecked {
backIndex = deque._end - 1;
}
return deque._data[backIndex];
}
/**
* @dev Return the item at a position in the queue given by `index`, with the first item at 0 and last item at
* `length(deque) - 1`.
*
* Reverts with `OutOfBounds` if the index is out of bounds.
*/
function at(Bytes32Deque storage deque, uint256 index) internal view returns (bytes32 value) {
// int256(deque._begin) is a safe upcast
int128 idx = SafeCast.toInt128(int256(deque._begin) + SafeCast.toInt256(index));
if (idx >= deque._end) revert OutOfBounds();
return deque._data[idx];
}
/**
* @dev Resets the queue back to being empty.
*
* NOTE: The current items are left behind in storage. This does not affect the functioning of the queue, but misses
* out on potential gas refunds.
*/
function clear(Bytes32Deque storage deque) internal {
deque._begin = 0;
deque._end = 0;
}
/**
* @dev Returns the number of items in the queue.
*/
function length(Bytes32Deque storage deque) internal view returns (uint256) {
// The interface preserves the invariant that begin <= end so we assume this will not overflow.
// We also assume there are at most int256.max items in the queue.
unchecked {
return uint256(int256(deque._end) - int256(deque._begin));
}
}
/**
* @dev Returns true if the queue is empty.
*/
function empty(Bytes32Deque storage deque) internal view returns (bool) {
return deque._end <= deque._begin;
}
}