Add CircularBuffer data structure (#4913)

Co-authored-by: ernestognw <ernestognw@gmail.com>

.changeset/cold-cheetahs-check.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': minor
+---
+
+`CircularBuffer`: Add a data structure that stores the last `N` values pushed to it.

contracts/mocks/Stateless.sol

@@ -10,6 +10,7 @@ import {AuthorityUtils} from "../access/manager/AuthorityUtils.sol";
 import {Base64} from "../utils/Base64.sol";
 import {BitMaps} from "../utils/structs/BitMaps.sol";
 import {Checkpoints} from "../utils/structs/Checkpoints.sol";
+import {CircularBuffer} from "../utils/structs/CircularBuffer.sol";
 import {Clones} from "../proxy/Clones.sol";
 import {Create2} from "../utils/Create2.sol";
 import {DoubleEndedQueue} from "../utils/structs/DoubleEndedQueue.sol";
@@ -24,6 +25,7 @@ import {ERC721Holder} from "../token/ERC721/utils/ERC721Holder.sol";
 import {Math} from "../utils/math/Math.sol";
 import {MerkleProof} from "../utils/cryptography/MerkleProof.sol";
 import {MessageHashUtils} from "../utils/cryptography/MessageHashUtils.sol";
+import {Panic} from "../utils/Panic.sol";
 import {Packing} from "../utils/Packing.sol";
 import {SafeCast} from "../utils/math/SafeCast.sol";
 import {SafeERC20} from "../token/ERC20/utils/SafeERC20.sol";

contracts/utils/Arrays.sol

@@ -455,6 +455,7 @@ library Arrays {
      * WARNING: this does not clear elements if length is reduced, of initialize elements if length is increased.
      */
     function unsafeSetLength(address[] storage array, uint256 len) internal {
+        /// @solidity memory-safe-assembly
         assembly {
             sstore(array.slot, len)
         }
@@ -466,6 +467,7 @@ library Arrays {
      * WARNING: this does not clear elements if length is reduced, of initialize elements if length is increased.
      */
     function unsafeSetLength(bytes32[] storage array, uint256 len) internal {
+        /// @solidity memory-safe-assembly
         assembly {
             sstore(array.slot, len)
         }
@@ -477,6 +479,7 @@ library Arrays {
      * WARNING: this does not clear elements if length is reduced, of initialize elements if length is increased.
      */
     function unsafeSetLength(uint256[] storage array, uint256 len) internal {
+        /// @solidity memory-safe-assembly
         assembly {
             sstore(array.slot, len)
         }

contracts/utils/README.adoc

@@ -21,6 +21,7 @@ Miscellaneous contracts and libraries containing utility functions you can use t
  * {EnumerableMap}: A type like Solidity's https://solidity.readthedocs.io/en/latest/types.html#mapping-types[`mapping`], but with key-value _enumeration_: this will let you know how many entries a mapping has, and iterate over them (which is not possible with `mapping`).
  * {EnumerableSet}: Like {EnumerableMap}, but for https://en.wikipedia.org/wiki/Set_(abstract_data_type)[sets]. Can be used to store privileged accounts, issued IDs, etc.
  * {DoubleEndedQueue}: An implementation of a https://en.wikipedia.org/wiki/Double-ended_queue[double ended queue] whose values can be removed added or remove from both sides. Useful for FIFO and LIFO structures.
+ * {CircularBuffer}: A data structure to store the last N values pushed to it.
  * {Checkpoints}: A data structure to store values mapped to an strictly increasing key. Can be used for storing and accessing values over time.
  * {MerkleTree}: A library with https://wikipedia.org/wiki/Merkle_Tree[Merkle Tree] data structures and helper functions.
  * {Create2}: Wrapper around the https://blog.openzeppelin.com/getting-the-most-out-of-create2/[`CREATE2` EVM opcode] for safe use without having to deal with low-level assembly.
@@ -95,6 +96,8 @@ Ethereum contracts have no native concept of an interface, so applications must
 
 {{DoubleEndedQueue}}
 
+{{CircularBuffer}}
+
 {{Checkpoints}}
 
 {{MerkleTree}}

contracts/utils/structs/CircularBuffer.sol

@@ -0,0 +1,130 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.20;
+
+import {Math} from "../math/Math.sol";
+import {Arrays} from "../Arrays.sol";
+import {Panic} from "../Panic.sol";
+
+/**
+ * @dev A fixed-size buffer for keeping `bytes32` items in storage.
+ *
+ * This data structure allows for pushing elements to it, and when its length exceeds the specified fixed size,
+ * new items take the place of the oldest element in the buffer, keeping at most `N` elements in the
+ * structure.
+ *
+ * Elements can't be removed but the data structure can be cleared. See {clear}.
+ *
+ * Complexity:
+ * - insertion ({push}): O(1)
+ * - lookup ({last}): O(1)
+ * - inclusion ({includes}): O(N) (worst case)
+ * - reset ({clear}): O(1)
+ *
+ * * The struct is called `Bytes32CircularBuffer`. Other types can be cast to and from `bytes32`. This data structure
+ * can only be used in storage, and not in memory.
+ *
+ * Example usage:
+ *
+ * ```solidity
+ * contract Example {
+ *     // Add the library methods
+ *     using CircularBuffer for CircularBuffer.Bytes32CircularBuffer;
+ *
+ *     // Declare a buffer storage variable
+ *     CircularBuffer.Bytes32CircularBuffer private myBuffer;
+ * }
+ * ```
+ */
+library CircularBuffer {
+    /**
+     * @dev Counts the number of items that have been pushed to the buffer. The residuo modulo _data.length indicates
+     * where the next value should be stored.
+     *
+     * 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.
+     *
+     * The last item is at data[(index - 1) % data.length] and the last item is at data[index % data.length]. This
+     * range can wrap around.
+     */
+    struct Bytes32CircularBuffer {
+        uint256 _count;
+        bytes32[] _data;
+    }
+
+    /**
+     * @dev Initialize a new CircularBuffer of given size.
+     *
+     * If the CircularBuffer was already setup and used, calling that function again will reset it to a blank state.
+     *
+     * NOTE: The size of the buffer will affect the execution of {includes} function, as it has a complexity of O(N).
+     * Consider a large buffer size may render the function unusable.
+     */
+    function setup(Bytes32CircularBuffer storage self, uint256 size) internal {
+        clear(self);
+        Arrays.unsafeSetLength(self._data, size);
+    }
+
+    /**
+     * @dev Clear all data in the buffer without resetting memory, keeping the existing size.
+     */
+    function clear(Bytes32CircularBuffer storage self) internal {
+        self._count = 0;
+    }
+
+    /**
+     * @dev Push a new value to the buffer. If the buffer is already full, the new value replaces the oldest value in
+     * the buffer.
+     */
+    function push(Bytes32CircularBuffer storage self, bytes32 value) internal {
+        uint256 index = self._count++;
+        uint256 modulus = self._data.length;
+        Arrays.unsafeAccess(self._data, index % modulus).value = value;
+    }
+
+    /**
+     * @dev Number of values currently in the buffer. This value is 0 for an empty buffer, and cannot exceed the size of
+     * the buffer.
+     */
+    function count(Bytes32CircularBuffer storage self) internal view returns (uint256) {
+        return Math.min(self._count, self._data.length);
+    }
+
+    /**
+     * @dev Length of the buffer. This is the maximum number of elements kepts in the buffer.
+     */
+    function length(Bytes32CircularBuffer storage self) internal view returns (uint256) {
+        return self._data.length;
+    }
+
+    /**
+     * @dev Getter for the i-th value in the buffer, from the end.
+     *
+     * Reverts with {Panic-ARRAY_OUT_OF_BOUNDS} if trying to access an element that was not pushed, or that was
+     * dropped to make room for newer elements.
+     */
+    function last(Bytes32CircularBuffer storage self, uint256 i) internal view returns (bytes32) {
+        uint256 index = self._count;
+        uint256 modulus = self._data.length;
+        uint256 total = Math.min(index, modulus); // count(self)
+        if (i >= total) {
+            Panic.panic(Panic.ARRAY_OUT_OF_BOUNDS);
+        }
+        return Arrays.unsafeAccess(self._data, (index - i - 1) % modulus).value;
+    }
+
+    /**
+     * @dev Check if a given value is in the buffer.
+     */
+    function includes(Bytes32CircularBuffer storage self, bytes32 value) internal view returns (bool) {
+        uint256 index = self._count;
+        uint256 modulus = self._data.length;
+        uint256 total = Math.min(index, modulus); // count(self)
+        for (uint256 i = 0; i < total; ++i) {
+            if (Arrays.unsafeAccess(self._data, (index - i - 1) % modulus).value == value) {
+                return true;
+            }
+        }
+        return false;
+    }
+}

scripts/generate/templates/Arrays.js

@@ -356,6 +356,7 @@ const unsafeSetLength = type => `
  * WARNING: this does not clear elements if length is reduced, of initialize elements if length is increased.
  */
 function unsafeSetLength(${type}[] storage array, uint256 len) internal {
+    /// @solidity memory-safe-assembly
     assembly {
         sstore(array.slot, len)
     }

test/utils/structs/CircularBuffer.test.js

@@ -0,0 +1,79 @@
+const { ethers } = require('hardhat');
+const { expect } = require('chai');
+const { loadFixture } = require('@nomicfoundation/hardhat-network-helpers');
+const { PANIC_CODES } = require('@nomicfoundation/hardhat-chai-matchers/panic');
+
+const { generators } = require('../../helpers/random');
+
+const LENGTH = 4;
+
+async function fixture() {
+  const mock = await ethers.deployContract('$CircularBuffer');
+  await mock.$setup(0, LENGTH);
+  return { mock };
+}
+
+describe('CircularBuffer', function () {
+  beforeEach(async function () {
+    Object.assign(this, await loadFixture(fixture));
+  });
+
+  it('starts empty', async function () {
+    expect(await this.mock.$count(0)).to.equal(0n);
+    expect(await this.mock.$length(0)).to.equal(LENGTH);
+    expect(await this.mock.$includes(0, ethers.ZeroHash)).to.be.false;
+    await expect(this.mock.$last(0, 0)).to.be.revertedWithPanic(PANIC_CODES.ARRAY_ACCESS_OUT_OF_BOUNDS);
+  });
+
+  it('push', async function () {
+    const values = Array.from({ length: LENGTH + 3 }, generators.bytes32);
+
+    for (const [i, value] of values.map((v, i) => [i, v])) {
+      // push value
+      await this.mock.$push(0, value);
+
+      // view of the values
+      const pushed = values.slice(0, i + 1);
+      const stored = pushed.slice(-LENGTH);
+      const dropped = pushed.slice(0, -LENGTH);
+
+      // check count
+      expect(await this.mock.$length(0)).to.equal(LENGTH);
+      expect(await this.mock.$count(0)).to.equal(stored.length);
+
+      // check last
+      for (const j in stored) {
+        expect(await this.mock.$last(0, j)).to.equal(stored.at(-j - 1));
+      }
+      await expect(this.mock.$last(0, stored.length + 1)).to.be.revertedWithPanic(
+        PANIC_CODES.ARRAY_ACCESS_OUT_OF_BOUNDS,
+      );
+
+      // check included and non-included values
+      for (const v of stored) {
+        expect(await this.mock.$includes(0, v)).to.be.true;
+      }
+      for (const v of dropped) {
+        expect(await this.mock.$includes(0, v)).to.be.false;
+      }
+      expect(await this.mock.$includes(0, ethers.ZeroHash)).to.be.false;
+    }
+  });
+
+  it('clear', async function () {
+    const value = generators.bytes32();
+    await this.mock.$push(0, value);
+
+    expect(await this.mock.$count(0)).to.equal(1n);
+    expect(await this.mock.$length(0)).to.equal(LENGTH);
+    expect(await this.mock.$includes(0, value)).to.be.true;
+    await this.mock.$last(0, 0); // not revert
+
+    await this.mock.$clear(0);
+
+    expect(await this.mock.$count(0)).to.equal(0n);
+    expect(await this.mock.$length(0)).to.equal(LENGTH);
+    expect(await this.mock.$includes(0, value)).to.be.false;
+    await expect(this.mock.$last(0, 0)).to.be.revertedWithPanic(PANIC_CODES.ARRAY_ACCESS_OUT_OF_BOUNDS);
+  });
+});