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docs: Describe the new message box model (#4485)
Fixes #4250 and writes out an initial implementation of a solidity frontier tree.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,9 @@ | ||
| // SPDX-License-Identifier: Apache-2.0 | ||
| // Copyright 2023 Aztec Labs. | ||
| pragma solidity >=0.8.18; | ||
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| interface IFrontier { | ||
| function insertLeaf(bytes32 _leaf) external; | ||
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| function root() external view returns (bytes32); | ||
| } |
85 changes: 85 additions & 0 deletions
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l1-contracts/src/core/messagebridge/frontier_tree/Frontier.sol
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,85 @@ | ||
| // SPDX-License-Identifier: Apache-2.0 | ||
| // Copyright 2023 Aztec Labs. | ||
| pragma solidity >=0.8.18; | ||
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| import {IFrontier} from "../../interfaces/messagebridge/IFrontier.sol"; | ||
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| contract FrontierMerkle is IFrontier { | ||
| uint256 public immutable DEPTH; | ||
| uint256 public immutable SIZE; | ||
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| uint256 internal nextIndex = 0; | ||
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| mapping(uint256 level => bytes32 node) public frontier; | ||
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| // Below can be pre-computed so it would be possible to have constants | ||
| // for the zeros at each level. This would save gas on computations | ||
| mapping(uint256 level => bytes32 zero) public zeros; | ||
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| constructor(uint256 _depth) { | ||
| DEPTH = _depth; | ||
| SIZE = 2 ** _depth; | ||
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| zeros[0] = bytes32(0); | ||
| for (uint256 i = 1; i <= DEPTH; i++) { | ||
| zeros[i] = sha256(bytes.concat(zeros[i - 1], zeros[i - 1])); | ||
| } | ||
| } | ||
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| function insertLeaf(bytes32 _leaf) external override(IFrontier) { | ||
| uint256 level = _computeLevel(nextIndex); | ||
| bytes32 right = _leaf; | ||
| for (uint256 i = 0; i < level; i++) { | ||
| right = sha256(bytes.concat(frontier[i], right)); | ||
| } | ||
| frontier[level] = right; | ||
| nextIndex++; | ||
| } | ||
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| function root() external view override(IFrontier) returns (bytes32) { | ||
| uint256 next = nextIndex; | ||
| if (next == 0) { | ||
| return zeros[DEPTH]; | ||
| } | ||
| if (next == SIZE) { | ||
| return frontier[DEPTH]; | ||
| } | ||
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| uint256 index = next - 1; | ||
| uint256 level = _computeLevel(index); | ||
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| // We should start at the highest frontier level with a left leaf | ||
| bytes32 temp = frontier[level]; | ||
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| uint256 bits = index >> level; | ||
| for (uint256 i = level; i < DEPTH; i++) { | ||
| bool isRight = bits & 1 == 1; | ||
| if (isRight) { | ||
| if (frontier[i] == temp) { | ||
| // We will never hit the case that frontier[i] == temp | ||
| // because this require that frontier[i] is the right child | ||
| // and in that case we started higher up the tree | ||
| revert("Mistakes were made"); | ||
| } | ||
| temp = sha256(bytes.concat(frontier[i], temp)); | ||
| } else { | ||
| temp = sha256(bytes.concat(temp, zeros[i])); | ||
| } | ||
| bits >>= 1; | ||
| } | ||
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| return temp; | ||
| } | ||
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| function _computeLevel(uint256 _leafIndex) internal pure returns (uint256) { | ||
| // The number of trailing ones is how many times in a row we are the right child. | ||
| // e.g., each time this happens we go another layer up to update the parent. | ||
| uint256 count = 0; | ||
| uint256 index = _leafIndex; | ||
| while (index & 1 == 1) { | ||
| count++; | ||
| index >>= 1; | ||
| } | ||
| return count; | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,30 @@ | ||
| // SPDX-License-Identifier: Apache-2.0 | ||
| // Copyright 2023 Aztec Labs. | ||
| pragma solidity >=0.8.18; | ||
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| import {Test} from "forge-std/Test.sol"; | ||
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| import {NaiveMerkle} from "./Naive.sol"; | ||
| import {FrontierMerkle} from "./../../src/core/messagebridge/frontier_tree/Frontier.sol"; | ||
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| contract MerkleTest is Test { | ||
| NaiveMerkle internal merkle; | ||
| FrontierMerkle internal frontier; | ||
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| uint256 public constant DEPTH = 10; | ||
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| function setUp() public { | ||
| merkle = new NaiveMerkle(DEPTH); | ||
| frontier = new FrontierMerkle(DEPTH); | ||
| } | ||
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| function testFrontier() public { | ||
| uint256 upper = frontier.SIZE(); | ||
| for (uint256 i = 0; i < upper; i++) { | ||
| bytes32 leaf = sha256(abi.encode(i + 1)); | ||
| merkle.insertLeaf(leaf); | ||
| frontier.insertLeaf(leaf); | ||
| assertEq(merkle.computeRoot(), frontier.root(), "Frontier Roots should be equal"); | ||
| } | ||
| } | ||
| } |
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