tornado-core/contracts/MerkleTreeWithHistory.sol

// https://tornado.cash
/*
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 */

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

interface Hasher {
  function MiMCSponge(uint256 in_xL, uint256 in_xR) external pure returns (uint256 xL, uint256 xR);
}

contract MerkleTreeWithHistory {
  uint256 public constant FIELD_SIZE = 21888242871839275222246405745257275088548364400416034343698204186575808495617;
  uint256 public constant ZERO_VALUE = 21663839004416932945382355908790599225266501822907911457504978515578255421292; // = keccak256("tornado") % FIELD_SIZE

  uint32 public levels;

  // the following variables are made public for easier testing and debugging and
  // are not supposed to be accessed in regular code
  bytes32[] public filledSubtrees;
  bytes32[] public zeros;
  uint32 public currentRootIndex = 0;
  uint32 public nextIndex = 0;
  uint32 public constant ROOT_HISTORY_SIZE = 100;
  bytes32[ROOT_HISTORY_SIZE] public roots;
  Hasher public immutable hasher;

  constructor(uint32 _treeLevels, Hasher _hasher) public {
    require(_treeLevels > 0, "_treeLevels should be greater than zero");
    require(_treeLevels < 32, "_treeLevels should be less than 32");
    levels = _treeLevels;

    hasher = _hasher;

    bytes32 currentZero = bytes32(ZERO_VALUE);
    zeros.push(currentZero);
    filledSubtrees.push(currentZero);

    for (uint32 i = 1; i < levels; i++) {
      currentZero = hashLeftRight(_hasher, currentZero, currentZero);
      zeros.push(currentZero);
      filledSubtrees.push(currentZero);
    }

    roots[0] = hashLeftRight(_hasher, currentZero, currentZero);
  }

  /**
    @dev Hash 2 tree leaves, returns MiMC(_left, _right)
  */
  function hashLeftRight(
    Hasher _hasher,
    bytes32 _left,
    bytes32 _right
  ) public pure returns (bytes32) {
    require(uint256(_left) < FIELD_SIZE, "_left should be inside the field");
    require(uint256(_right) < FIELD_SIZE, "_right should be inside the field");
    uint256 R = uint256(_left);
    uint256 C = 0;
    (R, C) = _hasher.MiMCSponge(R, C);
    R = addmod(R, uint256(_right), FIELD_SIZE);
    (R, C) = _hasher.MiMCSponge(R, C);
    return bytes32(R);
  }

  function _insert(bytes32 _leaf) internal returns (uint32 index) {
    uint32 currentIndex = nextIndex;
    require(currentIndex != uint32(2)**levels, "Merkle tree is full. No more leafs can be added");
    nextIndex += 1;
    bytes32 currentLevelHash = _leaf;
    bytes32 left;
    bytes32 right;

    for (uint32 i = 0; i < levels; i++) {
      if (currentIndex % 2 == 0) {
        left = currentLevelHash;
        right = zeros[i];

        filledSubtrees[i] = currentLevelHash;
      } else {
        left = filledSubtrees[i];
        right = currentLevelHash;
      }

      currentLevelHash = hashLeftRight(hasher, left, right);

      currentIndex /= 2;
    }

    currentRootIndex = (currentRootIndex + 1) % ROOT_HISTORY_SIZE;
    roots[currentRootIndex] = currentLevelHash;
    return nextIndex - 1;
  }

  /**
    @dev Whether the root is present in the root history
  */
  function isKnownRoot(bytes32 _root) public view returns (bool) {
    if (_root == 0) {
      return false;
    }
    uint32 i = currentRootIndex;
    do {
      if (_root == roots[i]) {
        return true;
      }
      if (i == 0) {
        i = ROOT_HISTORY_SIZE;
      }
      i--;
    } while (i != currentRootIndex);
    return false;
  }

  /**
    @dev Returns the last root
  */
  function getLastRoot() public view returns (bytes32) {
    return roots[currentRootIndex];
  }
}