tornado-core/contracts/ERC20Mixer.sol

// https://tornado.cash
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pragma solidity ^0.5.8;

import "./Mixer.sol";

contract ERC20Mixer is Mixer {
  address public token;

  constructor(
    IVerifier _verifier,
    uint256 _denomination,
    uint32 _merkleTreeHeight,
    address _operator,
    address _token
  ) Mixer(_verifier, _denomination, _merkleTreeHeight, _operator) public {
    token = _token;
  }

  function _processDeposit() internal {
    require(msg.value == 0, "ETH value is supposed to be 0 for ETH mixer");
    _safeErc20TransferFrom(msg.sender, address(this), denomination);
  }

  function _processWithdraw(address payable _receiver, address payable _relayer, uint256 _fee, uint256 _refund) internal {
    require(msg.value == _refund, "Incorrect refund amount received by the contract");

    _safeErc20Transfer(_receiver, denomination - _fee);
    if (_fee > 0) {
      _safeErc20Transfer(_relayer, _fee);
    }
    if (_refund > 0) {
      _receiver.transfer(_refund);
    }
  }

  function _safeErc20TransferFrom(address _from, address _to, uint256 _amount) internal {
    (bool success, bytes memory data) = token.call(abi.encodeWithSelector(0x23b872dd /* transferFrom */, _from, _to, _amount));
    require(success, "not enough allowed tokens");

    // if contract returns some data lets make sure that is `true` according to standard
    if (data.length > 0) {
      require(data.length == 32, "data length should be either 0 or 32 bytes");
      success = abi.decode(data, (bool));
      require(success, "not enough allowed tokens. Token returns false.");
    }
  }

  function _safeErc20Transfer(address _to, uint256 _amount) internal {
    (bool success, bytes memory data) = token.call(abi.encodeWithSelector(0xa9059cbb /* transfer */, _to, _amount));
    require(success, "not enough tokens");

    // if contract returns some data lets make sure that is `true` according to standard
    if (data.length > 0) {
      require(data.length == 32, "data length should be either 0 or 32 bytes");
      success = abi.decode(data, (bool));
      require(success, "not enough tokens. Token returns false.");
    }
  }
}