// SPDX-License-Identifier: agpl-3.0
pragma solidity 0.6.12;

/**
 * @title VersionedInitializable
 *
 * @dev Helper contract to implement versioned initializer functions. To use it, replace
 * the constructor with a function that has the `initializer` or `initializerRunAlways` modifier.
 * The revision number should be defined as a private constant, returned by getRevision() and used by initializer() modifier.
 *
 * WARNING: Unlike constructors, initializer functions must be manually
 * invoked. This applies both to deploying an Initializable contract, as well
 * as extending an Initializable contract via inheritance.
 *
 * ATTN: When used with inheritance, parent initializers with `initializer` modifier are prevented by calling twice,
 * but can only be called in child-to-parent sequence.
 *
 * WARNING: When used with inheritance, parent initializers with `initializerRunAlways` modifier
 * are NOT protected from multiple calls by another initializer.
 */
abstract contract VersionedInitializable {
  /**
   * @dev Indicates that the contract has been initialized.
   */
  uint256 private lastInitializedRevision = 0;

  /**
   * @dev Indicates that the contract is in the process of being initialized.
   */
  uint256 private lastInitializingRevision = 0;

  /**
   * @dev Modifier to use in the initializer function of a contract.
   */
  modifier initializer(uint256 localRevision) {
    uint256 topRevision = getRevision();
    (bool initializing, bool skip) = _preInitializer(localRevision, topRevision);

    if (!skip) {
      lastInitializingRevision = localRevision;
      _;
      lastInitializedRevision = localRevision;
    }

    if (!initializing) {
      lastInitializedRevision = topRevision;
      lastInitializingRevision = 0;
    }
  }

  modifier initializerRunAlways(uint256 localRevision) {
    uint256 topRevision = getRevision();
    (bool initializing, bool skip) = _preInitializer(localRevision, topRevision);
    _;

    if (!skip) {
      lastInitializingRevision = localRevision;
    }
    _;
    if (!skip) {
      lastInitializedRevision = localRevision;
    }

    if (!initializing) {
      lastInitializedRevision = topRevision;
      lastInitializingRevision = 0;
    }
  }

  function _preInitializer(uint256 localRevision, uint256 topRevision)
    private
    returns (bool initializing, bool skip)
  {
    require(localRevision > 0, 'incorrect initializer revision');
    require(localRevision <= topRevision, 'incorrect contract revision');

    initializing = lastInitializingRevision > 0 && lastInitializedRevision < topRevision;
    require(
      initializing || isConstructor() || topRevision > lastInitializedRevision,
      'Contract instance has already been initialized'
    );

    if (initializing) {
      require(lastInitializingRevision > localRevision, 'incorrect order of calls to initializers');
    }

    if (localRevision <= lastInitializedRevision) {
      // prevent calling of parent's initializer when it was called before
      if (initializing) {
        // Can't set zero yet, as it is not a top-level call, otherwise "initializing" will become false.
        // Further calls will fail with the 'incorrect order' assertion above.
        lastInitializingRevision = 1;
      }
      return (initializing, true);
    }
    return (initializing, false);
  }

  function isRevisionInitialized(uint256 localRevision) internal view returns (bool) {
    return lastInitializedRevision >= localRevision;
  }

  function REVISION() public pure returns (uint256) {
    return getRevision();
  }

  /**
   * @dev returns the revision number of the contract
   * Needs to be defined in the inherited class as a constant.
   **/
  function getRevision() internal pure virtual returns (uint256);

  /**
   * @dev Returns true if and only if the function is running in the constructor
   **/
  function isConstructor() private view returns (bool) {
    // extcodesize checks the size of the code stored in an address, and
    // address returns the current address. Since the code is still not
    // deployed when running a constructor, any checks on its code size will
    // yield zero, making it an effective way to detect if a contract is
    // under construction or not.
    uint256 cs;
    //solium-disable-next-line
    assembly {
      cs := extcodesize(address())
    }
    return cs == 0;
  }

  // Reserved storage space to allow for layout changes in the future.
  uint256[4] private ______gap;
}