// SPDX-License-Identifier: MIT
pragma solidity ^0.7.0;

/**
 * @title The ConfirmedOwner contract
 * @notice A contract with helpers for basic contract ownership.
 */
contract ConfirmedOwner {

  address private s_owner;
  address private s_pendingOwner;

  event OwnershipTransferRequested(
    address indexed from,
    address indexed to
  );
  event OwnershipTransferred(
    address indexed from,
    address indexed to
  );

  constructor(address newOwner) {
    s_owner = newOwner;
  }

  /**
   * @notice Allows an owner to begin transferring ownership to a new address,
   * pending.
   */
  function transferOwnership(address to)
    external
    onlyOwner()
  {
    require(to != msg.sender, "Cannot transfer to self");

    s_pendingOwner = to;

    emit OwnershipTransferRequested(s_owner, to);
  }

  /**
   * @notice Allows an ownership transfer to be completed by the recipient.
   */
  function acceptOwnership()
    external
  {
    require(msg.sender == s_pendingOwner, "Must be proposed owner");

    address oldOwner = s_owner;
    s_owner = msg.sender;
    s_pendingOwner = address(0);

    emit OwnershipTransferred(oldOwner, msg.sender);
  }

  /**
   * @notice Get the current owner
   */
  function owner() public view returns (address) {
    return s_owner;
  }

  /**
   * @notice Reverts if called by anyone other than the contract owner.
   */
  modifier onlyOwner() {
    require(msg.sender == s_owner, "Only callable by owner");
    _;
  }

}