// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import { IOwnable } from '../interfaces/IOwnable.sol';

/**
 * @title Ownable
 * @notice A contract module which provides a basic access control mechanism, where
 * there is an account (an owner) that can be granted exclusive access to
 * specific functions.
 *
 * The owner account is set through ownership transfer. This module makes
 * it possible to transfer the ownership of the contract to a new account in one
 * step, as well as to an interim pending owner. In the second flow the ownership does not
 * change until the pending owner accepts the ownership transfer.
 */
abstract contract Ownable is IOwnable {
    // keccak256('owner')
    bytes32 internal constant _OWNER_SLOT = 0x02016836a56b71f0d02689e69e326f4f4c1b9057164ef592671cf0d37c8040c0;
    // keccak256('ownership-transfer')
    bytes32 internal constant _OWNERSHIP_TRANSFER_SLOT =
        0x9855384122b55936fbfb8ca5120e63c6537a1ac40caf6ae33502b3c5da8c87d1;

    /**
     * @notice Initializes the contract by transferring ownership to the owner parameter.
     * @param _owner Address to set as the initial owner of the contract
     */
    constructor(address _owner) {
        _transferOwnership(_owner);
    }

    /**
     * @notice Modifier that throws an error if called by any account other than the owner.
     */
    modifier onlyOwner() {
        if (owner() != msg.sender) revert NotOwner();

        _;
    }

    /**
     * @notice Returns the current owner of the contract.
     * @return owner_ The current owner of the contract
     */
    function owner() public view returns (address owner_) {
        assembly {
            owner_ := sload(_OWNER_SLOT)
        }
    }

    /**
     * @notice Returns the pending owner of the contract.
     * @return owner_ The pending owner of the contract
     */
    function pendingOwner() public view returns (address owner_) {
        assembly {
            owner_ := sload(_OWNERSHIP_TRANSFER_SLOT)
        }
    }

    /**
     * @notice Transfers ownership of the contract to a new account `newOwner`.
     * @dev Can only be called by the current owner.
     * @param newOwner The address to transfer ownership to
     */
    function transferOwnership(address newOwner) external virtual onlyOwner {
        _transferOwnership(newOwner);
    }

    /**
     * @notice Propose to transfer ownership of the contract to a new account `newOwner`.
     * @dev Can only be called by the current owner. The ownership does not change
     * until the new owner accepts the ownership transfer.
     * @param newOwner The address to transfer ownership to
     */
    function proposeOwnership(address newOwner) external virtual onlyOwner {
        if (newOwner == address(0)) revert InvalidOwnerAddress();

        emit OwnershipTransferStarted(newOwner);

        assembly {
            sstore(_OWNERSHIP_TRANSFER_SLOT, newOwner)
        }
    }

    /**
     * @notice Accepts ownership of the contract.
     * @dev Can only be called by the pending owner
     */
    function acceptOwnership() external virtual {
        address newOwner = pendingOwner();
        if (newOwner != msg.sender) revert InvalidOwner();

        _transferOwnership(newOwner);
    }

    /**
     * @notice Internal function to transfer ownership of the contract to a new account `newOwner`.
     * @dev Called in the constructor to set the initial owner.
     * @param newOwner The address to transfer ownership to
     */
    function _transferOwnership(address newOwner) internal virtual {
        if (newOwner == address(0)) revert InvalidOwnerAddress();

        emit OwnershipTransferred(newOwner);

        assembly {
            sstore(_OWNER_SLOT, newOwner)
            sstore(_OWNERSHIP_TRANSFER_SLOT, 0)
        }
    }
}