// File: ERC1132.sol



pragma solidity 0.5.2;


/**
 * @title ERC1132 interface
 * @dev see https://github.com/ethereum/EIPs/issues/1132
 */

contract ERC1132 {
    /**
     * @dev Reasons why a user's tokens have been locked
     */
    mapping(address => bytes32[]) public lockReason;

    /**
     * @dev locked token structure
     */
    struct lockToken {
        uint256 amount;
        uint256 validity;
        bool claimed;
    }

    /**
     * @dev Holds number & validity of tokens locked for a given reason for
     *      a specified address
     */
    mapping(address => mapping(bytes32 => lockToken)) public locked;

    /**
     * @dev Records data of all the tokens Locked
     */
    event Locked(
        address indexed _of,
        bytes32 indexed _reason,
        uint256 _amount,
        uint256 _validity
    );

    /**
     * @dev Records data of all the tokens unlocked
     */
    event Unlocked(
        address indexed _of,
        bytes32 indexed _reason,
        uint256 _amount
    );


 function getListLockedAddrs()
        public view returns( address  [] memory);
        
        function getListResons()
        public view returns(bytes32 [] memory);
        
        function getAddrByReason(bytes32 my_reason) 
        public view returns(address);

    
    /**
     * @dev Locks a specified amount of tokens against an address,
     *      for a specified reason and time
     * @param _reason The reason to lock tokens
     * @param _amount Number of tokens to be locked
     * @param _time Lock time in seconds
     */
    function lock(bytes32 _reason, uint256 _amount, uint256 _time)
        public returns (bool);
  
    /**
     * @dev Returns tokens locked for a specified address for a
     *      specified reason
     *
     * @param _of The address whose tokens are locked
     * @param _reason The reason to query the lock tokens for
     */
    function tokensLocked(address _of, bytes32 _reason)
        public view returns (uint256 amount);
    
    /**
     * @dev Returns tokens locked for a specified address for a
     *      specified reason at a specific time
     *
     * @param _of The address whose tokens are locked
     * @param _reason The reason to query the lock tokens for
     * @param _time The timestamp to query the lock tokens for
     */
    function tokensLockedAtTime(address _of, bytes32 _reason, uint256 _time)
        public view returns (uint256 amount);
    
    /**
     * @dev Returns total tokens held by an address (locked + transferable)
     * @param _of The address to query the total balance of
     */
    function totalBalanceOf(address _of)
        public view returns (uint256 amount);
    
    /**
     * @dev Extends lock for a specified reason and time
     * @param _reason The reason to lock tokens
     * @param _time Lock extension time in seconds
     */
    function extendLock(bytes32 _reason, uint256 _time)
        public returns (bool);
    
    /**
     * @dev Increase number of tokens locked for a specified reason
     * @param _reason The reason to lock tokens
     * @param _amount Number of tokens to be increased
     */
    function increaseLockAmount(bytes32 _reason, uint256 _amount)
        public returns (bool);

    /**
     * @dev Returns unlockable tokens for a specified address for a specified reason
     * @param _of The address to query the the unlockable token count of
     * @param _reason The reason to query the unlockable tokens for
     */
    function tokensUnlockable(address _of, bytes32 _reason)
        public view returns (uint256 amount);
 
    /**
     * @dev Unlocks the unlockable tokens of a specified address
     * @param _of Address of user, claiming back unlockable tokens
     */
    function unlock(address _of)
        public returns (uint256 unlockableTokens);

    /**
     * @dev Gets the unlockable tokens of a specified address
     * @param _of The address to query the the unlockable token count of
     */
    function getUnlockableTokens(address _of)
        public view returns (uint256 unlockableTokens);

}

// File: ERC20.sol



pragma solidity 0.5.2;

import "./IERC20.sol";
import "./SafeMath.sol";

/**
 * @dev Implementation of the `IERC20` interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using `_mint`.
 * For a generic mechanism see `ERC20Mintable`.
 *
 * *For a detailed writeup see our guide [How to implement supply
 * mechanisms](https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226).*
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an `Approval` event is emitted on calls to `transferFrom`.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard `decreaseAllowance` and `increaseAllowance`
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See `IERC20.approve`.
 */
contract ERC20 is IERC20 {
    using SafeMath for uint256;

    mapping (address => uint256) private _balances;

    mapping (address => mapping (address => uint256)) private _allowances;

    uint256 private _totalSupply;

    /**
     * @dev See `IERC20.totalSupply`.
     */
    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See `IERC20.balanceOf`.
     */
    function balanceOf(address account) public view returns (uint256) {
        return _balances[account];
    }

    /**
     * @dev See `IERC20.transfer`.
     *
     * Requirements:
     *
     * - `recipient` cannot be the zero address.
     * - the caller must have a balance of at least `amount`.
     */
    function transfer(address recipient, uint256 amount) public returns (bool) {
        _transfer(msg.sender, recipient, amount);
        return true;
    }

    /**
     * @dev See `IERC20.allowance`.
     */
    function allowance(address owner, address spender) public view returns (uint256) {
        return _allowances[owner][spender];
    }

    /**
     * @dev See `IERC20.approve`.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function approve(address spender, uint256 value) public returns (bool) {
        _approve(msg.sender, spender, value);
        return true;
    }

    /**
     * @dev See `IERC20.transferFrom`.
     *
     * Emits an `Approval` event indicating the updated allowance. This is not
     * required by the EIP. See the note at the beginning of `ERC20`;
     *
     * Requirements:
     * - `sender` and `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `value`.
     * - the caller must have allowance for `sender`'s tokens of at least
     * `amount`.
     */
    function transferFrom(address sender, address recipient, uint256 amount) public returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, msg.sender, _allowances[sender][msg.sender].sub(amount));
        return true;
    }

    /**
     * @dev Atomically increases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to `approve` that can be used as a mitigation for
     * problems described in `IERC20.approve`.
     *
     * Emits an `Approval` event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
        _approve(msg.sender, spender, _allowances[msg.sender][spender].add(addedValue));
        return true;
    }

    /**
     * @dev Atomically decreases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to `approve` that can be used as a mitigation for
     * problems described in `IERC20.approve`.
     *
     * Emits an `Approval` event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `spender` must have allowance for the caller of at least
     * `subtractedValue`.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
        _approve(msg.sender, spender, _allowances[msg.sender][spender].sub(subtractedValue));
        return true;
    }

    /**
     * @dev Moves tokens `amount` from `sender` to `recipient`.
     *
     * This is internal function is equivalent to `transfer`, and can be used to
     * e.g. implement automatic token fees, slashing mechanisms, etc.
     *
     * Emits a `Transfer` event.
     *
     * Requirements:
     *
     * - `sender` cannot be the zero address.
     * - `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     */
    function _transfer(address sender, address recipient, uint256 amount) internal {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _balances[sender] = _balances[sender].sub(amount);
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }

    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
     * the total supply.
     *
     * Emits a `Transfer` event with `from` set to the zero address.
     *
     * Requirements
     *
     * - `to` cannot be the zero address.
     */
    function _mint(address account, uint256 amount) internal {
        require(account != address(0), "ERC20: mint to the zero address");

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }

     /**
     * @dev Destoys `amount` tokens from `account`, reducing the
     * total supply.
     *
     * Emits a `Transfer` event with `to` set to the zero address.
     *
     * Requirements
     *
     * - `account` cannot be the zero address.
     * - `account` must have at least `amount` tokens.
     */
    function _burn(address account, uint256 value) internal {
        require(account != address(0), "ERC20: burn from the zero address");

        _totalSupply = _totalSupply.sub(value);
        _balances[account] = _balances[account].sub(value);
        emit Transfer(account, address(0), value);
    }

    /**
     * @dev Sets `amount` as the allowance of `spender` over the `owner`s tokens.
     *
     * This is internal function is equivalent to `approve`, and can be used to
     * e.g. set automatic allowances for certain subsystems, etc.
     *
     * Emits an `Approval` event.
     *
     * Requirements:
     *
     * - `owner` cannot be the zero address.
     * - `spender` cannot be the zero address.
     */
    function _approve(address owner, address spender, uint256 value) internal {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = value;
        emit Approval(owner, spender, value);
    }

    /**
     * @dev Destoys `amount` tokens from `account`.`amount` is then deducted
     * from the caller's allowance.
     *
     * See `_burn` and `_approve`.
     */
    function _burnFrom(address account, uint256 amount) internal {
        _burn(account, amount);
        _approve(account, msg.sender, _allowances[account][msg.sender].sub(amount));
    }
}

// File: ERC20Detailed.sol



pragma solidity 0.5.2;

import "./IERC20.sol";

/**
 * @dev Optional functions from the ERC20 standard.
 */
contract ERC20Detailed is IERC20 {
    string private _name;
    string private _symbol;
    uint8 private _decimals;

    /**
     * @dev Sets the values for `name`, `symbol`, and `decimals`. All three of
     * these values are immutable: they can only be set once during
     * construction.
     */
    constructor (string memory name, string memory symbol, uint8 decimals) public {
        _name = name;
        _symbol = symbol;
        _decimals = decimals;
    }

    /**
     * @dev Returns the name of the token.
     */
    function name() public view returns (string memory) {
        return _name;
    }

    /**
     * @dev Returns the symbol of the token, usually a shorter version of the
     * name.
     */
    function symbol() public view returns (string memory) {
        return _symbol;
    }

    /**
     * @dev Returns the number of decimals used to get its user representation.
     * For example, if `decimals` equals `2`, a balance of `505` tokens should
     * be displayed to a user as `5,05` (`505 / 10 ** 2`).
     *
     * Tokens usually opt for a value of 18, imitating the relationship between
     * Ether and Wei.
     *
     * > Note that this information is only used for _display_ purposes: it in
     * no way affects any of the arithmetic of the contract, including
     * `IERC20.balanceOf` and `IERC20.transfer`.
     */
    function decimals() public view returns (uint8) {
        return _decimals;
    }
}

// File: IERC20.sol



pragma solidity 0.5.2;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP. Does not include
 * the optional functions; to access them see `ERC20Detailed`.
 */
interface IERC20 {
    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a `Transfer` event.
     */
    function transfer(address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through `transferFrom`. This is
     * zero by default.
     *
     * This value changes when `approve` or `transferFrom` are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * > Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an `Approval` event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `sender` to `recipient` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a `Transfer` event.
     */
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to `approve`. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

// File: Migrations.sol



pragma solidity 0.5.2;

contract Migrations {
  address public owner;
  uint public last_completed_migration;

  constructor() public {
    owner = msg.sender;
  }

  modifier restricted() {
    if (msg.sender == owner) _;
  }

  function setCompleted(uint completed) public restricted {
    last_completed_migration = completed;
  }

  function upgrade(address new_address) public restricted {
    Migrations upgraded = Migrations(new_address);
    upgraded.setCompleted(last_completed_migration);
  }
}

// File: OBSR.sol



pragma solidity 0.5.2;


import './ERC1132.sol';
import './ERC20.sol';
import './ERC20Detailed.sol';




/**
 * @title SimpleToken
 * @dev Very simple ERC20 Token example, where all tokens are pre-assigned to the creator.
 * Note they can later distribute these tokens as they wish using `transfer` and other
 * `ERC20` functions.
 */
contract OBSR is  ERC1132, ERC20, ERC20Detailed {
    uint8 public constant DECIMALS = 8;
    uint256 public constant INITIAL_SUPPLY = 1500000000000000000;

/**
    * @dev Error messages for require statements
    */


    string internal constant ALREADY_LOCKED = 'Tokens already locked';
    string internal constant NOT_LOCKED = 'No tokens locked';
    string internal constant AMOUNT_ZERO = 'Amount can not be 0';

    address[] lockedAddrs;
    bytes32[] reasons;
    
   

    /**
     * @dev Constructor that gives msg.sender all of existing tokens.
     */

    constructor () public ERC20Detailed("OBSR", "OBSR", DECIMALS) {
        _mint(msg.sender, INITIAL_SUPPLY);
    }

function getListLockedAddrs()public view returns( address  [] memory)
{
     return lockedAddrs;
}
 

function getListResons()public view returns(bytes32 [] memory)
{
     return reasons;
}


//returns addrlocked by reason





function getAddrByReason(bytes32 my_reason) public view returns(address)
{
    for (uint256 j = 0; j < lockedAddrs.length; j++)
        {
             uint256 amountLocked =   tokensLockedAtTime(lockedAddrs[j],my_reason,now);
             
             if(amountLocked!=0)
             {
                return   lockedAddrs[j];     
             }
        }
}

function tokensValidityLockedAtTime(address _of, bytes32 _reason, uint256 _time)
        public
        view
        returns (uint256 validity)
    {
        if (locked[_of][_reason].validity > _time)
            validity = locked[_of][_reason].validity;
    }




/**
     * @dev Locks a specified amount of tokens against an address,
     *      for a specified reason and time
     * @param _reason The reason to lock tokens
     * @param _amount Number of tokens to be locked
     * @param _time Lock time in seconds
     */
     
 
     
    function lock(bytes32 _reason, uint256 _amount, uint256 _time)
        public
        returns (bool)
    {
        uint256 validUntil = now.add(_time); //solhint-disable-line

        // If tokens are already locked, then functions extendLock or
        // increaseLockAmount should be used to make any changes
        require(tokensLocked(msg.sender, _reason) == 0, ALREADY_LOCKED);
        require(_amount != 0, AMOUNT_ZERO);

        if (locked[msg.sender][_reason].amount == 0)
            lockReason[msg.sender].push(_reason);

        transfer(address(this), _amount);

        locked[msg.sender][_reason] = lockToken(_amount, validUntil, false);
        
        lockedAddrs.push(msg.sender);
        
        
        
        reasons.push(_reason);
        
        emit Locked(msg.sender, _reason, _amount, validUntil);
        return true;
    }
    
    /**
     * @dev Transfers and Locks a specified amount of tokens,
     *      for a specified reason and time
     * @param _to adress to which tokens are to be transfered
     * @param _reason The reason to lock tokens
     * @param _amount Number of tokens to be transfered and locked
     * @param _time Lock time in seconds
     */
    function transferWithLock(address _to, bytes32 _reason, uint256 _amount, uint256 _time)
        public
        returns (bool)
    {
        uint256 validUntil = now.add(_time); //solhint-disable-line

        require(tokensLocked(_to, _reason) == 0, ALREADY_LOCKED);
        require(_amount != 0, AMOUNT_ZERO);

        if (locked[_to][_reason].amount == 0)
            lockReason[_to].push(_reason);

        transfer(address(this), _amount);

        locked[_to][_reason] = lockToken(_amount, validUntil, false);
        
        emit Locked(_to, _reason, _amount, validUntil);

        lockedAddrs.push(_to);
        reasons.push(_reason);

        return true;
    }

    /**
     * @dev Returns tokens locked for a specified address for a
     *      specified reason
     *
     * @param _of The address whose tokens are locked
     * @param _reason The reason to query the lock tokens for
     */
    function tokensLocked(address _of, bytes32 _reason)
        public
        view
        returns (uint256 amount)
    {
        if (!locked[_of][_reason].claimed)
            amount = locked[_of][_reason].amount;
    }
    
    /**
     * @dev Returns tokens locked for a specified address for a
     *      specified reason at a specific time
     *
     * @param _of The address whose tokens are locked
     * @param _reason The reason to query the lock tokens for
     * @param _time The timestamp to query the lock tokens for
     */
    function tokensLockedAtTime(address _of, bytes32 _reason, uint256 _time)
        public
        view
        returns (uint256 amount)
    {
        if (locked[_of][_reason].validity > _time)
            amount = locked[_of][_reason].amount;
    }

    /**
     * @dev Returns total tokens held by an address (locked + transferable)
     * @param _of The address to query the total balance of
     */
    function totalBalanceOf(address _of)
        public
        view
        returns (uint256 amount)
    {
        amount = balanceOf(_of);

        for (uint256 i = 0; i < lockReason[_of].length; i++) {
            amount = amount.add(tokensLocked(_of, lockReason[_of][i]));
        }   
    }    
    
    /**
     * @dev Extends lock for a specified reason and time
     * @param _reason The reason to lock tokens
     * @param _time Lock extension time in seconds
     */
    function extendLock(bytes32 _reason, uint256 _time)
        public
        returns (bool)
    {
        require(tokensLocked(msg.sender, _reason) > 0, NOT_LOCKED);

        locked[msg.sender][_reason].validity = locked[msg.sender][_reason].validity.add(_time);

        emit Locked(msg.sender, _reason, locked[msg.sender][_reason].amount, locked[msg.sender][_reason].validity);
        return true;
    }
    
    /**
     * @dev Increase number of tokens locked for a specified reason
     * @param _reason The reason to lock tokens
     * @param _amount Number of tokens to be increased
     */
    function increaseLockAmount(bytes32 _reason, uint256 _amount)
        public
        returns (bool)
    {
        require(tokensLocked(msg.sender, _reason) > 0, NOT_LOCKED);
        transfer(address(this), _amount);

        locked[msg.sender][_reason].amount = locked[msg.sender][_reason].amount.add(_amount);

        emit Locked(msg.sender, _reason, locked[msg.sender][_reason].amount, locked[msg.sender][_reason].validity);
        return true;
    }

    /**
     * @dev Returns unlockable tokens for a specified address for a specified reason
     * @param _of The address to query the the unlockable token count of
     * @param _reason The reason to query the unlockable tokens for
     */
    function tokensUnlockable(address _of, bytes32 _reason)
        public
        view
        returns (uint256 amount)
    {
        if (locked[_of][_reason].validity <= now && !locked[_of][_reason].claimed) //solhint-disable-line
            amount = locked[_of][_reason].amount;
    }

    /**
     * @dev Unlocks the unlockable tokens of a specified address
     * @param _of Address of user, claiming back unlockable tokens
     */
    function unlock(address _of)
        public
        returns (uint256 unlockableTokens)
    {
        uint256 lockedTokens;

        for (uint256 i = 0; i < lockReason[_of].length; i++) {
            lockedTokens = tokensUnlockable(_of, lockReason[_of][i]);
            if (lockedTokens > 0) {
                unlockableTokens = unlockableTokens.add(lockedTokens);
                locked[_of][lockReason[_of][i]].claimed = true;
                emit Unlocked(_of, lockReason[_of][i], lockedTokens);
		 for (uint256 k = 0; k < reasons.length; k++) {
                if(lockReason[_of][i]==reasons[k])
                 delete reasons[k];
               }    
            }
        }  

        if (unlockableTokens > 0)
            this.transfer(_of, unlockableTokens);
    }

    /**
     * @dev Gets the unlockable tokens of a specified address
     * @param _of The address to query the the unlockable token count of
     */
    function getUnlockableTokens(address _of)
        public
        view
        returns (uint256 unlockableTokens)
    {
        for (uint256 i = 0; i < lockReason[_of].length; i++) {
            unlockableTokens = unlockableTokens.add(tokensUnlockable(_of, lockReason[_of][i]));
        }  
    }

}



/*

function append(string memory a, string memory b, string memory c) internal pure returns (string memory ) {

    return string(abi.encodePacked(a, b, c));

}


function toString(address x) internal pure returns (string memory) {
        bytes memory b = new bytes(20);
        for (uint i = 0; i < 20; i++)
            b[i] = byte(uint8(uint(x) / (2**(8*(19 - i)))));
        return string(b);
    }


function uint2str(uint _i) internal pure returns (string memory _uintAsString) {
    if (_i == 0) {
        return "0";
    }
    uint j = _i;
    uint len;
    while (j != 0) {
        len++;
        j /= 10;
    }
    bytes memory bstr = new bytes(len);
    uint k = len - 1;
    while (_i != 0) {
        bstr[k--] = byte(uint8(48 + _i % 10));
        _i /= 10;
    }
    return string(bstr);
}
*/


/*
   bytes32[]  bts;

function getInfoByReason(bytes32 my_reason) public   returns(bytes32[] memory)
{
      
       bts =0;
        for (uint256 j = 0; j < lockedAddrs.length; j++)
        {
             uint256 amountLocked =   tokensLockedAtTime(lockedAddrs[j],my_reason,now);
             
             if(amountLocked!=0)
             {
                    //  string memory abcde = new string(_ba.length + _ 
               //  return  append(toString(lockedAddrs[j]), "", uint2str(amountLocked));
              
                bts.push(bytes32(uint256(msg.sender)));
                bts.push(bytes32(amountLocked));
               
               return bts;
               // return "123";
             }
              
          
        }
        
        
 

}

*/






// File: SafeMath.sol



pragma solidity 0.5.2;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a, "SafeMath: subtraction overflow");
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        // Solidity only automatically asserts when dividing by 0
        require(b > 0, "SafeMath: division by zero");
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b != 0, "SafeMath: modulo by zero");
        return a % b;
    }
}