pragma solidity ^0.6.0;

// SPDX-License-Identifier: UNLICENSED

/**
 * @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) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        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-contracts/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) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message 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, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        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) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message 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, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
    
    function ceil(uint256 a, uint256 m) internal pure returns (uint256 r) {
        require(m != 0, "SafeMath: to ceil number shall not be zero");
        return (a + m - 1) / m * m;
    }
}


// ----------------------------------------------------------------------------
// ERC Token Standard #20 Interface
// https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20-token-standard.md
// ----------------------------------------------------------------------------
/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
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.
     *
     * IMPORTANT: 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);
}

// ----------------------------------------------------------------------------
// Owned contract
// ----------------------------------------------------------------------------
contract Owned {
    address payable public owner;

    event OwnershipTransferred(address indexed _from, address indexed _to);

    constructor() public {
        owner = msg.sender;
    }

    modifier onlyOwner {
        require(msg.sender == owner, "only allowed by owner");
        _;
    }

    function transferOwnership(address payable _newOwner) public onlyOwner {
        owner = _newOwner;
        emit OwnershipTransferred(msg.sender, _newOwner);
    }
}

// ----------------------------------------------------------------------------
// 'Gundam' token contract

// Symbol      : Gundam
// Name        : GUNDAM SEED
// Total supply: 1,000,000,000
// Decimals    : 18

// ----------------------------------------------------------------------------
// ERC20 Token, with the addition of symbol, name and decimals and assisted
// token transfers
// ----------------------------------------------------------------------------
contract GundamSeed is IERC20, Owned {
    using SafeMath for uint256;
   
    string public symbol = "Gundam";
    string public  name = "GUNDAM SEED";
    uint256 public decimals = 18;
    uint256 _totalSupply = 1000000000 * 10 ** (18); // 1,000,000,000 
    
    mapping(address => uint256) balances;
    mapping(address => mapping(address => uint256)) allowed;
    
    uint256 preSaleStart = 1605128400; // 11-nov-2020 9pm gmt
    uint256 preSaleEnd = 1605560400; // 16-nov-2020 9pm gmt
    uint256 tokenRatePerEth = 1250000 ; // 1 Ethereum = 1,250,000  GUNDAM Tokens
    
    modifier saleOpen{
        require(block.timestamp >= preSaleStart && block.timestamp <= preSaleEnd, "sale is close");
        _;
    }

    // ------------------------------------------------------------------------
    // Constructor
    // ------------------------------------------------------------------------
    constructor() public {
        
        owner = 0xeB557d22bdFfBe0588bb9632A53883F6E011A15D;
        
        balances[address(owner)] =   5e8  * 10 ** (18); // 5,00,000,000 (500 million)
        emit Transfer(address(0), address(owner), 5e8  * 10 ** (18));
        
        // keep 500 million inside contract for presale
        balances[address(this)] =   5e8  * 10 ** (18); // 5,00,000,000 (500 million)
        emit Transfer(address(0), address(this), 5e8  * 10 ** (18));
    }

   
    /** ERC20Interface function's implementation **/
    
    // ------------------------------------------------------------------------
    // Get the total supply of the token
    // ------------------------------------------------------------------------
    function totalSupply() external override view returns (uint256){
       return _totalSupply;
    }
   
    // ------------------------------------------------------------------------
    // Get the token balance for account `tokenOwner`
    // ------------------------------------------------------------------------
    function balanceOf(address tokenOwner) external override view returns (uint256 balance) {
        return balances[tokenOwner];
    }
    
    // ------------------------------------------------------------------------
    // Token owner can approve for `spender` to transferFrom(...) `tokens`
    // from the token owner's account
    // ------------------------------------------------------------------------
    function approve(address spender, uint256 tokens) external override returns (bool success){
        allowed[msg.sender][spender] = tokens;
        emit Approval(msg.sender,spender,tokens);
        return true;
    }
    
    // ------------------------------------------------------------------------
    // Returns the amount of tokens approved by the owner that can be
    // transferred to the spender's account
    // ------------------------------------------------------------------------
    function allowance(address tokenOwner, address spender) external override view returns (uint256 remaining) {
        return allowed[tokenOwner][spender];
    }

    // ------------------------------------------------------------------------
    // Transfer the balance from token owner's account to `to` account
    // - Owner's account must have sufficient balance to transfer
    // - 0 value transfers are allowed
    // ------------------------------------------------------------------------
    function transfer(address to, uint256 tokens) public override returns (bool success) {
        // prevent transfer to 0x0, use burn instead
        require(address(to) != address(0));
        require(balances[msg.sender] >= tokens );
        require(balances[to] + tokens >= balances[to]);
        
        balances[msg.sender] = balances[msg.sender].sub(tokens);
        
        balances[to] = balances[to].add(tokens);
        emit Transfer(msg.sender, to, tokens);
        return true;
    }
    
    // ------------------------------------------------------------------------
    // Transfer `tokens` from the `from` account to the `to` account
    //
    // The calling account must already have sufficient tokens approve(...)-d
    // for spending from the `from` account and
    // - From account must have sufficient balance to transfer
    // - Spender must have sufficient allowance to transfer
    // - 0 value transfers are allowed
    // ------------------------------------------------------------------------
    function transferFrom(address from, address to, uint256 tokens) external override returns (bool success){
        require(tokens <= allowed[from][msg.sender]); //check allowance
        require(balances[from] >= tokens);
        balances[from] = balances[from].sub(tokens);
        allowed[from][msg.sender] = allowed[from][msg.sender].sub(tokens);
        balances[to] = balances[to].add(tokens);
        emit Transfer(from, to, tokens);
        return true;
    }
    
    // ------------------------------------------------------------------------
    // Transfer `tokens` from the `from` contract to the `to` account
    // ------------------------------------------------------------------------
    function _transfer(address to, uint256 tokens) private returns(bool){
        // prevent transfer to 0x0, use burn instead
        require(address(to) != address(0));
        require(balances[address(this)] >= tokens );
        
        balances[address(this)] = balances[address(this)].sub(tokens);
        balances[to] = balances[to].add(tokens);
        
        emit Transfer(address(this),to,tokens);
        return true;
    }
    
    // ------------------------------------------------------------------------
    // Get ethers to buy tokens during pre-sale
    // ------------------------------------------------------------------------
    receive() external payable saleOpen{
        
        uint256 tokens = getTokenAmount(msg.value);
        
        require(_transfer(msg.sender, tokens), "Insufficient balance of sale contract!");
        
        // send received funds to the owner
        owner.transfer(msg.value);
    }
    
    // ------------------------------------------------------------------------
    // Calculate tokens based on ethers sent
    // ------------------------------------------------------------------------
    function getTokenAmount(uint256 amount) private view returns(uint256){
        return (amount.mul(tokenRatePerEth));
    }
    
    // ------------------------------------------------------------------------
    // Send the unsold tokens back to owner
    // ------------------------------------------------------------------------
    function getUnSoldTokens() external onlyOwner{
        require(block.timestamp > preSaleEnd, "Sale is not close yet");
        require(_transfer(msg.sender, balances[address(this)]), "Insufficient balance of sale contract!");
    }
}