/*

    Copyright 2020 UBI.city
    SPDX-License-Identifier: Apache-2.0

*/
pragma solidity 0.6.9;
pragma experimental ABIEncoderV2;


/**
 * @title SafeMath
 * @author CITYMAN
 *
 * @notice Math operations with safety checks that revert on error
 */
library SafeMath {
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "MUL_ERROR");

        return c;
    }

    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b > 0, "DIVIDING_ERROR");
        return a / b;
    }

    function divCeil(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 quotient = div(a, b);
        uint256 remainder = a - quotient * b;
        if (remainder > 0) {
            return quotient + 1;
        } else {
            return quotient;
        }
    }

    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a, "SUB_ERROR");
        return a - b;
    }

    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "ADD_ERROR");
        return c;
    }

    function sqrt(uint256 x) internal pure returns (uint256 y) {
        uint256 z = x / 2 + 1;
        y = x;
        while (z < y) {
            y = z;
            z = (x / z + z) / 2;
        }
    }
}



/**
 * @title UBI.city Token
 * @author CITYMAN
 */
contract UBIcityERC20 {
    using SafeMath for uint256;

    string public symbol = "CITY";
    string public name = "UBICITY";

    uint256 public decimals = 18;
    uint256 public totalSupply = 100000000 * 10**18; // 100 million

    mapping(address => uint256) internal balances;
    mapping(address => mapping(address => uint256)) internal allowed;

    // ============ Events ============

    event Transfer(address indexed from, address indexed to, uint256 amount);

    event Approval(address indexed owner, address indexed spender, uint256 amount);

    // ============ Functions ============

    constructor() public {
        balances[msg.sender] = totalSupply;
    }

    /**
     * @dev transfer token for a specified address
     * @param to The address to transfer to.
     * @param amount The amount to be transferred.
     */
    function transfer(address to, uint256 amount) public returns (bool) {
        require(amount <= balances[msg.sender], "BALANCE_NOT_ENOUGH");

        balances[msg.sender] = balances[msg.sender].sub(amount);
        balances[to] = balances[to].add(amount);
        emit Transfer(msg.sender, to, amount);
        return true;
    }

    /**
     * @dev Gets the balance of the specified address.
     * @param owner The address to query the the balance of.
     * @return balance An uint256 representing the amount owned by the passed address.
     */
    function balanceOf(address owner) external view returns (uint256 balance) {
        return balances[owner];
    }

    /**
     * @dev Transfer tokens from one address to another
     * @param from address The address which you want to send tokens from
     * @param to address The address which you want to transfer to
     * @param amount uint256 the amount of tokens to be transferred
     */
    function transferFrom(
        address from,
        address to,
        uint256 amount
    ) public returns (bool) {
        require(amount <= balances[from], "BALANCE_NOT_ENOUGH");
        require(amount <= allowed[from][msg.sender], "ALLOWANCE_NOT_ENOUGH");

        balances[from] = balances[from].sub(amount);
        balances[to] = balances[to].add(amount);
        allowed[from][msg.sender] = allowed[from][msg.sender].sub(amount);
        emit Transfer(from, to, amount);
        return true;
    }

    /**
     * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
     * @param spender The address which will spend the funds.
     * @param amount The amount of tokens to be spent.
     */
    function approve(address spender, uint256 amount) public returns (bool) {
        allowed[msg.sender][spender] = amount;
        emit Approval(msg.sender, spender, amount);
        return true;
    }

    /**
     * @dev Function to check the amount of tokens that an owner allowed to a spender.
     * @param owner address The address which owns the funds.
     * @param spender address The address which will spend the funds.
     * @return A uint256 specifying the amount of tokens still available for the spender.
     */
    function allowance(address owner, address spender) public view returns (uint256) {
        return allowed[owner][spender];
    }
}