erc20.sol

pragma solidity ^0.5.0;

contract ERC20 {
  // Safemath
  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    require(c >= a, "SafeMath: addition overflow");

    return c;
  }

  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    require(b <= a, "SafeMath: subtraction overflow");
    uint256 c = a - b;

    return c;
  }

  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 uint256(0);
    }

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

    return c;
  }

  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    require(b > 0, "SafeMath: division by zero");
    uint256 c = a / b;
    return c;
  }

  function mod(uint256 a, uint256 b) internal pure returns (uint256) {
    require(b != 0, "SafeMath: modulo by zero");
    return a % b;
  }

  // // real contract
  mapping (address => uint256) private _balances;

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

  uint256 private _totalSupply;

  function totalSupply() public view returns (uint256) {
    return _totalSupply;
  }

  function balanceOf(address account) public view returns (uint256) {
    return _balances[account];
  }

  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] = sub(_balances[_sender], amount);
    _balances[recipient] = add(_balances[recipient], amount);
  }

  function transfer(address recipient, uint256 amount) public returns (bool) {
    _transfer(msg.sender, recipient, amount);
    return true;
  }

  function allowance(address owner, address spender) public view returns (uint256) {
    return _allowances[owner][spender];
  }

  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;
  }

  function approve(address spender, uint256 value) public returns (bool) {
    _approve(msg.sender, spender, value);
    return true;
  }

  function transferFrom(address _sender, address recipient, uint256 amount) public returns (bool) {
    _transfer(_sender, recipient, amount);
    _approve(_sender, msg.sender, sub(_allowances[_sender][msg.sender], amount));
    return true;
  }

  function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
    _approve(msg.sender, spender, add(_allowances[msg.sender][spender], addedValue));
    return true;
  }

  function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
    _approve(msg.sender, spender, sub(_allowances[msg.sender][spender], subtractedValue));
    return true;
  }

  function _mint(address account, uint256 amount) internal {
    require(account != address(0), "ERC20: mint to the zero address");

    _totalSupply = add(_totalSupply, amount);
    _balances[account] = add(_balances[account], amount);
  }

  function _burn(address account, uint256 value) internal {
    require(account != address(0), "ERC20: burn from the zero address");

    _balances[account] = sub(_balances[account], value);
    _totalSupply = sub(_totalSupply, value);
  }

  function _burnFrom(address account, uint256 amount) internal {
    _burn(account, amount);
    _approve(account, msg.sender, sub(_allowances[account][msg.sender], amount));
  }

  constructor() public {
    _mint(msg.sender, 1000);
  }

}