SimpleToken.sol

pragma solidity ^0.4.23;

contract SimpleToken {
    string public name = "SimpleToken";
    string public symbol = "SMPT";
    uint8 public decimals = 18;

    uint256 public totalSupply = 1e24;
    mapping (address => uint256) public balanceOf;
    mapping (address => mapping (address => uint256)) public allowance;

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

    constructor() public {
        balanceOf[msg.sender] = totalSupply;
        emit Transfer(0, msg.sender, totalSupply);
    }

    function transfer(address _to, uint256 _value) external returns (bool success) {
        _transfer(msg.sender, _to, _value);
        return true;
    }

    function transferFrom(address _from, address _to, uint256 _value) external returns (bool success) {
        require(allowance[_from][msg.sender] >= _value);
        _transfer(_from, _to, _value);
        allowance[_from][msg.sender] -= _value;
        return true;
    }

    function approve(address _spender, uint256 _value) external returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
        return true;
    }

    function _transfer(address _from, address _to, uint256 _value) private {
        require(balanceOf[_from] >= _value);
        balanceOf[_from] -= _value;
        balanceOf[_to] += _value;
        emit Transfer(_from, _to, _value);
    }
}