ERC20.sol

//SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.5.0 <0.9.0;

interface ERC20Interface {
    function totalSupply() external view returns (uint);
    function balanceOf(address tokenOwner) external view returns (uint balance);
    function transfer(address to, uint tokens) external returns (bool success);
    
    function allowance(address tokenOwner, address spender) external view returns (uint remaining);
    function approve(address spender, uint tokens) external returns (bool success);
    function transferFrom(address from, address to, uint tokens) external returns (bool success);
    
    event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
}

contract Cryptos is ERC20Interface {
    string public name = "Cryptos";
    string public symbol = "CRPT";
    uint public decimals = 0;
    uint public override totalSupply;

    address public founder;
    mapping(address => uint) public balances; 

    mapping(address => mapping(address => uint)) allowed;

    constructor(){
        totalSupply = 1000000;
        founder = msg.sender;
        balances[founder] = totalSupply;
    }

    function balanceOf(address tokenOwner) public view override returns (uint balance){
        return balances[tokenOwner];
    }

    function transfer(address to, uint tokens) public override returns (bool success) {
            require(balances[msg.sender] >= tokens);

            balances[to] += tokens;
            balances[msg.sender] -= tokens; 
            emit Transfer(msg.sender, to, tokens);

            return true;

    }

    function allowance(address tokenOwner, address spender) public view override returns (uint remaining) {
         return allowed[tokenOwner][spender];
    }

    function approve(address spender, uint tokens) public override returns (bool success) {
        require(balances[msg.sender] >= tokens);
        require(tokens > 0);
        allowed[msg.sender][spender] = tokens;
        emit Approval(msg.sender, spender, tokens);
        return true;
    }

    function transferFrom(address from, address to, uint tokens) public override returns (bool success) {
        require(allowed[from][to] >= tokens);
        require(balances[from] >= tokens);

        balances[from] -= tokens;
        balances[to] += tokens;
        allowed[from][to] -= tokens;

        return true;
    }
}