SimpleERC20WithOnchainData.sol

// SPDX-License-Identifier: Apache-2.0
// Author: Stephen Hodgkiss
// https://stephenhodgkiss.com

pragma solidity ^0.8.0;

/**
 * @title ERC20 Standard Token
 * @dev This is a standard ERC20 token with support for custom on-chain data.
 */
contract ERC20CustomData {
    
    // Token info
    string public name;
    string public symbol;
    uint8 public decimals;
    uint256 public totalSupply;
    
    // Custom on-chain data
    mapping (address => uint) public balances;
    mapping (address => mapping (address => uint)) public allowed;
    mapping (address => mapping (string => string)) public customData;
    
    // Events
    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
    event CustomDataUpdate(address indexed owner, string key, string value);
    
    // Constructor
    constructor (string memory _name, string memory _symbol, uint8 _decimals, uint256 _totalSupply) {
        name = _name;
        symbol = _symbol;
        decimals = _decimals;
        totalSupply = _totalSupply;
        balances[msg.sender] = totalSupply;
    }
    
    /**
     * @dev Function to transfer tokens
     * @param _to Address to which tokens are transferred
     * @param _value Amount of tokens to transfer
     */
    function transfer(address _to, uint256 _value) public {
        require(balances[msg.sender] >= _value);
        require(_to != address(0));
        balances[msg.sender] -= _value;
        balances[_to] += _value;
        emit Transfer(msg.sender, _to, _value);
    }
    
    /**
     * @dev Function to approve tokens for transfer
     * @param _spender Address to which tokens are approved
     * @param _value Amount of tokens to approve
     */
    function approve(address _spender, uint256 _value) public {
        require(_spender != address(0));
        allowed[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
    }
    
    /**
     * @dev Function to transfer tokens from one address to another
     * @param _from Address from which tokens are transferred
     * @param _to Address to which tokens are transferred
     * @param _value Amount of tokens to transfer
     */
    function transferFrom(address _from, address _to, uint256 _value) public {
        require(_from != address(0));
        require(_to != address(0));
        require(allowed[_from][msg.sender] >= _value);
        allowed[_from][msg.sender] -= _value;
        balances[_from] -= _value;
        balances[_to] += _value;
        emit Transfer(_from, _to, _value);
    }
    
    /**
     * @dev Function to update custom data on the chain
     * @param _owner Address of the owner of the data
     * @param _key Key for the data
     * @param _value Value for the data
     */
    function updateCustomData(address _owner, string memory _key, string memory _value) public {
        require(_owner != address(0));
        customData[_owner][_key] = _value;
        emit CustomDataUpdate(_owner, _key, _value);
    }
    
    /**
     * @dev Function to get custom data from the chain
     * @param _owner Address of the owner of the data
     * @param _key Key for the data
     * @return Value for the data
     */
    function getCustomData(address _owner, string memory _key) public view returns (string memory) {
        return customData[_owner][_key];
    }
    
    /**
     * @dev Function to get balance of an account
     * @param _owner Address of the account
     * @return Balance of the account
     */
    function getBalance(address _owner) public view returns (uint) {
        return balances[_owner];
    }
}