// SPDX-License-Identifier: MIT

pragma solidity >=0.4.22 <0.7.0;

contract TokenERC20 {
    string public name;
    string public symbol;
    uint8 public decimals = 18;  
    uint256 public totalSupply;


    mapping (address => uint256) public balanceOf;

    mapping (address => mapping (address => uint256)) public allowance;


    event Transfer(address indexed from, address indexed to, uint256 value);


    event Burn(address indexed from, uint256 value);


    constructor(uint256 initialSupply, string memory tokenName, string memory tokenSymbol) public {
        totalSupply = initialSupply * 10 ** uint256(decimals);  
        balanceOf[msg.sender] = totalSupply;                
        name = tokenName;
        symbol = tokenSymbol;
    }


    function _transfer(address _from, address _to, uint _value) internal {

        require(_to != address(0x0));

        require(balanceOf[_from] >= _value);

        require(balanceOf[_to] + _value > balanceOf[_to]);


        uint previousBalances = balanceOf[_from] + balanceOf[_to];
        // Subtract from the sender
        balanceOf[_from] -= _value;
        // Add the same to the recipient
        balanceOf[_to] += _value;
        emit Transfer(_from, _to, _value);

      
        assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
    }


    function transfer(address _to, uint256 _value) public {
        _transfer(msg.sender, _to, _value);
    }

   
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowance[_from][msg.sender]);     // Check allowance
        allowance[_from][msg.sender] -= _value;
        
        _transfer(_from, _to, _value);
        return true;
    }

   
    function approve(address _spender, uint256 _value) public returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        return true;
    }

    

    function burn(uint256 _value) public returns (bool success) {
        require(balanceOf[msg.sender] >= _value);   // Check if the sender has enough
        balanceOf[msg.sender] -= _value;            // Subtract from the sender
        totalSupply -= _value;                      // Updates totalSupply
        emit Burn(msg.sender, _value);
        return true;
    }


    function burnFrom(address _from, uint256 _value) public returns (bool success) {
        require(balanceOf[_from] >= _value);                // Check if the targeted balance is enough
        require(_value <= allowance[_from][msg.sender]);    // Check allowance
        balanceOf[_from] -= _value;                         // Subtract from the targeted balance
        allowance[_from][msg.sender] -= _value;             // Subtract from the sender's allowance
        totalSupply -= _value;                              // Update totalSupply
        emit Burn(_from, _value);
        return true;
    }
}