// SPDX-License-Identifier: MIT

pragma solidity 0.7.6;

import "@openzeppelin/contracts/utils/Context.sol";

import "./interfaces/IVeERC20.sol";

/// @title VeERC20
/// @notice Modified version of ERC20 where transfers and allowances are disabled.
/// @dev Only minting and burning are allowed. The hook `_beforeTokenOperation` and
/// `_afterTokenOperation` methods are called before and after minting/burning respectively.
contract VeERC20 is Context, IVeERC20 {
    mapping(address => uint256) private _balances;

    uint256 private _totalSupply;

    string private _name;
    string private _symbol;

    /// @dev Emitted when `value` tokens are burned and minted
    event Burn(address indexed account, uint256 value);
    event Mint(address indexed beneficiary, uint256 value);

    /**
     * @dev Sets the values for {name} and {symbol}.
     *
     * The default value of {decimals} is 18. To select a different value for
     * {decimals} you should overload it.
     *
     * All two of these values are immutable: they can only be set once during
     * construction.
     */
    constructor(string memory name_, string memory symbol_) {
        _name = name_;
        _symbol = symbol_;
    }

    /**
     * @dev Returns the name of the token.
     */
    function name() public view virtual returns (string memory) {
        return _name;
    }

    /**
     * @dev Returns the symbol of the token, usually a shorter version of the
     * name.
     */
    function symbol() public view virtual returns (string memory) {
        return _symbol;
    }

    /**
     * @dev Returns the number of decimals used to get its user representation.
     * For example, if `decimals` equals `2`, a balance of `505` tokens should
     * be displayed to a user as `5.05` (`505 / 10 ** 2`).
     *
     * Tokens usually opt for a value of 18, imitating the relationship between
     * Ether and Wei. This is the value {ERC20} uses, unless this function is
     * overridden;
     *
     * NOTE: This information is only used for _display_ purposes: it in
     * no way affects any of the arithmetic of the contract, including
     * {IERC20-balanceOf} and {IERC20-transfer}.
     */
    function decimals() public view virtual returns (uint8) {
        return 18;
    }

    /**
     * @dev See {IERC20-totalSupply}.
     */
    function totalSupply() public view virtual override returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See {IERC20-balanceOf}.
     */
    function balanceOf(address account) public view virtual override returns (uint256) {
        return _balances[account];
    }

    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
     * the total supply.
     *
     * Emits a {Transfer} event with `from` set to the zero address.
     *
     * Requirements:
     *
     * - `account` cannot be the zero address.
     */
    function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenOperation(address(0), account, amount);

        _totalSupply += amount;
        _balances[account] += amount;
        emit Mint(account, amount);

        _afterTokenOperation(account, _balances[account]);
    }

    /**
     * @dev Destroys `amount` tokens from `account`, reducing the
     * total supply.
     *
     * Emits a {Transfer} event with `to` set to the zero address.
     *
     * Requirements:
     *
     * - `account` cannot be the zero address.
     * - `account` must have at least `amount` tokens.
     */
    function _burn(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");

        _beforeTokenOperation(account, address(0), amount);

        uint256 accountBalance = _balances[account];
        require(accountBalance >= amount, "ERC20: burn amount exceeds balance");
        _balances[account] = accountBalance - amount;
        _totalSupply -= amount;

        emit Burn(account, amount);

        _afterTokenOperation(account, _balances[account]);
    }

    /**
     * @dev Hook that is called before any minting and burning.
     * @param from the account transferring tokens
     * @param to the account receiving tokens
     * @param amount the amount being minted or burned
     */
    function _beforeTokenOperation(
        address from,
        address to,
        uint256 amount
    ) internal virtual {}

    /**
     * @dev Hook that is called after any minting and burning.
     * @param account the account being affected
     * @param newBalance the new balance of `account` after minting/burning
     */
    function _afterTokenOperation(address account, uint256 newBalance) internal virtual {}
}