// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
  /**
    * @dev Returns the amount of tokens in existence.
    */
  function totalSupply() external view returns (uint256);

  /**
    * @dev Returns the amount of tokens owned by `account`.
    */
  function balanceOf(address account) external view returns (uint256);

  /**
    * @dev Moves `amount` tokens from the caller's account to `recipient`.
    *
    * Returns a boolean value indicating whether the operation succeeded.
    *
    * Emits a {Transfer} event.
    */
  function transfer(address recipient, uint256 amount) external returns (bool);

  /**
    * @dev Returns the remaining number of tokens that `spender` will be
    * allowed to spend on behalf of `owner` through {transferFrom}. This is
    * zero by default.
    *
    * This value changes when {approve} or {transferFrom} are called.
    */
  function allowance(address owner, address spender) external view returns (uint256);

  /**
    * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
    *
    * Returns a boolean value indicating whether the operation succeeded.
    *
    * IMPORTANT: Beware that changing an allowance with this method brings the risk
    * that someone may use both the old and the new allowance by unfortunate
    * transaction ordering. One possible solution to mitigate this race
    * condition is to first reduce the spender's allowance to 0 and set the
    * desired value afterwards:
    * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
    *
    * Emits an {Approval} event.
    */
  function approve(address spender, uint256 amount) external returns (bool);

  /**
    * @dev Moves `amount` tokens from `sender` to `recipient` using the
    * allowance mechanism. `amount` is then deducted from the caller's
    * allowance.
    *
    * Returns a boolean value indicating whether the operation succeeded.
    *
    * Emits a {Transfer} event.
    */
  function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

  /**
    * @dev Emitted when `value` tokens are moved from one account (`from`) to
    * another (`to`).
    *
    * Note that `value` may be zero.
    */
  event Transfer(address indexed from, address indexed to, uint256 value);

  /**
    * @dev Emitted when the allowance of a `spender` for an `owner` is set by
    * a call to {approve}. `value` is the new allowance.
    */
  event Approval(address indexed owner, address indexed spender, uint256 value);
}

/**
 * @dev Interface for the optional metadata functions from the ERC20 standard.
 *
 * _Available since v4.1._
 */
interface IERC20Metadata is IERC20 {
  /**
    * @dev Returns the name of the token.
    */
  function name() external view returns (string memory);

  /**
    * @dev Returns the symbol of the token.
    */
  function symbol() external view returns (string memory);

  /**
    * @dev Returns the decimals places of the token.
    */
  function decimals() external view returns (uint8);
}

/*
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
  function _msgSender() internal view virtual returns (address) {
    return msg.sender;
  }

  function _msgData() internal view virtual returns (bytes calldata) {
    this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
    return msg.data;
  }
}

/**
 * @dev Implementation of the {IERC20} interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using {_mint}.
 * For a generic mechanism see {ERC20PresetMinterPauser}.
 *
 * TIP: For a detailed writeup see our guide
 * https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How
 * to implement supply mechanisms].
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an {Approval} event is emitted on calls to {transferFrom}.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard {decreaseAllowance} and {increaseAllowance}
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See {IERC20-approve}.
 */
contract ERC20 is Context, IERC20, IERC20Metadata {
  mapping (address => uint256) private _balances;

  mapping (address => mapping (address => uint256)) private _allowances;

  uint256 private _totalSupply;

  string private _name;
  string private _symbol;

  /**
    * @dev Sets the values for {name} and {symbol}.
    *
    * The defaut 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 override returns (string memory) {
    return _name;
  }

  /**
    * @dev Returns the symbol of the token, usually a shorter version of the
    * name.
    */
  function symbol() public view virtual override 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 override 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 See {IERC20-transfer}.
    *
    * Requirements:
    *
    * - `recipient` cannot be the zero address.
    * - the caller must have a balance of at least `amount`.
    */
  function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
    _transfer(_msgSender(), recipient, amount);
    return true;
  }

  /**
    * @dev See {IERC20-allowance}.
    */
  function allowance(address owner, address spender) public view virtual override returns (uint256) {
    return _allowances[owner][spender];
  }

  /**
    * @dev See {IERC20-approve}.
    *
    * Requirements:
    *
    * - `spender` cannot be the zero address.
    */
  function approve(address spender, uint256 amount) public virtual override returns (bool) {
    _approve(_msgSender(), spender, amount);
    return true;
  }

  /**
    * @dev See {IERC20-transferFrom}.
    *
    * Emits an {Approval} event indicating the updated allowance. This is not
    * required by the EIP. See the note at the beginning of {ERC20}.
    *
    * Requirements:
    *
    * - `sender` and `recipient` cannot be the zero address.
    * - `sender` must have a balance of at least `amount`.
    * - the caller must have allowance for ``sender``'s tokens of at least
    * `amount`.
    */
  function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {
    _transfer(sender, recipient, amount);

    uint256 currentAllowance = _allowances[sender][_msgSender()];
    require(currentAllowance >= amount, "ERC20: transfer amount exceeds allowance");
    _approve(sender, _msgSender(), currentAllowance - amount);

    return true;
  }

  /**
    * @dev Atomically increases the allowance granted to `spender` by the caller.
    *
    * This is an alternative to {approve} that can be used as a mitigation for
    * problems described in {IERC20-approve}.
    *
    * Emits an {Approval} event indicating the updated allowance.
    *
    * Requirements:
    *
    * - `spender` cannot be the zero address.
    */
  function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
    _approve(_msgSender(), spender, _allowances[_msgSender()][spender] + addedValue);
    return true;
  }

  /**
    * @dev Atomically decreases the allowance granted to `spender` by the caller.
    *
    * This is an alternative to {approve} that can be used as a mitigation for
    * problems described in {IERC20-approve}.
    *
    * Emits an {Approval} event indicating the updated allowance.
    *
    * Requirements:
    *
    * - `spender` cannot be the zero address.
    * - `spender` must have allowance for the caller of at least
    * `subtractedValue`.
    */
  function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
    uint256 currentAllowance = _allowances[_msgSender()][spender];
    require(currentAllowance >= subtractedValue, "ERC20: decreased allowance below zero");
    _approve(_msgSender(), spender, currentAllowance - subtractedValue);

    return true;
  }

  /**
    * @dev Moves tokens `amount` from `sender` to `recipient`.
    *
    * This is internal function is equivalent to {transfer}, and can be used to
    * e.g. implement automatic token fees, slashing mechanisms, etc.
    *
    * Emits a {Transfer} event.
    *
    * Requirements:
    *
    * - `sender` cannot be the zero address.
    * - `recipient` cannot be the zero address.
    * - `sender` must have a balance of at least `amount`.
    */
  function _transfer(address sender, address recipient, uint256 amount) internal virtual {
    require(sender != address(0), "ERC20: transfer from the zero address");
    require(recipient != address(0), "ERC20: transfer to the zero address");

    _beforeTokenTransfer(sender, recipient, amount);

    uint256 senderBalance = _balances[sender];
    require(senderBalance >= amount, "ERC20: transfer amount exceeds balance");
    _balances[sender] = senderBalance - amount;
    _balances[recipient] += amount;

    emit Transfer(sender, recipient, amount);
  }

  /** @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:
    *
    * - `to` cannot be the zero address.
    */
  function _mint(address account, uint256 amount) internal virtual {
    require(account != address(0), "ERC20: mint to the zero address");

    _beforeTokenTransfer(address(0), account, amount);

    _totalSupply += amount;
    _balances[account] += amount;
    emit Transfer(address(0), account, amount);
  }

  /**
    * @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");

    _beforeTokenTransfer(account, address(0), amount);

    uint256 accountBalance = _balances[account];
    require(accountBalance >= amount, "ERC20: burn amount exceeds balance");
    _balances[account] = accountBalance - amount;
    _totalSupply -= amount;

    emit Transfer(account, address(0), amount);
  }

  /**
    * @dev Sets `amount` as the allowance of `spender` over the `owner` s tokens.
    *
    * This internal function is equivalent to `approve`, and can be used to
    * e.g. set automatic allowances for certain subsystems, etc.
    *
    * Emits an {Approval} event.
    *
    * Requirements:
    *
    * - `owner` cannot be the zero address.
    * - `spender` cannot be the zero address.
    */
  function _approve(address owner, address spender, uint256 amount) internal virtual {
    require(owner != address(0), "ERC20: approve from the zero address");
    require(spender != address(0), "ERC20: approve to the zero address");

    _allowances[owner][spender] = amount;
    emit Approval(owner, spender, amount);
  }

  /**
    * @dev Hook that is called before any transfer of tokens. This includes
    * minting and burning.
    *
    * Calling conditions:
    *
    * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
    * will be to transferred to `to`.
    * - when `from` is zero, `amount` tokens will be minted for `to`.
    * - when `to` is zero, `amount` of ``from``'s tokens will be burned.
    * - `from` and `to` are never both zero.
    *
    * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
    */
  function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }
}

/**
 * @title ERC20Decimals
 * @dev Implementation of the ERC20Decimals. Extension of {ERC20} that adds decimals storage slot.
 */
abstract contract ERC20Decimals is ERC20 {
  uint8 immutable private _decimals;

  /**
    * @dev Sets the value of the `decimals`. This value is immutable, it can only be
    * set once during construction.
    */
  constructor (uint8 decimals_) {
    _decimals = decimals_;
  }

  function decimals() public view virtual override returns (uint8) {
    return _decimals;
  }
}

/**
 * @title PrivilegeERC20
 * @dev Implementation of the PRVG ("Privilege") ERC20 token contract
 * with facilities for redeeming the configured ERC20 token for an
 * equivalent amount of PRVG "Privilege" token.
 *
 * All 150,000,000 UBT can be approved and redeemed by calling `redeem(uint256 amount)`
 * prior to July 4, 2022 when the redeemable period expires. Any PRVG
 * which remain unredeemed by the outstanding circulating supply of UBT as
 * of July 4, 2022 will be burned.
 *
 * PRVG is required to make use of the permissionless provide.network.
 */
contract PrivilegeERC20 is ERC20Decimals {

  /**
   * @dev Emitted when `amount` of redeemable ERC20 tokens are exchanged
   * for the equivalent number of ERC20 tokens allocated as part of the
   * underlying `totalSupply` of this token contract.
   */
  event Redeem(address indexed sender, uint256 amount);

  // ERC20 token address of the redeemable token
  IERC20 public redeemableToken;

  // Expiry timestamp at which no further ERC20 token redemptions will be processed and the
  // unclaimed balance of the underlying `totalSupply` of this token contract will be burned
  uint256 public redemptionExpiryTimestamp;

  constructor (address _redeemableToken, uint256 _expiry)
    ERC20("Privilege", "PRVG")
    ERC20Decimals(8)
  {
    redeemableToken = IERC20(_redeemableToken);

    require(_expiry > block.timestamp + 2592000 seconds); // 1 month minimum redemption period
    redemptionExpiryTimestamp = _expiry;

    uint256 _totalSupply = 20000000000000000;
    _mint(address(this), redeemableToken.totalSupply());
    _mint(msg.sender, _totalSupply - redeemableToken.totalSupply());
  }

  /// @dev require the contract call to occur after the redeemable period
  modifier afterRedeemablePeriod() {
    require(block.timestamp > redemptionExpiryTimestamp, "redeemable period has not expired");
    _;
  }

  /// @dev require the contract call to occur during the redeemable period
  modifier duringRedeemablePeriod() {
    require(block.timestamp <= redemptionExpiryTimestamp, "redeemable period has expired");
    _;
  }

  /**
   * @dev Burn the unclaimed portion of the `totalSupply` of the underlying
   * token contract after the redeemable period has ended
   */
  function burnUnredeemed() external afterRedeemablePeriod {
    _burn(address(this), this.balanceOf(address(this)));
  }

  function decimals() public view virtual override returns (uint8) {
    return super.decimals();
  }

  /**
   * @dev Redeem the given `amount` of redeemable ERC20 tokens 1:1 for
   * PRVG and "burn" the redeemable token by transferring and forever
   * locking it up in this underlying token contract.
   */
  function redeem(uint256 amount) external duringRedeemablePeriod {
    require(redeemableToken.transferFrom(msg.sender, address(this), amount), "tx failed during attempted transfer from swappable token contract");
    require(this.transfer(msg.sender, amount), "tx failed during attempted transfer to sender");
    emit Redeem(msg.sender, amount);
  }
}