// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

// Importing the ERC20 interface from OpenZeppelin, a library of secure smart contracts.
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

// Extending the IERC20 interface with a function to get the decimals of the token.
interface IExtendedERC20 is IERC20 {
    function decimals() external view returns (uint8);
}

// Contract for selling tokens in exchange for USDT and USDC.
contract SaleToken {
    // Token being sold in this contract.
    IExtendedERC20 public immutable token;
    // USDC token contract address.
    IERC20 public immutable usdcToken;
    // USDT token contract address.
    IERC20 public immutable usdtToken;
    // Admin address - only this address can perform certain administrative functions.
    address public immutable admin;
    // Price of the sale token in terms of USD.
    uint256 public tokenPriceInUSD;
    // Decimals of the sale token.
    uint8 private immutable tokenDecimals;
    // Constant representing the number of decimals in USDT and USDC (assumed to be 6).
    uint8 private constant USD_DECIMALS = 6;

    // Event to log the sale of tokens.
    event Sell(address indexed buyer, uint256 amount);

    // Modifier to restrict function execution to only the admin.
//    modifier onlyAdmin() {
//        require(msg.sender == admin, "Only admin can perform this action");
//        _;
//    }

    // Constructor to set initial values of the contract.
    constructor(IExtendedERC20 _token, IERC20 _usdtToken, IERC20 _usdcToken, uint256 _tokenPriceInUSD) {
        admin = msg.sender;
        token = _token;
        usdtToken = _usdtToken;
        usdcToken = _usdcToken;
        tokenPriceInUSD = _tokenPriceInUSD;
        tokenDecimals = _token.decimals();
    }

//    // Function to update the token price, only callable by the admin.
//    function setTokenPriceInUSD(uint256 _newPrice) external onlyAdmin {
//        tokenPriceInUSD = _newPrice;
//    }

    // Function to buy tokens with USDT.
    function buyTokensInUSDT(uint256 _numberOfTokens) external {
        _buyTokens(usdtToken, _numberOfTokens);
    }

    // Function to buy tokens with USDC.
    function buyTokensInUSDC(uint256 _numberOfTokens) external {
        _buyTokens(usdcToken, _numberOfTokens);
    }

    // Internal function to handle the purchase of tokens.
    function _buyTokens(IERC20 usdToken, uint256 _numberOfTokens) private {
        uint256 requiredUSD = calculateRequiredUSD(_numberOfTokens);
        validateTransferRequirements(usdToken, requiredUSD);
        performTokenTransfer(usdToken, requiredUSD, _numberOfTokens);
        emit Sell(msg.sender, _numberOfTokens);
    }

    // Function to calculate the required amount of USD for the given number of tokens.
    function calculateRequiredUSD(uint256 _numberOfTokens) private view returns (uint256) {
        return (_numberOfTokens * tokenPriceInUSD * 10 ** uint256(tokenDecimals)) / (10 ** uint256(tokenDecimals) * 10 ** uint256(USD_DECIMALS));
    }

    // Function to validate if the buyer has enough balance and allowance to buy tokens.
    function validateTransferRequirements(IERC20 usdToken, uint256 requiredUSD) private view {
        require(usdToken.balanceOf(msg.sender) >= requiredUSD, "Insufficient balance");
        require(usdToken.allowance(msg.sender, address(this)) >= requiredUSD, "Transfer not allowed");
    }

    // Function to perform the transfer of tokens from buyer to admin and from contract to buyer.
    function performTokenTransfer(IERC20 usdToken, uint256 requiredUSD, uint256 _numberOfTokens) private {
        require(token.balanceOf(address(this)) >= _numberOfTokens, "Not enough tokens");
        require(usdToken.transferFrom(msg.sender, admin, requiredUSD), "Transfer failed");
        require(token.transfer(msg.sender, _numberOfTokens), "Token transfer failed");
    }
}
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