Exchange Rate System Contract

// SPDX-License-Identifier: Apache-2.0
pragma solidity >=0.4.9 <0.9.0;

interface IExchangeRate {
    // Given a value in tinycents (10^-8 US cents), returns the equivalent
    // value in tinybars at the current network exchange rate.
    function tinycentsToTinybars(uint256 tinycents) external returns (uint256);

    // Given a value in tinybars, returns the equivalent value in tinycents
    // at the current network exchange rate.
    function tinybarsToTinycents(uint256 tinybars) external returns (uint256);
}

// SPDX-License-Identifier: Apache-2.0
pragma solidity ^0.8.22;

interface IExchangeRate {
    function tinycentsToTinybars(uint256 tinycents) external returns (uint256);
}

contract UsdPricedStore {
    // The Exchange Rate system contract address.
    address constant EXCHANGE_RATE = address(0x168);

    // 10 cents = 10 * 10^8 tinycents = 1_000_000_000.
    uint256 constant TEN_CENTS_IN_TINYCENTS = 10 * 10**8;

    event Purchased(address indexed buyer, uint256 paidTinybars);

    function purchase() external payable {
        // Ask the network how many tinybars are equivalent to $0.10 right now.
        uint256 requiredTinybars =
            IExchangeRate(EXCHANGE_RATE).tinycentsToTinybars(TEN_CENTS_IN_TINYCENTS);

        // msg.value is in 18-decimal wei. Convert tinybars (8 decimals) to wei
        // by multiplying by 10^10 so the units line up.
        uint256 requiredWei = requiredTinybars * 10**10;
        require(msg.value >= requiredWei, "insufficient HBAR for $0.10");

        emit Purchased(msg.sender, requiredTinybars);
    }
}