MatchingDonations.sol

//SPDX-License-Identifier: AFL-3.0 (Academic Free License v3.0) https://opensource.org/licenses/afl-3.0
//code by Harsh Strongman from lifemathmoney.com, the #1 self-improvement website for men

//solidity version specifier
pragma solidity 0.8.1;

//A philanthropist has agreed to send matching contributions for all amounts received from the public
//Given: the address of the philanthropist
//philanthropist sends his maximum donation first, and only then money is accepted from the public
//contract can be ended after 30 days, where any remaining balance of the philanthropist is refunded to him

contract donation {

    //setting up global variables
    // DO NOT SEND ETH TO THESE ADDRESS - THE PRIVATE KEYS HAVE BEEN BURNED AND YOUR ETH WILL BE LOST FOREVER
    // if you deploy this code, make sure you change these addresses
    address payable addressOfPhilanthropist = payable(0x6ed0B157827d1F04EcE69e4BB19FBEB652f23A0d);
    address payable addressOfCharity = payable(0x92893c3fEb851bc5D75f976bfA7713BEdD1E0Eb3);
    uint receiptFromPhilanthropist;
    uint receiptFromOthers;
    uint startTime;

    //constructor to set the start time when the contract is created
    constructor() {
        startTime = block.timestamp;
    }


    //receive ether and maintain separate totals for receipts from the philanthropist and from everyone else
    receive() external payable {
        if (msg.sender == addressOfPhilanthropist) {
            receiptFromPhilanthropist += msg.value;
        }
        else {
            require((receiptFromOthers + msg.value) > receiptFromOthers); //protecting against overflow
            receiptFromOthers += msg.value;
            require((receiptFromOthers * 2) > receiptFromOthers); //protecting against overflow
        }
    }

    function withdraw() public {

        //check if 30 days have passed
        require((block.timestamp - startTime) > 30 days, "30 days have not yet passed");

        //send matching donation and refund the balance to the philanthropist
        if (receiptFromOthers < receiptFromPhilanthropist) {
            addressOfCharity.transfer((receiptFromOthers * 2));
            addressOfPhilanthropist.transfer(address(this).balance);
        }
        else {
            addressOfCharity.transfer(address(this).balance);
        }
    }
}