BuyPairsWithSignedOrders.sol

/*
    This contract is a POC that allows anyone to buy a crypto pair like DAI:ETH via a signed order from a market maker.

    Example:
    Alice is a crypto whale with a lot of ETH and is willing to sell that ETH for DAI
    Bob is a HODLER that wants to buy DAI:ETH once it hits a certain price.

    - Alice and Bob are willing to trade at 1 ETH to 200 DAI so Bob sends a request to alice to buy 1 eth for 200 dai
    - Alice's bot automatically accepts this offer as it hits her desired price range so she signs a message that states "Sell 1 ETH from my balance to Bob for 200 DAI, expiring in 5 minutes"
    - Bob receives the approved request and submits it to this contract within the expiry. 200 DAI is deducted from his balance and 1 ETH is deducted from Alice's balance.

*/

pragma solidity ^0.5.11;

interface Token {
    function allowance(address _owner, address _spender) external view returns (uint256 remaining);
    function transferFrom(address _from, address _to, uint256 _value) external returns (bool success);
}

contract BuyPairsWithSignedOrders {

    constructor() public {

    }

    /*
       Swap a pair like ETH:DAI via a signed order
       @Requires this contract is approved to move funds from the buyer and seller
       @Param pair1 the address of the asset that the seller wishes to sell
       @Param pair2 the address of the asset that the buyer is willing to use to buy the sellers asset
       @Param the amount of pair1 that the seller is willing to sell
       @Param the price of pair1 in relation to pair2
       @Param expiry, the timestamp for which this deal must be executed within (in block time)
       @Param v: signature param for signed message
       @Param r: signature param for signed message
       @Param s: signature param for signed message
    */
    function swapPair(
        address pair1,
        address pair2,
        uint256 amount,
        uint256 price,
        uint256 expiry,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) public returns(bool) {
        //order cannot be expired
        require(expiry >= block.timestamp);
        bytes32 messageHash = keccak256(abi.encodePacked(pair1, pair2, price, msg.sender));
        address seller = ecrecover(messageHash, v, r, s);
        Token tokenPair1 = Token(pair1);
        Token tokenPair2 = Token(pair2);
        //check that the seller has enough to sell and has approved this contract to move the funds
        require(tokenPair1.allowance(seller, address(this)) >= amount);
        //check that the buyer has enough to buy and has approved this contract to move the funds
        require(tokenPair2.allowance(msg.sender, address(this)) >= (amount * price));
        //do trade
        require(tokenPair1.transferFrom(seller, msg.sender, amount));
        require(tokenPair2.transferFrom(msg.sender, seller, (amount * price)));
        return true;
    }

}