Gas optimisation for my art experiment spacecans.com

Question

so I was microdosing some research chems and the thought came to me that I need to do some artsy blockchain stuff for once. It's two tin cans floating in space that you can advertise your so-called shitcoins or whatever else on. It's currently running on the Rinkeby testnet. You can buy parts of the labels of these tin cans with Ether, put a custom image on them, and link it to a website.

However my smart contract still needs some optimisation regarding gas and security. Currently the largest area one can buy in one transaction is 8x8 parts out of 32x32 total, because the buy function apparently consumes too much gas otherwise. (is this because of the block size limit?)

Any tips for a solidity newbie regarding gas optimisation? Thanks!

Here's my smart contract on etherscan: https://rinkeby.etherscan.io/address/0x51d40569c946355b8e6c72ef0bcc85991ec273d2#code

Here's the website: https://spacecans.com

pragma solidity >=0.4.22 <0.6.0;

contract Main {

    address payable chairperson;
    uint8 public constant N = 32;
    uint8 public constant M = 32;
    uint256 public constant initialTabPrice = 1 finney;
    uint256 public constant feePercent = 15;
    uint256 public constant referralPercent = 30;
    uint256 public constant appreciationPercent = 200;
    Tab[N][M] public tabs;
    mapping (bytes32 => address payable) referralLinks;

    struct Tab {
        address payable owner;
        uint256 price; /// Price for next purchase
        bytes32[2] linkUrl;
        bytes32[3] imageUrl;

        /// Used for images spanning over multiple tabs with dimensions X,Y
        uint8 x; /// x index [0, X-1]
        uint8 X; 
        uint8 y; /// y index [0, Y-1]
        uint8 Y;
    }

    struct ToPay {
        address payable owner;
        uint256 amount;
    }

    constructor() public {
        chairperson = msg.sender;
    }

    function uint2str(uint i) internal pure returns (string memory){
        if (i == 0) return "0";
        uint j = i;
        uint length;
        while (j != 0){
            length++;
            j /= 10;
        }
        bytes memory bstr = new bytes(length);
        uint k = length - 1;
        while (i != 0){
            bstr[k--] = byte(uint8(48 + i % 10));
            i /= 10;
        }
        return string(bstr);
    }

    function bytes32ToString(bytes32 x) internal pure returns (string memory) {
        bytes memory bytesString = new bytes(32);
        uint charCount = 0;
        for (uint j = 0; j < 32; j++) {
            byte char = byte(bytes32(uint(x) * 2 ** (8 * j)));
            if (char != 0) {
                bytesString[charCount] = char;
                charCount++;
            }
        }
        bytes memory bytesStringTrimmed = new bytes(charCount);
        for (uint j = 0; j < charCount; j++) {
            bytesStringTrimmed[j] = bytesString[j];
        }
        return string(bytesStringTrimmed);
    }

    function getTabs(uint8 x1, uint8 y1, uint8 x2, uint8 y2) external view returns(string memory) {
        string memory res = "";
        for (uint8 yt = y1; yt <= y2; yt++) {
            for (uint8 xt = x1; xt <= x2; xt++) {
                 res = string(abi.encodePacked(res, ",", uint2str(getTabPrice(xt,yt)),  ",", bytes32ToString(tabs[xt][yt].linkUrl[0]), bytes32ToString(tabs[xt][yt].linkUrl[1]), ",", bytes32ToString(tabs[xt][yt].imageUrl[0]), bytes32ToString(tabs[xt][yt].imageUrl[1]), bytes32ToString(tabs[xt][yt].imageUrl[2]), "#", uint2str(tabs[xt][yt].x), ".", uint2str(tabs[xt][yt].X), ".", uint2str(tabs[xt][yt].y), ".", uint2str(tabs[xt][yt].Y)));
            }
        }  
        return res;
    }

    function getTabPrice(uint8 x, uint8 y) internal view returns(uint256) {
        if (tabs[x][y].price == 0) {
            return initialTabPrice;
        }
        return tabs[x][y].price;
    }

    function getTabOwner(uint8 x, uint8 y) internal view returns(address payable) {
        if (tabs[x][y].price == 0) {
            return chairperson;
        }
        return tabs[x][y].owner;
    }
    // string calldata linkUrl, string calldata imageUrl, string calldata referral

    function buyTabs(uint8[4] calldata z, bytes32[6] calldata s) external payable {
        /// z: x1, y1, x2, y2
        /// Calculate combined price for asked tabs
        uint8 xt;
        uint8 yt;
        uint256 price = 0;
        uint256 tabPrice;
        address payable tabOwner;

        for (xt = z[0]; xt <= z[2]; xt++) {
            for (yt = z[1]; yt <= z[3]; yt++) {
                price += getTabPrice(xt,yt);
            }
        }

        /// Check if payment is sufficient
        assert(msg.value >= price);

        ToPay[1024] memory toPay;

        /// Pay previous owner and assign new owner and new price
        for (xt = z[0]; xt <= z[2]; xt++) {
            for (yt = z[1]; yt <= z[3]; yt++) {
                tabPrice = getTabPrice(xt,yt);
                tabOwner = getTabOwner(xt,yt);

                //tabOwner.transfer((tabPrice * (100-feePercent)) / 100);
                for(uint i=0; i<=toPay.length;i++) {
                    if (toPay[i].amount == 0) {
                        //toPay[i] = ToPay(tabOwner, (tabPrice * (100-feePercent)) / 100);
                        toPay[i].owner = tabOwner;
                        toPay[i].amount = (tabPrice * (100-feePercent)) / 100;
                        break;
                    }

                    if (toPay[i].owner == tabOwner) {
                        toPay[i].amount += (tabPrice * (100-feePercent)) / 100;
                        break;
                    }
                }

                tabs[xt][yt] = Tab(msg.sender, tabPrice * appreciationPercent / 100, [s[0], s[1]] , [s[2], s[3], s[4]], xt-z[0], z[2]-z[0]+1, yt-z[1], z[3]-z[1]+1);
            }
        }

        for(uint i=0; i<=toPay.length;i++) {
            if (toPay[i].amount == 0) {
                break;
            }
            toPay[i].owner.transfer(toPay[i].amount);
        }

        if (s[5][0] != 0) {
            referralLinks[s[5]].transfer(address(this).balance * referralPercent / 100);
        }

        /// Remaining funds are fees
        chairperson.transfer(address(this).balance);
    }

    function setReferralLink(bytes32 linkName) external {
        referralLinks[linkName] = msg.sender;
    }

    function retrieveLostEther() public {
        assert(msg.sender == chairperson);
        msg.sender.transfer(address(this).balance);
    }
}

Answer 1

Loops often increase the use of gas, moreover if you need to perform searches. In your contract, there are several of them, including a nested one in function getTabs. Try to change to data structure to avoid using them at all.