Too much gas consumption

Question

I'm programming my first Eth Contract, I'm facing a problem, the gas consumption ( estimated ) of the buy method is going really to high ( Quickly > N million gas before getting the "Maximum gas allowance exceeded" error. ) To be quick, the idea is the following: There is a map ( 2D map ) with multiple zones that you can own ( called units here, this is why I maintain a "unitsToState" obvious mapping ). You can purchase multiple adjacent zones at once, so a "Block" is created. So when you purchase a new Block the contract must check if all the units inside are empty ( unitsToState[x] == 0 ). When the block is purchased, these states are set to 1. I don't explain too much details here, because I guess the problem is mainly "Solidity" bad algorithm programming from me. This method can be executed with arround 500k gas for fromX, fromY, toX, toY that represent a small zone, but when those are far from each other, I got the "Maximum gas allowance exceeded" error during my gas estimation .. So really I guess there is a problem .. ``` ... struct Block { address owner; uint fromX; uint fromY; uint toX; uint toY; string imageUrl; string redirectUrl; string text; bool removed; } uint size = 100; mapping (uint => uint) unitsToState; Block[] public blocks; uint public areaPrice; uint public areaPerUnit; ... function buy(uint fromX, uint fromY, uint toX, uint toY, string imageUrl, string redirectUrl, string text) payable public { require(fromX >= 0); require(fromY >= 0); require(fromX <= toX); require(fromY <= toY); require(toX < size); require(toY < size); // Here do check of collisions. for (uint i = fromX; i <= toX; i++) { for (uint j = fromY; j <= toY; j++) { require(getUnitsToState(i*size*size + j) == 0); } } uint width = toX - fromX + 1; uint height = toY - fromY + 1; uint areaCount = width * height * areaPerUnit; uint price = areaCount * areaPrice; require(msg.value >= price); Block memory b = Block( msg.sender, fromX, fromY, toX, toY, imageUrl, redirectUrl, text, false ); blocks.push(b); // Registrer units states. for (i = fromX; i <= toX; i++) { for (j = fromY; j <= toY; j++) { unitsToState[i*size*size + j] = 1; } } } ... EDIT: The problem seems to be in greatest part from the part after "// Registrer units states.". But ... it doesn't really help me as long as I needs to do that ..

Rob Hitchens · Answer

Your iterative processes are increasing the cost of interactions relative to scale. This won't do. It's too expensive and will ultimately fail. More on why this is anti-pattern: https://blog.b9lab.com/getting-loopy-with-solidity-1d51794622ad

This part

// Here do check of collisions.
for (uint i = fromX; i <= toX; i++) {
    for (uint j = fromY; j <= toY; j++) {
        require(getUnitsToState(i*size*size + j) == 0);
    }    
}

can be refactored to complete in a single step (at any scale) by setting up a data structure to support a one-step lookup.

// x => y => value
mapping(uint => mapping(uint => uint)) public xy;

Set the non-zero values as you go.

This part can be engineered out of the design.

// Registrer units states.
for (i = fromX; i <= toX; i++) {
    for (j = fromY; j <= toY; j++) {
        unitsToState[i*size*size + j] = 1;
    }    
}

This is setting everything to a default value of 1. You automatically get a default value of 0 at no cost, so use that and create your own getter functions to implement the offset.

function setThing(uint value) public {
  thing = value - 1;
}

function getThing() public returns(uint) {
  return thing + 1;
}

Hope it helps.

user19510 · Answer

I suspect this is simply the cost of doing what you're doing. IIRC, the gas cost for changing a zero value to a non-zero is 20,000. A 10x10 block would require storing 10*10=100 ones, and 100*20000 is 2,000,000 gas.

Assuming that's roughly consistent with what you're seeing, that's simply what it costs to store this data.

Too much gas consumption

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