Is the gas usage O(1) for this code?

Question

Is the gas usage constant for this function regardless of mapping/array size (assuming non-zero array entries)?

items is an array, and presence is a mapping

function clear() public {
    for (uint i = 0; i < items.length; i++) {
        presence[items[i]] = 0;
    }

    delete items;
}

In other words, is the gas refund from deleting the data greater than the gas consumed when iterating and querying the data?

This function is a snippet from this Set implementation.

Answer 1

A quick analysis shows that the function call gas usage isn't O(1) and also isn't close to O(1). After more than 2 items, the gas usage starts rising for every additional item.

Here is a table of gas usage for clearing a varying number of items (tested using https://remix.ethereum.org):

Number of items	Gas used to clear items
0	23823
1	18644
2	23976
3	29308
4	34640