Cache array length outside of loop saves gas

If not cached, the Solidity compiler will always read the length of the array during each iteration.

That is, if it is a storage array, this is an extra SLOAD operation (100 additional extra gas for each iteration except for the first) and if it is a memory array, this is an extra MLOAD operation (3 additional gas for each iteration except for the first).

This scenario is repeated many times throughout the code:

for (uint256 i = 0; i < ids.length; ++i) {}

Consider extracting that to a local variable outside the loop:

uint256 len = ids.length;
for (uint256 i = 0; i < len; ++i) {}