Hash of address pair combination where order is irrelevant

Question

I have a contract where I often set stuff for a combination of 2 addresses. Because the parameters addressA and addressB are interchangeable I currently save the information redundantly.

Example:

    mapping ( address => mapping ( address => uint256 )) public maxNextUpdateInBlocks;

    function configureCurrencyPair(address _currencyA, address _currencyB, uint256 maxNextUpdateInBlocks) public onlyOwner {
        maxNextUpdateInBlocks[_currencyA][_currencyB] = maxNextUpdateInBlocks;
        maxNextUpdateInBlocks[_currencyB][_currencyA] = maxNextUpdateInBlocks;
    } 

Can this be improved? I am thinking about a hash of the address combination where it doesn't matter if I hash currencyA+currencyB or currencyB+currencyA

And just set in one line:

maxNextUpdateInBlocks[currencyAB] = maxNextUpdateInBlocks;

Answer 1

You could order the addresses first before storing them:

mapping(bytes32 => uint256) maxNextUpdateInBlocks;

function configureCurrencyPair(address _currencyA, address _currencyB, uint256 _maxNextUpdateInBlocks) public onlyOwner {
    bytes32 key;
    if(_currencyA > _currencyB)
        key = keccak256(_currencyA, _currencyB);
    else
        key = keccak256(_currencyB, _currencyA);

    maxNextUpdateInBlocks[key] = _maxNextUpdateInBlocks
} 

This can probably be done in a cleaner way, but you'll get the idea.

Gas cost: 20593

==EDIT==

As Raghav Sood commented, it's also possible to skip hashing:

mapping(address => mapping(address=> uint256)) maxNextUpdateInBlocks;

function configureCurrencyPair(address _currencyA, address _currencyB, uint256 _maxNextUpdateInBlocks) public onlyOwner {

    if(_currencyA > _currencyB)
        maxNextUpdateInBlocks[_currencyA][_currencyB] = _maxNextUpdateInBlocks;
    else
        maxNextUpdateInBlocks[_currencyB][_currencyA] = _maxNextUpdateInBlocks;
} 

This is a bit less expensive: 20525 Gas