Gas: Improve validateWeights uniqueness check

[code423n4]

Handle

cmichel

Vulnerability details

The Basket.validateWeights function checks the uniqueness of the tokens by iterating over all tokens for each token.
This runs in O(_tokens.length^2) and is very inefficient as the number of tokens increases.

Recommended Mitigation Steps

Sort the tokens off-chain and provide them already in a sorted (ascending) way.
Then the validateWeights function only needs to verify that the tokens are indeed strictly sorted which runs in linear time:

for (uint i = 0; i < length; i++) {
    require(_tokens[i] != address(0));
    require(_weights[i] > 0);

    // check sorted to ensure uniqueness
    if (i > 0) {
        require(_tokens[i] > _tokens[i - 1]);
    }
}

[GalloDaSballo]

Agree with the findings, but want to point that the suggested improvement requires full trust of the input, which may not be a good idea

[GalloDaSballo]

Duplicate of #160