Implementation of Fisher-Yates Shuffle in solidity.

Shuffle.sol

pragma solidity ^0.8.0;

contract Shuffle {

    uint256[] public shuffle = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47];
    uint256 public entropy = block.timestamp;

    function shuffleArray() public {

        for(uint256 i = shuffle.length -1 ; i > 0; i--) {

            uint256 swapIndex = entropy % (shuffle.length - i);

            uint256 currentIndex = shuffle[i];
            uint256 indexToSwap = shuffle[swapIndex];

            shuffle[i] = indexToSwap;
            shuffle[swapIndex] = currentIndex;
        }
    }

}

Very clever! But I'm rather certain that there's a bug here.

As of now in the first iteration i == shuffle.length - 1, so

entropy % (shuffle.length - i)
==
entropy % (shuffle.length - shuffle.length + 1)
==
entropy % 1
==
0

It means that the last array item is always swapped with the first one, but it should be swapped with a random item in the array, including itself.

I think that the fix is to:

- uint256 swapIndex = entropy % (shuffle.length - i);
+ uint256 swapIndex = entropy % (i + 1);