What is the "^=" operator?

Question

I found this weird gist online for generating random numbers. Please remember that, despite the code's comments, generating random numbers on-chain is insecure for values more than 2.5 ETH.

// this labyrinth contract as a public utility can be used as an "entropy crawler", 
// using the state itself as a source of entropy (generated entirely from human social coordination 
// that is highly random. )

contract Labyrinth {

  uint entropy;

  function getRandomNumber() public returns (uint) {
    entropy ^= uint(blockhash(entropy % block.number));
    return entropy;
  }

}

What is the "^=" operator and what does it do? I tried searching in javascript, solidity, and other languages through Google but nothing is coming up.

Source: https://gist.github.com/resilience-me/8e877bc39a9f8d86f17f08bc62d71d70

Answer 1

This is perhaps more of a general programming question than anything specific to Solidity...

It's a bitwise exclusive OR (XOR - ^), combined with an assignment operator (=).

a ^= b is equivalent to a = a ^ b, in a similar way to a = a + 1 being written a += 1.