UniswapV2Pair.sol

contract UniswapV2Pair {
  // Contract Storage Variables:
  uint public price0CumulativeLast;
  uint public price1CumulativeLast;
...
  // The only place these storage variables are updated:
  function _update(uint balance0, uint balance1, uint112 _reserve0, uint112 _reserve1) private {
    uint32 timeElapsed = blockTimestamp - blockTimestampLast;
    if (timeElapsed > 0 && _reserve0 != 0 && _reserve1 != 0) {
      price0CumulativeLast += uint(UQ112x112.encode(_reserve1).uqdiv(_reserve0)) * timeElapsed;
      price1CumulativeLast += uint(UQ112x112.encode(_reserve0).uqdiv(_reserve1)) * timeElapsed;
    }
    blockTimestampLast = blockTimestamp;
  }
}