// src/core/contracts/libraries/AdaptiveFee.sol
  function exp(
    uint256 x,
    uint16 g,
    uint256 gHighestDegree
  ) internal pure returns (uint256 res) {
    // calculating:
    // g**8 + x * g**7 + (x**2 * g**6) / 2 + (x**3 * g**5) / 6 + (x**4 * g**4) / 24 + (x**5 * g**3) / 120 + (x**6 * g^2) / 720 + x**7 * g / 5040 + x**8 / 40320

    // x**8 < 152 bits (19*8) and g**8 < 128 bits (8*16)
    // so each summand < 152 bits and res < 155 bits
    uint256 xLowestDegree = x;
    res = gHighestDegree; // g**8

    gHighestDegree /= g; // g**7
    res += xLowestDegree * gHighestDegree;

    gHighestDegree /= g; // g**6
    xLowestDegree *= x; // x**2
    res += (xLowestDegree * gHighestDegree) / 2;

    gHighestDegree /= g; // g**5
    xLowestDegree *= x; // x**3
    res += (xLowestDegree * gHighestDegree) / 6;

    gHighestDegree /= g; // g**4
    xLowestDegree *= x; // x**4
    res += (xLowestDegree * gHighestDegree) / 24;

    gHighestDegree /= g; // g**3
    xLowestDegree *= x; // x**5
    res += (xLowestDegree * gHighestDegree) / 120;

    gHighestDegree /= g; // g**2
    xLowestDegree *= x; // x**6
    res += (xLowestDegree * gHighestDegree) / 720;

    xLowestDegree *= x; // x**7
    res += (xLowestDegree * g) / 5040 + (xLowestDegree * x) / (40320);
  }
